COMPOUND FORMATION AT 175 °C IN CALCIUM HYDROXIDE AND SILICA DIOXIDE SUSPENSION
Compound formation peculiarities in the Ca(OH)2–SiO2∙nH2O–H2O system with the molar ratio of CaO/SiO2 equal to 1.5 have been examined. The hydrothermal synthesis of the compounds has been carried out in unmixed suspensions at a temperature of 175 °C. The duration of isothermal curing was 4, 8, 16, 24, 48, and 72 hours. In unstirred Ca(OH)2 and SiO2∙nH2O suspensions with CaO/SiO2 = 1.5, within 4 hours of isothermal curing at 175 °C, dicalcium silicate hydrates – a-C2S hydrate, C-S-H(II) and a low base semi-crystalline C-S-H(I) type calcium silicate hydrate – were formed. Within 48 h of hydrothermal synthesis, Ca(OH)2 was fully reacted, and semi-crystalline C-S-H(II) became metastable and recrystallized into stable compounds – a-C2S hydrate and C-S-H(I). The typical destruction temperatures of a-C2S hydrate and Ca(OH)2 were possible to determine by using differential thermogravimetric analysis data: the destruction of Ca(OH)2 intensively proceeded at a temperature of ~456 ºC when a sample lost 1.85 % of its mass. Meanwhile, a-C2S hydrate destruction occurred at a higher temperature (~478 ºC), and weight loss was equal to 0.58 %. After 72 h of synthesis, a-C2S hydrate prevailed in the products.