Effectiveness of rice husk ash-derived alkali activator in fresh, mechanical, and microstructure properties of geopolymer mortar at ambient temperature curing

Conventional geopolymers are proven to be eco-friendly compared to Portland cement-based concrete (PC). However, the used alkali activator, i.e. sodium silicate is associated with high carbon emission and cost, making the geopolymers not really a sustainable alternative to PC. This experimental investigation was carried out to understand the potential of rice husk ash (RHA)-based alkali activator in the synthesis of fly ash-blast furnace slag (FA-GGBFS)-based geopolymers at ambient temperature. Three different concentrations of sodium hydroxide (by wt. %) solutions, i.e. 20%, 24%, and 27%, were used to synthesize an RHA-based alkali activator. A commercial-grade sodium silicate solution was used to compare the results of geopolymer mortars (GPM) with the prepared RHA-based alkali activator. Fresh, mechanical, and microstructural investigations were carried out for both the RHA and commercial-grade alkali activator-based FA-GGBFS GPM specimens. The compressive strength of RHA-based optimum GPM was found to be 41 MPa at 28 days of the curing period, which was close to the control sample made with the commercial activator; similar observations were found for the flow table test. Microstructural investigation (XRD and SEM) confirmed that the GPM prepared with the RHA-based alkali activator has a similar microstructure as the GPM with the commercial-grade alkali activator.