Properties of fly ash geopolymer modified with red mud and silica fume: a comparative study

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.

Properties of red-mud-based geopolymers in the light of their chemical composition

“…Every 5% red mud increase corresponds to a 17% decrease in flow diameter (Eq. 2) [14]. These results are not surprising because red mud has a high percentage of LOI.…”

“…Ten different sample compositions are categorized as low calcium RM collected from the literature [7,[10][11][12][13][14][15][16][17][18]. From a statistical point of view, the average can be taken as the mean or median value depending on the existence of outliers [19].…”

“…Therefore, the optimum percentage is 15%. The reduced initial setting time has reached 196 minutes, and the final setting time has come 300 minutes with the 15% red mud added to fly ash geopolymers [10,14], as shown in Figure 2. The final setting time has been tested for the high calcium red mud combined with other geopolymer precursors, fly ash and ground granulated blast furnace slag.…”

“…The effect of low calcium red mud addition on the workability of fly ash geopolymers has been investigated by S.Y. Kim et al [14] and R. Bajpai et al [34]. The increase of red mud percentage reduces geopolymer workability almost in a linear relationship.…”

Review on the Progress of Treatment Methods of Heavy Metals in Municipal Solid Waste Incineration Fly Ash

“…The TCLP test and the comprehensive test show that more than 99.6% of heavy metals in geopolymer could be immobilized when the geopolymer precursor exceeds 60%. Bajpai et al, [64] did a comparative study by investigating the activation of fly ash-base geopolymer paste modified by red mud and silica fumes. The results revealed that the water absorption of fly ash geopolymer increase with red mud.…”