Microcrystalline calcium hydroxide in pozzalanic cement pastes

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Pergamon

Cement and Concrete Research, Vol. 2~, No. 6, pp. 1191-1196,1994 Copyright © 1994 Elsevier Scionc¢ lad Printed in the USA. All fights reserved 0008-8846/94 $6.00+.00

0008-8846(94) 00063-8 MICROCRYSTALLINE CALCIUM HYDROXIDE IN POZZOLANIC CEMENT PASTES G.W. Groves and I.G. Richardson University of Oxford, Department of Materials, Parks Road, Oxford OX1 3PH, U.K.

(Refereed) (Received December 10, 1993;in f'malform May 11, 1994)

ABSTRACT A microcrystalline form of calcium hydroxide consisting of dusters of lamallae, -10rim in thickness, intimately mixed with C-S-H gel, has been observed in a lime-silica cement paste by transmission electron microscopy. The volumes of calcium hydroxide and C-S-H in the dusters are comparable. The implications of this are discussed. Introduction The observation of calcium hydroxide (CH) in microcrystalline form in hardened ordinary Portland cement pastes of low water/cement ratio has previously been reported (1). In contrast to the commonly observed form of calcium hydroxide in hardened pastes, namely, large relatively perfect crystals, the microcrystalline form, observable only by transmission electron microscopy (TEM), consisted of plates ~10nm thick, parallel to the basal plane of the calcium hydroxide crystal. These plates tended to lie parallel to one another in clusters in local regions but were variously oriented about an approximately common c-axis. No determinations of the composition of these clusters or the surrounding C-S-H gel were at that time made, although a positive identification of crystalline calcium hydroxide was made from selected area electron diffraction patterns (1). In a current study (2) of the pozzolanic reaction, hardened pastes have been prepared from a mixture of calcium oxide and silica, having the overall composition 3CaO.SiO2. Upon examining sections of the paste by TEM regions of microcrystalline calcium hydroxide very similar to those previously observed in Portland cement paste were found. Similar regions have been observed in a blast-furnace cement activated by calcium hydroxide. This paper presents a study of these regions using microanalysis as well as electron diffraction in the TEM. 1191

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Experimental A paste was m a d e up of a mixture of calcium oxide and amorphous silica particles having a surface area of 56m2g-1, in the proportions to give a paste of composition 3CaO.SiO2. Because of the large surface area and consequent high water d e m a n d it was necessary to use a high water/solids ratio of 0.8 to obtain a sufficiently fluid mix (superplasticizer was not added). The paste was cured in sealed tubes for 54 days at ambient temperatures (approximately 20°C). Slices of the hardened paste were then cut and TEM specimens prepared by ion-beam thinning, using a cold stage to limit the temperature rise during thinning. The hardened paste was noticeably weak, as might be expected from the high water/solids ratio used. A blast furnace slag paste, activated by calcium hydroxide and hydrated for 3.3 years was also examined in the TEM. The microscope used was a JEOL 2000FX operating at a voltage of 200KV. The microscope was equipped with a Tracor Northern X-ray analyser. The elements Ca and Si were analysed for, referring to standards of C3S and Wollastonite to derive Ca:Si atomic ratios for the regions analysed. Results a)

Morphology and Crystallography

Figure 1 shows a region of microcrystalline calcium hydroxide in the CaO/SiO2 paste together with the electron diffraction pattern from the region, which lies in the lower left hand area of the micrograph. In the micrograph, which gives the view of an essentially 2-dimensional section, this region has the appearance of lines of contrast which are roughly parallel to the preferred orientation of the basal planes of calcium hydroxide, as indicated by the innermost reflection of the diffraction l~attern. This reflection, together with the three outer reflections, 10i0, 10~1 and 1120, which are visible, indicate a fibre-texture about the crystallographic c-axis. The conclusion, as in previous work (1) is that the microcrystalline region contains thin plates parallel to the basal plane which tend to lie roughly parallel to one another but which display a wide range of orientations corresponding to various rotations about the c-axis. Inspection of the diffraction pattern indicates that a substantial proportion of the plates have their caxis at some angle to the plane of the specimen or in other words that m a n y of the plates are not seen exactly edge-on. The plates appear to be about 10nm in thickness, although this m a y exaggerate slightly their true thickness due to a projection effect. Clear diffraction contrast from the microcrystals is seldom evident but fig.2 displays a few strongly diffracting regions, showing intense black contrast. These are no more than 20-30nm in length suggesting either that the plates are not continuously crystalline or that misorientations about the c-axis occur frequently within individual plates as well as between plates. A further area showing regions of strong diffraction contrast is seen in fig.3. Unusually, this area gave uniform rings in diffraction, indicating a chaotic range of orientations of crystallites in the area. In all other microcrystalline areas examined, at least some degree of texture was evident. In order to show more clearly the parts within the microcrystalline regions giving crystalline calcium hydroxide reflections, dark field micrograp_hs were obtained using an objective aperture admitting about one quarter of the 1010 and 1011 reflection

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LIME-SILICA CEMENT, MICROCRYSTALLINE Ca(OH)2, TEM

Fig.1

Fig.3

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Fig.2

Fig.4

rings. An example is s h o w n in fig.4. In a few cases, the bright areas (i.e. areas diffracting strongly into the aperture) appear as lines -10nm thick and ~100nm long, consistent with plate-like crystals of thickness -10nm. In other cases the bright areas are specks ~10nm in size, but often lined up suggesting a plate-like crystal with either many breaks in its structure or many changes in crystallographic orientation on a 10nm scale within it. These inferences are consistent with those d r a w n from bright-field micrographs such as fig.2.

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Compositions

Areas of C-Soil gel were distinguished by a random foil-like appearance as for example in the region to the upper right of fig.l, and by a diffraction pattern showing only two very diffuse rings, characteristic of C-S-H viewed in the TEM (3). The composition of C-S-H regions was analysed and was found to be significantly different in two different though nominally identical specimens. In one specimen calcium hydroxide was not seen (albeit in a very limited thin area) and the C / S ratio of the gel was 1.50 + 0.08 (see Table 1). In the second specimen in which the microcrystalline calcium hydroxide was found, the C/S ratio of the C-S-H gel was 1.11 + 0.06. Whether the difference in specimens resulted from inhomogeneity in the original paste, or had some other cause, is not known. All C-S-H gels had C/S ratios below the C / S ratio of about 1.8 which is typical for a C3S paste (4). The analysis of the composition of microcrystalline calcium hydroxide clusters showed that considerable quantities of silicon were present in the clusters. If this were found in a few cases only it might be attributed to C-S-H gel present in the thin section above or below the cluster, but in view of the substantial extent of the clusters, the limited thickness of the electron-transparent region of the specimen, and the fact that all 11 areas analysed showed substantial quantities of silicon, it must be concluded that the silicon is present within the clusters themselves. The average Ca:Si ratio found for the microcrystalline calcium hydroxide regions was 3.42 + 1.17 (Table 1). The most likely source of the silicon is C-S-H gel intimately mixed on a nanometre scale with the calcium hydroxide. Assuming the mean Ca:Si ratio of 3.42, a density for the CS-H gel of 2000kg/m 3 and a composition with a Ca:Si ratio equal to that most commonly found in the C-S-H gel adjacent to the microcrystalline clusters i.e. CI.ISHz, gives the result that the volumes of calcium hydroxide and C-S-H within the clusters are r o u g h l y the same, although with considerable variability from one cluster to another. The diffraction patterns from the microcrystalline regions are consistent with the_presence of C-S-H since they exhibit weak diffuse scattering in the region of the 1010 calcium hydroxide reflection, as does C-S-H resulting from cement hydration (3). Table 1 Area ca:si ratio number of analyses

Ca:Si atom ratios in analysed areas C-Soil gel

.

C-S-H gel

. microcrystals

microcrystals

specimen I

specimen 2

CaO/SiO2 paste

sla~ paste

1.50+0.08 10

1.11+0.06 9

3.42+1.17 11

5.67+3.12 11

.Discussion The clusters of microcrystalline calcium h y d r o x i d e seen in the present pozzolanic cement appear similar in terms of morphology and crystallography to those seen previously in OPC hardened pastes of low w a t e r / c e m e n t ratio (1). The new finding is that the clusters consist of an intimate, nanometre-scale mixture of calcium

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LIME-SILICA CEMENT, MICROCRYSTALLINE Ca(OH)2, TEM

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hydroxide and C-S-H. Since microanalysis was not performed in (1), there is no direct proof that this finding applies also to the clusters in (1). However, because of the strong similarities otherwise, it appears reasonable to suggest that this is indeed the case. In reviewing previous TEM work by the authors (unpublished) it was noted that similar clusters of calcium hydroxide with substantial silicon content had been observed also in a hardened paste of blast furnace slag activated by calcium hydroxide addition. In this paste, the C / S ratio of the C-S-H gel was 1.33 + 0.19 (Table 1) and the C / S ratio of the microcrystalline calcium hydroxide clusters was 5.67 + 3.12 (Table 1). The volume of CS-H inferred to exist within the clusters is substantial, comparable to the volume of calcium hydroxide itself. The morphology of the C-S-H within a cluster of calcium hydroxide microcrystals cannot be identified with certainty. The diffuse scattering of electrons by C-S-H has been found to be too weak to allow effective dark-field images of the C-S-H alone to be produced. However the distribution of CH microcrystals revealed by dark-field (fig.4) is fairly homogeneous, within a cluster, suggesting that the C-S-H is intermingled with plates of calcium hydroxide on a very fine scale. This in turn suggests a co-deposition of C-S-H and calcium hydroxide. It has been proposed that the structure of C-S-H is based on extremely distorted layers of calcium hydroxide, with OH groups replaced by SiO4 tetrahedra (5). It may be that calcium hydroxide and C-S-H have similar nuclei, leading to the possibility of intimate co-deposition. Theories of the retarding action of organic additives on cement hydration appear to support this view in proposing that good retarders are able to poison nuclei of both calcium h y d r o x i d e and C-S-H by adsorption (6-8). The resolving p o w e r of our analytical technique does not allow the composition of the C-S-H within the cluster to be analysed directly. However, it seems reasonable to assume that it is of similar C/S ratio to that of the surrounding C-S-H, i.e. of relatively low C/S ratio. The presence of layers of crystalline calcium hydroxide intermingled with C-S-H of low C/S ratio adds credibility to a model for C-S-H of higher C / S ratio in which layers of calcium hydroxide are combined on an atomic scale with tobermorite-like layers (9).

The conditions leading to the formation of microcrystalline calcium hydroxide are not clear. In previous work (1), it was observed in Portland cement pastes of relatively low w a t e r / c e m e n t ratio, but a high water/solid ratio of 0.8 was used to form the pastes reported on here. It is likely that, during the curing time of 54 days, reaction of the lime-silica mixtures was incomplete and that further reaction w o u l d lead to a mixture of calcium hydroxide with a C-S-H of higher Ca:Si ratio than that reported here. In this process, microcrystalline calcium hydroxide might react to form additional C-S-H. Further study of the progressive development of microstructure in pozzolanic cements would be of interest. Conclusions Microcrystalline calcium hydroxide, similar in crystallography and morphology to that observed previously in Portland cement pastes, has been observed in pozzolanic cement pastes. Compositional analysis of clusters of the microcrystals indicates that the microcrystals within the clusters are intermixed on an extremely fine scale with a comparable volume of C-S-H gel.

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Acknowledgements The support of SERC under grant GR/H64972 is acknowledged. References 1. 2. 3. 4. 5. 6. 7. 8. 9.

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