Understanding the impact of synthesis parameters on the pore structure properties of fly ash-based geopolymers

Abstract

This paper systematically investigated the effects of alkali activator modulus (AM), alkali dosage (AD), and water-fly ash ratio (W/FA) on the pore volume, pore size distribution, pore volume fractal dimension (Dv), and pore surface fractal dimension (Ds) of fly ash-based geopolymer (FABGs) by orthogonal test and mercury intrusion porosimetry test. Dv and Ds of FABG can be calculated by the Menger sponge model and Zhang and Li's models, respectively. The relationship between multiple synthesis parameters and pore characteristics of FABGs was detailedly studied through range analysis and correlation detection analysis, which provides a basic understanding of the influence of multiple parameters on the pore characteristics of FABG, as well as the relationship between pore characteristics and mechanical properties. The results show that AM has the greatest effect on the volume proportion of gel pores and transition pores, and AD and W/FA have the greatest effect on the volume proportion of capillary pores and large pores, respectively. W/FA has the greatest effect on the porosity, and the porosity and the most probable aperture increase with the increment of W/FA and decrease with a rise in AD. It is worth noting that AM, AD, and W/FA have little effect on the Dv of larger pores, but have a greater on the Dv of smaller pores, with the influence order of AD-W/FA-AM. Ds of the large pores and capillary pores range between 2.70–2.90, and the Ds of the transition pores and gel pores are mostly greater than 3.00, which suggests that there may be more ink-bottle pores in transition pores and gel pores. In addition, there was a strong linear relationship between the porosity and compressive strength of FABGs, and the compressive strength of FABGs showed a decreasing trend with the increase of porosity. Compared with Dv, there is a stronger correlation between Ds and compressive strength, indicating that the roughness of the pore surface and the pore size distribution have a greater influence on the compressive strength of FABGs.