Impact of surface impurity on phase transitions in amorphous micro silica

In this work we study three types of spherically shaped micron and submicron sized amorphous micro silica (MS) as common raw material for production of porous calcium silicate products used for insulation, which are selected on basis of chemical composition and production method. Two of them have silica content of 96% (from silicon production) and one has that of 92% (from ferro-silicon production). In order to achieve high quality calcium silicate products, which strongly depends on the characteristics of the raw MS, it is crucial to study the chemical and physical properties of the raw MS obtained from different sources. We find that the surface impurities of raw MS lower its crystallization temperature determined from differential scanning calorimetry (DSC). The thermodynamic origin of this correlation is discussed. From the surface crystallization behaviour of raw silica, it is inferred that impurities are located at the surface of the MS, and this is further verified by acid treatment of raw MS. The acid treatment leads to an increase in crystallization temperature by 50°C. The DSC proves to be an effective tool for the calcium silicate industry to detect and monitor the chemical fluctuation of raw MS.