Effect of Magnesite Addition and Mechanical Activation on the Synthesis of Fly Ash-Based Geopolymers

Abstract

Ca/Mg carbonate minerals, such as calcite and dolomite, play an increasingly important role in the development of alkali-activated binders or geopolymers, which are regarded as promising sustainable cement materials. In contrast to studies on calcite and dolomite, the effect on geopolymer properties of the addition of natural magnesite (magnesium carbonate) to aluminosilicate raw materials has not been investigated. The aim of this study is to investigate the influence of mechanical activation (MA) and natural magnesite addition to fly ash (FA) on the compressive strength of geopolymers based on the natural magnesite–FA blend. Magnesite substitutes FA in amounts of up to 20 wt.%. Geopolymers were prepared using NaOH solution as an alkaline agent. XRD, FT-IR spectroscopy, thermogravimetry, SEM, and a dissolution test are used to investigate the geopolymerization process. The major reaction product was sodium-containing aluminosilicate hydrogel. Magnesite is found to transform, to a minor degree, to hydrotalcite. MA of the blend significantly improves geopolymer strength. For geopolymers based on (FA + magnesite) blends mechanically activated for 180 s, the strength is on average 8.0 ± 1.5, 3.0 ± 0.9, 1.5 ± 0.2, and 1.7 ± 0.5 times higher than that for the geopolymers based on the blends mechanically activated for 30 s at the age of 7, 28, 180, and 360 d, respectively. Although blending FA with magnesite does not increase geopolymer strength, for the mixtures containing 1%–10% magnesite, in general, the strength is either not reduced or it is reduced to a small degree compared to the geopolymers based on 100% FA. Using previously obtained data, for the first time, the effect of the addition of three Ca/Mg carbonate minerals to FA and MA on geopolymer performance is compared. Under similar conditions, geopolymer strength decreases in the order calcite > dolomite > magnesite. The main factors affecting the strength of geopolymers based on the mechanically activated blends of FA with magnesite including filler, dilution, and chemical effects are discussed.