Setting and nanostructural evolution of metakaolin geopolymer

Geopolymers are being promoted as a sustainable alternative to the ordinary cements mainly because their production is associated with much less CO2 emissions. They also show advantages including compressive strength comparable to portland cements, high temperature resistance, and stability under acid attack. However, before geopolymers can be widely used in practice, certain other behaviors must be understood and controlled, among which is setting, the transition from fluid to solid. Geopolymer setting is repeatedly seen to be substantially accelerated by addition of calcium. The objective of this study is to understand the setting at the nanostructural level for calcium and non-calcium geopolymers using a relatively pure aluminosilicate metakaolin precursor. Prior to probing nanostructural evolution, a combination of water treatment to extract soluble species and solvent treatment to extract water was developed for use to stop the geopolymer formation for early-age geopolymers with and without calcium, allowing enough time for nuclear magnetic resonance (NMR) tests. Additionally, a protocol to quantify structures of early-age geopolymers was developed using NMR deconvolution and validated by intensity analysis of NMR spectra and by quantitative chemical extractions. The nanostructural evolution during formation of geopolymer was investigated and correlated with setting. In the non-calcium mix, aluminum was released rapidly right after mixing and immediately condensed with silicates to form aluminosilicate units with growing sizes on metakaolin surface. Set occurred as the remaining aqueous silicates began attaching to these units to form a gel with an interconnected network structure. The structural connectivity during this evolution, for the first time, was monitored experimentally. Additionally, accelerated setting by calcium was investigated. With calcium, enhanced rate and extent of metakaolin dissolution were observed and were found to decrease Si/Al ratio available for geopolymer formation and thus to further enhance geopolymer gel formation. These observed effects caused the faster setting. The Al-substituted calcium silicate hydrate (C-A-S-H) was identified in the calcium mix, but no evidence showed that it is directly involved in setting. Considering faster dissolution by calcium noted above, the reaction extent after set was examined. A higher amount of calcium resulted in a higher reaction extent. A higher reaction extent resulted in a higher compressive strength, a relationship also observed in the non-calcium mixes when reaction extent was controlled independently of the Si/Al ratios of geopolymer gel