Fresh and Hardened Properties of High Calcium Fly Ash-Based Geopolymer Matrix with High Dosage of Borax
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RESEARCH PAPER
Fresh and Hardened Properties of High Calcium Fly Ash‑Based Geopolymer Matrix with High Dosage of Borax Antoni Antoni1 · Austin A. T. Purwantoro1 · Widya S. P. D. Suyanto1 · Djwantoro Hardjito1 Received: 5 April 2019 / Accepted: 13 December 2019 © Shiraz University 2019
Abstract Geopolymer is synthesized by mixing material rich in alumina and silica, such as fly ash, with a highly alkaline solution to form a hardened matrix. However, when using high calcium fly ash as a precursor, a flash set frequently occurs, i.e., the mixture hardens very rapidly before casting can be completed. The use of borax as an additive has been reported due to its potential to prolong the setting time. In this study, the use of a significantly higher dosage of borax is explored. The results show that the addition of borax up to 20% of fly ash, by mass, into the alkaline solution prolongs the setting time by up to 90 min. Conversely, the addition of higher amounts of borax tends to decrease the compressive strength of the geopolymer, whereas adding a small amount of calcium oxide into the mixture increases the strength marginally, especially when the borax content is small. However, the amount of calcium oxide should be limited because at higher content, the effectiveness of borax to prolong the setting time is reduced. Keywords Geopolymer · High calcium fly ash · Workability · Flash set · Borax · Calcium oxide · Compressive strength
1 Introduction The use of fly ash as cement substitute material has become common in the construction world, especially for partial substitution (Huang et al. 2013; Rashad 2015). The application of fly ash can be increased even further when fly ash is used as the main binder by geopolymeric reaction. Fly ash, either with low calcium (class F) or high calcium (class C) content, is a material rich in silica and alumina, and can be used as a precursor in making geopolymer. The activation of the source material occurs in the presence of a highly alkaline solution, commonly a combination of sodium hydroxide and sodium silicate (Wattimena et al. 2017; Yildirim et al. 2011). The ratio and content of the two alkalis determine the characteristics of the geopolymer concrete (Arioz et al. 2012; Hardjito et al. 2004; Mustafa et al. 2011; Rangan 2010; Rattanasak and Chindaprasirt 2009; Thunuguntla and Gunneswara Rao 2018). The use of high calcium fly ash as a source material results in a high-strength geopolymer (Antoni et al. 2017; * Antoni Antoni [email protected] 1
Petra Christian University, Jl. Siwalankerto 121‑131, Surabaya 60236, Indonesia
Chindaprasirt et al. 2007; Guo et al. 2010). The geopolymerization and hydration reactions may occur concurrently in the matrix when high calcium fly ash is used (Antoni et al. 2016a; Li et al. 2013). The high calcium fly ash used had similar calcium content with ground-granulated blastfurnace slag (GGBFS), which can be beneficial in increasing the compressive strength of the geopolymer (Sethi et al. 2018). Both reactions can improve the quality of the
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