Thermal conductivity of several geopolymer composites and discussion of their formulation

We fabricated 50.8-mm cube-shaped samples of metakaolin geopolymer (GP) composites with various additives chosen to increase or decrease the thermal conductivity of the composite. Sodium-based GP (NaGP) and GP composites were more conductive than potassium-based GP (KGP) composites for a given phase fraction of filler, but the maximum amount of filler phase was higher with KGP due to the lower viscosity of the KGP mixture. The highest thermal conductivity achieved was about 8 W/m K by KGP + 44-vol% graphite flakes, whereas NaGP + 27 vol% graphite flakes reached 4.7 W/m K. The thermal conductivity was strongly affected by the moisture remaining in the composite, which appeared to have a greater effect at higher filler content. On the other hand, the size of alumina particles (6, 40, or 120 μm) did not have any apparent effect on thermal conductivity for the same filler content. Larger particles caused less change in mixture viscosity, though, thus permitting incorporation of higher filler phase fractions and therefore higher thermal conductivity.