Properties of fly ash geopolymer modified with red mud and silica fume: a comparative study
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Properties of fly ash geopolymer modified with red mud and silica fume: a comparative study Rishabh Bajpai1 · Anshuman Shrivastava1 · Manpreet Singh2 Received: 17 June 2020 / Accepted: 12 October 2020 © Springer Nature Switzerland AG 2020
Abstract Alkali-activated geopolymer has emerged as a sustainable alternative to highly polluting conventional cement. The present paper investigates the activation of fly ash-based geopolymer paste modified by red mud and silica fume. Three geopolymer mixes are considered: (1) fly ash; (2) fly ash—red mud; and (3) fly ash—silica fume. Mass proportions of modifiers in the respected mixes varied by 0, 5%, 10%, 15%, 20%, and 25% of total binder. Heat curing (60 °C) provided for 24 h just after casting. Alkaline activator consists of sodium hydroxide and sodium silicate solutions. Workability results reveal that both red mud and silica fume triggered the reduction in workability with the later one had a greater impact. A flash set was obtained with the silica fume content of more than 15%. Both the modifiers cause a decrease in the setting time at ambient temperature, and silica fume proved to be more productive with the reduction of more than 65% of setting time. Mixes containing red mud exhibit slightly higher density. Water absorption of fly ash geopolymer increases with red mud. Compressive strength increases with the addition of modifiers, and geopolymer containing silica fume possesses the highest compressive strength among the three mixes considered. Compressive strength exhibited a linear correlation with molar ratios (Si/Al and Na/Si) of source materials. XRD and SEM methods showed that the geopolymerization reaction is affected by the red mud and silica fume. It is recommended that the flash set and setting time at higher contents of modifiers must be examined for the design of a modified geopolymer mix. Keywords Flash set · Fly ash · Geopolymer · Red mud · Setting time · Silica fume
1 Introduction Climate change due to 7% of global equivalent carbon dioxide generated from cement-based activities needs to be addressed to save the precious earth [1]. The use of industrial by-products such as coal fly ash in alkaliactivated aluminosilicate-based geopolymer is an attractive sustainable approach, due to the reduction of overall carbon emissions by 64% compared with cement [2]. Aluminosilicate based geopolymers are inorganic polymers in nature [3]. The evaluation of alkali-activated fly ash geopolymers has been a major field of research for the
past three decades [4–6] since it possesses considerable cementitious properties [7]. However, the limitations such as longer setting time, unpredictable workability, efflorescence, and low strength (when cured at ambient conditions) of geopolymer have restrained its field use. The utilization of several additives in fly ash-based geopolymer has been practised to improve the strength and durability [8, 9]. However, differences in particle size, strength activity index, chemical and mineralogical compositions of these additives have
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