Effect of Ca(OH)2 Addition on the Engineering Properties of Sodium Sulfate Activated Slag

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1. Introduction

Alkali-activated cements (AACs) or geopolymers produced from the reaction of an alkali metal source (solid or dissolved) with a solid aluminosilicate powder [1] are more environmentally-friendly and require less energy compared to conventional ordinary Portland cement (OPC) based cementitious materials [2]. AACs, as one of the most promising alternatives to the OPC, have equivalent or better performance than conventional cementitious binders. It has been stated that AACs can have less greenhouse gas emission (CO2), superior mechanical properties and better durability performance against high temperature, acid and sulfate attacks when compared to several types of existing OPC-based concrete [3,4,5,6,7,8]. Among all AACs, ground granulated blast furnace slag (GGBFS) activated by sodium silicate and/or sodium hydroxide is the most intensively studied since it provides the best formulation for high strength and other advantageous properties. However, both sodium silicate and sodium hydroxide do not exist naturally and must be obtained through an energy-intensive manufacturing process. This is particularly true for sodium silicate which is made by melting sand and sodium carbonate at 1350-1450 °C and then dissolving it in an autoclave at 140-160 °C under appropriate steam pressure [1]. Consequently, sodium silicate and sodium hydroxide activation may not be the best solution for achieving a sustainable cementitious system.

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Sodium sulfate (Na2SO4) is suggested as a cheaper, cleaner, less harmful and more environmentally friendly alternative activator to sodium silicate and sodium hydroxide for the activation of slag [9]. This is due to sodium sulfate being found naturally as thenardite (anhydrous Na2SO4) and mirabilite (Na2SO4·10H2O) [10]. As reported by Mobasher et al. [10], the main reaction products in Na2SO4 activation are calcium-aluminosilicate-hydrate (C-A-S-H) type phases with a low Ca/Si ratio, providing the main contribution to the strength of the AAC and ettringite (3CaO·Al2O3·3CaSO4·32H2O) as a secondary reaction product [11]. In this activation, ettringite is stable even at high alkalinity due to the high sulfate contents of the system [11,12]. The Na2SO4 sodium sulfate activated slag has been considered as a possible binder for coping with certain radioactive wastes containing reactive metals due to its lower pH, heat of hydration and lower free water content [12]. However, there have been limited studies on the activation of slag with sodium sulfate as compared to sodium silicate and sodium hydroxide. Possibly, this is owing to the sodium sulfate-activated slag mixtures exhibiting low early strength [9,10,13]. A higher early strength can be obtained in these systems by grinding the slag to a finer particle size, curing at higher temperatures, or using a combined chemical activator to obtain a higher early strength in these systems. Rashad et al. [9] pointed out that increasing the slag fineness is a more effective method than increasing the concentration of Na2SO4 for optimizing early and long-term strength of Na2SO4 activated slag mixtures. Fu et al. [13] also reported that the addition of Na2SO4 in conjunction with Portland cement improved slag hydration through a complex process. Gijbels et al. [14] also investigated the effect of the sodium hydroxide content on alkali/sulfate-activated binders from 90 wt% GGBFS and 10 wt% phosphogypsum, and found that a high ettringite content can increase the compressive strength. Zhang et al. [15] pointed out that using ultra-fine GGBFS improves the mechanical performance of Na2SO4 activated slag mixtures at ambient temperature.

To date, the studies concerning rheological and mechanical properties of sodium sulfate activated GGBFS mixtures are very limited. Although sodium sulfate-activated GGBFS mixtures can significantly reduce the carbon footprint, their drawbacks of lower compressive strength, longer setting time and higher porosity, severely limit their applications. This study aims to investigate the influence of Ca(OH)2 addition on the engineering properties of sodium sulfate activated slag such as rheological properties using a rheometer, mechanical properties using compressive and flexural strength tests, porosity using mercury intrusion porosimetry (MIP), microstructure using a scanning electron microscope/backscattered electron (BSE) imaging, chemical groups in reaction products using Fourier transform infrared spectroscopy (FTIR) and phase identification using X-ray diffraction (XRD). Test results showed that the engineering properties of the Na2SO4 activated slag cements can be significantly improved by the addition of Ca(OH)2.

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