Effects of calcium on setting mechanism of metakaolin‐based geopolymer

Abstract

Geopolymer setting is seen to be substantially accelerated by addition of calcium and the objective of this study was to determine the mechanism for this effect by examining metakaolin geopolymers with and without calcium. Solid‐state 27Al NMR tests were used to examine the dissolution extent both qualitatively and quantitatively. Solid‐state 29Si NMR tests were conducted to determine the amount and structure of each phase. Prior to the quantitative tests, chemical extractions were used to facilitate assignment of peaks in each spectrum. On addition of calcium, it was found that both the rate and the extent of metakaolin dissolution were enhanced. Accelerating dissolution increases the Al concentration in solution, thus reducing Si/Al available for geopolymer gel formation and further accelerating the gel formation to cause faster setting. Although C‐A‐S‐H was observed in the calcium mix, no evidence indicated that it is directly involved in setting.

The Compressive Strength and Microstructure of Alkali-Activated Binary Cements Developed by Combining Ceramic Sanitaryware with Fly Ash or Blast Furnace Slag

“…This indicates that Ca(OH) 2 was consumed during the alkali-activation reactions. In line with the obtained results, Chen et al [4] observed that adding calcium to metakaolin geopolymers enhanced the rate and extent of MK dissolution. The higher dissolution rates of the precursor increased the Al concentration in the solution as Al tends to dissolve faster than Si [4].…”

“…In line with the obtained results, Chen et al [4] observed that adding calcium to metakaolin geopolymers enhanced the rate and extent of MK dissolution. The higher dissolution rates of the precursor increased the Al concentration in the solution as Al tends to dissolve faster than Si [4]. As explained by Ranjbar et al [7], the bigger amounts of Al(OH) 4 − species available for condensation at early ages improve the condensation rate between silicate and aluminate species, rather than only between silicate species.…”

“…The higher dissolution rates of the precursor increased the Al concentration in the solution as Al tends to dissolve faster than Si [4]. As explained by Ranjbar et al [7], the bigger amounts of Al(OH) 4 − species available for condensation at early ages improve the condensation rate between silicate and aluminate species, rather than only between silicate species. Since the condensation between silicate species is slower than that between silicate and aluminate species [42], this accelerates the reaction kinetics, and originates higher initial strength and shorter setting times [4,7].…”

“…While N-A-S-H gel mainly forms in the former, the activation of high-calcium systems principally brings about C-A-S-H gel [2,3]. Both gels may co-exist in the presence of calcium, with calcium silicate hydrate (C-A-S-H) being favored at a high pH (>12) if sufficient calcium is provided [4].…”

Influence of calcium content on the atomic structure and phase formation of alkali‐activated cement binder

“…The alkali‐activated material can be mainly composed of the calcium aluminosilicate hydrate (CASH) or the sodium aluminosilicate hydrate (NASH) based on the calcium content. Currently, it is generally considered that the CASH gel has a layer structure which is similar to the calcium silicate hydrate (CSH) gel and the NASH gel possess the 3D network structure formed by aluminate and silicate tetrahedrons linked with shared oxygen atoms . It has also been reported that the aluminum normally can be introduced into the structure of CSH by partially substituting the silicate tetrahedrons at the bridging sites to form CASH gel, but the detailed structure examination at the atomic scale for the CASH gel is still limited.…”