a review on applications of sol-gel science in cement

Construction and Building Materials 291 (2021) 123065

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Construction and Building Materials

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Review

A review on applications of sol-gel science in cement

https://doi.org/10.1016/j.conbuildmat.2021.1230650950-0618/� 2021 Elsevier Ltd. All rights reserved.

⇑ Corresponding author.E-mail addresses: [email protected] (P.A.K. Nair), [email protected] (W.L. Vasconcelos), [email protected] (K. Paine), [email protected] (J. Calabria-Hol

Pooja Anil Kumar Nair a,c, Wander Luiz Vasconcelos b, Kevin Paine a,c,Juliana Calabria-Holley a,c,⇑aDepartment of Architecture and Civil Engineering, Faculty of Engineering and Design, University of Bath, BA2 7AY Bath, United KingdombDepartment of Metallurgical and Materials Engineering, Universidade Federal de Minas Gerais (UFMG), Av. Antônio Carlos, 6627, Pampulha, Belo Horizonte, Minas Gerais, BrazilcBRE Centre for Innovative Construction Materials (BRE CICM), Department of Architecture and Civil Engineering, Faculty of Engineering and Design, University of Bath, BA2 7AYBath, United Kingdom

h i g h l i g h t s

� The benefits of using sol-gel technology to advance cement science.� The effects of silanes on the C-S-H structure.� Sol-gel technology applied to cement science and chemistry.� Sol-gel technology as a tool to tailor C-S-H and other hydration products at the nano-level.� The benefits of using sol-gel technology to advance cement science.� Future applications of sol-gel technology on cementitious materials.

a r t i c l e i n f o

Article history:Received 14 September 2020Received in revised form 4 March 2021Accepted 12 March 2021Available online 22 April 2021

Keywords:Sol-gel processCalcium-Silicate-Hydrate (C-S-H)HydrationSilanes

a b s t r a c t

This is the first review that presents an outlook on sol-gel science’s potential in different areas of cementscience and chemistry. Progress in nanotechnology has shown sol-gel science possibilities to advanceknowledge and modify the C-S-H structure at the nanolevel. There is a longstanding work in several areasof cement and concrete research to understand cement hydration mechanisms and to study the structureof C-S-H. Researchers have made attempts to tailor C-S-H through the addition of silanes, which has aidedunderstanding of its structure. Research on sol-gel science in various fields of cement science has beenincreasing recently because of promising results. Overall it is shown that a change in sol-gel parametershas a significant impact on cement hydration. This review indicates how sol-gel technology can be furtherexploited to investigate and improve the hydrated cement microstructure.

� 2021 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22. C-S-H structure and cement hydration mechanism. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.1. Complexity of cement hydration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.2. Silica polymerisation within the C-S-H structure during hydration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.2.1. Influence of Ca/Si on the mechanical properties of the C-S-H. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.2.2. Influence of variation in pH on the degree of Si polymerisation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.3. The effect of admixtures and silane based coupling agents on the C-S-H structure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Sol-gel science . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Introduction to sol-gel science . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63.2. Sol-gel mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63.3. Influence of nature of catalysts on the structure of sol-gel silicates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73.4. Influence of water/alkoxide (r) ratio on the sol-gel silicate structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

4. Sol-gel science in cement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.1. General mechanism of sol-gel silicates in cement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
ley).

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4.2. Applications of sol-gel science in cement and concrete structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.2.1. Re-evaluation of cement hydration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94.2.2. Improvement of physical properties of cement composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94.2.3. Repair of cement and concrete structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
5. Conclusion and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Declaration of Competing Interest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

1. Introduction

Although nanotechnological advances have led to an increase inunderstanding of cement hydration mechanisms, research in sev-eral areas of cement, is still carried out to understand hydrationmechanisms better, improve the cement hydrate microstructure,and to develop new types of cement. The main hydration productof Portland cement (PC), C-S-H (calcium silicate hydrate) is a nano-porous inorganic gel. Although C-S-H constitutes more than half ofthe entire volume of the hydrated paste and plays a crucial role inproviding cementitious composites with strength, its structure andmorphology is still not yet fully understood [1–5]. Silica plays acrucial role in the C-S-H framework, and nanotechnological exam-inations like XRD and NMR have revealed the presence of bridgingsilicates in the C-S-H structure [6–8]. This has led to the addition ofsilica or siliceous compounds in the cement matrix by replacing PCto improve the cement hydration mechanics and C-S-H structureand subsequently the cement structural properties. Furthermore,the addition of silica or siliceous compounds in the cement matrixhas been shown to densify the C-S-H network and to provide therequired strength with less PC content [3–5]. The extent to whichthis affect the C-S-H structure will differ, however, depending onthe form of silica (nano-silica, silica fume, alkyl silanes) [8] andthere is some uncertainty as to the extent of some of the contribu-tions made by silica additions. Both micro and nano-silica serve asnucleation sites in addition to their pozzolanic contribution [9–11].From the above, it is clear that the addition of silica-based con-stituents alters the hydration reaction and hence subsequentlythe C-S-H structure. When additional silica sources are incorpo-rated into the cement matrix, there is difficulty in accuratelyunderstanding the hydration mechanisms due to the aggravationof the existing complexity of the cement matrix [8]. Therefore,the hydration reaction, improvement in strength, and densificationof the C-S-H microstructure, on silica addition may not be fullyunderstood. Through careful control of the synthesis conditions,it is possible to bring clarity to the hydration mechanism and sub-sequently, the C-S-H structure [11]. The recent advances in sol-geltechnology have shown potential to control and tailor silica-basedstructures, and this method is being used as a smart tool in thecement matrix [12–15].

Studies on sol-gel processing of silica-based gel started in themid-1800s as it allowed a degree of control on the molecular struc-ture of composites and the formation of numerous inorganic net-works; which is not possible via traditional methods such aschemical vapour deposition, ball milling, and flame spray[15–21]. Sol-gel technology provides a high level of homogeneityto the colloidal gel and this technology can be used to achieve highpurity minerals at low processing temperatures [17]. Sol-gel tech-nology has a wide application in various fields, from ceramic toanti-corrosion coatings, hybrid materials for optical fields, as cou-pling agents, for glass production at a very low temperature, and tografting agents [22–25]. It is also used in the synthesis of zeolites,geopolymers, and other aluminosilicates, that can be utilised asadditions to Portland cement composites or as alternative cemen-titious materials [15,26–29]. Sol-gel technology allows the

2

chemistry of the silicate network to be conveniently manipulatedby altering the synthesis conditions (starting precursors, watercontent, catalyst, synthesis temperature), which makes this tech-nology one of the most recommended procedures for the synthesisof inorganic and hybrid (inorganic–organic) compounds [30]. The -sol-gel processing is easy to modify or replicate [17]. Among themethods commonly used in the preparation of silica samples,the sol-gel process provides a flexible chemical route to obtainmaterials with tailored properties such as particle size, surfacechemistry, and pore structure [12,21]. Additionally, sol–gel tech-nology exhibits lower synthesis temperatures and shorter reactiontimes than conventional processing methods, such as flame spraypyrolysis and combustion synthesis [13]. Sol-gel silicates can besynthesised as thin films, powders, monoliths and fibres by chang-ing, for example, the ageing and drying conditions from the sameinitial starting chemicals [14,15,28].

Although sol-gel science has been compared with cementhydration and C-S-H structure in the past, research on the applica-tions of sol-gel technology in cement have gained momentum overthe past few years because of the promising results mentionedabove. Sol-gel technology has been applied to cement science invarious fields, from re-examination of cement hydration and studyof C-S-H structure to repair of concrete and cement structures[14,29–35]. It should be noted that sol-gel synthesis of silicates fol-lows a similar pathway to cement hydration from hydrolysis to sil-icate polymerisation followed by nucleation, ageing, anddensification. The use of sol-gel silicates to tailor the C-S-H struc-ture can be regarded as a separate branch in the polymerisationof inorganic materials which needs further research. Cement scien-tists are now using the sol-gel technology to modify the cementmatrix at the nano level. Recent studies have been focused onunderstanding the compatibility and capabilities of sol–gel tech-nology to develop ordered silica nanostructures that, in turn, canyield a more ordered C-S-H network [12]. Among the severalapproaches to add organic molecules in the C-S-H framework toimprove the mechanical properties of hardened cement structures,the sol-gel approach was considered to have good repeatabilitywhich was not found otherwise [34,36].

This paper is the first to review the applications of sol-gel tech-nology in the fields of cement science. This review provides anoverview of cement hydration establishing the relationshipbetween the C-S-H structure, hydration mechanism and themechanical performance of the C-S-H structure. The factors thatinfluence the silica polymerisation within C-S-H structure are alsoreviewed followed by an overview of sol-gel science and the influ-ence of specific synthesis parameters on the chemistry of silanes.The chemistry of both sciences will provide a ground to understandthe similarity between the two processes. In turn, this will providea better understanding of the silicate-based polymerisation mech-anism in cement hydration, and how it can be crafted using sol-geltechnology to tailor the micro and nanostructural properties ofC-S-H. This review examines the applications of sol-gel science inthree fields of cement: (1) cement hydration mechanism; (2)improvement of the physical properties of cement-based struc-tures; (3) repair of cement and concrete structures.


2. C-S-H structure and cement hydration mechanism

C-S-H formed during cement hydration is the backbone ofstrength and other mechanical properties of cement and concrete[37,38]. Previous research has unravelled the C-S-H structure frompolydisperse nanoscale units [37] to mesoscopic disk-like blocks[39]. To date, the control of C-S-H structure and its propertieshas been a challenge due to the complexity of the hydration mech-anism and products [37]. In the past, researchers have showninsights regarding how heterogeneities developed during hydra-tion can be retained within the C-S-H structure and its subsequentimpacts on mechanical performances [37,40]. Even though it hasbeen identified that heterogeneities may be controlled [37], it isunclear how this control can be used in smart mix design. In thissection, the C-S-H structure and the cement hydration mechanismare reviewed based on silica polymerisation, to focus specificallyon the changes in the C-S-H structure. There are a wealth of excel-lent reviews describing cement hydration and C-S-H structure[1,6,41–49]. In this section two key points are highlighted: (i) thecomplexity of the hydration mechanics and, (ii) the changes thattake place within the C-S-H structure during silica hydrolysis andcondensation. Both help in understanding how sol–gel technologycan be exploited in crafting the identified heterogeneities such asthe colloidal nature of the C-S-H gel [47] and the macroscopicmaterial properties (e.g. density and porosity).

Furthermore, the scope of this review on the C-S-H structure isto revive the understanding of the C-S-H structure from atomic tomesoscopic level. This is to emphasise the importance of unravel-ling ways to tailor this in cement chemistry as this is critical inevaluating macroscopic properties such as mechanical strength,porosity, and this knowledge will guide readers on understandingthe cement chemistry in the perspective of sol-gel science thatfollows.

2.1. Complexity of cement hydration

Cement hydration is a chaotic and complex process [49–53].Hydration of PC may be considered as a collection of combinedchemical processes, all of which occur at a rate determined bythe constituents in the cement matrix. The four main componentsin Portland Cement (PC) are tricalcium silicate (C3S), dicalcium

Fig. 1. Transport and boundary nucleation grow

3

silicate (C2S), tricalcium aluminate (C3A), and calcium aluminofer-rite (C4AF) [7,54,55]. Although it is possible to form and isolate thehydration of individual clinker constituents, the hydration mecha-nism observed for each of these clinker constituents is not neces-sarily representative within the cement paste. This is due to theinteractivity of the phase formation in the cement hydration pro-cess. Consequently, the study of cement hydration is difficultbecause of its structural and chemical complexity and simplifiedassumptions have been necessary [46,49]. The induction periodduring the hydration process is a consequence of the disorderednucleation and growth of C-S-H upon the dissolution of C3S andother mineral sources in the hydration process [49]. Studies con-ducted by Thomas et al. [56] showed that the addition of reactiveC-S-H eliminated the induction period in cement hydration.

Concerning the dissolution and precipitation mechanism, it canbe inferred that hydration of cement particles comprises hydrolysisof the clinker compounds to form hydroxides which then furtherreact and condense to form C-S-H precipitates that grow and forman interconnected network of C-S-H. The hydrolysis (of the clinkerparticles) and condensation (of the hydration products) occursconcurrently, ultimately covering the remaining unreacted clinkerparticles. Both hydrolysis and condensation are reversible reac-tions and proceed further when either the dissolved ions transportinwards and precipitate in the pores or the water transports out-ward to react with the unreacted cement [49,55]. The termshydrolysis and condensation process will be used interchangeablywith cement hydration in this paper. Fig. 1 represents the variousstages of the cement hydration process mentioned above high-lighting mainly the diffusion mechanisms through the assumedspherical C-S-H shells and growth of C-S-H as the hydration reac-tion progresses. Although the general representation of cementhydration is based on a few reactions, the reality is that those reac-tions and products are far more complex and very difficult to con-trol [37]. Furthermore, although it has been universally accepted toresemble the mineral tobermorite or jennite, the C-S-H structureremains a focal point of interest and investigation in the field ofcement science. The basic chemical reactions in the hydration pro-cess are not given in this paper and readers are guided to the keypapers as mentioned above [1,6,41,43–46].

Due to the complexity of this dissolution and precipitationmechanism, it is difficult to control the structure and properties

th mechanisms of cement hydration [49].

Table 1Approaches developed to unravel the hydration mechanism and understand theC-S-H structure.

Model/Tests Results Inference Ref.

Poroelastic model Deconstructed the C-S-H intorepresentativevolume element(RVE) from paste tonanometric scalelength

& Interconnectedsolid C-S-H and gelpore relation(Fig. 2).

& Role of mesoscalenature of C-S-Hstructure onmechanicalbehaviour

[59]

Micromechanicalmodel

Macroproperties ofcement structurewere result ofmicrostructure andhydration degree ofcement

Overall loadtransfer as well asthe material relatedcompressive stresswas dependent ondeviatric stresspeaks withinhydrate phases

[58]

Conceptual basedhomogenisingtheory

Translated hydrationcharacteristics of co-existence of solid C-S-H block without gelporosity anddensifying C-S-H.

& Gel porosity islinked to specificprecipitation space

& Densification of C-S-H translated tothree scale modelfor poroelasticity ofcement paste.

[59]

NMR surfacerelaxivitycalibration

Densification of C-S-Hstructure.

Idea of uniform gelporosity resultingfrom constant massdensity was ruledout.

[48,60]


of the C-S-H phase [37]. Furthermore, the intrinsic multiscale prop-erties of C-S-H, which spans from nanometres to micrometres, hasmade control of the C-S-H structure and its resulting properties achallenge [38]. Due to the availability of both nuclear magnetic res-onance (NMR) relaxometry and nanoindentation data, severalmodels from realistic molecular models [57] to continuummicromechanics models [58,59] have been developed to resolvethe mechanisms of hydration process and the C-S-H structure. Sev-eral models and concepts were developed to understand thecement hydration mechanisms and link the relation betweenC-S-H structure, hydration mechanisms and mechanical properties(strength in particular). Table 1 shows some of the approachesdeveloped in unravelling the relation between the nature of thedegree of hydration and influence of these properties on themechanical performance.

Through knowledge of hydration mechanisms complementedwith recent poroelastic modelling results, it has been possible tobreakdown the C-S-H structure and observe the precipitation anddensification mechanisms on a hierarchical observation scale.Although the approaches in Table 1 are reliable tools for translatingthe property-quantitative microstructure, questions as how to con-trol the C-S-H structure and hydration mechanisms remain [53].Indeed, despite the success of many developed models in reflectingthe dissolution, precipitation and densification of events at differ-ent observational scales, the parameterisation of the interparticleinteractions are based on the mechanical properties measuredusing nanoindentation experiments, NMR and atomic force micro-scopy (AFM) [53,61].

Furthermore, the complexity of the atomic structure of C-S-Hmakes it challenging to interlink the relationship between thenanoscale mechanical properties of C-S-H and its composition[38]. This knowledge gap has likely limited the potential toimprove the mechanical properties of C-S-H [38].

4

2.2. Silica polymerisation within the C-S-H structure during hydration

The evolution of C-S-H models has developed over the yearsbased on the silica deficient solid solution of colloidal or layeredmodels [42,50]. The C-S-H structure is generally considered a lay-ered structure depending on the Ca/Si ratio and comparable tominerals like tobermorite and jennite [62–65]. The colloidal gel-like properties of C-S-H gels have been identified since the1950s; however, to date, studies on these properties have receivedless attention than structural interpretations like the deviatoricstress, gel porosity, poroelasticity within the C-S-H structure. How-ever, the complex atomic structure of C-S-H makes it challengingto identify the correlation between the C-S-H composition and itsmechanical properties at nano-level. The silicate layer in theC-S-H undergoes polymerisation and studies indicate the presenceof dimer units Q1 [7,65]. As hydration reactions proceed a decreasein Q1 signals and an increase of Q2 signals (29Si NMR) indicatespolymerisation of silicate within the C-S-H and the developmentof a layered structure. Furthermore, the degree of polymerisationof Si is usually determined based on the Ca/Si ratio. For instance,the lower Ca/Si ratio in the C-S-H structure, the higher is the poly-merisation degree of silica in C-S-H [8,66–68]. In context to the lay-ered model, water in the C-S-H gel pores are mobile or immobilebased on the location of water molecules within the C-S-H struc-ture (interlayer or intralayer within the C-S-H sheets respectively)[69]. Furthermore, the mobile water molecules within the C-S-Hgel dissociates into H+ and OH–, which moves to the silicate oxygenatoms on the silicate chains which are not bonded, forming Si-OH(silanol) groups. Hydroxyl groups (OH–) will also move to Ca2+ ionsto form Ca-OH at the interlayer. The formed Si-OH acts as an activesite for silica polymerisation. The readers are guided to a wealth ofreferences. [70–76] to understand the detailed dissociation ofwater in the C-S-H gel on varying the Ca/Si ratios. From an under-standing of silica chemistry and knowledge of the pH conditions ofthe pore water, the C-S-H resembles a particulate gel structure.Parameters such as the interlayer water and pH influence the silicapolymerisation within the C-S-H structure during hydration. In thefollowing section, the effect of Ca/Si ratio, the influence of watercontent and pH on the structure of C-S-H at the nanoscale are high-lighted as they can be controlled using sol-gel technology. Thescope of the review on the influence of the parameters on theC-S-H structure is to apprehend ways in which sol-gel technologyand its application in cement science can provide a new aspect forthe altercation of the C-S-H chemistry to achieve the mechanicalperformance required for reducing cement consumption.

2.2.1. Influence of Ca/Si on the mechanical properties of the C-S-HThe mechanical properties of C-S-H can be related to Ca/Si ratio

[38]. Recently, X-ray diffraction revealed that at lower Ca/Si ratio,the densification of C-S-H could increase the mechanical propertiesof C-S-H despite the presence of defects [38]. The hardness andindentation modulus of C-S-H decrease with increase in Ca/Si(Fig. 3). Furthermore, at low Ca/Si, C-S-H polymorphs are foundto be more stable [38]. Therefores the stability of the C-S-H struc-ture can be improved by the control of silica polymerisation. Froma sustainability view, altering the chemistry of C-S-H by loweringCa/Si could provide strength and stiffness of composites at lowercement contents [38]. However, targeted improvements in themechanical properties of the C-S-H relies on the kinetics of silicapolymerisation and the assimilated disorder associated with theC-S-H structure [62].

An analogy of glass science with cementitious materials hasrecently resulted in new insights regarding the relationshipbetween the engineering properties and the C-S-H atomic struc-ture/composition [38]. Topological constraint theory (TCT) is a

Fig. 2. Heirarchical representation of hydration mechanisms [58].

Fig. 3. Nanoscale influence of the Ca/Si ratio on the mechanical properties. (a) Atomistic simulation data of C-S-H indendation modulus M (orange squares) and hardness, H(brown squares) as a function of Ca/Si and compared to the experimental nanoindentation hardness (grey squares). (b). Computed indentation modulus parallel (M1) andperpendicular (M3) to the C-S-H layer [62]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 4. Ternary map predicting the C-S-H hardness on varying C-S-H composition using TCT theory [38].


powerful technique used to resolve the complexity of disorderedglass based on the composition [38].

TCT explains the atomic topology, which influences macro-scopic properties without masking the relevant structural details[38]. TCT was applied in cement science to establish a relationbetween composition and structure and was successful in the pre-diction of the hardness of C-S-H. Based on the TCT knowledge aternary diagram was developed correlating the relationshipbetween hardness and C-S-H composition (Fig. 4). The success ofthe novel silica based theory of TCT has probably encouraged theuse of other smart silica-based engineering tools like sol-gel tech-nology to control the structural applications of the cement-basedmaterials.

5

2.2.2. Influence of variation in pH on the degree of Si polymerisationThe cement hydration medium is highly alkaline due to the

presence of Ca(OH)2 and other alkali hydroxides (mainly sodiumand potassium). As cement hydration reaction is a non-equilibrium process with simultaneous precipitation and conden-sation, the silica in C-S-H polymerises in a basic medium atapproximately pH 12. At an elevated pH medium, the rapid reac-tion of aluminates and calcium in cement paste occurs along withsilicate ions [77]. In this alkaline medium, the C-S-H attains nega-tive charges that prevent the aggregation of C-S-H particles. Whenthe hardened cement paste is cured in a basic environment, theunhydrated silica in cement forms particulates. These SiO2

particulates act as nucleation points forming more C-S-H or


electrostatically attract positive ions dissociated in the medium[78] which can result in higher strength. This theory is the princi-ple of an increase in strength of cement paste under a highly basic(pH � 11) environment [79,80].

2.3. The effect of admixtures and silane based coupling agents on theC-S-H structure

Admixtures and silane coupling agents have been used to con-trol the hydration kinetics of cement and modify the C-S-H struc-ture. Several models have been focusing on studying the impactsof various types of admixtures on hydration [81]. Bishop and Bar-ron [82] have explained the possible mechanism of the use ofretarders on the hydration mechanisms. Among these mecha-nisms, the considered possibilities were of surface adsorption ofretarder on the anhydrous clinker particle surface and also nucle-ation and seeding effects of hydration products. It is understoodthat the admixture interacts with the aluminate phases maskingthe silicate hydration [81]. For example, polycarboxylate super-plasticisers (PCE) adsorb onto aluminate phases that intervenewith the fluidity retention. The residual amount of PCE that isnon-adsorbed is not enough to compensate for the increase of solidsurface due to hydration [83]. It is understood that PCE or organosilicate modified PCE and other silylated hydrosoluble polymersextend the dormant stage of hydration kinetics due to the adsorp-tion on the aluminate phases. However, the organo silane-modifiedgroup in PCE can covalently bond to silicates in the C-S-H, improv-ing the performance and densifying the microstructure of hard-ened cement pastes. Furthermore, triethanolamine, when used asan admixture in cement-fly ash systems, promotes the pozzolanicactivity; however, the effect of triethanolamine admixtures likepolycarboxylate, on hydration kinetics is unclear [84]. The under-standing of the interaction mechanisms of these chemicals onthe kinetic rate is still in its infancy. Silane-based coupling agentsare silica-based polymers that forms covalent bond with thecement surface. They act as hydrophobic barriers by reacting withthe silica in the C-S-H structure and preventing water from dis-rupting this interfacial bond. These silica-based coupling agentscan be produced by sol-gel technology. It is vital to investigatethe mechanism of interaction of these organic ions in the chemicaladditives (polycarboxylate, alkoxysilanes) with the different ionsin the pore solution. These aspects that actually impart sol-geltechnology onto the C-S-H structure are explained in detail in alater section (Section 4.2). It has been stated that the rate of disso-lution and interaction of the organic additives is faster when com-pared to the cement hydration hence it is complex to detect thereaction of these compounds on the formed C-S-H [83]. This con-centration of ions keeps increasing and ultimately the solutionbecomes supersaturated. Therefore it becomes energeticallyfavourable for ions to combine and form a solid phase [55]. Hencethe use of admixtures and silane based agents can provide an alter-native route of controlling the hydration kinetics and the C-S-Hstructure.

3. Sol-gel science

This section deals with basic concepts of sol-gel science for thepreparation and processing of silica-based nanomaterial to high-light the controllability sol-gel process offers, hence allowing tai-loring and modification of colloidal silica. Additionally, thissection of the review focuses on alkoxysilanes (Si(OR)4) generallyknown as silanes, where R represents the alkyl group (e.g. CH3

for methyl, CH2CH3 for ethyl). Silanes are the most used precursorsfor silica-based networks in both sol-gel science and cementstructures. Silanes are silicon compounds with four substituents

6

including an organic group such as organosilane (for exampleethoxy silane (Si(OC2H5)4), tetramethylsilane Si(CH3)4); whereas,a siloxane is a functional group of silanes or organosilane withSi-O-Si linkage. Silanes that have undergone hydrolysis and con-densation and consists of (-Si-O-Si-)n, where n > 1, is called a silanederivative (e.g., silane oligomer, sol-gel silicate).

3.1. Introduction to sol-gel science

According to the conventional definition by Brinker and Scherer[19], the sol-gel process is a technology where the sol (or solution)evolves gradually and forms a gel-like network that contains both aliquid and solid phase. This process involves the reaction of precur-sors which are either a metal alkoxide or a metal salt in a solvent toform a colloidal suspension called a sol. The sol then forms a con-tinuous network by hydrolysis and condensation in the liquidphase called gel [21,85]. Once the gel is formed, it can be furtherdried, chemically stabilised or densified at higher temperaturesdepending upon the application. With the advancement in nan-otechnology, there are several techniques for preparing silicananoparticle (SNPs) such as chemical vapour deposition, ballmilling, flame spray pyrolysis and sol-gel technology on [86–88].The interest in sol-gel technology has grown in recent years dueto its repeatability, controllability, and low-temperature require-ments. Sol-gel technology is regarded as a unique method to pre-pare particles of the desired morphology and size by controllingthe physico-chemical properties of the compounds. Furthermore,this technique opens the possibility to tune the intrinsic properties(the fundamental chemical composition and the structure of thematerial) of the synthesised silica materials. Furthermore, unlikeother methods that require high temperature for developing purematerial, sol-gel technology enables the same ceramic qualitymaterial, at a low temperature.

Iller [89] stated that silica gel is formed either by discrete par-ticle growth or by the formation of a 3-D network, which is con-trolled by precursors, pH, processing time, and molar ratiosbetween the reacting agents, and these parameters influence thefinal product formed during the sol-gel reaction [90]. In general,precursors that form reactive inorganic surface charges (e.g.,sodium metasilicate, alkoxysilanes, or even natural materials likeclay) do not cause coagulation or precipitation, should be used asthe starting material in the sol-gel process [91].

3.2. Sol-gel mechanism

Alkoxysilanes undergo partial or complete hydrolysis based onthe amount of water available for hydrolysis. During the hydrationof the alkoxysilane, colloidal particles will be formed, which fur-ther undergo condensation and form larger molecules and oligo-mers. Hydrolysis and condensation are not consecutive reactionsrather condensation can be initiated before the completion of thehydrolysis.

During hydrolysis, alkoxides like tetraethyl orthosilicate (TEOS,Si(OC₂H₅)₄) create reactive groups, –OH, which are necessary forcondensation reactions (Eq.1). Depending on the amount of water,the silicon alkoxide undergoes partial or complete hydrolysis. Astoichiometric value of water/alkoxide, r = 2 is enough to convertSi(OR)4 to oxide. When r = 4 complete hydrolysis occurs, all theOR will be substituted by reactive –OH sites (Eq. (4)). The alcoholformed during hydrolysis in (Eq. (1) and Eq. (2)) plays an activepart in the sol-gel reaction and can react with the silanol and formthe initial reactant, alkoxide. This reverse process is called re-esterification (reverse of Eq. (1)). Condensation reactions are initi-ated as soon as the first reactive –OH group is formed [92]. In thesilica system, the condensation involves silanol groups and pro-duces siloxane bonds plus by-products like alcohol and water


(Eq. (2) and Eq. (3)). Unlike other organic polymerisation reactions,in the sol-gel process, the first step is creating a reactive site (–OH)that initiates the condensation reaction.

Furthermore, condensation is dependent on the degree ofhydrolysis and the presence of at least one silanol group (Si-OH),which is needed at the centre of the Si atom. The hydrolysis andcondensation of the intermediate species are discussed by Mileaet al. [91]. The kinetics of hydrolysis and condensation is depen-dent mainly on the pH and the catalyst in the medium. Forinstance, alkoxysilanes react very slowly with alcohol and water,and hence the addition of a catalyst is necessary.

� Si� OCH2CH3 þ H � OH $� Si� OH þ CH3CH2 � OH ð1Þ

� Si� OCH2CH3 þ HO� Si �$� Si� O� Si � þCH3CH2 � OH

ð2Þ

� Si� OH þ HO� Si �$� Si� O� Si � þH � OH ð3Þ

Si� OCH2CH3ð Þ4 þ 4H � OH $ Si� OHð Þ4 þ 4CH3CH2 � OH ð4Þ

Si� OHð Þ4 þ Si� OHð Þ4 $ OHð Þ4 � Si� O� Si� OHð Þ4 þ H � OH

ð5ÞThe progress of polycondensation leads to the formation of an

extended SiO2 network. The SiO2 structure, growth, and gel timeare dependent on the process parameters such as water to alkox-ide, i.e., r-value, nature, the concentration of catalyst, synthesistime, and drying temperature. Controlling the process parametersenables tailoring the structure and properties of the sol-gel sili-cates [93]. For instance, when monomers are completely hydrol-ysed (Eq. 4 and Eq. 5), then a more complex branching of thesiloxane network can be obtained when compared to partialhydrolysis [92]. A detailed explanation of the effect of theseparameters on the silicate network can be found in the literature[19,91–93].

3.3. Influence of nature of catalysts on the structure of sol-gel silicates

The final structure of the gel depends largely on the choice ofthe catalyst which will lead to an acidic or basic route. In general,in an acid catalysed system, microporous chain-like structures areformed with a pore size < 2 nm [91,92]. Furthermore, acid catal-ysed medium with low r-value develops a weakly branched poly-meric network [93]. On the other hand in basic conditions, amore connected and branched network of colloidal particles isobtained [19,91–93]. A detailed explanation of the mechanisms

Fig. 5. Growth of silicate structures on the variation of pH [90] Published by TheRoyal Society of Chemistry.

7

by which pH influences the kinetics of hydrolysis and condensationand subsequent polymeric structure, can be found elsewhere[19,91–93]. The hydrolysis rate is accelerated in acidic conditionswhereas the rate of condensation is faster under a basic medium.These rates are strongly influenced by the steric effect of alkylgroups as well [93]. Danks et al. [90] have explained clearly themechanisms and subsequent structure formation. Under basic con-ditions, the rate of condensation is faster as mentioned abovehence before the completion of hydrolysis, condensation occurs.However, if r = 4, complete hydrolysis occurs (Eq.4) before the firstcondensation and thus the resulting species, (OH)3Si–O–Si(OH)3has 6 active OH sites (Eq. (5)) for condensation reactions. This ser-ies of condensation step leads to small particulates which link witheach other and form highly branched polymers dispersed in a fluidmedium (sol). Eventually, the polymers are fully linked, and nolonger mobile, forming a non-fluid structure called gel [88]. Thisindicates that in basic medium there is a competition betweenthe R and the pH. Whereas in an acidic medium, as hydrolysis isfaster, condensation proceeds before the completion of hydrolysisdue to the presence of the reactive –OH sites. This results in linearchain-like structures [88]. Fig. 5 clearly shows the effect of pH onthe structure of the sol-gel silicate chain.

3.4. Influence of water/alkoxide (r) ratio on the sol-gel silicatestructure

From Eq. 1, Eq. 2 and Eq. 3, the water in the sol-gel reaction hasa strong influence on the kinetics of hydrolysis and condensation.The influence of catalysts with a low amount of water has a pro-nounced effect on the structure of the silicates. As water increases,at constant solvent content, beyond r > 5, it reduces the silicateconcentration and leads to dilution effect which affects the rateof hydrolysis and condensation and hence increases the gel time[92].

Fig. 6 shows a strong correlation between gel time and silicaconcentration. From both the techniques (scattering light and tilt-ing method) it can be seen that the gelation time increases as silicacontent increases (decrease in water content) [94]. Thus, increasedgel time is an indication of silica polymerisation. This polymerisa-tion of silicate structure is due to the mentioned dilution effect atr > 5. Furthermore, as the alcohol content which is used for the sol-gel reaction or formed as a by-product during the hydrolysis andcondensation increases the gel time which can cause retardationof the cement hydration when these sol-gel silicates are usedwithin the cement matrix. A more detailed explanation of themechanism of silica polymerisation on variation with r ratio andsolvent content is available in the mentioned references [92,94].

4. Sol-gel science in cement

This section describes the compatibility of sol-gel science withcement hydration and discusses the existing applications of sol-gel science in cement. Applications of sol-gel technology in cementand concrete structures can be broadly divided into three researchareas: (1) cement hydration mechanisms; (2) improvement ofphysical and chemical properties of hardened cement structures;and (3) repair of cement and concrete structures. The sectionintends to draw attention to the promise of sol-gel science as anarea of further necessary cement and concrete research. Severalstudies have been conducted on the use of silanes in the cementmatrix as surface coatings to admixtures to improve the freshand mechanical properties [23,95–100]. However, silanes tend toextend the dormant stage of cement hydration in several studies[98,101], and certain cases, there is lack of repeatability of resultswhich is highlighted in the studies of C-S-H structure and repair

Fig. 7. Mechanism of sol-gel silicate interaction with cement particle [83].


of concrete structures [13,34]. When these silanes are modifiedusing sol-gel technology they tend to overcome the formerly men-tioned drawbacks to a great extent [13,34,101]. The literature onsol-gel modified silanes in various fields of cement is scant and thisreview projects the need for more research in sol-gel technology inthe respective cement areas.

4.1. General mechanism of sol-gel silicates in cement

In the cement hydration mechanism, the silicates undergohydrolysis which is followed by condensation and formation ofproducts (e.g. C-S-H). However, these two stages in cement hydra-tion are uncontrollable. The idea of incorporating sol-gel silicatesin the cement matrix is to use the primary goal of the sol-gel pro-cess suggested by Hench and West [18], which is to say that thesol-gel silicates have a proven ability to act as collateral in control-ling other materials. This is because the sol-gel silicates can be con-trolled at the surface and interface levels. Based on sol-gel science(section 3) the possible reaction, on the addition of sol-gel silicatesto cement, is the hydrolysis of the silanes and sorption of the sila-nols on the cement surface. These silanols can further undergo con-densation to form Si-O-Si bonds or remain as silanol groupattached to cement particle unreacted for further condensationwith the –OH sites within the C-S-H structure [98]. The silica net-work within the C-S-H structure can link with other silica-basedmaterials. Thus, in cement paste modified with sol-gel silicatesthe condensation and polymerisation to form Si-O-Si networkcan take place in three ways: (1). between the silanols on the sur-face of the cement particles; (2) between silanol on the surface andunhydrolysed silanes during the sol-gel process; (3) between theunreacted silanol and silica in cement clinker and sol-gel andcement and clinker. This is different from the silanes which havelesser reactive –OH sites when compared to sol-gel modifiedsilanes [101]. The crosslinking mentioned in the above mechanismis via hydrolysis and condensation, which are reversible due to thepresence of an aqueous medium with the cement paste. The reac-tion speed depends on the environmental conditions in which thehydrolysis and condensation take place. Hydrolysis and condensa-tion are the stages in the cement hydration process (Section 2.1).Sol-gel synthesis of silica-based species is also done throughhydrolysis and condensation, and these two stages are fully

Fig. 6. Influence of silica concentration on the gel time determined by scattering(red dots) and tilting method (green dots) [94].

8

controllable through the sol-gel synthesis parameters. Thus, theuse of sol-gel silicates can, potentially, be used to control the envi-ronmental conditions of the hydrolysis and condensation andhence, understand and tailor the C-S-H product. Sol-gel silicatesconsist of silicon molecule which can form a covalent bond withorganic and inorganic material [95]. Due to the great variety ofthe functional groups on the sol-gel silicate, its incorporation intocementitious material has recently been attracting attention due toits effects on the workability, the hydration regulation, themechanical property and the durability [13,34,101]. The literaturedoes not reveal changes with variation in pH as the pore solution incement being highly alkaline it is obvious that the sol-gel silicate,when added to cement, undergoes hydrolysis and condensationin a basic medium. However, the difference in the basicity of thesolvent used during sol-gel synthesis directly affects the condensa-tion process during sol-gel synthesis [102,103]. In a basic mediumof sol-gel silicates, the condensation is more predominant resultingin a more linear structure, and in an acidic medium, hydrolysis isfaster hence it develops a highly branched structure [19,104]. Incement hydration the pore water being basic leads to interlinkedlinear structure (Fig. 7).

4.2. Applications of sol-gel science in cement and concrete structures

This section congregates the existing literature that uses sol-geltechnology in various applications of cement science. In sol-gelscience, metal silicates, silanes (tetraethyl orthosilicate (TEOS)) orfunctionalised organo-silanes such as (1) amine functionalisedsilanes (3-aminopropyltriethoxysilane (APTES), N-2-aminoethyl-3-aminopropyltrimethoxysilane (AEAPTMS)); (2) alkyl function-alised (methoxy terminated polydimethyl siloxane (PDMS)); (3)epoxy functionalised (3-glycidyloxypropyltrimethoxy silane(GPTMS)), are examples of silica-based precursors. In the cementindustry, silanes are often used as surface coating and cementadditives like nano-silica, and water-reducing admixtures. Silaneshave also been used in the synthesis of cement hydration phases[23,95–99,105]. Although these same silanes and metal silicatescan be directly used in cement, to improve the physical and chem-ical properties of concrete structures [98,101], it is important tonotice that they are used in their unhydrolysed form. In the

Fig. 9. Identical pathway of sol–gel technology and cement hydration.

Fig. 8. Structural difference between the forms of APTES.


sol-gel process, however, they will undergo hydrolysis and conden-sation to deliver more complex silica-based materials with differ-ent properties. Fig. 8 shows the structural differences betweenthe forms of APTES when used as the silane. In the sol-gel processthere are several parameters to control the synthesis of silica-based materials, these sol-gel modified silicates provide a morecontrollable environment within the cement matrix and tailoredfinal properties. The ability to control the cement matrix environ-ment and its final properties cannot be achieved with the directuse of the unhydrolysed forms of silanes and silicates.

4.2.1. Re-evaluation of cement hydrationScherer [86] has described the C-S-H phase of the hydrated

cement matrix as a colloidal precipitate and has compared it toinorganic gels developed through sol-gel synthesis. Furthermore,Thomas and Jennings [1] have compared C-S-H to sol-gel silica pre-cipitate formed under reversible conditions. According to sol-gelscience, base catalysed synthesis at high water/alkoxide, (r) formhighly branched clusters which will eventually precipitate. Thismechanism is similar to cement hydration and C-S-H precipitation[35]. Researches based on the evaluation of inorganic polymerisa-tion during cement hydration with sol-gel started since 1999,though the literature of the use of sol-gel in cement is scarce[63]. There have been several research attempts to study the struc-ture of C-S-H and also the subsequent changes in the degree ofpolymerisation upon C-S-H aging [35,63]. The inorganic gel formedduring condensation of the sol-gel process can continue to poly-merise long after gelation, though, at a slow rate because of thepresence of water or OH– groups. The continuous nature of thepolymerisation of the silica structure is similar to the hydrationprocess in cement, in which it continues indefinitely at an everdecreasing state [35]. Careful examination of sol-gel science (sec-tion 3 and review of Wu [35]), shows that the formation of basicsilicate species follows an identical pathway to PC hydration andother blended cement mixtures. Both sol-gel silicates and cementhydration undergo hydrolysis and condensation of the starting sil-ica precursors followed by formation of colloidal silicate frame-work which in cement is C-S-H and finally ageing (see Fig. 9).

In several contexts, the cement hydration has been looked intosol–gel science literature however, this has been implicitly[13,35,63,96]. A comprehensive understanding of the silica chem-istry is given by Iller [89]. Iller’s findings has paved the way foradvances in the field of silica-based sol-gel science and the under-standing of fundamental properties of the C-S-H structure forma-tion. This further highlights the common aspects of bothdisciplines: sol-gel science and a new approach of evaluation oncement hydration [64]. This furthermore highlights the need forresearch on the sol-gel process to develop ways to control themechanism of cement hydration.

9

4.2.2. Improvement of physical properties of cement compositesThe physical properties of the hardened cement and concrete

are strongly influenced by its C-S-H structure [12,32,34,66,106].The sol-gel process has been used in cement not only for theenhancement of the C-S-H structure but also as a means to betterunderstand the complex nature of the cement hydration. Studiesbased on the sol-gel process in improving the physical propertiesof hardened cement structures can be broadly divided into tworesearch fields: (1) studies on synthetic C-S-H outside cementmatrix; (2) use of sol-gel silicates within the cement matrix. Inboth cases, the silanes and metal silicates commonly used incement technology are mentioned in the introduction of thissection.

4.2.2.1. Studies on synthetic C-S-H outside cement matrix. Althoughthe structure of C-S-H may be compared to tobermorite and/or jen-nite, it can accommodate substantial defects within the structures[34]. There is an increasing interest in altering the structure ofC-S-H at the nanoscale to improve the macroscale properties ofhardened cement and concrete structures [32,34,36]. Furthermore,the incorporation of polymer composites within the C-S-H struc-ture has been shown to improve the mechanical properties ofcement paste [32,34,36,98,101]. However, there seems to be a lackof consistency in results when it comes to the intercalation of poly-mer composites (poly ethylene glycol (PEG), poly acrylic acid(PAA), and poly vinyl alcohol (PVA) [107]) within the C-S-H struc-ture [32,34]. Sol-gel technology is considered as a simple and alter-native method for the intercalation of a small organic group withinthe C-S-H via Si–C bonds [13,34]. Furthermore, due to the possibil-ity to control different parameters in the sol-gel process, it enablesthe synthesis of pure hydrate species, which in turn facilitates thecharacterisation and investigation of the C-S-H structures [14,33].Sol-gel technology has enabled organic moieties to be incorporatedin the C-S-H structures via covalent bonds (Si-C) [34]. It was possi-ble to carry out studies in a more controlled environment whenC-S-H has been synthesised via the sol-gel process, unlike the cases

Fig. 10. Homogeneously dispersion of nano-silica in cement paste when prepared by sol-gel synthesis when compared to commercially available nano-silica:(a) agglomeration of commercially available nano-silica [109]; (b) dispersion of nano-silica synthesised by sol-gel technology [110].


during cement hydration [8,14,33]. Furthermore, research by Zhuet al. [13] had also confirmed the ability of the sol-gel process tocontrol the nucleation and growth process of nano C-S-H gel byavoiding its agglomeration. This has shown the ability of the syn-thesis procedure to modify the C-S-H structure either by intercala-tion or interaction of these organic or inorganic species. It can beseen from 29Si NMR that as the organic content is increased withinthe C-S-H structure the tetrahedral Q species shifts to T speciesindicating the linking of organic content to the silica network[34]. One advantage of the use of sol-gel technology for intercala-tion of organic moieties was that they were added into the C-S-Hframework at room temperature by co-precipitation of two ormore silanes [34]. This further highlights the sol-gel processabilityto modify the C-S-H structure at a lower temperature. Thus, thereview of the mentioned literature further shows the possibilityof sol-gel science to be used within the cement matrix to controlthe hydration mechanics.

4.2.2.2. Use of sol-gel silicates within the cement matrix. The use ofsol-gel silicates within the cement matrix alters the cement hydra-tion mechanism and thus the mechanical properties of hardenedcement pastes and mortars. Silica additives of the nanometricrange are widely used in cement however, mixing of these materi-als in cement is always a challenge [108]. Nano-silica generallytends to form agglomerates which make it difficult to trace theactual reactivity as they can act as fillers or push the unreactedclinker and hydrated cement particles away. Once in the shape oflarge agglomerates they absorb free water and contribute to thefluidity of the pastes [109]. Sol-gel based nano-silica, on the otherhand, enhances the homogeneity of the mixes due to charged sur-faces that create a well-dispersed system (Fig. 10).

In general, silicates synthesised via sol-gel aid particle disper-sion within the cement matrix [12,101,111]. Well dispersed parti-cles are evidence of highly controlled mechanisms in terms of silicachemistry, the details of which are out of the scope of this reviewbut are available elsewhere [2,30]. Improved dispersibility of silicananoparticles has been shown to increase the mechanical proper-

10

ties of the cement matrix. For instance, improved dispersion ofnano-silica synthesised by the sol–gel mechanism has shown a30% higher compressive strength than the control sample withthe addition of just 0.01% of nano-silica by mass of the total solidconstituents [111]. Singh et al [108] revealed in the SEM micro-graphs that, with the addition of nano-silica synthesised by sol-gel technology, the cement microstructure became more compact,denser and uniform when compared to the blank sample (Fig. 11).Furthermore, the compressive strength results had shown animprovement in strength by 26% more than the control on theaddition of this powdered nano-silica [108].

Several studies have been conducted on the use of silanes asadmixtures in cement pastes and mortars. Although the basic reac-tion mechanism of silane with the C-S-H is similar to sol-gel sili-cates (Section 4.1), there tend to be more reactive OH sites in thehydrated sol-gel silicates. Furthermore, these silanes, when usedunhydrolysed in cement, follow a basic route due to the high alka-linity of cement pastes. However, when silanes are tailored by thesol-gel process the silicate network can be altered to sols orbranched structure along with an increased number of reactivesites. Silanes act as promising materials to enhance the cementpaste and mortar cohesion and toughness [101]. Whilst silanesimprove compressive strength and consistence, they also tend toincrease the dormant period of cement hydration. However, Fenget al [101] have shown that organo-silane when used as sol-gelderivative (oligomer or nanoparticle (Fig. 13) does not retardcement hydration or affect the heat of hydration. This is probablybecause of the controllability of the reaction and hence less alcoholby-products are produced which would otherwise retard thehydration (reflecting Section 3.4, where an increase in gel timewas observed with an increase in alcohol). A combination of thesereasons might have led to the improvement in physical and chem-ical properties when used as sol-gel silicates rather than unhydrol-ysed silanes.

Fig. 12(a) shows how an unhydrolysed silane, when used incement, undergoes hydrolysis and condensation in a basic routewhereas Fig. 12(b) shows how the same silane, when used as a pre-

Fig. 12. Difference in silicate structure on the use of silane and sol–gel silicate in cement paste: (a) unhydrolysed TEOS; (b) sol–gel modified silane with TEOS as the startingprecursor in cement paste.

Fig. 11. Change in the morphology on the addition sol-gel nano-silica: (a and b) blank sample without sol-gel nano-silica addition at 1 day and 28 days respectively; (c and d)sample with sol-gel nano-silica addition at 1 day and 28 days respectively [108].


11

Fig. 13. Conversion to silane to different derivatives where RO is the alkoxy group and X is the functional group [101].

Fig. 14. Reaction of silanes in different forms: (a) three types of silanes used in cement [98]; (b) variation in heat of evolution during cement hydration showing noretardation on use of sol–gel nanoparticles from the same precursor amino silane (AS) [101].


cursor in sol-gel process, can be tailored perhaps before addinginto the cement matrix.

Furthermore, nano C-S-H when added to concrete by replacing0.75% of cement by mass acted has acted as an early strength agent[12]. Whilst the precise mechanism is not fully understood, it maybe theoretically explained by the fact that silane monomers havefewer reactive hydroxyl groups than sol-gel silica derivatives. Thecontrollability and good dispersion yield by the sol-gel processare therefore likely to influence the cement hydration rate andthe mechanical properties of the cement mortars.

When TEOS was added as an admixture in the fresh paste, stud-ies have shown a decrease in the degree of hydration and compres-sive strength even at later ages [96,98,99]. However, researchshows that TEOS improves the polymerisation of C-S-H in a hard-ened cement structure when used as a consolidant (not hydrolysedvia sol–gel process) [98]. If this was the case, then TEOS when usedin fresh paste should not have decreased the degree of hydration orcompressive strength. Zhu et al [12] developed nano C-S-H by sol-gel process using TEOS as the initial precursor and found anenhancement in compressive strength with 0.75% replacement ofcement by mass.

When amine functionalised silanes are added to PC pastes andmortar, they extend the dormant period as shown by isothermalconduction calorimetry (Fig. 14a). However, the total heat ofhydration is normally higher than control cement pastes and 28-day strengths are also higher. The improved 28-day compressivestrength is because the electron-rich nitrogen atom, in the aminegroup, enters into hydrogen bonding with the hydrogen donatinggroups like hydroxyl or other amines, which results in increaseddispersion of particles within the cement paste. Research hasshown improvement in the later age strengths for cement mixesusing amino silanes. This could be because of the in-situ polymeri-

12

sation of the silanes within the cement matrix as the alkoxy groupundergoes hydrolysis and condensation with the hydroxyl groupswithin the cement matrix to form siloxane bonds. Svegl et alshowed that amino silane improved the mechanical propertieswhen compared to cement samples without any modification[23]. However, the results obtained were not optimised; therewas an increase in compressive strength up to 0.6% replacementof cement by mass, but after this further replacement of PC withamino silanes reduced the compressive strength.

The hydrolysis and condensation of amino silanes are relativelyslow and hence amino silanes are the rate-determining step in theprocess of hydration of cement when incorporated within thecement matrix. The amine groups are mainly located on the surfaceof the siloxane matrix because of the hydrophilicity of the groupwhen compared to the hydrophobic tail of the organo-silane. Adetailed mechanism of the reaction of amino silanes is given inthe literature and Ottebbrite et al have elaborated on the reactionmechanism of amino silane-based nanoparticles [112].

Unlike the amino silanes, epoxide-based silanes like GPTMS,have very different chemistry. Here, there is a competitionbetween the polymerisation of both the organic and inorganicchains. The faster polycondensation of one chain can affect thecondensation of other chains [22,98]. GPTMS in a highly basic envi-ronment re-assembles and undergoes in situ polymerisation form-ing nanoparticles [22,113]. This opening of epoxide of GPTMSwhen used in cement during in situ polymerisation makes thecement surface hydrophobic.

This brief chemistry of silanes (TEOS, APTES, and GPTMS) withcement matrix has been included to highlight that, different silanesbehave uniquely within cement matrices based on the functionalgroup on the silane. For instance, the amine on the silane createsa hydrophilic nature whereas the epoxide on GPTMS makes the

Table 2Variation in mechanical properties on using different silicate composition additions under curing condition of approximately 20 ± 2℃, RH = 95%, cement/sand = 3.

SR. NO. Material Type of silicate used in cement mortar Dosage (% of cement by mass) Water to cement ratio (w/c) Compressive strength(MPa)

Ref.

7 days 28 days

1 Blank � � � 46.3 ± 1.7 61.9 ± 1.4 [82]2 APTES Silane 0.5 0.485 52.9 ± 2.2 67.8 ± 3.03 VTES 51.6 ± 1.4 68.5 ± 1.04 GPTMS 1 54.8 ± 1.4 68.0 ± 1.15 APTES 46.5 ± 1.6 63.1 ± 1.46 VTES 46.8 ± 2.1 68.2 ± 2.37 GPTMS 53.9 ± 2.4 66.8 ± 2.58 APTES Sol-gel Nanoparticle 0.5 51.5 ± 2.5 67.3 ± 2.69 VTES 46.4 ± 1.5 60.2 ± 1.510 GPTMS 53.3 ± 2.0 57.5 ± 1.611 Blank � � 0.42 46 57.8 [23]12 AEAPTMS Silane 1 36.5 6413 Blank � � 0.5 32 45 [95]14 TEOS Silane 1 25 3515 APTES 32 5016 AEAPTMS 38 5517 Blank � � 0.4 38 40 [88]18 Sodium silicate Sol-gel Nanoparticle 1 � 50.4

Table 3Use of sol-gel technology in various fields of cement and the effects in the system.

Mode of Involvement ofsol-gel in cement

Mode of Involvement of sol-gel incement

Ref.

Nano C-S-H & Improved early age strength.

& Reduced the nucleation barrier.

[13]

Nano-silica & Enhanced pozzolanic reaction, thusincreased C-S-H.

& Improved mechanical properties.& Increased solution reactivity

[91]

Comparison of C-S-H to sol-gel based inorganicnetwork

& Similarity of silica polymerisation inC-S-H structure and sol-gel synthesisof silicates.

& C-S-H structure is postulated as acolloidal precipitates as in case of sol-gel silicates in basic environment.

[1]

Nano-silica & Improved mechanical properties.

& Higher pozzolanicity increased C-S-Hby 30%

[88]

Admixture by varying sol-gel parameters

& Acidic sols delayed hydration.

& Solvent free sols accelerated cementhydration.

& Dense morphology.

[64]

Nano-silica by varyingtemperature parameter

& Sol-gel prepared nano-silica calcinedat 400 ℃ showed increase inperformance.

& No delay on dormant stage.

[66]

Nanoparticle with 3 silane(GPTMS, VTES, APTES)

&No delay in cement hydration.

& Increased compressive strength.

& Alteration of C-S-H structure bylinking surface hydroxyl groups.

[82]

Hybrid organic-inorganicC-S-H

& Modification of the C-S-H structure. [31]


cement surface hydrophobic. Hence the silanes for the sol-gel pro-cess must be selected based on the application of cement and con-crete structures.

Table 2 shows how different silicate sources affect the mechan-ical properties of cement composites. Two conclusions can beinferred from Table 2 Silanes and sol-gel modified silanes generallytend to enhance the mechanical properties of cement mortars,however, sol-gel silicates not only increase the compressivestrength but also generally do not tend to extend the dormantstage (Fig. 14 and Table 2).

Shakil et al. [96] have emphasised the similarities between sol-gel synthesis of silicates and cement hydration and have studiedthe alterations in the degree of silica polymerisation of C-S-H bycarefully tuning the sol-gel parameters. Instead of sol-gel basednanoparticle, the authors studied sol-gel silicates in the network-ing stage to alter the degree of polymerisation of silica in the C-S-H structure. Several sol-gel routes were carried out: acidic route,basic route, a combination of two silanes, and solvent-free synthe-sis [64]. The findings suggested that the silicate species introducedby the sol-gel process created additional nucleation sites whencompared to the control samples. Shakil et al. [96] observed howa change in parameters caused significant variation in the cementhydration indicating the possibility of control of hydration usingthe sol-gel process. Furthermore, Shakhmenko et al. [97] showedthat sol-gel nano-silica improved the initial day strength four timeshigher than the control, which brings benefit over the silanes thatcaused a delay in retardation during cement hydration. Recently,sol-gel based aluminosilicates were synthesised that replacedabout 15% of the mass of cement [114]. Results from the finding[96] indicated well-structured sol-gel based aluminosilicates actedas a seeding agent and had created reactive nucleation sites thatformed nanoporous C-S-H.

Table 3 shows the different fields in which sol-gel has beenapplied to improve the physical and chemical properties of cementand concrete structures.

4.2.3. Repair of cement and concrete structuresA considerable amount of resources is used in the repair and

rehabilitation of hardened cement and concrete structures[13,99,100,115–118]. As a result, the cost of repair is sometimeshigher than the initial investment. Construction budget of £40 bil-lion/year is being spent on the repair and maintenance of mainlyUK concrete structures [119]. Several nano-silica dispersions, avail-

13

able on the market are employed for the protection of concretestructures. However, these treatments despite their small particlesize find it hard to penetrate the pores of the repair structure[13]. There is a growing use of silanes as a surface treatment agentwhen compared to other metal silicates and epoxy resins due totheir improved dispersion [100]. Several studies have shown thatsilanes improve the mechanical properties of cement composites[13,100] Barberena-Fernández et al. [99], have shown that unhy-drolysed TEOS improved the polymerisation of C-S-H by increasingthe chain length. In situ hydrolysis and condensation of TEOS in the


pores of hardened cement have been reported to form nano-silica,which has further reacted with portlandite in the presence of waterto produce C-S-H [99,120,121]. Consequently, this has proved tooptimise the structural and chemical properties of hardened sur-faces of the cement mortar and hence are used widely as surfaceconsolidant. When compared to silanes and other nano-silica, arecent study [14] was conducted on the use of sol-gel modified sil-icates as repair agents on the hardened cement structures. Theresults showed an increase in penetration of the sol-gel silicatewithin the cement structure when compared to nano-silica. Thereaction mechanism of these sol-gel silicates where similar to thatmentioned by Barberena-Fernández et al however, a distinct differ-ence was the controllability of the formation of C-S-H which wasabsent when unhydrolised silanes were used [13]. Additionally,the sol–gel process allowed to control and simulate the conditionsof concrete pore solution which eliminated the interference of C-S-H and other silica phases within the hardened cement pastes [13].This further shows that sol-gel technology can be a promisingmethod in the repair of cement and concrete structures and neces-sitates further research in this area.

5. Conclusion and outlook

This is the first review paper that projects a new outlook on theuse of sol-gel technology in cement and concrete research. In thisreview, the complexity of cement hydration providing relationbetween C-S-H structure, hydration mechanisms, and mechanicalproperties, in particular strength is covered in a more comprehen-sive fashion. The review also explained the silica polymerisation ofC-S-H, and the principles of sol-gel science.

Furthermore, the ability of sol-gel technology to develop con-trolled silicates are highlighted. Identifying the ability to tailor sil-icates at the nano level using sol-gel technology, the idea was toextend this controllability into the complex silica-based cementhydration.

The review focussed on three main applications of sol-gel pro-cess in cement and concrete structures: (1) re-examining cementhydration; (2) improvement of physical properties of cement com-posites; and (3) repair of cement and concrete structures. Cementhydration follows an identical pathway as that of synthesis of sol-gel silicates in an alkaline route. It was also seen that there is enor-mous potential for modified silanes via sol-gel technology to con-trol complex systems like cement hydration by having control overthe formation of hydration products. This will open doors to a bet-ter understanding of the C-S-H structure. From the review onapplications of sol-gel science in various cement fields, a commonconclusion could be drawn that the reaction of unhydrolisedsilanes was quite different from sol-gel modified silicates withthe same silane as the parent precursor. For instance, in somecases, it was observed that sol-gel modified nanoparticles over-come the problem of retardation in cement pastes which was notthe case when these unhydrolised silanes were used directly intocement pastes. From the above, it can be concluded that sol-geltechnology can certainly control the degree of hydration withinthe cement structure. This is evident as the same starting materialcan yield different results by just altering certain sol-gel parame-ters such as the pH and solvent content.

It is clearly demonstrated in this review that sol-gel technologyis a promising field in the future in various fields of cement sciencelike the repair of concrete structures, cement hydration mecha-nisms, and development of carbon capture cement compositesbecause of its controllability, repeatability, and compatibility withcementitious matrix.

The following areas of sol-gel with cement must be researchedand explored.

14

1) There needs to be more research on the influence of severaldifferent parameters of sol-gel synthesis on the final struc-ture of the hydration products as well as the physical andchemical properties of cement composites. Each of theparameters (amount and type of the silane, pH, temperature,and time of synthesis, drying conditions) have a role to playin the final structure of the hydration product. This reviewprovided an overview on how different starting precursorsused in sol-gel synthesis affect the cement hydration kinet-ics; however, very few synthesis parameters were focusedon these optimisation studies. Research in this direction ofdevelopment of sol-gel silicates as a novel and smart mate-rial can be used in the coming decades as a structural mod-ifying tool in Portland cement clinker and other sustainableand alternative binders.

2) More research should be conducted on synthesis of clinkerconstituents using sol-gel synthesis to understand the reac-tion environment during cement hydration.

3) Further investigation on the use of sol-gel science on alter-native binders, waste materials, natural clays or low-costsynthetic materials like sodium silicates to develop econom-ical solutions.

4) Applying the sol-gel science of CO2 capture within cementcomposites to develop innovative carbon capture cementi-tious materials which can be a promising area of researchin the coming years.

Declaration of Competing Interest

The authors declare that they have no known competing finan-cial interests or personal relationships that could have appearedto influence the work reported in this paper.

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a review on applications of sol-gel science in cement

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