Calcium silicate hydrates, formed hydrothermally

A sodium metasilicate modified high-alumina cement may be used in geothermal wells at temperatures up to 300°C (Sugama and Garciello, 19%). Sodium calcium silicate hydrate and boehmite are formed as products of reaction in the hydrothermal reaction. 

Calcium Silicates. Cements aie hydiated at elevated tempeiatuies foi the commercial manufacture of concrete products. Using low pressure steam curing or hydrothermal treatment above 100°C at pressures above atmospheric, the products formed from calcium siUcates are often the same as the hydrates formed from their oxide constituents. Hence lime and siUca ate ftequendy used in various proportions with or without Portland cement in the manufacture of calcium siUcate hydrate products. Some of these compounds are Hsted in Table 6. 

In [27-29], hydrothermal, mechanochemical and solid-phase syntheses of calcium silicate from anhydrous and hydrated oxides were compared. Initial components were taken at Ca/Si ratio equal to 0.8 1.0 1.2. According to X-ray analysis, the interaction in the mixtures of anhydrous oxides under mechanical activation is not completed. The product being formed is X-ray amorphous. When heated at 600-800 C, it is crystallized in the form of a -Ca2Si04 (Fig. 3.6a). At higher temperatures, Ca3Si207 is formed P-CaSiOj is crystallized at 860 C. The observed sequence of stages is similar to those observed in solid-phase synthesis of wollastonite. The amount of p-CaSiOj at 900°C does not exhibit any substantial dependence on the initial fractions of the reagents. 

---