Thermal treatment of C-S-H gel at 1 bar H2O pressure up to 200 °C

Homogeneous CaO-SiO2-H2O gels were prepared at Ca/Si molar ratios 0,83, 1.01, 1.21, 1,50, 1.83 and 2.02. These were aged for 1224 months at 25degreesC and subsequently treated in steam, I bar total pressure, at 130 or 200degreesC; also in water at 55 and 85degreesC. Gels with low Ca/Si ratios partially crystallised at 85degreesC. At 130degreesC in steam, crystalline products included 11 and 14 Angstrom tobermorite, xonotlite, afwillite, portlandite and another incompletely characterised phase. At 200degreesC, the gels retained much water but remained amorphous to X-ray powder diffraction (XRD). However, electron microscopy, coupled with diffraction and analysis, disclosed that the "amorphous" product obtained at 85-200degreesC had undergone crystallisation with domains typically 10-1000 nm. At higher bulk Ca/Si ratios, 1.83 and 2.02, much nanoscale precipitation of Ca(OH)(2) occurs, probably by exsolution, such that the residual C-S-H product has a Ca/Si ratio in the range 1.4- 1.5. The complex thermal history of the products is reflected in their pH conditioning ability, measured at 25 degreesC. The results are applied to predict the evolution of pH in a cement-conditioned nuclear waste repository which experiences a prolonged thermal excursion. (C) 2002 Elsevier Science Ltd. All rights reserved.