Geopolymerization of the Estonian oil shale solid heat carrier retorting waste ash: characterization of structural changes through infrared (ATR-FTIR) and nuclear magnetic resonance spectroscopic (29Si MAS-NMR) analysis

Abstract

In the current thesis, three different activator solutions alongside water were used to synthesise geopolymeric binders from black ash. It was proven that some extent of geopolymerisation takes place in every sample of activated black ash, indicating that black ash could be used to produce geopolymeric materials. Producing cement from geopolymers instead of Portland cement would be 60% energy-efficient and emit up to 80% less carbon dioxide. Producing cements from black ash would be a sustainable utilization for otherwise useless hazardous waste. In sodium disilicate (SiO2/Na2O 2,72) activated sample calcium silicate hydrate gel is formed, which is the phase connected to the strength of Portland cement. The 29Si MAS-NMR and ATR-FTIR spectra indicate that the gel contains polymeric parts in which the polymer is mainly formed of silicate middle groups (Q2) and the silicate chains are lengthening in time. These factors are responsible for the strength and further strengthening of the material. The samples of black ash activated with sodium disilicate SiO2/Na2O 1,5 show similar characteristics. However the NMR spectrum reveals that the amorphous geopolymeric phase (C–S–H gel) is accompanied by a two dimensional silicate chain structure which is also characteristic of C–S–H phase. In NaOH activated black ash samples, it was observed that it contains silicate structures with short chain lengths and also a content of undissolved amorphous phase from the source material. This indicates that NaOH solution was not sufficient in dissolving the amorphus phase present in source material and the depolymerized silicate species have not substantially polycondensed. Based on the results of this thesis the synthesis of geopolymeric products from black ash can be achieved with activator solutions containing soluble silica. Altough further research is needed to establish the optimal ratios of activator solutions the results show a proof of concept.