Material design of geopolymers using calcined allophane

Abstract

As kaolinite, the main material of geopolymers, is not produced in East Asia and elsewhere, it is necessary to find alternative materials to replace kaolinite. The authors therefore investigated the preparation conditions and optimum curing temperature of geopolymers using allophane, which is not used as a cementitious material. Based on compressive strength and internal temperature history results, specimens were prepared by mixing allophane calcined at 800 °C for 3 h with an alkaline solution adjusted to a Na2O/SiO2 ratio of one at a water-to-powder ratio of two. Powder X-ray diffraction and nuclear magnetic resonance spectral analyses of specimens indicated that amorphous geopolymers with a three-dimensional structure were produced from calcined allophane. As for the optimum curing temperature, the samples cured at 20 °C showed the best strength development properties. On the other hand, specimens cured at 80 °C showed a temporary decrease in compressive strength as zeolite formation progressed with increasing curing time. Since 20 °C is the optimal curing condition, we conclude that geopolymers that can be used for on-site curing can be produced by using calcined allophane.