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

Abstract

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 PO4 3can 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. In the work reported here, the microstructure and the cold crushing strength (CCS) of the materials produced by alkaline-bonding and phosphoric acid-bonding of MK have been studied as a preliminary effort to develop these materials as replacements in some current uses and for future applications. Batches of 100 g of different compositions were made as listed in Table 1. The approximate chemical compositions of the nominal batch compositions based on the chemical analyses of the precursors from the suppliers resulted in molar ratios of Si/Al = 2 and Na/Al = 1 for MK-based geopolymers (MKGP). Similarly, the phosphoric acid-bonded MK (MKP) had molar ratios for Si/Al = 1 and Al/P = 1. MKGP (see Table 1) was prepared by adding MK to the sodium silicate solution. MKP was made by first mixing the deionised water (DIW) with 85 mass% H3PO4, then adding MK to the acid. Minimum amount of water was added to achieve the required workability, because any excess of water would increase the porosity and thus decrease strength. All the batches were mixed in a dental mixer (Renfert, Dental mixer, UK) for *5 min under vacuum at 300 rpm. To the MKGP and MKP batch compositions (see Table 1), 40 mass% sand was added before mixing (i.e. MKSGP and MKSP). Ordinary washed beach sand of size fractions, 81 mass% 250–500 lm, 17 mass% 125–250 lm and 2 mass%\125 lm, was used. Generally, sand is added in making mortar for building applications; hence, it was added for comparison with ordinary Portland cement mortar. The slurries produced in each instance were cast to form 25 mm dia 9 40 mm long cylindrical specimens in sealed polycarbonate containers for subsequent physical and mechanical testing. The cast items were kept at room temperature for 2 h before curing at 60 C for 24 h in an oven. After removing the seals, the samples were left at ambient temperature for 4 days before demoulding for MKGP. The MKP and MKSP samples were demoulded after 14 days when they were dry (they were still wet after D. S. Perera (&) J. V. Hanna J. Davis M. G. Blackford B. A. Latella Y. Sasaki E. R. Vance Australian Nuclear Science and Technology Organisation, PO Box 1, Menai, NSW 2234, Australia e-mail: pereradan@gmail.com; dsp@ansto.gov.au