Novel synthesis of lightweight geopolymer matrices from fly ash through carbonate-based activation

Abstract

This paper describes a novel synthesis route for porous geopolymers based on carbonate-based activation of fly ash as the primary source material. The aqueous dissolution of sodium carbonate supplies NaOH which provides the alkaline medium for fly ash geopolymerization (along with externally supplied sodium silicate also, if needed). The CO2 released from the low temperature (∼100 °C) decomposition of carbonates formed as reaction products, acts as the pore-forming agent. The dosage of the alkaline agents is tailored to obtain a porous material with large enough porosity (∼50%), and sufficient mechanical strength (∼15 MPa) and thermal conductivity (∼0.3 W/m-K) so as to be used as structural/insulating elements in buildings. Detailed analysis of microstructure and reaction products reveals the presence of sodium carbonate crystals in the matrix, recrystallized from the initial carbonates added. Determination of CO2-equivalent emissions shows that these matrices, despite releasing some amount of CO2 in the pore-forming process, are more sustainable than similar matrices synthesized from portland cement or geopolymerized using same starting materials and other activating agents.