Investigating experimentally the potency of divergent sodium hydroxide and sodium silicate molar proportions on silica fume and obsidian-based geopolymer mortars

Abstract

The search for and development of new binders that can replace traditional Portland cement is of vital importance from an environmental perspective. Geopolymers are one of the most important solutions in this regard. Geopolymers are composed of various types of binders containing sufficient Al and Si sources and activated with alkaline activators. In addition to their high mechanical and durability features beside Portland cement, they are one of the most important alternatives for reducing carbon emissions. Although obsidian (OB) and silica fume (SF), which are waste materials, have sufficient Al and Si resources for the development of new geopolymers, there have been very few studies on these materials. The purpose of this study is to experimentally investigate the properties of geopolymer mortars with OB-based geopolymers with different proportions of SF substituted in ranging from 2.5 to 10 weight percent. In addition to sodium silicate (Na2SiO3), sodium hydroxide (NaOH) has also been used as an alkaline activator with different molar properties such as 6, 8, 10, 12, and 14 M. The alkali activator/binder proportion was standardized as 0.45 and the curing temperature as 150°C. The experimental research on geopolymer mortars with diverse mixing ratios consisted of 3 stages. In the first stage, the effects of sodium hydroxide (SH) alkali activator with a constant concentration of 12 M were measured for all mixtures. In the second stage, the effects of SH and sodium silicate (SS) alkali activators with different molar characteristics on the performance of the mortars were studied. In the third stage, mechanical tests were carried out to determine the compressive strength (CS) and durability tests such as water absorption, sorptivity, and UPV were carried out to determine the durability properties of the geopolymer mortars. In addition, various microstructural tests such as x-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetric analysis, and scanning electron microscope-EDS were carried out to determine the changes in the microstructure. The peak CS of 66.24 MPa was acquired from sample S1. SF substitution does not have a significant positive effect on mechanical properties. However, SF substitution has a positive effect on important durability properties such as water absorption and sorptivity. The maximum improvement in water absorption tendency increases in direct proportion to the SF substitution rate. Although the water absorption rate of the control S1 sample was 5.95%, this rate decreased to 4.33% in the S5 sample with 10% SF substitution. SEM images clearly showed that the mortar specimens had different structures such as geopolymer matrix, capillary cracks, and microvoids, and the matrices contributed positively to the strengths. Based on the results of this study, geopolymers manufactured from OB and SF can be used to develop environmentally friendly sustainable building materials.