Abstract
Currently, there are a very limited number of studies on the effect of admixtures on properties of ‘one-part’ geopolymers. This paper reports the effects of different superplasticizers and retarders on fresh and hardened properties of one-part fly ash-slag blended geopolymers made by different solid activators. Two different grades of sodium silicate, namely anhydrous sodium metasilicate powder (nSiO2/nNa2O = 0.9) and GD Grade sodium silicate powder (nSiO2/nNa2O = 2.0) were used as the solid activators. Five different commercially available superplasticizers, including three modified polycarboxylate-based superplasticizers (denoted as PC1, PC2, and PC3) and two naphthalene-based superplasticizers (denoted as N1 and N2), as well as three different retarders, including sucrose, anhydrous borax and a commercially available retarder, were investigated. Workability, setting time and compressive strength of the mixtures without and with addition of each ‘individual’ admixture were measured. The results showed the effect of admixtures on the properties of the one-part geopolymers significantly depended on the type of solid activator and the type of admixture used. When GD Grade sodium silicate powder was used as the solid activator, all investigated admixtures not only had no positive effect on the workability and setting time, but also significantly reduced the compressive strength of the mixture. However, when anhydrous sodium metasilicate powder was used as the solid activator, the PC1 and sucrose were the best performing superplasticizer and retarder, respectively, causing no reduction in the compressive strength, but significant increase in the workability (up to + 72%) and setting time (up to + 111%), respectively as compared to the mixture with no admixture. In addition, the results also showed that addition of ‘combined’ admixtures (i.e., PC1 in the presence of sucrose) significantly increased the workability (up to + 39%) and setting time (up to + 141%), but slightly reduced the compressive strength (−16%) of the mixture activated by anhydrous sodium metasilicate powder, as compared to the mixture with no admixture.
Highlights
Ordinary Portland cement (OPC) has been widely used as the main binder for production of concrete in the construction industry
Geopolymer is an alternative binder to OPC, which can be manufactured from materials of geological origin or industrial
Based on the aforementioned results, it can be concluded that among the SPs investigated in this study, PC1, N2 and N1 were chemically stable in the anhydrous sodium metasilicate solution, provided up to 40% increase in workability of the geopolymer paste [32,57]
Summary
Ordinary Portland cement (OPC) has been widely used as the main binder for production of concrete in the construction industry. Geopolymer is an alternative binder to OPC, which can be manufactured from materials of geological origin (such as metakaolin) or industrial. Materials 2019, 12, 3410 by-products (such as fly ash and slag that contain a considerable amount of silica and alumina) with high alkaline activators [3]. The utilization of industrial by-products such as fly ash as the source material in geopolymer production is beneficial as disposition of these industrial by-products occupies large areas in landfills, which could be used for other purposes [4]. According to previous studies, using fly ash as the source material for production of geopolymer consumes up to 60% less energy and emits up to 80% less carbon dioxide as compared to production of OPC [5,6]
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