Porous structure optimisation of flash-calcined metakaolin/fly ash geopolymer foam concrete

Abstract

This study reports the production and characterisation of geopolymer foam concrete (GFC). This material is foreseen for use as a self-bearing insulation material. In order to identify an optimal paste composition, eight mixtures were made and are presented in a ternary diagram (dry extract of alkaline solution, flash-calcined metakaolin (MK) and fly ash (FA)). The characterisation of these pastes (initial setting time (IST), shrinkage and compressive strength) indicated an optimal composition corresponding to 25% of activator, 62.5% of MK and 12.5% of FA. The GFCs were then produced by inserting variable amounts of H₂O₂ (1, 1.5 and 2%) into the geopolymer paste. The fresh GFC porous structure was stabilised with surfactant. The GFCs produced had low densities (225 < ρ < 506 kg/m³) associated with low thermal conductivities (0.07 < λ < 0.12 W/(m.K)) and acceptable compressive strength (0.5 < R_c < 1.85 MPa, for samples cured at 20 °C). A significant influence of the surfactant content on the porous structure was demonstrated. The lower surfactant content led to a porous structure made of larger bubbles separated by wider matrix walls promoting GFC compressive strength.

Keywords:

• Geopolymer foam concrete (GFC)
• metakaolin (MK)
• fly ash (FA)
• compressive strength
• thermal conductivity
• porous structure