Mature tricalcium silicate cement pastes were examined in ion-beam-thinned sections. The microstructure consisted of very homogeneous amorphous hydrate gel surrounding residual unhydrated cores of tricalcium silicate, these regions beiig linked by a fibrillar outer hydration product. Carbonation had produced microcrystals of calcite within the outer product only. X-ray microanalysis gave Ca : Si ratios for the hydrate regions which showed considerable deviations but were on average in fairly good agreement with indirectly determined Ca: Si ratios of pastes of the same material. There was no significant difference in composition between inner and outer hydration products.
Materials of very high flexural strengths (> IOOMPa) can be made by high shear mixing of cement pastes with small amounts of water soluble polymer. Two such systems - high alumina cement/partially hydrolysed polyvinyl acetate and OPC/polyacrylamide - have been examined using a variety of techniques including isothermal calorimetry, infra-red spectroscopy and X-ray diffraction. In both systems the polymer addition appears to become cross-linked by ions released by the cement powder leading to an increase in stiffness of the material. In particular the reactions involved in the OPC system are analogous to those occurring in certain dental cements. The importance of removing excess water from the pastes is demonstrated.
A number of oxides, including MgO, ZnO, La203, A1203, TiOz, and F were reacted with C3S, either individually or in combination, in order to stabilize a variety of polymorphic forms. The resultant phases were examined by both XRD and TEM. It was found that the results obtained by the conventional means of observing groups of reflections in powder X-ray diffraction patterns were not in agreement with the structure types identified by electron diffraction. In particular, the rhombohedral form of alite, synthesized by the addition of either ZnO or A1203 + F to C3S, was not observed by TEM although the corresponding X-ray patterns all displayed single peaks Characteristic of the rhombohedral cell. Electron diffraction of C3S doped with 2.02 and 4.43% ZnO gave monoclinic patterns, whereas C3S doped with A1 + F gave several forms, all of them different from the form produced by ZnOdoped C3S. The addition of La203 produced a new hexagonal lattice with cell dimensions of a = 2.575 and c = 2.50 nm. When Al + F was incorporated in C3S, the modifications found included the triclinic lattice characteristic of pure C3S, the hexagonal lattice characteristic of La-doped C3S, and a complex lattice distinguished by the presence of incommensurate doublet reflections and sometimes extra weak spots. A similar incommensurate superlattice was found in alites containing Mg, Al, and a large amount of Ti (0.13 mole). The Jeffery cell for the Mtll modification of C3S doped with 1.63% MgO was confirmed.
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