Impact of amorphous micro silica on the C-S-H phase formation in porous calcium silicates

Amorphous micro silica (MS) is used as a raw material in the production of porous calcium silicate. MS is a by-product from the silicon and silicon-alloy production, and strongly influences the properties of porous calcium silicate products. We investigate the kinetics of the formation of calcium silicate hydrate (C-S-H) gel from the reaction between quicklime and MS based on two types of MS (MS97.5 and MS92.8, which have 97.5 and 92.8 wt% SiO2, respectively). The C-S-H gel is found to be amorphous, but with short- and medium range order similar to tobermorite for the two types of MS. MS97.5 is found to react faster than MS92.8. The activation energy is determined to be 76 kJ/mol for MS97.5 and 103 kJ/mol for MS92.8. For comparable surface area of MS, the C-S-H formation mechanism is strongly affected by metallic or metal oxide impurities in two ways. First, the positively charged metal ions are attached to the negatively charged hydroxyl groups at the MS surface, resulting in less available hydroxyl groups for MS-quicklime reaction, and in lower solubility of MS. Second, impurities react with the calcium silicate network, making less raw material available for the reaction.