Semi-empirical AM1 calculations on 6-memebered alumino-silicate rings model: implications for dissolution process of metakaoline in alkaline solutions

In order to investigate the dissolution process of metakaline in alkaline solutions, two 6-memebered rings models consisting of AlO₄ tetrahedron and SiO₄ tetrahedron, respectively are firstly proposed to represent the structure of metakaoline in this paper. Analysis of the dissolution mechanism of the two 6-memebered rings models in strongly solution reveals that the dissolution process of metakaoline is composed of ring breakage for releasing HOTO ₃ ^≡ anion, formation of HO–T(OM)₃ by ion-pairing reaction between HOTO ₃ ^≡ anion and M⁺ cation, and further interaction between the remaining broken ring cluster and MOH solutions. A computational chemistry method: Semi-empirical AM1 calculation is then conducted on the two models to obtain the details of three steps involved in dissolution process. The calculated results showed that 6-member ring model consisting of AlO₄ tetrahedron is more reactive than 6-member ring model consisting of SiO₄ tetrahedron. Compared with local environment, strongly alkaline accelerated the dissolution of 6-member ring model consisting of SiO₄ tetrahedron. Na⁺ has stronger ion-pairing interaction than K⁺. The further reaction between the remaining broken ring cluster and strongly alkaline solution depended on the types of the remaining broken ring cluster and alkaline solution. The above results enhanced our understanding of dissolution mechanisms of metakaoline in highly alkaline solutions, which is especially important to geopolymerization reaction.