Construction and Building Materials 196 (2019) 681–691

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Construction and Building Materials

journal homepage: www.elsevier .com/locate /conbui ldmat

Enhancing the performance of metakaolin blended cement mortarthrough in-situ production of nano to sub-micro calcium carbonateparticles

https://doi.org/10.1016/j.conbuildmat.2018.11.1340950-0618/� 2018 Elsevier Ltd. All rights reserved.

⇑ corresponding author.E-mail address: jwang@eng.ua.edu (J. Wang).

Xin Qian a, Jialai Wang a,⇑, Liang Wang a,b, Yi Fang a

aDepartment of Civil, Construction, and Environmental Engineering, The University of Alabama, Tuscaloosa, AL 35487, USAb School of Civil Engineering and Architecture, Anhui University of Science and Technology, Huainan 232001, PR China

h i g h l i g h t s

� CO2 is added into metakaolin blended cement mortar through Pre-carbonation method.� Nano to micro CaCO3 particles have been produced in fresh metakaolin blended cement mortar.� In situ produced CaCO3 particles can react with the aluminate phases to alter the hydration products of the cement.� Both the mechanical properties and durability of the mortar have been improved by the pre-carbonation method.

a r t i c l e i n f o

Article history:Received 11 April 2018Received in revised form 14 November 2018Accepted 16 November 2018Available online 26 November 2018

Keywords:Ordinary Portland cementIn situ productionMetakaolinCalcium carbonateNanoparticlesHydration products

a b s t r a c t

Metakaolin has emerged as a popular supplementary cementitious material to partially replace ordinaryPortland cement (OPC) due to its high pozzolanic reactivity and surface area. In this study, a new method,pre-carbonation method, is proposed to exploit the synergistic effect between the metakaolin blendedcement and calcium carbonate so that the performance of the cement mortar can be further improved.This new method in-situ produces nano to sub-micro calcium carbonate particles in a slaked lime slurrythrough bubbling carbon dioxide. The produced carbonated slurry is then mixed with other ingredients ofthe mortar. The in-situ produced nano to sub-micro calcium carbonate particles possess high reactivitydue to their high surface area, leading to faster and more complete reaction between the cement andthe calcium carbonate. Experimental studies show both the mechanical properties and durability ofthe metakaolin blended mortar have been improved by the proposed method. The calorimetry resultsshow that the carbonated slaked lime slurry can accelerate the hydration of the metakaolin blendedcement due to the extra heterogenous nucleation sites provided by the calcium carbonate nano tosub-micro particles. TGA/XRD results suggest that more ettringite is produced in the mortar/concretewith pre-carbonated slaked lime slurry, which increases the total volume of the hydration products, lead-ing to a denser microstructure of the mortar/concrete. All these findings suggest that in-situ productionof nano to submicro calcium carbonate particles provides a viable way to improve the performance ofmetakaolin blended cement.

� 2018 Elsevier Ltd. All rights reserved.

1. Introduction

The use of supplementary cementitious materials (SCMs) suchas fly ash, metakaolin, and silica fume in ordinary Portland cement(OPC) based concrete has attracted very high interest in last fewdecades [1–7]. This technology can effectively cut the carbonfootprint of the concrete by reducing the amount of OPC in con-crete. Some other potential benefits, such as better mechanical

properties and durability, can also be achieved by replacing OPCwith SCMs [8]. Metakaolin, which is produced by controlledcalcination of kaolin clay, is a representative SCM with very highpozzolanic reactivity [9]. By partially replacing the OPC with themetakaolin, the mechanical properties of the produced concretecan be significantly improved because more binding material,calcium silicate hydrates (C-S-H), can be produced through thepozzolanic reaction between the calcium hydroxide (CH) and themetakaolin. In addition, the large surface area of the metakaolinprovides more nucleation sites for the hydration of the OPC [10].

Table 1Chemical compositions of OPC and metakaolin (%).

Oxide Composition OPC Metakaolin

SiO2 22.94 59.41CaO 64.85 0.13TiO2 0.21 0.51Al2O3 4.89 30.83Fe2O3 2.57 1.42MgO 3.52 2.21Na2O 0.20 2.71K2O 0.81 1.34LOI 2.52 0.68

682 X. Qian et al. / Construction and Building Materials 196 (2019) 681–691

The ground limestone mainly consists of calcite (CaCO3) andhave typical particle sizes comparable to that of OPC. Ground lime-stone is usually treated as an inert mineral filler [11]. The strengthof the concrete can be severely reduced with high dose of groundlimestone replacement due to the dilution effect. However, addingsmall amount of ground limestone to concrete can slightly enhancethe compressive strength of the concrete due to the limited reac-tivity of the limestone powders with the aluminate phases in theOPC. The reaction between the aluminate phases and the calciumcarbonate in the limestone can produce more ettringite and stabi-lize them, which has a much larger volume than other hydrationproducts of the OPC. As a result, the microstructure of the hard-ened concrete is denser, leading to higher compressive strengthof the concrete.

Antoni et al. have shown that the properties of metakaolinblended cement based concrete can be further improved byexploiting the synergistic effect between ground limestone andthe metakaolin [12]. The addition of CaCO3 can influence the distri-bution of lime, alumina and sulfate in the concrete and therebyoptimizes the mineralogy of the hydrated cement pastes. It isreported that CaCO3 could react with monosulfate (AFm) to pro-duce monocarbonate, which is more stable than AFm, as shownin Eq. (1) [13]:

3 CaOð Þ3 Al2O3ð Þ � CaSO4 � 12H2Oþ 2CaCO3 þ 18H2O

! 2 CaOð Þ3 Al2O3ð Þ � CaCO3 � 11H2Oþ CaOð Þ3 Al2O3ð Þ� 3CaSO4 � 32H2O ð1Þ

Current study shows monocarbonate has a bulk modulus of54 GPa [14], which is higher than any other hydration product ofcement. With the presence of carbonate, less sulfate is consumedto produce monosulfate so that more sulfate can be left to produceettringite. According to thermodynamic simulation [15], formationof ettringite and hemicarbonate produces more solid products andreduces the porosity of the hydrated concrete, leading to a strongerconcrete. Meanwhile, by introducing hemicarbonate and monocar-bonate, the conversion of ettringite to AFm is suppressed ordelayed [16]. When metakaolin was used to replace portion ofcement, the aluminate phase of the mix is increased, and therebymore ettringite can be expected if calcium carbonate is available.This is the major mechanism responsible for the synergistic effectbetween the metakaolin and the ground limestone.

However, this synergistic effect cannot be fully realized due tothe low reactivity of the ground limestone. The reactivity of groundlimestone mainly depends on its size, and smaller limestone parti-cles provide more surface area for the reaction between limestoneand C3A [15]. The strength of the produced concrete is significantlyweakened by a high-level replacement of limestone particles atmicrometer size. At this size, most limestone powder can onlyact as an inert filler. To overcome this problem, nano-sized lime-stone powders has been used in metakaolin blended mortar,resulting much improved mechanical properties of the producedmortar [17]. However, practical application of limestone nanopar-ticles in concrete is limited by its low-cost efficiency. The cost ofnanoparticles itself is usually 100–1000 times higher than that ofthe OPC or other conventional raw-materials used in OPC-basedconcrete [18]. In addition, dispersing nanoparticles in concrete isnot trivial, especially for large scale application.

Aiming to enable practical application of calcium carbonatenanoparticles in concrete, Qian et al. [19] proposed a pre-carbonation process, which can in-situ produce CaCO3 particlesat nano to sub-micro size in fresh concrete. In this technique,calcium-rich mineral such as calcium hydroxide is first added intothe mixing water to form a diluted calcium-rich slurry. Then CO2 isbubbled into this slurry to carbonate the mineral, which produceCaCO3 precipitate and possiblely some Ca(HCO3)2 due to excessive

carbonation in the slurry. The unique feature of this pre-carbonation method is that nano to sub-micro particles of CaCO3

are in situ produced. Compared to ground limestone, the CaCO3

produced through the pre-carbonation method possesses very highsurface area (more than 40 times higher than that of OPC) obtainedfrom Brunauer–Emmett–Teller (BET) method. This methodeliminates all the cost associated with grinding and dispersing oflimestone nanoparticles, effectively removing all major barrierspreventing practical application of nanoparticles in concrete.

In this study, the pre-carbonation method is extended to meta-kaolin blended cement based concrete. It is anticipated that in-situproduced nano to sub-micro CaCO3 particles can quickly react withaluminate phases in the blended cement, and therefore, enhancethe performance of the produced mortar/concrete.

2. Materials and methods

2.1. Materials

CO2 gas with a purity of 100% was purchased from Airgas, Uni-ted States. Slaked lime, used as the calcium-rich mineral, was pro-duced by the Montevallo Plant (owned by Lhoist North America).The type I/II OPC and metakaolin was manufactured from SakreteInc. and Powerpozz company, respectively. Their chemical compo-sitions obtained from X-ray fluorescence (XRF) are presented inTable 1. The ground limestone (94% CaCO3) with maximum parti-cles size less than 44 mm was produced by Dudadiesel. The fineaggregate used in this research is natural river sand with specificgravity of 2.70 and water absorption capacity of 0.95%. It wasoven-dried for 12 h at 110 �C before mixed with other ingredientsof the mortar.

2.2. Pre-carbonating the slaked lime slurry

Two concentrations of the slaked lime were used to make thecarbonated slaked lime slurry: 0.14 mol/L (1% by weight of water)and 0.52 mol/L (3% by weight of water). After mixing the slakedlime with deionized water for 3 min, CO2 with a flow rate of 2 L/min was bubbled into the slaked lime slurry at the ambient tem-perature (23 �C), as shown in the Fig. 1. An air stone was used tohelp homogenously disperse CO2 gas in the slurry. During carbon-ation process, a magnet stir was used to stir the slurry with a stir-ring rate of 800 rpm. The carbonation duration for 1% and 3%slaked lime slurry were chosen as 10 min and 15 min to fully car-bonate the slaked lime slurry, respectively. These carbonationdurations were determined by monitoring the pH value of theslaked lime slurry during the carbonation process. As illustratedin our previous study [19], the pH value of the slaked lime slurryreduces with carbonation time. Once all slaked lime is fully carbon-ated, the pH value of the slurry will reach a minimum value andmaintain at this value regardless whether CO2 is ingested intothe slurry. The carbonation durations (10 min and 15 min) arechosen such that the pH value of the slurry already reaches the

Fig. 1. Set-up used to carbonate the slaked lime slurry.

X. Qian et al. / Construction and Building Materials 196 (2019) 681–691 683

minimum value. The pre-carbonated slaked lime slurry was thenmixed with other ingredients of the sample immediately afterthe carbonation process.

2.3. Heat evolution analysis

Isothermal calorimetry test was conducted according to ASTMC1679 and ASTM C1702 [20,21] to understand how pre-carbonation alters the hydration kinetics of the metakaolinblended cement pastes. To this end, a few pre-carbonated pastesamples were prepared with the carbonated slaked lime slurry.The 1% and 3% pre-carbonated slaked lime slurries were preparedbased on the set-up shown in Fig. 1. Before mixed with the carbon-ated slurry, metakaolin blended cements were prepared by par-tially replacing the cement with the metakaolin and mixing for3 min. Three percentages of metakaolin replacement were used:10%, 20%, and 30%. The control paste specimen with a water to bin-der (w/b) ratio of 0.5 was mixed for 2 min before placed in thecalorimeter. The pre-carbonated blended cement paste sampleswere produced in the same way as the control group except thatthe pre-carbonated slaked lime slurries were used to replace themixing water. Same initial mixing temperature was used to makethe control and the pre-carbonated specimens. The hydration heatproduced in the first 30 min was not used to minimize the effectinduced by the mixing of the cement pastes. In addition to thepre-carbonated groups, two groups specimens with limestone(1% LS and 3% LS) were also prepared to compare the effectsinduced by the ground limestone and the CaCO3 produced by thepre-carbonation method. The amounts of limestone added to thesetwo groups were equal to the amounts of calcium carbonate pre-cipitates produced by carbonating 1% and 3% slaked slurry, respec-tively. Another two control groups with slaked lime (1% SL and 3%SL) were also prepared with the same mix as the pre-carbonatedgroups without pre-carbonating the slaked lime slurries.

2.4. Hydration products examination

The cement paste samples were prepared with the same pro-portions as those used in the calorimetry test. The blended cementpastes were ground with a ceramic mortar before analysis. X-raydiffraction (XRD) patterns of all cement paste samples wereobtained by scanning between 8� and 23� with a Bruker D8 Dis-cover at a scanning rate of 10�2h per minute using Cu Ka radiationat 35 kV and 20 mA. Thermogravimetric analysis (TGA) was alsoused to examine the hydration products at a heating rate of

10 �C/min from 30 �C to 1000 �C by a Simultaneous Thermal Ana-lyzer 8000 from PerkinElmer.

The morphologies and microstructures of the cement pasteswere obtained by using a JOEL 7000 FE scanning electron micro-scope (SEM). Before being placed in the SEM chamber, all the spec-imens were coated with a layer of gold (Au) to prevent charging.

2.5. Mechanical property testing

Seven groups of 20% metakaolin blended cement mortar sam-ples were manufactured with w/b ratio of 0.5 to investigate theeffect of pre-carbonating the slaked lime on the mechanical prop-erties of the produced mortars. As shown in Table 2, two pre-carbonated groups, 1% PreC and 3% PreC, were made by replacingthe OPC with same amount of CaCO3 particles produced in thepre-carbonated 1% and 3% slaked lime slurries, respectively.Groups 1% LS and 3% LS were prepared by replacing the OPC withthe ground limestone at the same amount as the CaCO3 producedby carbonating 1% and 3% slaked lime slurries, respectively. As dis-cussed before, the carbonation duration was chosen to ensure thatthe slaked lime can be fully carbonated. Therefore, the amount ofcalcium carbonate precipitates can be calculated based on thechemical reaction between carbonic acid and the slaked limeadded in the slurry. Groups 1% SL and 3% SL were produced byusing the same proportions as the pre-carbonated groups but with-out carbonating the slaked lime slurries. To evaluate the effective-ness of the pre-carbonation method on the blended cement mortarwith different metakaolin replacements (10% and 30%), six moregroups of mortar samples were made, as shown in Table 3. Super-plasticizer was used to tune the workability of the mortar samplesto ensure the same workability was achieved by all the groups.

50 � 100 mm cylinder mortars were manufactured to test thecompressive strength based on ASTM C109 [22]. After kept insealed plastic molds for 24 h, these cylinder specimens weredemolded and cured in lime-saturated water at 23 �C until desig-nated ages of 3 d, 7 d, and 28 d, respectively. At each age, compres-sion test was conducted on three duplicated specimens, and theaverage values were reported.

Three groups, Control, 3% LS, and 3% PreC, were prepared toevaluate the effect of pre-carbonation on the flexural strength ofmetakaolin blended mortars using the same mix design shown inTable 2 based on ASTM C348 [23]. The fresh mortar samples werecast into 40 � 40 � 160 mm steel prismmolds and compacted witha vibration table. The molds were sealed with plastic sheets for24 h before demolding. All the specimens were then cured in amoist chamber at ambient temperature until the testing age. At7 d and 28 d, the flexural strength tests were carried out on threeduplicated specimens with a MTS QTest/25.

2.6. Durability test

2.6.1. Water sorption testSince there is a strong correlation between the rate of water

absorption and the permeability of the concrete [24], the watersorption test was chose to examine the effect of pre-carbonationmethod on the durability of metakaolin blended mortar. The watersorption rate of the Control, 3% LS, and 3% PreC groups, shown inTable 2, were obtained using ASTM C1403 [25]. Three duplicatedspecimens with size of 50 � 50 � 50 mm were cast and stored ina moist chamber covered by plastic sheet to prevent drippingwater for 24 h before demolding. After demolding, all the speci-mens were cured in a sealed plastic bag at 23 �C until 28 d. Thespecimens were then dried in an oven at 110 �C for at least 24 huntil the difference between two successive measurements of theweight of the specimen was less than 0.2%. After cooled in an ambi-ent environment, the width and length of the soaking surface and

Table 2Mix design for 20% metakaolin blended mortars produced with/without using pre-carbonation method (kg/m3).

Constituent Control 1% LS 3% LS 1% SL 3% SL 1% PreC 3% PreC

Cement 288.2 286.3 282.4 286.3 282.4 286.3 282.4Metakaolin 72.0 71.5 70.5 71.5 70.5 71.5 70.5Water 180.1 180.1 180.1 180.1 180.1 180.1 180.1River sand 907.7 907.7 907.7 907.7 907.7 907.7 907.7Slaked lime – – – 1.8 5.4 1.8 5.4Limestone – 2.4 7.3 – – – –Superplasticizer 1.9 1.9 1.9 2.4 2.4 3.0 3.8Pre-carbonation duration – – – – – 10 min 15 min

X%LS: limestone slurry with equivalent CaCO3 as fully carbonated X% slaked lime slurry.X%SL: X% slaked lime slurry.X%PreC: Pre-carbonated X% slaked lime slurry.

Table 3Mix design for blended mortars with different metakaolin replacements (kg/m3).

Metakaolin replacement 10% 30%

Constituent Control 1% PreC 3% PreC Control 1% PreC 3% PreC

Cement 324.2 322.0 317.6 252.1 250.0 245.6Metakaolin 36.0 35.8 35.3 108.1 107.8 107.3Water 180.1 180.1 180.1 180.1 180.1 180.1River sand 907.7 907.7 907.7 907.7 907.7 907.7Slaked lime – 1.8 5.4 – 1.8 5.4Superplasticizer – – – 2.8 3.5 3.8Pre-carbonation duration – 10 min 15 min – 10 min 15 min

Group codes are the same as in Table 2.

684 X. Qian et al. / Construction and Building Materials 196 (2019) 681–691

the initial weight of the specimens were measured and recorded.Then all the specimens were immersed 3 mm into water, and theweight of these immersed mortars was measured at 0.25 h, 1 h,4 h, and 24 h.

2.6.2. Sulfate resistance testSulfate resistance was tested on the metakaolin blended mortar

bars prepared based on ASTM C1012 with dimension of25 � 25 � 280 mm [26]. The w/b ratio of the mortar is 0.485, andthe ratio between river sand and cement is 2.75 for all control,limestone, and pre-carbonation specimens. After casting, the spec-imens were initially cured in a sealed container and placed into anoven at 35 �C for 24 h. Then the specimens were demolded andcured in saturated lime water at 23 �C until an average compres-sive strength of 20 Mpa measured by cubic specimens made insame batch was reached. After recording the initial length, all thesemortar bars were submerged in the 50 g/L sodium sulfate solutionwhich was produced by dissolving anhydrous sodium sulfate witha purity of 99% into deionized water. The lengths of these mortarswere measured using a comparator after 7 d, 14 d, 21 d, 28 d, and56 d.

3. Results and discussions

3.1. Heat evolution

The calorimetry results of the 20% metakaolin blended cementpastes prepared with and without using pre-carbonation methodare shown in Fig. 2. Both pre-carbonated metakaolin blendedcement pastes exhibit higher and stronger peaks of the hydrationof tricalcium silicate (C3S) and tricalcium aluminate (C3A) com-pared to that of other groups in both 1% and 3% cases. As shownin Fig. 2(a), the hydration of the paste is actually slightly sloweddown by replacing OPC with limestone powder or slaked limebecause of the dilution effect of these two replacements. It alsosuggests that the size of the ground limestone must be smallenough in order to accelerate the hydration of the cement throughthe seeding effect. The size of the ground limestone used in this

study is clearly too big to achieve this accelerating effect. Thiscan be further confirmed by Fig. 2(b) in which more OPC wasreplaced by the ground limestone. Once again, the hydration ofthe blended cement paste was retarded. Unlike in Fig. 2(b), 3% SLgroup exhibits higher hydration heat release rate than the controlgroup. This is because more slaked lime is added, which improvethe pH value of the cement paste, leading to higher dissolution rateof the cement. Fig. 2(c) and (d) show the accumulated releasedhydration heat per gram of all cement pastes. It can be seen thatthe accumulated hydration heat of the paste at 60 h was improvedup to 5% and 7% by precarbonating 1% and 3% carbonated slakedlime slurries, respectively.

All these figures clearly suggest that the pre-carbonationmethod is effective on promoting the hydration of the cement;while simply adding the ground limestone or the slaked limemay not, and sometime, retard the hydration of the cement. Thepromotion effect induced by the pre-carbonation method can beattributed to the higher surface area provided by the nano tosub-micro CaCO3 particles in-situ produced by the pre-carbonation method. For this reason, better accelerating effect onthe hydration was achieved by the 3% PreC group because morenucleation sites were produced in this group than in the 1% PreCgroup. More nucleating sites also lead to a shorter dormant stageof the 3% PreC group, as can be observed in Fig. 2( b).

Similar promotion effect induced by the pre-carbonationmethod were also observed on the hydration of the 10% and 30%metakaolin blended cement pastes, as shown in Fig. 3. It showsthat the accelerating effect of the pre-carbonation method is morepronounced with 3% carbonated slaked lime slurry in bothpastes, which is in agreement with Fig. 2. Clearly, this is becausemore nano to sub-micro CaCO3 particles are produced by pre-carbonating 3% slaked lime slurry.

3.2. Hydration products

XRD analyses were carried out to study the effect of pre-carbonation method on the hydration products of the metakaolinblended cement and to verify the speculation mentioned above.

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Fig. 2. Isothermal calorimetry curves of 20% metakaolin blended cement pastes produced with and without using pre-carbonation method: (a) Thermal power for metakaolinblended cement paste with 1% replacement; (b) Thermal power for metakaolin blended cement paste with 3% replacement; (c) Accumulated released hydration heat ofmetakaolin blended paste with 1% replacement; (d) Accumulated released hydration heat of metakaolin blended paste with 3% replacement.

X. Qian et al. / Construction and Building Materials 196 (2019) 681–691 685

The XRD patterns of the metakaolin blended cement paste at 4 dare presented in Fig. 4. Fig. 4(a) and (b) show that pre-carbonation may slightly enhance the content of ettringite in 20%metakaolin blended samples. Similar trends were also observedin 10% and 30% metakaolin replacement groups, as shown inFig. 4(c) and (d). It should be pointed out that quantitative analysisof the hydration product based on these XRD is not reliable. There-fore, TGA was carried out.

TGA results are shown in Fig. 5. Three main peaks appear on thederivative mass loss graphs of all these samples, corresponding tothe decomposition of ettringite, CH, and calcite, respectively. Asmarked in these figures, the typical decomposition temperatureranges for ettringite, AFm, CH, and calcite are 80–105 �C,180–200 �C, 400–500 �C, and 680–760 �C, respectively [27–30].The calculated weight losses of ettringite, AFm, portlandite (CH),and calcite of 20% metakaolin blended cement produced usingdifferent methods are presented in Table 4 based on thetemperature range mentioned above. More ettringite was foundin two pre-carbonated samples, which is in good agreement withthe XRD results shown in Fig. 4. Among seven groups, the oneproduced by pre-carbonating 3% slaked lime has the lowestcontent of CH because more CH was consumed by the reactionbetween the CH and the metakaolin accelerated by the pre-carbonation method. This is in agreement with the calorimetry

results, which shows the paste produced by pre-carbonating 3%slaked lime has the strongest peak of C3A. The amount of calcitein the 1% PreC sample is lower than the control one, due to reactionbetween the calcium carbonate and aluminate phase to producemonocarbonate or hemicarbonate phases. On the contrary, thecontent in 3% PreC group has more calcite than the control group.This is because more calcite was produced by carbonating 3%slaked lime slurry.

Two groups prepared by adding the ground limestone possesshigher amount of calcite than other groups, suggesting that themajority of the added ground limestone remain unreacted. Fig. 5and Table 4 also shows that the pastes prepared using the un-carbonated slaked lime slurries have more CH than other pastes.This is anticipated because extra slaked lime was added into thepaste to replace small portion of the OPC. No significant differenceon the content of AFm (including both monocarbonate and mono-sulfate) was found among these samples. TGA analysis clearly indi-cates that the CaCO3 particles produced by the pre-carbonationmethod can alter the mineral composition of the hydrated meta-kaolin blended cement, even at an early age.

The representative SEM images of 20% metakaolin basedcement paste made with 3% ground limestone (Fig. 6(a,c)) andusing carbonated 3% slaked lime slurry (Fig. 6(b,e,f)) were obtained.Comparing Fig. 6(a) and (b) suggests that the microstructure of the

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Fig. 3. Isothermal calorimetry curves of blended cement pastes produced with different metakaolin replacements: (a) blended cement paste with 10% metakaolin; (b)blended cement paste with 30% metakaolin.

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Fig. 4. XRD patterns of the metakaolin blended cement pastes produced with and without using pre-carbonation method: (a) blended cement mortar with 20% metakaolinand 1% replacement; (b) blended cement mortar with 20% metakaolin and 3% replacement; (c) blended cement mortar with 10% metakaolin; (d) blended cement mortar with30% metakaolin.

686 X. Qian et al. / Construction and Building Materials 196 (2019) 681–691

paste prepared with the carbonated slaked lime slurry is denserthan the one produced by adding ground limestone. Fig. 6(c) andFig. 6(d) show that large unreacted CaCO3 particle can be foundin the specimen made with ground limestone. However, no signof CaCO3 particle, like the one shown in Fig. 6(c), was found in

the cement paste made with the carbonated slaked lime slurry.As shown in Fig. 6(e), some hexagonal shape particles, were foundin the pre-carbonated sample, which is similar to hemicarbonate[31]. Fig. 6(f) presents needle-like ettringite found in the cementpaste made by the pre-carbonation method.

Fig. 5. TGA results of metakaolin blended cement pastes produced with and without using pre-carbonation method at 4 d: (a) 20% metakaolin blended cement paste with 1%replacement; (b) 20% metakaolin blended cement paste with 3% replacement; (c) 10% metakaolin blended cement paste; (d) 30% metakaolin blended cement paste.

Table 4The calculated weight losses of 20% metakaolin blended cement pastes at 4 d.

Specimen Weight loss (%) related to the decomposition of

Ettringite AFm CH Calcite

Control 3.34 0.65 2.33 0.841% LS 3.21 0.65 2.24 1.063% LS 3.17 0.66 2.17 1.391% SL 3.04 0.63 2.31 0.913% SL 3.20 0.59 2.48 0.771% PreC 3.78 0.64 2.30 0.693% PreC 3.58 0.65 2.0 0.93

X. Qian et al. / Construction and Building Materials 196 (2019) 681–691 687

3.3. Mechanical properties

Calorimeter results suggest that the hydration of the metakao-lin blended cement is promoted by using the carbonated slakedlime slurry. This suggests that the mechanical properties of themetakaolin blended cement mortars should be enhanced by the

pre-carbonation method. Fig. 7 compares the compressivestrengths of the metakaolin blended cement mortars preparedwith different methods mentioned in Section 2.5. For the 20%metakaolin replacement, the compressive strengths of the mortarsamples with pre-carbonated 3% slaked lime slurry were enhancedby 21%, 19%, and 13%, at ages of 3 d, 7 d, and 28 d, respectively. This

Spectrum 1

(a) (b)

(c) Spectrum 1 (d)

(e) (f)

Fig. 6. Representative SEM images of 20% metakaolin blended cement pastes: (a) and (c) 3% ground limestone specimen; (b), (e), and (f) 3% pre-carbonated specimen; (d)corresponding EDX spectrum of (c).

688 X. Qian et al. / Construction and Building Materials 196 (2019) 681–691

improvement in strength is higher than that of the 1% pre-carbonated slaked lime slurry. This difference can be attributedto more nucleation sites for the hydration of the cement providedby carbonating more slaked lime, agreeing well with the calorime-try study (Fig. 2). This significant improvement cannot be achievedby simply replacing the cement with slaked lime or limestonepowders, as revealed by Fig. 7(a). It shows that the compressivestrength of the mortar sample was actually reduced by replacingthe OPC with 3% ground limestone, which can be attributed tothe low reactivity of the ground limestone.

Similar strength improvement was also achieved in the 10% and30% metakaolin blended cement mortars by the pre-carbonationmethod. Higher strength improvement can be achieved by theblended cement with higher metakaolin replacement. This isbecause more aluminate is available in the cement with highercontent of metakaolin. Aluminate can react with CaCO3 to producemore ettringite and AFm phases, as shown in Eq. (1), leading todenser microstructure and more improvement of the strength ofthe mortar. Similar trend was also observed by Antoni et al. [12].

They found that limestone addition is more effective in the blendedcement mortar with higher metakaolin replacement.

The flexural strengths of the control mortars, the one producedthrough carbonating 3% slaked lime slurry, and the one producedwith ground limestone replacement are shown in Fig. 8. The pre-carbonated method improves the flexural strength of themetakaolin blended cement mortar by 10% and 9% at 7 d and28 d, respectively, which is not observed in the group with groundlimestone. This improvement on the flexural strength of the meta-kaolin blended cement mortar is induced by the CaCO3 particlesproduced by the pre-carbonation method.

3.4. Durability

The durability tests, including water sorption test and sulfateresistance test, were carried out on the metakaolin blended cementmortar produced by the pre-carbonation method. Fig. 9 comparesthe rate of water absorption of three groups of 20% metakaolinblended cement mortars. Compared with the control group, the

Control 1% LS 3% LS 1% SL 3% SL 1% PreC 3% PreC0

10

20

30

40

50

60

100%

100%

100%

Com

pres

sive

Stre

ngth

(Mpa

)

Groups

3 d 7 d 28 d

101%

103%

104%

96%

96%

98%

98%

100%

102%

103%

102%

107%

107%

116%

116%

111%

119%

121%

20% Metakaolin replacement

(a)

Control 1% PreC 3% PreC0

10

20

30

40

50

60

100%

100%

Com

pres

sive

Stre

ngth

(Mpa

)

Groups

3 d 7 d 28 d

100%

106%

107%

110%

107%

111%

118%

10% Metakaolin replacement

(b)

Control 1% PreC 3% PreC0

10

20

30

40

50

60

Com

pres

sive

Stre

ngth

(Mpa

)

Groups

3 d 7 d 28 d

100%

100%

100%

113%

111%

111%

113%

115%

128%

30% Metakaolin replacement

(c)

Fig. 7. Compressive strengths of metakaolin blended cement mortars preparedby using different methods: (a) 20% metakaolin blended cement mortar; (b)10% metakaolin blended cement mortar; (c) 30% metakaolin blended cementmortar;

Control 3% LS 3% PreC0

1

2

3

4

5

6

109%

110%

97%

100%

100%

Flex

ural

Stre

ngth

(Mpa

)

Groups

7 d 28 d

100%

Fig. 8. Flexural strengths of metakaolin blended cement mortars produced withand without using pre-carbonation method.

Control 3% LS 3% PreC0

5

10

15

20

25

30

35

Wat

er a

bsor

ptio

n (g

/cm

2 )

Groups

0.25 h 1 h 4 h 24 h

Fig. 9. Water sorption of metakaolin blended cement mortars produced with andwithout using pre-carbonation method.

X. Qian et al. / Construction and Building Materials 196 (2019) 681–691 689

pre-carbonated group exhibits lower water absorption rate from0.25 h to 24 h. Water sorption at 24 h was reduced by 7% by thepre-carbonation method. This improvement is induced by the den-ser microstructure of the sample made by the pre-carbonationmethod. The water absorption rate of the 3% LS group is evenhigher than that of the control group, suggesting that directlyadding ground limestone powder cannot achieve the sameenhancement on the durability of the mortar as the pre-carbonation method.

The effect of pre-carbonation method on the sulfate resistanceof the produced metkaolin blended cement mortar was evaluatedby measuring the expansion of the metakaolin blended cementmortar bars immersed in 50 g/L sulfate solution. As shown inFig. 10, the expansion of the mortar bar produced with carbonating3% skaled lime slurry is lower than that of the control one, suggest-ing that pre-carbonating 3% slaked lime can improve the sulfateresistance of the mortar. This improvement is attributed to thelower permeability of the mortar reuslting from the densermicrostructure induced by the pre-carbonation method.

0 1 2 3 4 5 6 7 8 90.000

0.005

0.010

0.015

0.020

Expa

nsio

n (%

)

Time (weeks)

Control 3% LS 3% PreC

Fig. 10. Expansions of metakaolin blended cement mortars produced with andwithout using pre-carbonation method in 50 g/L sodium sulfate solution.

690 X. Qian et al. / Construction and Building Materials 196 (2019) 681–691

4. Conclusions

This study exploits the novel pre-carbonation method toimprove the performance of the metakaolin blended cement mor-tar. This new method can in-situ produce CaCO3 particles at nanoto sub-micro scale in the mixing water. These particles can pro-mote the hydration of the cement by providing extra seeding sitesfor the hydration products of the OPC, as evidenced by thecalorimetry testing. TGA analyses suggest that the in-situ producedCaCO3 particles can react with the aluminate phases in the cementto alter the hydration products of the cement, producing highervolume of the hydration products. This can densify the microstruc-ture of the hardened mortar. As a result, both the mechanicalproperties and durability of the mortar can be improved by thepre-carbonation method, as validated by the experimental studycarried out in this study.

Compared with ground limestone powder in micrometer scale,the CaCO3 particles produced by the pre-carbonation method ismuch more reactive. Therefore, the synergistic effect between themetakaolin and the calcium carbonate can be utilized by the pre-carbonation method to enhance the performance of the metakaolinblended cement mortar. This enhancement cannot be achieved byusing the ground limestone powders at micrometer size, asdemonstrated by the experimental program in this study. Sincenanoparticles can be produced in situ in the pre-carbonationmethod, the cost of grinding and dispersing of limestone nanopar-ticles is eliminated. The pre-carbonation method can also makebeneficial use of CO2 from industry exhaust and thereby reducethe carbon footprint of the produced concrete. More work will becarried out on other calcium-rich minerals, like slag and calciumrich fly ash, instead of slaked lime to capture CO2 to further explorethe potential of this technique.

Conflict of interest

None.

Acknowledgement

The financial supports from National Science Foundation, Uni-ted States (CMMI-1761872) and the Research Stimulation Programat the University of Alabama are highly appreciated.

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