Rice husk ash as a silica source for the production of autoclaved aerated concrete – A chance to save energy and primary resources

If the silica raw material used in autoclaved aerated concrete (AAC) requires a lower level of energy for dissolution, a lower autoclaving temperature could be applied to reach the required product performance. Accordingly, the raw material quartz sand in the AAC mixture was replaced with rice husk ash (RHA) which has a higher solubility of silica with increasing temperatures in alkaline binder systems. AAC samples were produced at autoclaving temperatures of 152 °C, 165 °C, 175 °C, and 192 °C with an autoclaving time of 6 h. Quantitative X-ray diffraction (XRD) analysis according to Rietveld refinement, qualitative XRD analysis, and scanning electron microscopy (SEM) were carried out to investigate the microstructure of the hardened AAC samples. Additionally, the compressive strength, dry bulk density, A-value, and total drying shrinkage were determined. In contrast to the quartz-based AAC, the properties of the RHA-based AAC were improved by applying a lower autoclaving temperature than the typical temperature used for industrial production of AAC, i.e. 192 °C. For the RHA-based AAC, a maximum increase of 22% in the compressive strength, 132% in the total content of tobermorite, 19% in the A-value, and a maximum decrease of 33% in the total value of drying shrinkage were observed as the applied autoclaving temperature was changed from 192 °C to 165 °C. Moreover, the material efficiency of the AAC product can be improved by using RHA as an agricultural waste product in the mixture. The RHA-based AAC autoclaved at 165 °C exhibited a slightly higher compressive strength to density ratio (A-value) than the quartz-based AAC autoclaved at 192 °C. However, a higher total drying shrinkage was observed for the RHA-based AAC autoclaved at 165 °C compared to the quartz-based AAC autoclaved at 192 °C. From a general point of view, the results showed that the optimum temperature for autoclaving AAC has a direct dependency on the dissolution properties of silica raw materials used in the mixture.