Design of porous Geopolymers for hygrothermal applications: role of nano and meso porosity

Soluble silica content and the concentration of hydrogen peroxide were used to achieve porous geopolymers with nano, meso and milliporosity. The use of rice husk ash as a partial replacement for metakaolin allows to maintain the SiO2/Al2O3 ratio between 1.93 and 2.71 and achieve layered bulks with the desired moisture buffering capacity. Stereo Optical Microscopy, Environmental Ecanning Electron Microscopy, Mercury Intrusion Porosimetry, and Microtomography were used with the aim of correlating the intrinsic characteristics of the porous network to the moisture buffering capacity. A detailed analysis of the results permitted to link the moisture buffering capacity to the SiO2/Al2O3 ratio and the gel pores/large pores ratio. The saturation regime of these layered porous systems was found between 20 and 27%, while the daily moisture buffering capacity varied from 0.37 to 4.82% considering cycles of desorption and absorption. It was revealed that specimens with a high volume of gel pores (nano and meso) and optimal aeration with millipores are ideal matrices for hygroscopic applications. The porous matrices were found promising for the design of building systems with passive thermal comfort.