Tailoring porosity and pore structure of cellular geopolymers for strength and thermal conductivity

Abstract

Cellular geopolymer is produced by entraining air in alkali-activated low calcium fly ash (AAF) paste. Rheology control with the use of Bentonite clay and surfactant creates stable aeration on adding Aluminum powder to AAF paste. The clay provides yield stress control in the AAF paste that is required for retaining the bubbles in the suspension. The surfactant is used to prevent the coalescence of the air bubbles in the AAF paste that produces a homogenous void network in the cellular geopolymer. The viscosity of the AAF paste is determined by the silica content and modulus (molar ratio of SiO2/Na2O) of the activating solution. Process variables such as Aluminum powder content and size, clay and surfactant dosages and composition of activating solution are considered to tailor the porosity and pore size in the cellular geopolymer. The total porosity of the cellular geopolymer depends on the viscosity of the AAF paste, which is controlled by the silica content and modulus of the activating solution. The mean pore size and its distribution depend on the Aluminum powder size. The mean pore size decreases with an increase in yield stress of the AAF paste and a decrease in the surface tension. Cellular geopolymer with density lower than 1000 Kg/m3 with thermal conductivity in the range of 0.25–0.20 W/m.K and compressive strengths of 11.5 to 8.5 MPa are produced by controlling the process variables. The strength of the cellular geopolymer depends on the mean pore size in addition to the total porosity. The thermal conductivity is dependent only on the total solidity and hence on the total porosity and is not affected by the pore size distribution. The existing models used for predicting the strength and thermal conductivity in relation to porosity and pore characteristics are explored and modifications are proposed for cellular geopolymers.