Hydrothermal reactions in the lime-rich part of the system CaO-SiO₂-H₂O

Abstract

Summary The mineralogy of autoclaved products from various forms of calcium silicate andfrom lime and silica has been investigated. The results are as follows. With a CaO/SiO 2 ratio of 1, the product is always well crystallized tobermorite. With a CaO/SiO 2 ratio of 2, the product varies: starting with lime and silica, dicalcium silicate alpha-hydrate is the first product; this eventually changes into dicalcium silicate beta-hydrate, the stable phase; starting with calcium silicate hydrate I and lime, the first product comprises Flint's CSH(A) and dicalcium silicate alpha-hydrate, and it seems likely that the Flint's CSH(A) will eventually convert to dicalcium silicate alpha-hydrate and the dicalcium silicate alpha-hydrate to dicalcium silicate beta-hydrate; if one starts with β-2CaO.SiO 2 , the product in the first place is dicalcium silicate gamma-hydrate; in the length of processing used in the experiments reported in this paper, no other phase was found, but it is considered that dicalcium silicate gamma-hydrate is only metastable. With a CaO/SiO 2 ratio of 3, again the products vary with the starting minerals: with mixes of lime and silica and with mixes of 2CaO.SiO 2 and lime, the first product is dicalcium silicate alpha-hydrate which is eventually converted to the beta-hydrate; with mixes of calcium silicate hydrate (1) and lime, the product is always Flint's CSH(A) with or without some dicalcium silicate alpha-hydrate; with 3CaO.SiO 2 as the starting material, the product is always 3CaO.SiO 2 .1½H 2 O. It is clear from this work that the form of the starting material has a considerable influence on the nature of the metastable phases produced, and also that, in this part of the system CaO-SiO 2 -H 2 O, even prolonged processing up to 90 days is insufficient to produce true equilibrium phases.