Reaction mechanisms of fly ash and metakaolin geopolymers and environmental compatibility

Sun, Zengqing; Vollpracht, Anya (Thesis advisor); Dehn, Frank (Thesis advisor)

Aachen (2020)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2020, Kumulative Dissertation

Abstract

Much is unknown about the reaction and environmental properties of geopolymers. In this work, the early age and long-term geopolymerization were studied, as well as the partitioning of main components, heavy metals and trace elements between geopolymer binders and aqueous phase at different leaching scenarios. In terms of the early age geopolymerizaiton, isothermal calorimeter and in-situ XRD were used to characterize the heat and mineral evolution of fly ash geopolymer (FAG) and metakaolin geopolymer (MKG). The results were compared to alkali activated binders (AAB) based on ground granulated blast-furnace slag (GGBS). Factors including activator concentration, liquid-to-solid ratio, and curing temperature were taken into consideration. FAG is more temperature dependent, with much higher activation energy than MKG and GGBS based AAB. The dissolution of reactive minerals contained by solid precursor and formation of new crystalline phases were rapid and took place during the initial dissolution period. In addition, XRD results were analysed using the partial or no known crystalline structure method to quantify the amorphous evolution, which was demonstrated to be powerful in quantifying early age geopolymerization. The evolutions of mechanical and chemical properties of geopolymers were profoundly influenced by curing protocol. Initial heat treatment contributes to the strength development and zeolite formation, in particular for the FAG. Meanwhile, this also leads to slightly worse pore characteristics. During sample handling and aging, natural carbonation took place in two ways: adsorption and reaction of CO2 with mobile alkalis on the one hand and with framework oxygen atoms of the alunimosilicate gels on the other hand. The partitioning of chemical species (Na, K, Al, Si, Ca, Ba, As, Cd, Co, Cr, Cu, Mo, Ni, Pb, Sb, Se, Tl, V, Zn, chloride and sulfate) between the solid and aqueous phase was characterized in a broad pH range (1-14). Chloride and sulfate are of virtually pH independent characteristic. The overall leaching of Na, K, Ca, Si, Al, Co, Cu and Ni can be classified as amphoteric leaching pattern, in which concentrations increase at acidic and alkaline conditions. In contrast, the leaching of Mg, Zn, Pb, Mn, Sr and Sb exhibits cationic pattern, the maxima concentration locates in acidic pH range and decrease as pH values increase. Oxyanionic pattern, with high releases taking place in extreme acidic conditions and natural pH, is found for the leaching of As, V, Mo, Cr and Se from FAG and MKG. Geochemical modelling was carried out, which offers a novel insight into the assemblages of solubility controlling phases. Meanwhile, limited by the not fully known chemistry of geopolymer, the current geochemical modelling is far from satisfactory, which deserves further investigation. The leaching of monolithic geopolymer mortars were conducted following the European specification CEN/TS 16637-2. Results show that the dynamic surface leaching of Na, K, Al, Si, Ca, Ba, As, Cd, Co, Cr, Cu, Mo, Ni, Pb, Sb, Se, Tl, V, Zn, chloride and sulfate is mainly controlled by diffusion, while surface wash-off, solubility and depletion also take place. A comparison with the leaching of cement based materials and the regulatory limits in Germany and in the Netherlands was conducted. Leaching of above heavy metals and trace elements (except B, V and Mo) from geopolymers are in a similar range as for cementitous binders. The cumulative releases of V are higher than for most cementitious materials but still meet the requirement in the Netherlands (in Germany there is no valid threshold to date). In addition, no changes in phase composition on the surface layer of geopolymers were observed after leaching. The GGBS based AAB possesses superior compressive strength, polishing- and skid-resistant property, which can be used for the construction of pavement surface layer. The flexural strength of GGBS based AAB is higher than cement counterpart of the same strength grade, suggesting a higher capacity to withstand bending damage. Moreover, equivalent CO2 emission of GGBS based AAB paving is around 40 % lower than its OPC counterpart. All these results indicate that GGBS based AAB can be used as an environmentally friendly pavement binder. Depending on the mix composition, the GGBS based AAB, MKG and their mixture synthesized in this thesis can be used to produce ambient temperature cured pervious concrete of high aggregate-to-binder ratio.

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