Abstract
In this study, sorptivity, setting time, resistance to sulfuric acid, and compressive strength of mortars that use alkali-activated GGBS and fly ash as binders, were evaluated experimentally. The activation of binders, was achieved at room temperature of 22 ± 2 °C using combinations of sodium silicates (Na2SiO3) and sodium hydroxide (NaOH) solutions in ratios of 1.5, 2.0, and 2.5. The parameters considered in terms of their effects on fresh and hardened properties include: NaOH molarity, activator ratio Na2SiO3/NaOH, mortar sample age, and relative amount of GGBS/fly ash in binder combination. Sorptivity, change in mass, and compressive strength were determined for mortar samples that were submerged in 10% sulfuric acid solution for 7 days, 28 days, and 90 days. The binder for mortar samples tested at each of the specified ages consisted of 100% GGBS (G100), 75%GGBS+25% fly ash (G75F25), or 50% GGBS + 50% fly ash (G50F50). The binder was activated using Na2SiO3 solution, combined with 10 M, 12 M, 14 M, or 16 M NaOH solution. It was found that sorptivity decreases with increase in curing age, for all activator ratios, concentrations, and relative amounts of GGBS/fly ash. Binder consisting of 75%GGBS + 25% fly ash with NaOH concentration of 12 M had the lowest sorptivity. Exposure of alkali-activated GGBS/fly ash mortar samples to sulfate attack did not cause loss in mass nor visible signs of damage/deterioration. All binder combinations experienced increase in compressive strength after curing in 10%sufluric acid solution, with the optimum G75F25 mix achieving a 28-day strength of 80.53 MPa when NaOH molarity is 10 M, which increased to 91.06 MPa after 90 days. Variation in concentration of NaOH didn’t cause significant change in the magnitudes of 28-day or 90-day compressive strengths of G50F50. However, despite slow dissolution of fly ash and immersion in 10% sulfuric acid solution, G50F50 developed 28-day compressive strength of 56.23 MPa and 90-day compressive of 86.73 MPa, which qualifies G50F50 as high strength mortar for practical purposes.
Highlights
Typical design considerations for suitability of concrete in a typical application include workability/flowability, setting time, durability, and compressive strength
One of the goals of this study is to evaluate the effects of two factors on the initial and final setting times: (1) the relative amounts of ground-granulated blast furnace slag (GGBS) and fly ash in the total binder content, and (2) the molarity of sodium hydroxide (NaOH) activator
This study evaluated selected factors influencing sorptivity, setting time, and sulfuric acid resistance, of alkali-activated mortar samples in which the binder is either GGBS or combination or GGBS and fly ash
Summary
Typical design considerations for suitability of concrete in a typical application include workability/flowability, setting time, durability, and compressive strength. These properties have been studied extensively for traditional concrete that uses ordinary Portland cement (OPC) as binder. Studies have shown that long-term exposure of concrete to aggressive environment, such as freezing and thawing, or alkali-aggregate reaction may cause significant loss of strength and impair load-carrying capacity of structural elements [1]. Studies have shown that ingress of water carrying harmful chlorides into concrete may lead to corrosion of reinforcing steel bars and jeopardize the integrity of the structural system [2]. Fly ash and ground-granulated blast furnace slag (GGBS), individually or combined have demonstrated ability to enhance mechanical properties and durability, when used as concrete binders to partially replace OPC [3,4]
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