Impact of Micro Silica Surface Hydroxyl Groups on the Properties of Calcium Silicate Products

Porous calcium silicates are widely used in insulating systems for high temperature applications. In the production of porous calcium silicates, quicklime and micro silica have been utilized as key raw materials. In the reaction between SiO2 and CaO, the dissolution of SiO2 has been proven to be the limiting step, and therefore large SiO2 surface area is desired. However, other SiO2 surface structure, such as surface impurities (metal ions) and hydroxyl groups might also influence the reaction. In this work, we investigate the influence of micro silica surface hydroxyl groups in the synthesis of calcium silicate. We increase the hydroxyl groups by dispersing the micro silica in 10wt% H2O2 (CS10%), and we remove the hydroxyl groups by drying the micro silica at 650°C and then disperse it in water (CS650). A reference sample is also prepared from as-received micro silica dispersed in water (CSref.). We find that the micro silica dispersed in 10% H2O2 decrease in average sizes, while the silica heated to 650°C increase the average sizes, compared to the as-received micro silica dispersed in demineralized water. From X-ray diffraction, we find that the crystal phase xonotlite does not change for sample CS10% and the crystal phase tobermorite increase compared to CSref. Sample CS650 is mainly composed of Ca(OH)2 after reaction, and hardly any xonotlite and tobermorite crystal phase found. The compressive strength is found to increase for CS10%. The linear shrinkage also increases, which is caused by the increase in tobermorite crystal structure. H2O2 increase the micro silica surface area, and the number of hydroxyl groups on the micro silica surface. By removing hydroxyl groups from the silica surface, we tremendously decrease the surface area and the reactivity of micro silica. The results show that the hydroxyl groups play an important role in the reaction between micro silica and quicklime.