Effects of Water on Pore Structure and Thermal Conductivity of Fly Ash-Based Foam Geopolymers

The influence of the water-to-solid ratio (W/S) on the viscosity, pore characteristics, bulk density, compressive strength, and thermal conductivity of foamed fly ash-based geopolymers with thermal conductivity less than 0.065W/(m center dot K) was investigated, and their properties and cost analysis were also compared with that of foamed ordinary Portland cement (OPC). When the W/S varied from 0.38 to 0.5, the apparent viscosity of geopolymer paste 15min after the preparation decreased significantly from 168Pa center dot s to 6Pa center dot s. The increasing W/S ratio contributed to the rise of the number of microcapillaries (phi < 50 nm) and macrocapillaries (50 nm < 50 mu m) but contributed to the decline of artificial air pores (phi > 50 mu m). The refinement of pore characteristics lowered the 28d thermal conductivity of foamed geopolymers from 0.06W/(m center dot K) to 0.048W/(m center dot K). Although the slight increase of total porosity of foamed geopolymers from 89% to 92% with the increase of the W/S ratio weakened their 28d compressive strength from 0.75 MPa to 0.45 MPa, this strength still meets the Ordinary Portland Cement (OPC) based Foam Insulation Board standard of JC/T2200-2013 (>0.4 MPa for 0.25g/cm(3)). The production cost of foamed geopolymers was slightly higher by 1.1-1.5 times than that of foamed OPC. However, considering the more beneficial effect of environmental load reductions and better mechanical and thermal properties of foamed geopolymers than those of foamed OPC, slightly higher cost would be acceptable for practical application.