Calcium silicate pastes

CH can be observed as areas darker than the unreacted clinker phases but brighter than the other hydration products. As in calcium silicate pastes, these appear to have grown in regions initially occupied by water. Although the areas appear discrete on two-dimensional sections, they are not necessarily so in the three-dimensional material. They can engulf small cement grains. 

The experimental considerations applying to calcium silicate pastes (Sections 5.1 and 5.2) are equally relevant to cement pastes. Of the methods so far used in attempts to determine the degrees of reaction of the individual clinker phases as a function of time, QXDA (C39,D12,T34,P28) has proved much the most satisfactory. Procedures are essentially as for the analysis of a clinker or unreacted cement (Section 4.3.2), but it is necessary to take account of overlaps with peaks from the hydration products, and especially, with the C-S-H band at 0.27-0.31 nm. The water content of the sample must be known, so that the results can be referred to the weight of anhydrous material. If a sample of the unhydrated cement is available, and its quantitative phase composition has been determined, it may be used as the reference standard for the individual clinker phases in the paste. 

As with calcium silicate pastes, the gelatinous nature of the principal reaction product renders any definition of chemically bound water somewhat arbitrary. The three definitions of water content described in Section... 

The loss above 550°C is due partly to CO2 and partly to the final stages of dehydration of C-S-H and the hydrated aluminate phases. It is not practicable to distinguish the contributions from TG evidence alone, and, unless evolved gas analysis is used, a separate determination of COj should be made. As with calcium silicate pastes, serious errors arise if TG determinations are carried out on material that has been treated with an organic liquid, e.g. to stop hydration. Losses above 550°C of more than about 3%, referred to the ignited weight, indicate serious carbonation either from this or other causes. 

Silicate anion structures in Portland cement pastes have been studied by the methods described in Section 5.3.2 for calcium silicate pastes. Trimethylsily- i lation (TMS) studies (L20,T12,S69,T36,L31,M43,M44) show that, as with C,S. the proportion of the silicon present as monomer decreases with age and that the hydration products contain dimer, which is later accompanied and eventually partly replaced by polymer (>5Si). Some results have i indicated that fully hydrated pastes of cement differ from those of CjS in that substantial proportions of the silicate occur as monomer (S69,L31), but the results of a study in which pastes of CjS, P-CjS and cement were compared (M44) suggest that the differences between the anion structures of cement and CjS pastes are probably within the considerable experimental errors inherent in the method. The recovery of monomer from unhydrated P-CjS was only 66% and results for cement pastes can only be considered semiquantitative. 

Other foreign ions in the anhydrous phases, and the extents to which they tend to pass into the pore solution on hydration. If the C-S-H constituent of the gel is assumed to have the same Ca/Si ratio as in calcium silicate pastes, one would expect that the ratio would be about 1.9 (Si/Ca = 0.53) in a gel with Al/Ca = 0.07. This agrees with some, but not all, of the data in Table 7.1. The hypothesis might explain the observations of Rayment and Lachowski (R29) on the bimodal distribution of Ca/Si ratio in the in situ gel and the relative constancy of its Ca/(Si + Al) ratio. It probably offers the most satisfactory explanation of the existing data but needs to be further tested. Continued studies by TEM of ion-thinned sections may be expected to yield valuable data in this respect. 

Studies on calcium silicate pastes show that the distribution of silicate anion size is shifted significantly upwards with a rise in curing temperature (Section 5.3.2). XRD gives no definite indication that the C-S-H formed at temperatures up to 100 C is more crystalline than that formed at ordinary temperatures, though the product of the 2-month treatment at 90"C mentioned earlier (T5) contained a little a-CjSH, a crystalline phase which is... 

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