Water Permeability, Strength Development And Microstructure of Activated Pulverized Rice Husk Ash Geopolymer Cement

Abstract

The activated pulverized rice husk ash (PRHA) as a pozzolanic material mixed with Portland cement (OPC) in various ratios was synthesized as geopolymer cement. The alkaline activator was prepared from a mixture of sodium hydroxide (NaOH) and sodium silicate (Na2SiO3). Effect of OPC replacement on the various properties of the synthesized geopolymers was evaluated. The results showed that the water of consistency sharply decreased with the increase of the OPC addition, while the setting times (initial and final) slightly decreased. The water absorption and apparent porosity reduced with OPC replacement at the expense of PRHA merely up to 18 wt. % content, and then increased. The bulk density as well as flexural and compressive strengths improved and enhanced also up to 18 wt. % OPC content, but then declined with any further increase. The free lime content increased up to 3 days of curing and then decreased onward due to its consumption during the pozzolanic reactions. The 18 wt. % OPC geopolymer mix is the optimum mix, where it achieved the better water absorption (6.89 %), bulk density (1.9099 g/cm3), apparent porosity (15.62 %), flexural strength (5.54 MPa) and compressive strength (58.25 MPa) at 90 days curing times. The ultrasonic pulse velocity test largely confirmed the obtained physical and mechanical characteristics. The FTIR spectra showed that the spectra of G18 geopolymer was more pronounced due to the formation of NASH geopolymer-gel needed to ensure better conformity between different constituents in the whole system that responsible for the improvements in the mechanical properties. The microstructure of the optimum G18 geopolymer mix is well-defined with no or little pores that responsible for the improving of physical properties and the enhancement of mechanical strengths, whereas the microstructure of G24 is more porous and heterogeneous matrix which was contributed to the reduction of mechanical strengths.