Relative strengths of phosphoric acid-reacted and alkali-reacted metakaolin materials

Excerpt

The use of binders such as phosphoric acid to consolidate aluminosilicates to form refractory products has been reported since the 1940s [1]. Another class of materials formed by reacting concentrated alkaline silicate solutions with metakaolin (MK) has been termed geopolymers by Davidovits [2]. MK is made by heating (~750 °C) of kaolin to render it X-ray amorphous and thus more reactive. In a geopolymer, the aluminosilicate is composed of cross-linked AlO₄⁻ and SiO₄ tetrahedra, charge balanced with Na⁺ or K⁺ ions. It was shown by Cao et al. [3] that PO₄³⁻ can be incorporated in the geopolymer structure. Derrien et al. [4] added calcium phosphates to geopolymers, but they did not show whether the phosphate was part of the geopolymer structure. MacKenzie et al. [5] showed that the P occupied tetrahedral sites in the geopolymer with a different chemical shift from that of the aluminium phosphate reactants. Cao et al. [3] made nine compositions with Si/P molar ratios of 0.13–0.63 by adding H₃PO₄ to metakaolin and the maximum strength obtained was 55 MPa for the composition with Si/P = 0.21 molar ratio. Mackenzie et al. [5] made one composition of molar ratio of Si/P = 25 by adding a small amount of aluminium phosphate to an MK-based geopolymer, having molar ratios of Si/Al = 1.6 and Si/Na = 2.6. …