Influence of SiO2 crystallinity and impurities on the reactivity of lime-silica mixtures

Abstract

Several industrial by-products (Hi-Silica, thermal silica densified) were characterized from the chemical and mineral viewpoint and investigated their interactions with portlandite at 180 and 200 oC. The molar ratio of primary mixture was CaO/SiO2 = 0.66. The duration of the reaction varies from 24 to 168 hours. The phase composition and properties of calcium silicate hydrates is strongly affected by crystallinity and impurities in SiO2. It was determined that silica fume – Hi-Sil contains a large quantity of absorbed water which retarded the reaction between lime and SiO2. Low-base calcium silicate hydrates form more heavily comparing with pure system. Carbon impurities in silica thermal densified influences reactivity of initial mixture and impede reaction between Ca2+ and Si4+ ions. It was found that reactivity of silica thermal densified significantly increased after burning it at 900 oC and became suitable as raw material for the synthesis of calcium silicate hydrates. Meanwhile, in the lime–quartz– H2O system, due to low quartz solubility rate even in 168 hours of synthesis at 200 oC the main products are 1.13 nm tobermorite and xonotlite. Obtained results were confirmed by XRD, DSC, IR and SEM analysis methods.