microstructural alterations of slag and...

MICROSTRUCTURAL ALTERATIONS OF SLAG AND SILICA FUME CONCRETES SUBJECTED TO SHORT TERM HEAT CURING

Manu Santhanam (1) and Karen Scrivener (2)

(1) Department of Civil Engineering, IIT Madras, Chennai, India

(2) LMC, EPFL, Lausanne, Switzerland

Abstract This paper describes the microstructural study of slag and silica fume concretes subjected

to short term heat curing. Qualitative and quantitative characterization of the microstructure was performed using backscatter electron imaging and image analysis. Significant differences were observed in the composition of the inner C-S-H as well as in the distribution of sulphate and aluminate in the outer C-S-H for the concretes with slag and silica fume as compared to plain OPC. Further, the degree of hydration measured using image analysis was substantially different. The study also explored the role of the initial pre-heating delay period on the microstructure of the heat cured concretes.

1. INTRODUCTION

Heat curing of concrete is performed to accelerate the rate of setting and strength gain of concrete, in various applications such as cold weather concreting and in precasting operations. Sustained high temperature curing of concrete is known to adversely affect its long term strength and durability. According to Verbeck and Helmuth (1968), heat curing of OPC concrete led to the development of a coarser microstructure, with a non-uniform distribution of hydration products. It has also been reported that differences between the microstructure of high temperature (50 – 80 oC) cured concretes and normal temperature (10 – 30 oC) cured concretes are more evident at higher degrees of hydration (Kjellsen et al. 1991). While there is some conflict amongst researchers as regards the comparative degrees of hydration between normal and high temperature cured concretes, it is generally agreed that strength reduction at later ages is because of increased coarse porosity at high temperatures (Kjellsen et al. 1990).

Microstructural studies on OPC concrete at high temperature indicate that the inner C-S-H at high temperatures has a higher relative brightness, and is thicker than at normal temperatures (Zhang 2007, Kjellsen et al. 1991). The difference in brightness is attributed to the difference in C-S-H composition (in terms of bound water content, and C/S ratio). It has also been reported that different segments of inner C-S-H may form depending on the temperature curing history of the concrete (Famy et al. 2002). When concrete is subjected first

Second International Conference on Microstructural-related Durability of Cementitious Composites, 11-13 April 2012, Amsterdam, The Netherlands

to heat curing and then to lower temperature moist curing, two distinct bands of C-S-H are seen to form, the lighter outer band at the high temperature and the darker inner band in the long term because of the normal curing. The calcium hydroxide also deposits in a more compact form at high temperatures (Kjellsen et al. 1991). Higher hollow shell porosity has also been observed at higher temperatures (Kjellsen 1996). More monosulphate forms as opposed to ettringite at the early ages for high temperature cured concrete (Zhang 2007), and the inner C-S-H also demonstrates higher affinity for sulphate at high temperatures (Zhang 2007, Famy et al. 2002).

Research on blended cement systems at high temperature (Escalante-Garcia and Sharp 1998) indicates that there is increased consumption of calcium hydroxide by the pozzolanic reaction. Furthermore, higher degree of hydration for all cement phases has also been reported at high temperatures. Maltais and Marchand (1997) reported that long term strength reductions observed for OPC mortars subjected to isothermal curing at 40 oC did not occur for fly ash mortars. In terms of durability, slag and silica fume have been shown to be beneficial in reducing the rate of chloride diffusion through concrete subjected to steam curing (Campbell and Detwiler 1993). The size and continuity of pores is reduced in concrete with slag and silica fume (Cao and Detwiler 1995).

Microstructural studies indicate that slag mixes at high temperatures produce large quantities of monosulphate and reduced amounts of calcium hydroxide (Escalante-Garcia and Sharp 2001). Slag hydration products are denser at high temperature, and higher Mg content is reported for the slag inner product at high temperature (Cao and Detwiler 1995, Escalante-Garcia and Sharp 2001). Large numbers of Hadley grains were also reported in silica fume paste at 70 oC (Cao and Detwiler 1995).

It is evident from literature that there is limited understanding of the effects of heat curing on the performance of concrete with mineral admixtures, from the viewpoint of microstructure. This research study outlines the microstructural alterations observed in slag and silica fume concrete subjected to short term heat curing and subsequent moist curing. Also evaluated in this study is the effect of the pre-heating (or delay) period on the performance of the heat cured concretes.

2. EXPERIMENTAL DETAILS

2.1 Materials, mixture design, specimens, and tests Binders used included CEM I cement, Ground granulated blast furnace slag, and silica

fume. 20 mm maximum size coarse aggregate (river gravel) and river sand were used, along with a sulphonated naphthalene formaldehyde based superplasticizer to maintain the workability of the mixes in the same range (25 – 50 mm slump). Three categories of concretes were prepared, each with a total binder content of 350 kg/m3 and w/c of 0.5. For the OPC series, only CEM I was used as binder, while for the PCSL series, CEM I and slag were used in the ratio 60:40, and for the PCSF series, CEM I and silica fume were used in the ratio 90:10. 11 x 22 cm cylinder specimens were prepared for each mix and stored in conditions described in section 2.2.

Representative samples of concrete were removed from cylinder specimens at the testing age, and stored in isopropanol for a week, followed by air drying for at least 2 days. These samples were then impregnated with a low viscosity epoxy resin, and polished on a series of


grits, down to 1 µm fineness. The polished epoxy-impregnated specimens were then subjected to carbon coating and used for scanning electron microscopy (SEM) studies.

The SEM used for the study was equipped with stage control software, which enabled the stage to move to different positions within the selected user-defined area so as to obtain photomicrographs of specimen at the same magnification and light settings. The microscope used for the study was operated at an accelerating voltage of 15kV. For BSE imaging the spot size was chosen to have a good resolution of image, while for the EDS analysis spot size was a little higher to generate desired X-ray results. The count rate for EDS was about 20 kcps and counting time was real time of 10 s. Microanalyses were performed with ZAF corrections. Analyses were made for Na, Mg, Al, Si, S, K, Ca, and Fe. Oxygen was stoichiometrically calculated, to provide the analysis totals.

2.2 Curing For each concrete mix, specimens were divided equally among four curing regimes,

including the control curing at 20 oC. The high temperature regimes, outlined below, are explained schematically in Figure 1.

(i) ‘601’: referring to 1 hour pre-heating period (ii) ‘604’: referring to 4 hours pre-heating period (iii) ‘608’: referring to 8 hours pre-heating period

A large bowl of water was placed inside the heating chamber to ensure that no drying took place from the surface of the specimens. The temperature rise period (from 20 to 60 oC) was approximately 1 hour, the high temperature period (at 60 oC) was 5 hours, and the cooling period (from 60 to 20 – 30 oC) was 5 hours.

Moulds were removed after 24 hours, the tests at 1 day were performed, and remaining specimens were transferred to the moist room until the day of testing.

Figure 1. Details of the high temperature curing regimes followed in the study


3. RESULTS

3.1 General appearance In the case of OPC concrete, even at 20 oC, there is evidence of substantial hydration at 1

day, which results in low open porosity. AFt deposits are observed at 1 day (Figure 2), but there are no discernable AFt deposits at later ages. On the other hand, heat cured OPC concretes show substantial AFm deposits at 1 day (see Figure 2), and there is some recrystallization of ettringite in voids at later ages (as early as 7 days – see Figure 3). With increase in the curing duration, the matrix porosity was seen to reduce, as expected.

Figure 2. OPC20 showing AFt deposits (left) and OPC608 showing AFm deposits (right) at 1 day

Figure 3. OPC608 showing AFt deposition in voids at 7 (left) and 28 (right) days

The high temperature specimens exhibit a thick layer of inner C-S-H as early as 1 day, and some evidence of two toning is seen as early as 7 days. As reported in literature (Zhang 2007, Famy et al. 2002), the inner C-S-H in heat cured specimens is brighter than in the control specimens, at all ages. Qualitatively, there is not much difference between the heat cured mixes with different delay periods (1 – 8 hours).


PCSL specimens at 20 oC showed very high porosity and less hydration as compared to the OPC control specimens. The heat cured slag concretes show significantly greater AFm deposits compared to OPC, as seen in Figure 4. However, there is hardly any evidence of ettringite deposition in the voids at later ages, such as occurred in the OPC concretes. Hydration of slag particles is seen by a rim forming around them as early as 1 day for the heat cured mixes (see Figure 4). Small slag particles are also completely hydrated for the heat cured mixes as shown in Figure 4. On the other hand, the mix at 20 oC shows evidence of slag hydration only at later ages (28 days and beyond). In the presence of slag, hydration of alite particles is extensive, with sufficient inner C-S-H forming at 1 day (Figure 5).

In contrast with OPC, two-tone inner C-S-H is not clearly observed for the heat cured slag concretes. By 90 days, all slag concretes showed a dense microstructure, with a number of slag particles showing evidence of hydration. There was no discernible difference between the control and heat cured specimens.

Concrete with silica fume exhibited a microstructure similar to OPC. There was evidence of a greater degree of hollow shell porosity in the case of heat cured silica fume mixes (example in Figure 6), which agrees with the observations in literature (Kjellsen 1996). In a trend similar to the slag concretes, the brightness of the inner C-S-H was not markedly different for the heat cured mixes compared to the control concrete. Thus, evidence of two tone C-S-H could not be detected. Also, unlike OPC and slag concretes, there were no significantly large deposits of AFt or AFm. At 28 days, all concretes showed a compact and dense microstructure (Figure 7), with little porosity, which is expected in the case of silica fume concretes. By 90 days, there was no discernible difference between the control and heat cured concretes.

Figure 4. PCSL604 at 1 day with AFm deposits and evidence of hydrating slag particle (left) and PCSL608 at 7 days showing completely hydrated small slag particles (right)


Figure 5. Thick inner C-S-H in PCSL604 at 1 day

Figure 6. PCSF601 at 1 day, showing many hollow shell hydration grains

Figure 7. PCSF604 at 28 days, showing dense microstructure, and several fully hydrated cement grains


3.2 Composition of inner and outer C-S-H Energy dispersive X-ray analysis was performed on specimens for determining the

chemical composition of the inner and outer C-S-H products of the hydrated cement paste. Microanalyses were performed only in the bulk paste phase of the concrete and only specimens hydrated for 90 days were selected for inner C-S-H analyses. This was because only the 90-day specimens formed inner C-S-H of sufficient thickness for the EDS spot analysis with reliable results to be performed. Analyses of outer product phase were made more arbitrarily but always at spots considered to be essentially of C-S-H. In each specimen, at least 200 analyses were performed for the inner and outer C-S-H. The composition of outer C-S-H was presented in the form of plots between S/Ca ratio and Al/Ca ratio to understand the nature of C-S-H, while the Ca/(Si+Al) and S/Ca ratios were determined for the inner C-S-H.

The composition of inner C-S-H for the different concretes is presented in Table 1. Figures in brackets indicate the standard deviation. Ca/(Si+Al) ratios varied between 1.8 and 2.1 for OPC concretes. This ratio was lower for slag concretes, in the range 1.55 – 1.65, and still lower for silica fume concretes, between 1.44 and 1.63. For all three concretes, the general trend observed was that this ratio decreased upon heat curing. Longer delay periods resulted in lower ratios, although this cannot be confidently concluded based on the available data as there are exceptions.

Table 1. Details of inner C-S-H composition

Mix Ca/(Si+Al) S/Ca

OPC20 2.036 (0.100) 0.040 (0.011)

OPC601 2.051 (0.097) 0.039 (0.012)

OPC604 1.797 (0.159) 0.039 (0.012)

OPC608 1.954 (0.160) 0.038 (0.017)

PCSL20 1.654 (0.163) 0.022 (0.010)

PCSL601 1.642 (0.162) 0.027 (0.013)

PCSL604 1.584 (0.165) 0.047 (0.028)

PCSL608 1.547(0.058) 0.034(0.018)

PCSF20 1.625(0.064) 0.027(0.016)

PCSF601 1.539(0.056) 0.034(0.011)

PCSF604 1.509(0.059) 0.044(0.015)

PCSF608 1.438(0.082) 0.036(0.028)


With respect to the S/Ca ratio, unlike reports in literature (Zhang 2007), the C-S-H in heat cured OPC concretes did not really show higher sulphate inclusions. This could be because of the short duration of heat curing, which did not allow for diffusion into the inner C-S-H. For slag concretes, although the S/Ca ratio is seen to increase for the heat cured specimens, conclusions cannot be clearly made since the standard deviations are also high for these concretes. Similar observations hold good for the silica fume concretes also. Overall, it can be said that short term heat curing does not significantly affect the sulphate content of inner C-S-H.

The elemental distribution for the matrix from OPC20 is presented in Figure 8. The compositions of AFm and AFt are also marked on the figure. The analysis shows that the aluminates in the matrix have a composition consistent with AFm. Similar inferences were drawn for all the concretes, and the summary is presented in Table 2. The results in Table 2 indicate that the behavior of silica fume concrete is similar to OPC concrete, with the exception of OPC20 at 1 day (where AFt is predominant in the outer C-S-H). For the heat cured OPC and silica fume concretes, the sulphate and aluminate distribution in the outer C-S-H is characteristic of AFm at early and later ages. In the case of slag concrete, the matrix shows little AFm, as the S/Ca ratios are low even at 1 day; however, there are abundant deposits of AFm in the heat cured slag concretes. At 90 days, the Mg content of outer C-S-H was markedly different (higher) for the heat cured slag concretes compared to the control slag concrete.

Figure 8. S/Ca and Al/Ca ratios for OPC20 matrix – compositions fall on the AFm ratio line

3.3 Degree of hydration The degree of hydration was determined from the analysis of the BSE images, after a series

of operations involving collection of near 200 images at 800x magnification, image processing to obtain correct control of brightness and contrast, selection of images with a majority of paste phase, segmentation of phases, and calculation of the phase amounts. The whole process was programmed in a software, but several difficulties were encountered because of the mixed nature of the aggregate, and the presence of slag particles (which have similar gray level as cement particles). The area fraction of anhydrous cement was calculated by comparing the number of pixels representing anhydrous cement to total number of pixels


of the image. The volume fraction of anhydrous phases relative to the cement paste in the concrete was calculated, and the degree of hydration was determined from the following equation:

Degree of hydration α = 1- (1)

where v0 is the initial volume fraction of the cement particles present in the fresh paste of concrete, and v1 is the volume fraction at the time period of consideration. The degrees of hydration for the different concretes as a function of age are presented in Tables 3 – 5. Please note that the highlighted cells show inconsistent measurements, and certain data points are not available due to problems in specimen preparation.

Table 2. Summary of EDX on outer C-S-H – observations of the composition, in terms of the type of aluminate

Mix Outer C-S-H at 1 day Outer C-S-H at 90 days

OPC20 AFt AFm

OPC601 AFm AFm

OPC604 AFm AFm

OPC608 AFm AFm

PCSL20 Low S/Ca Low S/Ca; Mg/Ca less than 0.05

PCSL601 Low S/Ca; but plenty of AFm deposits Low S/Ca; Mg/Ca up to 0.15



PCSF20 AFt and AFm mix AFm

PCSF601 AFm AFm

PCSF604 AFm AFm

PCSF608 AFm AFm

Table 3. Degrees of hydration for OPC concretes

Specimen 1 day 7 day 28 day 90 day OPC 20 63.2 (7.7) 74.1 (9.0) 80.4 (9.9) 88.2 (5.6) OPC 601 80.5 (6.9) 84.0 (6.4) 87.0 (6.2) 87.7 (6.5) OPC 604 78.9 (8.0) 69.2 (11.8) 87.0 (6.5) 89.9 (6.5) OPC 608 70.0 (9.4) 81.4 (8.6) 83.7 (6.5) 87.7 (5.4)


Table 4. Degrees of hydration for PCSL concretes

Specimen 1 day 7 day 28 day 90 day PCSL 20 76.1 (5.9) 86.4 (5.4) 90.9 (5.1) 91.3 (6.0) PCSL 601 75.6 (7.1) 87.0 (4.6) 82.6 (5.5) Not Available PCSL 604 77.3 (7.5) 83.6 (5.8) 86.0 (4.8) Not Available PCSL 608 82.5 (7.1) 84.8 (7.3) 86.8 (4.9) 90.3 (4.9)

Table 5. Degrees of hydration for PCSF concretes

Specimen 1 day 7 day 28 day 90 day PCSF 20 74.5 (8.0) 81.9 (9.0) 84.1 (7.9) 84.4 (6.6) PCSF 601 77.1 (6.2) 79.4 (6.6) 77.3 (7.0) 84.5 (7.5) PCSF 604 77.7 (7.8) 79.0 (7.8) 81.4 (5.9) 82.8 (6.5) PCSF 608 72.1 (8.6) 75.9 (7.2) Not Available Not Available

The data for degree of hydration, calculated using image analysis, suggest the following:

Hydration of cement phases is accelerated in the presence of slag or silica fume, since the degree of hydration in the PCSL and PCSF series (at 20 oC) is consistently higher than for OPC. This is consistent with reports in literature.

The heat cured slag and silica fume concretes have similar degrees of hydration as their respective control concretes, for almost all curing ages.

The silica fume mixes do not show substantial enhancement in the degree of hydration beyond 28 days, while there is significant increase on the degree of hydration for the control OPC mix (OPC20) and all the slag mixes between 28 and 90 days. The slag concretes reach the highest degree of hydration among all concretes at 90 days.

Of course, it must be understood that the technique used to find the degree of hydration rests on the amount of unhydrated cement only. The contributions of slag and silica fume are difficult to bring about in the image analysis process selected.

4. CONCLUSIONS

Microstructural alterations in concrete subjected to short term heat curing were discussed in this paper. The differences in microstructure between OPC concrete and slag / silica fume concretes were described based on a qualitative and quantitative study using backscattered scanning electron microscopy coupled with image analysis. Heat cured slag concretes show significantly greater AFm deposits as compared to OPC concrete, but there is no evidence of ettringite re-deposition in the long term, as is the case with OPC concrete. The relative amounts of calcium sulphoaluminates in silica fume concrete is lower, and these concretes show dense microstructure, with almost no difference between the heat cured and normally cured concretes at 90 days. Heat curing led to a decrease in the Ca/(Si+Al) ratio of the inner C-S-H for all three concretes. Hydration of cement phases was seen to be accelerated in the presence of slag or silica fume. Heat cured slag and silica fume concretes showed similar degrees of hydration as their respective control concretes at all ages, while heat cured OPC concrete showed higher degrees of hydration at early ages.


5. REFERENCES

[1]. G. J. Verbeck and R. A. Helmuth (1968), Structures and Physical Properties of Cement Pastes, Proceedings, Fifth International Symposium on the Chemistry of Cement, Vol. III, The Cement Association of Japan, Tokyo, pp. 1 - 31.

[2]. K.O. Kjellsen, R.J. Detwiler and O.E. Gjørv (1991), Development of microstructures in plain cement pastes hydrated at different temperatures, Cem. Concr. Res. 21, p. 179

[3]. K.O. Kjellsen, R.J. Detwiler and O.E. Gjørv (1990), Pore structure of plain cement pastes hydrated at different temperatures, Cem. Concr. Res. 20, p. 927 – 933.

[4]. X. Zhang (2007), Quantitative microstructural characterisation of concrete cured under realistic temperature conditions, PhD Dissertation, Swiss Federal Institute of Technology (EPFL) Lausanne.

[5]. C. Famy, K.L. Scrivener, A. Atkinson and A.R Brough (2002), Effects of an early or a late heat treatment on the microstructure and composition of inner C-S-H products of Portland cement mortars, Cem. Concr. Res. 32, p. 269-278.

[6]. K.O. Kjellsen (1996), Heat curing and post heat curing regimes of high performance concrete: Influence on microstructure and C-S-H composition. Cem. Concr. Res. 26, p. 295–307.

[7]. J. I. Escalante-Garcia and J. H. Sharp (1998), Effect of temperature on the hydration of the main clinker phases in portland cements: Part II, Blended Cements, Cem. Concr. Res. 28, p. 1259-1274.

[8]. Y. Maltais and J. Marchand (1997) Influence of curing temperature on cement hydration and mechanical strength development of fly ash mortars, Cem. Concr. Res. 27, p. 1009-1020.

[9]. G.M. Campbell and R.J. Detwiler (1993), Development of Mix Designs for Strength and Durability of Steam-Cured Concrete, Concrete International 15, p. 37.

[10]. Y. Cao and R.J. Detwiler (1995), Backscattered electron imaging of cement pastes cured at elevated temperature, Cem. Concr. Res. 25, pp. 627–638.

[11]. J.I. Escalante-Garcia and J.H. Sharp (2001), The microstructure and mechanical properties of blended cements hydrated at various temperatures, Cem. Concr. Res. 31, p. 695-702.


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