The Utilisation of Rice Husk Ash Leachates for The Synthesis of Eco-Friendly Geopolymers

Abstract

Geopolymers are inorganic polymers projected as feasible alternatives to Portland cement due to their lower CO2 emissions and high mechanical properties. In recent times, however, their impact on other environmental indices such as abiotic depletion, freshwater toxicity, marine ecotoxicity, terrestrial ecotoxicity, acidification, and human toxicity has become questionable, thus the need to investigate alternative eco-friendly precursors and activators. This work aims to manufacture an eco-efficient geopolymer by utilising biomass ash leachate as an alternative to the conventional alkali silicate solution. The leachate was obtained by soaking the rice husk ash (RHA) in a 10 M concentration of NaOH solution and filtering to obtain a clear solution. The effects of the calcination temperature of the kaolin and the RHA content in the alkaline solution were investigated, and a factorial design of experiments was developed considering three levels of calcination temperature (700°C, 800°C, and 900°C) and five levels of RHA content (0 g, 5 g, 10 g, 15 g, and 20 g). Physical and mechanical tests were performed on the synthesised geopolymer pastes, and chemical analysis was performed using X-ray diffraction and X-ray spectroscopy. The impacts of replacing the synthetic alkali silicates with rice husk ash-based activators were compared. The results showed that the calcination temperature of the kaolin and the content of the RHA both contributed significantly to the flexural and compressive strength at the 0.01 level of significance. A compressive strength of 10.4 MPa was obtained for the MK915 binders, which showed a 100% increase from samples without RHA content. This research proves that utilising RHA-based activators in metakaolin-based geopolymers can be feasible for less critical applications where a very high compressive strength will not be required.Keywords: alkaline activators, biomass, geopolymers, leachates, rice husk ash