Crystalline phase formation in metakaolinite geopolymers activated with NaOH and sodium silicate

The effect of the NaOH content and the presence of sodium silicate activators on the formation of crystalline phases from metakaolinite-based geopolymers were studied by X-ray powder diffraction (XRD), Rietveld quantitative XRD, solid-state MAS NMR and SEM in samples synthesized with varying NaOH contents and different curing times at 40 °C. Geopolymers activated with NaOH alone with Si/Na ratios of 4/4 or less formed the crystalline zeolite Na–A (Na₉₆Al₉₆Si₉₆O₃₈₄·216H₂O), but at ratios >4/4 nanosized crystals of another zeolite (Na₆AlSiO₄]₆·4H₂O) were formed. The Si/Na ratio of 4/4 produces a product of greatest crystallinity. The addition of sodium silicate in addition to NaOH significantly reduces crystallite formation. The network units of all the materials containing NaOH and sodium silicate are essentially the same, namely, tetrahedral SiO₄] units coordinated through four bridging oxygens to four aluminium atoms denoted as Q⁴ Si(4Al) units]. A templating function of the various silicate units of the sodium silicate molecules is suggested to occur in geopolymerization, which differs from the reaction route operating when NaOH alone is used as the activator. This templating function is responsible for the suppression of crystallization and the increase in strength of the geopolymers activated with sodium silicate.