Strength development in clay–fly ash geopolymer

This paper presents the role of influential factors on the strength development in a clay–fly ash geopolymer that 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 influential factors are Na2SiO3/NaOH ratio, L/FA ratio and heat conditions. The optimum ingredient for the clay–FA geopolymer is the Na2SiO3/NaOH ratio of 0.7 and the L/FA ratio of 0.6. The Na2SiO3/NaOH ratio required for the clay–FA geopolymer is less than that of the FA geopolymer because the clay has high cation absorption ability and then absorbs some of the input NaOH. For a given Na2SiO3/NaOH content, the strength increases with increasing the liquid alkaline activator. The excess input alkaline activator causes the precipitation at very early stage before the condensation process in geopolymerization and results in the cracks on the FA particles. The overheating (very high temperature) and excess heat duration cause the micro-cracks on the specimens. The relationship between the strength and heat energy is proposed to integrate the role of heat temperature and duration on the geopolymerization. The compressive strength increases with increasing heat energy up to a certain level. Beyond this level, the specimens shrink and crack due to the reduction in pore fluid, which results in the strength reduction. The relationship between strength and heat energy can be used as fundamental for further study on the strength development and the mix design method for the clay–FA geopolymer with different specimen dimensions, clay minerals, liquid alkaline activators, pozzolanic materials and clay:FA ratios.

► Performed strength and microstructure tests of geopolymer brick.
► Explain role of liquid alkaline activator on strength development.
► Explain role of heat temperature and duration on strength development.
► Analyze strength of the clay–fly ash geopolymer samples.
► Suggest a relationship between strength and heat energy.