The effects of inorganic salt contamination on the strength and durability of geopolymers

Two geopolymer systems were prepared by alkali activation of fly ash and kaolin at room temperature with alkaline silicate solutions. System I was synthesised using a less concentrated alkaline silicate solution than System II. Inorganic salts were dissolved or suspended in the mixing water and added to the early pastes before setting to simulate process contamination involving the activating solutions. The strength and durability of the resultant products were examined by comparing the compressive strengths, Fourier-transform transmission infrared spectroscopy (FTIR) spectra, X-ray diffraction (XRD) diffractograms and scanning electron microscopy coupled with energy dispersive spectrometer (SEM–EDS) analysis at different ages. It was found that chloride salts such as KCl, CaCl2 and MgCl2 were detrimental to System I geopolymer. They decreased the product durability by gradually causing precipitation and crystallisation in the aluminosilicate gel—the binding phase of geopolymer. On the other hand, carbonate salts, K2CO3 and CaCO3, were beneficial by lowering the dissolved water contents and preventing hydrolytic attacks on the gels. The greater alkali contents in the System II geopolymers were found to promote greater solid dissolution but also caused aluminosilicate gel precipitation at a very early age. This lowered the product compressive strength and masked the effects of the inorganic salt contamination in System II geopolymer.