Abstract
For geopolymers (usually composed of unreacted precursor and gel), the compressive strength is controlled by two factors. The first is the degree of reaction, or, equivalently, the amount of gel formed, including any calcium silicate hydrate gel in calcium-containing mixtures. The second factor is the gel composition, generally given by the Si/Al ratio. These two parameters are interrelated for typical silicate-activated metakaolin geopolymers. By separating out effects of Si/Al ratio and degree of reaction, this study quantitatively correlates the degree of reaction with the compressive strength of metakaolin-based geopolymers with and without calcium. Solid-state 29Si nuclear magnetic resonance (NMR) aided with chemical extractions was used to determine gel amounts and composition for several geopolymer mixtures. The compressive strength was also measured for each mixture at 7 days. Both the increase of Na/Al ratio in mixtures without calcium and addition of external calcium increased the degree of reaction, and compressive strength correlated linearly (R2 > 0.88) with the degree of reaction.
Highlights
Geopolymers are synthesized by activating aluminosilicate precursors with sodium/potassium hydroxides and/or silicates
In our previous study of metakaolin geopolymers before and around setting [15], we demonstrated that dissolution of metakaolin and setting of the mixtures were enhanced by calcium hydroxide (Ca(OH)2 )
This study quantified the relationship between degree of reaction and strength for metakaolin
Summary
Geopolymers are synthesized by activating aluminosilicate precursors with sodium/potassium hydroxides and/or silicates. Geopolymers have attracted significant research interest [1,2,3] in the last two decades They are, in principle, environmentally friendly and low-emission alternatives to conventional portland cement, as synthesis can be carried out at room temperature using industrial wastes such as fly ash, slag, incineration bottom ash, and red mud [2,4,5]. Metakaolin is a simple aluminosilicate precursor material that facilitates understanding of the composition–structure–property relationships for geopolymers. Unlike fly ashes, it is almost entirely amorphous and reacts relatively rapidly [10]. Metakaolin is dissolved in an alkaline or alkali-silicate solution to release silicon (Si) and aluminum (Al) ions, which condense with each other to form aluminosilicate oligomers, building blocks that further condense
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