Chemical interactions between siliceous aggregates and low-Ca alkali-activated cements

The chemical interactions between natural siliceous aggregates and a low-Ca alkali-activated cement (geopolymer) were studied. By leaching ideal aluminosilicate minerals such as kaolinite and albite in various alkaline solutions with or without soluble silicates, it was found that addition of 0.5 M SiO2 to a highly alkaline activating solution ([OH−]0 = 5 to 10 M) was responsible for the formation of an Al-enriched aluminosilicate surface through the initial non-stoichiometric and Si preferential dissolution of the parent aluminosilicates. This then facilitated soluble silicate deposition from the activating solutions onto the Al-rich surfaces, which resulted in the formation of a dense deposited aluminosilicate gel interfacial layer. This aluminosilicate interface formed during albite leaching ([OH−]0 = 5 to 10 M and [SiO2]0 = 0.5 M) was found to possess a similar Si/Al ratio to the real interface between a siliceous aggregate slice (basalt or siltstone) and a low-Ca fly ash/kaolinite geopolymer, activated with an activating solution of [OH−]0 = 10 M and [SiO2]0 = 2.5 M. Due to the similarities between the activating solutions used and the interfaces formed, it is postulated that a similar formation mechanism is shared between the deposited aluminosilicate interface formed from leaching and a ‘real’ geopolymer concrete synthesis. Without soluble silicate addition, or if a solution of low alkalinity ([OH−]0 = 0.6 M and [SiO2]0 = 0 and 0.5 M) was used, the Al-enriched reacting surface was not formed, and no deposited aluminosilicate interface was observed in these systems.