Development of engineered geopolymer composites containing low-activity fly ashes and ground granulated blast furnace slags with hybrid fibers

Engineered geopolymer composites, a green version of engineered cementitious composites, replace the cement by industry byproducts such as ground granulated blast furnace slag and/or fly ash with alkaline activators. However, fly ashes largely available in Kazakhstan termed here as low-activity fly ash, do not meet ASTM standard in terms of the particle size distribution and strength activity index due to lack of quality control during collection. Therefore, this paper aims to develop an engineered geopolymer composite containing low-activity fly ash and ground granulated blast furnace slag with hybrid fibers. Different combinations of fibers including polyvinyl alcohol, polypropylene, and steel fibers were investigated. The developed composite is targeted for multi-functional retrofitting on unreinforced masonry structures which requires high strain hardening capacity and low thermal conductivity. To achieve this target, the workability, dry shrinkage, compressive strength, tensile strength, strain hardening capacity, and thermal conductivity were examined through extensive experimental studies. From test results, it has been found that the geopolymer composite containing low-activity fly ash with a proper fiber combination can exhibit desirable mechanical and thermal properties with more than 20 MPa compressive strength, more than 2% of the tensile strain hardening capacity, and less than 0.25 W/mK thermal conductivity. Based on test results, micromechanical models were also developed for the composite to identify suitable fiber combinations for required strain hardening capacities.