Chiang Mai Journal of Science

Red clays from different sources have different compositions and properties. They are abundant natural resources in Thailand. However, there is no report on using red clays for studying geopolymer structures. In this work, geopolymers were synthesized from four sources of red clays in Thailand including Chiangmai, Korat, Ratchaburi and Angthong using kaolin clay from Lampang as a reference. The samples were prepared using 2:3 mixed alkaline solution of 10 M NaOH and Na₂SiO₃ with 60:40 solid/liquid ratio by weight. Among them, Chiangmai red clay generated the best properties of 38.85±1.66 MPa compressive strength and 11.40±1.24 MPa bending strength. The strengths were inversely proportional to the elemental Si/Al ratio of the raw materials, in which the Chiangmai red clay has the lowest value at 1.49. In addition, 29Si and 27Al MAS-NMR data showed that the structure of geopolymers from the Chiangmai red clay was in 3-dimensional network while the others were sheet-like or chain networks which resulted in their low strengths. It was found that the red clays could be used as a main raw material for synthesis geopolymers, especially Chiangmai red clay which had better strength than the Lampang kaolin. The red clays in Thailand have high potential to produce low-cost geopolymers. They have good mechanical properties and are plentiful in Thailand.