Rice Husk Ash Derived Sodium Silicate Using Hydrothermal and Convection Heating Methods

Geopolymers produced by using waste materials are more cost-effective and environmentally friendly. Commercial sodium silicate (sodium water glass) is currently being produced by direct fusion of pure silica sand and soda ash (Na₂CO₃) in oil, gas or electrically-fired furnaces at high temperatures (~1300 °C)—a production process that is energy intensive and which generates CO₂. One method of minimizing these environmental issues is to use supplementary sources of silica, such as rice husk ash (RHA), to produce sodium silicate at lower temperatures. Therefore, this project aims to compare the effect of sodium silicate solution made of rice husk ash sieved at various particle sizes (un-sieved, <45 μm, <300 μm) using two techniques of (a) hydrothermal and (b) convection heating on fly ash-based geopolymer mortars. The geopolymers synthesized using ‘in-house’ sodium silicates were then compared with those made using commercial sodium silicate. Physicochemical properties of raw RHA and synthesized sodium silicates were characterized using the Energy Dispersive X-ray Fluorescence Spectrometer (EDXRF), X-ray Powder Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR) and Field Emission Scanning Electron Microscope (FESEM), Energy Dispersive X-ray (EDX), and Inductively-Coupled Plasma (ICP). When using the convection heating method, finer particle sizes resulted in a higher conversion rate, while finer RHA resulted in the precipitation of crystalline silica during the hydrothermal process. Geopolymer mortars cured at a temperature of 70 °C were tested for compressive strength at 1, 7 and 28 days. At lower alkali dosages, the mortar strengths made out of both in-house and commercial sodium silicates had comparable strengths, while at higher alkali dosages the mortars prepared from commercial sodium silicates demonstrated significantly higher strengths.