Thermal Properties of Geopolymers

Abstract

The physics and testing methodology of thermal properties are introduced prior to a review of OPC and geopolymer thermal properties. The amorphous inorganic structure of the geopolymers lends itself to good thermal resistance which leads to potential applications such as thermal insulation. Thermal expansion can generate destructive internal stresses when structural parts are heated and restrained from moving. The thermal expansion measuring techniques commonly utilised are dilatometry, interferometry and thermomechanical analysis (TMA). Thermal expansion measurements of metakaolin and fly ash based geopolymers show several distinct regions as the temperature increases. The extent of these regions varies from system to system and the changes are attributed to dehydration, dehydroxylation, densification and crystallisation. Fillers and aggregates can be added to geopolymers to reduce the thermal expansion of the composite and extend the usable temperature range. Thermal conductivity determination is required to assess geopolymers’ suitability for potential applications in thermal barriers and construction structural members. The two approaches to measuring thermal conductivity: steady state and transient (non-steady state) techniques are compared. The microstructure of geopolymer profoundly influences thermal conductivity; particularly porosity which if increased leads to a reduction in thermal conductivity. The addition of aggregate influences the thermal conductivity of OPC and geopolymer concrete and at the same time decreases thermal durability due to mismatch of thermal conductivity between aggregate and matrix. Some geopolymers with low initial strength have been shown to gain strength after exposure to high temperatures. It has been hypothesised that unreacted precursor levels can convert to geopolymer at high temperature and increase the strength. Once again the importance of the geopolymer microstructure is highlighted.