Influence of rice husk ash on mechanical properties and fire resistance of recycled aggregate high-calcium fly ash geopolymer concrete

Abstract The use of recycled aggregates in geopolymer concrete provides an eco-friendly alternative for the construction industry. Since geopolymer concretes made with recycled aggregates generally have weaker internal structure compared with those using natural aggregates, SiO2-rich materials have therefore been used to improve the strength properties, one of which is the well-known nano-SiO2 (nS). However, because nS is not environmental friendly, other green SiO2-rich materials, such as rice husk ash (RHA) from agricultural wastes, are needed. The objective of this study is to investigate the effectiveness of replacing nano-SiO2 with rich husk ash for improving the performance of recycled aggregate geopolymer concrete (RAGC) made from high-calcium fly ash that is able to set and harden without heat curing. The mechanical properties and fire-resistant performance of RHA-added and nS-added geopolymer concretes were tested and compared. The results show that the addition of RHA is effective in improving the strengths of RAGCs, particularly when the SiO2/Al2O3 ratio is increased to 4.17. The 28-day compressive strengths of RAGCs containing RHAs ranged from 36.0 to 38.1 MPa due to the improved microstructure and denser matrix and were comparable to those of RAGCs made with nS. However, the inclusion of SiO2-rich materials had an adverse effect on the post-fire residual strength of geopolymer concretes made from recycled aggregates due primarily to the reduced porosity.