EFFECT OF CURING REGIME ON PROPERTIES OF GEOPOLYMER MIXTURES CONTAINING BASIC OXYGEN FURNACE SLAG (BOFS) AGGREGATES

Abstract

Basic oxygen furnace slag (BOFS) is a by-product material from the steel industry, which might be used as a substitute for the natural aggregates in the construction industry. The issue pertaining to the use of BOFS as an aggregate is the volume expansion in the concrete matrix by Ca(OH)2 (calcium hydroxide) or Mg(OH)2 (magnesium hydroxide) formed by free calcium oxide (f-CaO) and free magnesium oxide (f-MgO) components in BOFS. Mineral sequestration (CO₂ curing) and the geopolymerization technique are potential remedies for the BOFS expansion problem. This research studied the mechanical, durability, and microstructural characteristics of BOFS-based geopolymer mortar under different curing regimes and durations. While flowability and air content are among the fresh properties of geopolymer mixtures, compressive strength, dielectric constant, drying shrinkage, and expansion (1M NaOH solution and water expansion) are the primary focus of the hardened properties and durability of geopolymer mixtures. Scanning electron microscope (SEM) image, Fourier transform infrared (FTIR) spectroscopy, and X-ray diffraction (XRD) analysis were carried out to investigate the microstructural properties of geopolymer mortar. Four different curing regimes were evaluated: air, water, steam, and combined steam and CO₂ curing. It was looked into how various curing times affected the characteristics of the geopolymer mortar based on BOFS: 6- and 12-hours steam curing and 6-, 12- and 24-hours combined steam and CO₂ curing. Additionally, three different BOFS/Sand ratios of 100/0, 75/25, and 50/50 were compared. The test methods started with evaluating the mortar’s fresh properties: flowability and air content. The fresh property test results indicate that the mixture with a BOFS/Sand ratio of 75/25 has high flowability, and the air content of mortar mixtures increases with the decrease of BOFS content. Furthermore, compressive strength results indicate that steam curing and combined steam and CO₂ curing increase the compressive strength and have a higher dielectric constant value in comparison to ambient air-cured BOFS-based geopolymer mortar. BOFS/Sand ratio of 75/25 and the longer steam and CO₂ curing favored the compressive strength development. However, the BOFS/Sand ratio and duration of steam curing and CO₂ curing do not significantly affect the dielectric constant value. For drying shrinkage and 1M NaOH and water expansion tests, both steam curing and the combined steam and CO2 curing hindered the excessive drying shrinkage and expansion in 1M NaOH solution compared to air or water-cured mixtures. The water and 1M NaOH expansion characteristic curves underwent less fluctuation for longer steam and CO₂ curing duration. The drying shrinkage was favored by longer steam curing duration. However, it was not affected by CO₂ curing duration. FTIR spectroscopy, SEM image analysis and XRD results indicated that the geopolymerization of the mixture was more apparent during the steam curing and CO₂ curing in comparison to water curing and air curing regimes. The longer duration of steam and CO₂ curing also promoted the formation of more stable and dense geopolymer matrix structure, due to longer hydration and formation of stable carbonate compounds.