Distinctive Spectral and Microscopic Features for Characterizing the Three-Dimensional Local Aluminosilicate Structure of Perlites

Expanded perlite and raw perlites of various origins were examined using solid-state NMR and Fourier transform infrared (FTIR) spectroscopies, as well as scanning electron microscopy, in an attempt to correlate the characteristics of aluminosilicate framework with the material’s expansion properties. The 27Al magic angle spinning (MAS) NMR spectra indicated that aluminum atoms in perlite are mainly tetrahedrally coordinated in Q4 structures not participating in the expansion process; two different octahedral Al species were also identified in perlite. Owing to the tetrahedra−octahedra transformations, perlite may be classified among the metastable materials. The 29Si MAS NMR and 1H–29Si CPMAS NMR spectra showed that Q3 and Q4 units are mostly abundant, a fact that explains the low cation exchange capacity of perlites. Deconvoluted FTIR and 29Si MAS NMR spectra are consistent regarding the relative Q n intensities which, together with the network geometry, determine the expansibility of raw perlites. The calculated variations of the Si–O–Si angle are more significant than those of the Si–O bond length, remarkably affecting the structure of the aluminosilicate network. Thus, perlite samples of increasingly fragmentary framework exhibit the highest expansion ratios. SEM micrographs monitored the expansion process differentiating between the low- and high-expansibility perlites.