Effect of amorphous silica fume as active filler for rapid densification of the geopolymer products formed by warm pressing

Abstract

Geopolymers are attracting attention in the field of concrete building materials from the viewpoint of reducing CO2 emissions. This study demonstrates the effects of the addition of silica fume in the warm press method on the densification and mechanical properties of a geopolymer. First, geopolymer products hardened by conventional slurry curing, using fly-ash and sodium hydroxide solution as starting materials, were prepared. The hardened geopolymer products were then pulverized into small grains. Subsequently, various amounts of silica fume were mixed into the fine geopolymer particles to form the starting powder mixtures. A series of these mixtures (10 mass% water added) were placed into a steel mold, which was then inserted into a warm press device. An uniaxial pressure of 240 MPa was applied at room temperature and then the temperature was raised from 80 to 180 °C. When the warm press temperature was set at 80 °C, the compressive strength of the hardened bodies increased in line with the amount of added silica fume. Particularly, in the case of a hardened body with 30 wt% silica fume, the compressive strength reached 195 MPa, which was about three times that of a hardened body with 0 wt% silica fume. The added silica fume was found to improve the fine stacking of starting geopolymer particles in the early stages of warm pressing and help form objects with tightly packed voids between the original geopolymer particles. In contrast, for heat treatment at a warm press temperature of 180 °C, the compressive strength of the hardened bodies with 30 wt% silica fume decreased compared to those for the hardened bodies with 0, 10 and 20 wt% silica fume. From the results of nuclear magnetic resonance analysis, this decrease in strength was considered to be due to an increase in the silica-rich phase.