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.