A multinuclear MAS NMR study of calcium-containing aluminosilicate inorganic polymers

Abstract

Natural abundance 43Ca, 27Al and 29Si MAS NMR spectra were acquired for a series of aluminosilicate inorganic polymers (geopolymers) containing up to 50 wt% of Ca(OH)2, CaCO3, Ca3(PO4)2 and both amorphous and crystalline CaSiO3. The 27Al and 29Si MAS NMR spectra of these geopolymers were generally consistent with those of conventional materials, with their XRD diffractograms showing the presence of the added crystalline phases. Comparison of the 43Ca MAS NMR spectra with those of the calcium starting materials suggests that both the calcium silicates and Ca(OH)2 enter into the geopolymer network, but the spectra of geopolymers containing CaCO3 and Ca3(PO4)2 are identical to those of the original compound, suggesting that the role of these materials is simply as a filler. At the highest concentration of CaCO3, the 43Ca resonance of the aragonite polymorph appears in the geopolymer but disappears on heating at 400 °C for 36 h, suggesting that this is a metastable phase formed in situ under the highly alkaline conditions of geopolymer formation and in the presence of calcite nuclei.