Ceramics-Silikáty 66, (2) 236 - 244 (2022)

The work deals with the synthesis of metakaolin phosphate-based geopolymers (MKPGs) using calcined kaolin, phosphoric acid (PA), and mixing water. The metakaolin was thermally treated at 750 °C for different holding times. The different duration in the heating at 750 °C led to the gradual dehydroxylation of kaolin to metakaolin with a different structural water content (inter-layer and inner-layer residual water). Considering the temperature or duration of the thermal dehydroxylation, the inter-layer water evolved first, followed by the inner-layer water at a higher temperature or later time. The polymerisation degree is closely linked with the presence of the inter- or inner-layer water. It was found that the inter-layer residual water negatively influences the compressive strength, while the water in the inner-layer exerts a more negligible effect on the resistance. Indeed, the inter-layer water loss is linked to the structural changes in kaolinite; but the loss of water from the inner-layer is related to the structural changes in the metakaolinite. The holding time of dehydroxylation and water-to-metakaolin (W/M) ratio were optimised. A maximum strength of 57.6 MPa was achieved at the Al/P and W/M ratios of 1.0 and 0.40, respectively. The mass losses at the different temperatures showed that the additional excess mixing water increased the porosity of the MKPGs, resulting in a decrease in the compressive strength.