Influence of Quartz Particle Size and Ca/Si Ratio on Strength of Autoclaved Calcium Silicate Board

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The chemical reactions and strength of lime-quartz based autoclaved board samples were investigated. The samples were prepared using different particle sizes of quartz under saturated steam pressures at 180°C for various times from 1-80 h. The quartz particle sizes used were 2.6 (finer), 8.4 (middle), 15.5 (coarser), μm in average diameter. The starting materials of Ca (OH) <SUB>2</SUB> and quartz with Ca/Si = 0.6, 0.8 and 1.0 were mixed with cellulose fiber and water, followed by molding to a density of 1000 kg·m<SUP>-3</SUP>. The demolded boards were then hardened by autoclaving. After autoclaving for 1-4 h all the specimens, yielded hydrothermally formed Ca-rich C-S-H, which further reacted with silica to give a crystalline tobermorite with 1.1 nm spacing. The finer quartz with a lower Ca/Si ratio gave a higher strength after a short autoclaving time and a finer microstructure. With prolonged hydrothermal treatment, the specimens with a coarser quartz size and a Ca/ Si = 0.8, gave the highest strength, indicating higher tobermorite crystallinity. After 80 h of treatment though, the binding products of 1.1 nm tobermorite decomposed into gyrolite for the specimens with a 0.6 and 0.8 Ca/Si ratio and xonotlite for those with a Ca/Si of 1.0, resulting in a decrease in strength and a coarser microstructure.
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