Factors influencing strength development in clay–fly ash geopolymer

This paper investigates the strength development with different ingredients, specimen sizes, and heating conditions for the manufacturing of clay–fly ash geopolymer. A silty clay is used as fine aggregates and fly ash (FA) is used as a pozzolanic material. A liquid alkaline activator (L) is a mixture of sodium silicate solution (Na2SiO3) and sodium hydroxide solution (NaOH). The studied factors are FA/clay ratio, Na2SiO3/NaOH ratio, L/FA ratio, molding moisture content, specimen sizes, heat temperature and duration. The optimum ingredients for clay–FA geopolymer are Na2SiO3/NaOH ratio of 0.7 for all FA/clay ratios, L/FA ratios and specimen sizes tested. This Na2SiO3/NaOH ratio is thus considered as constant for making clay–FA geopolymer. The optimum L/FA ratio decreases as the FA/clay ratio increases. The optimum L/FA ratio is 0.6 for FA/clay ratio of 0.3 and is 0.5 for FA/clay ratio of 0.7. The molding moisture content providing the highest strength is at 1.0OMC for FA/clay ratio of 0.3, 0.8OMC for FA/clay ratio of 0.5, and 0.6OMC for FA/clay ratio of 0.7. The optimum ingredient (Na2SiO3/NaOH ratio, L/FA ratio and moisture content) is irrespective of sample size. The larger specimens with higher weight require higher heat temperature and longer duration for the geopolymerization development. The heat energy per weight (E/W) concept can integrate the role of heat temperature, duration and specimen weight on the geopolymerization. The specimens with high FA/clay ratio require low E/W. The optimum heat energy per weight is approximately 8.5 °C h/g for FA/clay ratios of 0.3 and 0.5 and 7.57 °C h/g for FA/clay ratio of 0.7. The relationship between strength and E/W is very useful for production industry to estimate the heat temperature and duration to attain the target strength for the required weight in making clay–FA geopolymer brick.