Reaction and the Solid Reation Products in the System CaO-Silica Gel-Water at 110°C

Abstract

As previously reported (J. Ceram. Assoc., Japan, 68 [3] 63 (1960)) the low temperature modifications of calcium hydrate silicates play an important roll in the bonding of cement at ordinary temperatures. And also the high temperature modifications formed hydrothermaly play a dominant part in the hardening of cement paste and relating secondary products cured at elevated temperature.The present paper contains the results of the investigations on the reaction and solid reaction products in the system CaO-SiO2 (gel)-water at 110°C by means of chemical analysis, electron microscope, X-ray and electron diffraction, differential thermal analysis, and differential thermo-balance.The starting mixtures of the C/S(=CaO/SiO2) mol. ratios 0.8-3.0 were kept at 110°C for 3 or 7 days in an autoclave with an electromagnetic stirrer.The results obtained are summarized as follows:(1) The C/S of the reaction products were in the range 0.76-1.69, increasing nearly proportional with the increase of C/S of the starting mixtures as long as C/S≤1.50, whereas C/S of the reaction products gave nearly the same values when the mol. ratios of the starting mixtures C/S≥1.50.(2) Practically no difference was found in the amount of CaO remaining unreacted whether process was continued 3 or 7 days. This means that the reaction stops before 3 days, although the growth of crystals continues farther with time.(3) The solid reaction products obtained were tobermorite phase in film, crumpled foil, fiber or plate, and afwillite in prismatic crystals.Tobermorite phase appeared in fiborous form from the starting mixture of C/S=0.80, and in plate form from C/S=1.00 or 1.20. Prismatic afwillite was the main solid phase produced from lime rich starting mixtures (C/S>1.20). In the case of C/S=1.2 the mixed crystals of tobermorite phass and afwillite was observed, in which the former is developing from the surface of the latter.(4) The filmy or fiborous tobermorite phase may be identical with CSH (B) or C2SH2, and the plate will be C4S5H5. Both fibers and plates showed the habit of developing in the direction of b-axis.The cementing properties of tobermorite phase and afwillite seems to be in the following order:tobermorite phase (plate)>tobermorite phase (film and fiber)>afwillite(5) The thermal curves showed an exothermic peak arising from a temperature between 782° and 823°C. The film or fiber crystal of tobermorite phase gave the higher peak than the plate crystal.Dehydration of combined water up to around 600°C was measured by a thermo-balance. Afwillite showed an endothermic peak starting at a temperature between 282° and 331°C (dehydration of combined water) and also an exothermic peak arising between 816° and 893°C (structural change).