SOLUBILITY OF PRECURSORS AND CARBONATION OF WATERGLASS-FREE GEOPOLYMERS
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SOLUBILITY OF PRECURSORS AND CARBONATION OF WATERGLASS-FREE GEOPOLYMERS N. WERLING1 *
, F. DEHN2, F. KRAUSE3, A. STEUDEL1, R. SCHUHMANN1, AND K. EMMERICH1
1
Competence Center for Material Moisture (CMM), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany 2 Institute for Concrete Structures and Building Materials (IMB), Karlsruhe Institute of Technology (KIT), Gotthard-Franz-Str. 3, 76131 Karlsruhe, Germany 3 Institute of Functional Interfaces (IFG), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 EggensteinLeopoldshafen, Germany
Abstract—Geopolymers have the potential to function as an environmentally friendly substitute for ordinary Portland cement, with up to 80% less CO2 emission during production. The effect is best utilized for geopolymers prepared with amorphous silica instead of waterglass (Na2xSiyO2y+x) to adjust the Si:Al ratio. The reactivity of the precursors with the alkaline activator affects the final mineralogical properties of the binder. The purpose of the present study was to investigate the amount of different phases formed during geopolymerization and to understand the quantitative evolution of carbonation during geopolymer synthesis by determining the solubility of metakaolinite and amorphous SiO2 in NaOH at various concentrations. The solubility was studied by ICP-OES measurements. X-ray diffraction was used for qualitative and quantitative phase analysis of the geopolymers. The solubility of the precursors increased with calcination temperature of metakaolinite, reaction time for amorphous SiO2, and at higher NaOH concentrations. Partial dissolution resulted in free Na+, which is a source for the formation of carbonates in the geopolymers. Thermonatrite occurred prior to trona formation in all samples. Keywords—Amorphous silica . Carbonation . Geopolymers . Metakaolinite INTRODUCTION Geopolymer binders are inorganic polymers with a 3-dimensional framework structure of oligomers with various ratios of Si, Al, O, and OH–. The negative charge created by Si/Al substitutions is balanced by the cation of the alkaline activator solution. As well as supplementary cementitious materials (SCM), geopolymer binders are potential substitutes for ordinary Portland cement (OPC), and emit up to 40–80% less CO2 than OPC during their production (McLellan et al. 2011; Davidovits 2013). The lower emissions are mainly due to the fact that the raw materials contain no structural CO2. In contrast to SCM, which replace only a certain amount of OPC, geopolymers are ideally OPC-free binders. OPC is a hydraulic binder, while geopolymers are alkaline-activated binders. High-Ca and low-Ca/Ca-free types of alkaline-activated binders differ from one another (Herrmann et al. 2018) in that geopolymers contain little or no Ca. Aluminosilicates (fly ash, furnace slag, silica fume, or calcined clays) function as precursors and are activated with a highly alkaline solution (waterglass and/or highly concentrated alkali brines), which le
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