Sulphate resistance of silane coupling agent reinforced metakaolin geopolymer composites

Abstract

Sulphate resistance is an important index for evaluating geopolymer durability. This study investigates the beneficial effects of silane coupling agent (SCA) addition on the sulphate corrosion resistance of metakaolin-based geopolymer composites via strength tests and various characterisation techniques such as XRD, FTIR, SEM-EDS, and 29Si NMR. The results show that the compressive and tensile strengths and corrosion resistance coefficient of the geopolymer first increased, then decreased with increasing SCA dosages. The geopolymer incorporated with the optimum dosage of 0.1 wt% SCA exhibits the highest strengths, fluidity, and sulphate resistance coefficient. SCA addition effectively compacts the pore structure, thus making it difficult for SO42− to invade the matrix. The SEM-EDS diagram reveals that before and after sulphate attack, the Na content of the pure geopolymer decreased from 7.37 wt% to 3.84 wt%, and the mass ratio decreased by 47%. However, the Na content of the geopolymer composite modified with SCA decreased from 5.64 wt% to 4.85 wt%, and the mass ratio only decreased by 14%, indicating that the addition of SCA reduced the Na precipitation rate of the geopolymer in the sulphate environment. The 29Si NMR spectrum also reveals that the addition of SCA inhibits the resonance peak shift from Q4 to Q3 after 56 d of sulphate attack, which indicates improvement in the stability of the three-dimensional network structure in the geopolymer.