Salicylic acid–methanol extraction of aluminosilicate hydrates

Abstract

AbstractThis study focuses on understanding how salicylic acid and methanol (SAM), commonly used to dissolve certain calcium silicates in portland cement, affect geopolymer gels and other sodium aluminosilicate hydrates. The technique has been used in the past to selectively dissolve hydrated calcium silicates formed through alkali activation of aluminosilicate mixtures containing calcium in the expectation that the extraction does not dissolve alkali aluminosilicate hydrates. However, the work presented here reveals that SAM solution does, in fact, dissolve aluminosilicate hydrates. Zeolite A, a three‐dimensional, low‐silica, crystalline sodium aluminosilicate hydrate, was fully dissolved upon SAM extraction, zeolite X was partially dissolved in the SAM extraction, and only zeolite Y was completely unaffected by SAM extraction. Similar behavior was observed with amorphous sodium hydroxide‐metakaolin geopolymer products—at an early age (1 day), SAM dissolved all the gel and left only unreacted metakaolin, and at a later age (7 days), SAM partially dissolved the geopolymer gel. Nuclear magnetic resonance spectroscopy indicated that dissolution requires the presence of silicon at the Q4(4Al) sites. Extraction was seen to involve some dealumination and complete removal of sodium. The mechanism of aluminosilicate dissolution in SAM appears to be acid attack followed by removal of aluminum via chelation with salicylic acid.