CeraMat

Analysis of the process used to hydrate orthorombic anhydrite (CaSO4-II) obtained by the heat treatment of gypsum has shown that its reactivity is closely related to its specific surface. If, however, the BET surface is greater than 6-7 m2/g, reactivity is also dependent on the degree of microstructural disorder. These two parameters in turn are closely dependent on the thermal treatment for synthesis.

In order to clarify the influence of grinding on anhydrite reactivity to water, the authors have ground natural and synthetic products in different ways and investigated the evolution of microstructure and the rate of the conversion: CaSO4 II → CaSO4·2H2O, as a function of grinding. Different parameters have been used to characterize the anhydrite microstructure; nevertheless, only the specific surface BET and very short-range disorder, defined by the 'disorder coefficient' (Cd), can affect the product reactivity.

The expansion of three solid solutions of the ferrite phase, C6A2F, C4AF and C6AF2, in the presence of CaSO4 · 2H2O, Ca(OH)2 and siliceous powder with water/solid ratio of 0.3, has been compared with that of two reference mortars: the first containing C3A, Ca(OH)2 and CaSO4 · 2H2O, the second C3A doped by 4,5% Fe2O3, Ca(OH)2 and CaSO4 · 2H2O. The expansion of mortars, based on the ferrite phase, has been observed to be a function of Al2O3 contents, and is much weaker than that of mortars based on C3A. For these last ones, the expansion decreases from C3A to doped C3A.

The employment of non-dolomitic calcareous aggregates in the composition of concrete may serve to cause degradation if the mother rock contains sufficiently disordered silicates in the form of finely dispersed microinclusions. This paper reports the observation of fissures due to the formation of gel as a result of ASR in aggregates with less than 4% of microinclusions and even when this value was less than 1%.

A sol-gel process produced microsphere of Al2O3 containing 10 wt% ZrO2 by coprecipitating Al and Zr hydroxides from the respective chlorides through the addition of NH4OH. In this paper, gel processing is examined as a function of temperature, pH, and residual Cl- content. The washed gel was peptized at 80°C with HCl and the sol concentrated to an Al3+ ion content of 4 mol/L. The concentrated sol is used to feed a pilot plant for microsphere production. The microspheres calcined at 1200°C are dense, with an average size of 20 μm and a BET surface area of 30 m2/g.