Effect of superplasticizers on properties of one-part Ca(OH)₂/Na₂SO₄ activated geopolymer pastes

Highlights

• Polycarboxylate is the most effective superplasticizer for Ca(OH)₂/Na₂SO₄ geopolymer.

• Reducing the w/b ratio improves the compressive strength of Ca(OH)₂/Na₂SO₄ geopolymer.

• Lowering the w/b using high superplasticizer content (i.e. 3%) delays the participation of reaction products.

Abstract

This paper investigates the effect of three different superplasticizers (SPs), Naphthalene (N), Melamine (M) and Polycarboxylate (PC), on the properties of one-part fly ash/slag geopolymer pastes activated by Ca(OH)₂/Na₂SO₄ powder combination. The flowability, setting time, and compressive strength of the achieved geopolymer pastes were assessed. It was found that the superplasticizers significantly improved the flowability, retarded the setting time, and increased the compressive strength of the one-part Ca(OH)₂/Na₂SO₄ geopolymer pastes. The most recommended superplasticizer was found to be polycarboxylate.

The use of polycarboxylate SP for decreasing the water content (i.e. w/b) in Ca(OH)₂/Na₂SO₄ geopolymer pastes has resulted in reducing the porosity and enhancing the compactness of the aluminosilicate gel, therefore, improved the compressive strength. It was found that the excessive use of polycarboxylate (i.e. 3%) for additional water reduction in Ca(OH)₂/Na₂SO₄ geopolymer pastes had an adverse effect; it extended the induction period and delayed the participation of reaction products, thus significantly prolonged the setting time.

Furthermore, a comparison between the achieved Ca(OH)₂/Na₂SO₄ geopolymer pastes and the conventional Na₂SiO₃-anhydrous one-part geopolymer pastes was carried out. It was found that Ca(OH)₂/Na₂SO₄ geopolymer exhibited significantly lower compressive strength, higher flowability and considerably longer setting time compared to Na₂SiO₃ geopolymer. Hence, such binder can be implemented as a green non-structural material.

Introduction

Consumption of ordinary Portland cement (OPC) has been undergoing an unprecedented increase worldwide due to the huge demand for municipal infrastructures and high-rise or large-space residential buildings [1]. However, the cement industry is facing great challenges due to its environmental aggressiveness caused by energy consumption and carbon dioxide emissions during the production process [2]. The implementation of energy/resource conservation techniques in cement production failed to reduce carbon dioxide emissions which are basically released from the decarbonation of limestone [3], [4]. Geopolymer, a clinker-free construction material, shows immense potentials in terms of environmental friendliness and durable performance [5], [6] which permit the partial alternation of traditional cement [7].

Geopolymer binders are usually derived from the activation of an aluminosilicate precursor such as fly ash, slag or metakaolin using chemical solutions including alkali-silicate [7], [8] or aluminum-phosphate [9], [10]. However, the alkali silicate-containing activators usually show high viscosity, corrosiveness, and pH-toxicity [11]. Further, the utilization of such alkaline solutions requires skilled workers for the safe production of alkali-activated materials, which are not always accessible in construction sites. Thus, a new category of geopolymers named “one-part” or “just-add-water” geopolymer was inspired [12]. The one-part geopolymer is a ready-mixed dry powder in which the solid activators and aluminosilicate precursors are pre-formulated together. Such a one-part geopolymer is a promising feasible material for in-situ construction applications due to its simple operation (using water-to-binder ratio only) alike the traditional OPC. This feature attractively pushes forward commercializing and standardizing [13] the geopolymer products from laboratories to “real” engineering fields.

Based on previous research [7], the alkaline activator in a one-part geopolymer mix can be any ingredient that provides alkali cations and raises the pH value during the geopolymerization reaction. Currently, the commercial solid anhydrous sodium metasilicate is preferentially selected for the preparation of one-part geopolymer [14], [15], [16], [17] due to the high strength of one-part geopolymer binders activated by such solid activator. However, from an environmental point of view, the life cycle assessment (LCA)-based mix design recommended by Habert et al [18] and Ouellet-Plamondon and Habert [19] may discourage the large-scale use of sodium silicates due to the environmental concerns companioned with their production. The production of synthetic alkali silicates can be described as a high energy intensity procedure due to the direct fusion of sand at around 900 ℃ or evaporation of metasilicate solution [20]. Therefore, relatively less hostile and more economic activators are expected to replace such highly corrosive alkali silicate activators for one-part geopolymer preparation.

Many studies have focused on the use of sodium carbonate (Na₂CO₃) as a less hostile and economic activator to replace the silicate-containing activators either partially [21] or fully [22], while relatively limited studies have used calcium hydroxide as a solid activator in geopolymer synthesis. Few studies have used calcium hydroxide (Ca(OH)₂) along with sodium carbonate as an activator combination in geopolymer [23], [24]. Further, part of the studies used calcium hydroxide as an additive to the raw aluminosilicate binders of the geopolymer [16], [25], [26]. On the other hand, calcium oxide was also considered as a possible activator [27] or additive [28] in the geopolymer systems. According to earlier research, calcium hydroxide has a relatively weak corrosiveness (pH < 14) and is considerably less expensive relative to silicate-containing activators [24]. Besides, the addition of sodium sulfate into calcium hydroxide can subsequently increase the pH environment due to the formation of sodium hydroxide via the chemical process shown in Eq. (1), which may contribute to the activation degree and strength development at early stage. It was reported that the resultant CaSO₄⋅2H₂O (i.e. gypsum) can act as an upstream material for the formation of ettringite which may microscopically densify the formed matrix and increase the mechanical strength [24]. Further, previous research reported the beneficial effect of gypsum addition to the fly ash geopolymer system in terms of strength development [29], [30]

$$N{a}_{2}S{O}_4+Ca{(OH)}_2+2H_{2}O\leftrightarrow CaS{O}_4·2H_{2}O\downarrow +2NaOH$$

Overall, the bulk of the available research has focused on the use of calcium hydroxide as an activator for the synthesis of alkali-activated slag pastes [31], [32], [33], [34]. It is worth mentioning that there is no available research hitherto that investigated fly ash/slag geopolymer pastes activated by calcium hydroxide. Furthermore, up to date no research has studied the effect of using superplasticizers (SPs) on the geopolymer using Ca(OH)₂/Na₂SO₄ as an activator except for Choi et al [33] who briefly reported the benefits of using SPs in the “one-part” alkali-activated slag composites. On the other hand, many studies, which explored the compatibility of liquid and/or powder commercial admixtures and retarders with one-part geopolymer binders, have focused on utilizing anhydrous sodium metasilicate [35], [36], [37], sodium silicate powder [37], and sodium hydroxide powder [38] as solid activators and did not consider the Ca(OH)₂/Na₂SO₄ combination. Moreover, the mechanism of SPs in two-part geopolymer binders (i.e. prepared using silicate-containing and/or sodium hydroxide liquid activators) was also delivered in several previous studies [39], [40], [41], [42].

Thus, considering the environmental and cost-related issues of the solid activators, this paper investigates the utilization of solid calcium hydroxide and sodium sulfate as an activator combination for one-part fly ash/slag geopolymer preparation. In this research, the effectiveness of three different types of superplasticizers including naphthalene (N), melamine (M) and Polycarboxylate (PC) on the compressive strength, flowability and setting of the one-part Ca(OH)₂/Na₂SO₄ geopolymer in addition to the superplasticizer stability behavior in Ca(OH)₂/Na₂SO₄ alkali medium are assessed. Furthermore, a comparison is carried out between the properties of the one-part Ca(OH)₂/Na₂SO₄ geopolymer in reference to the conventional Na₂SiO₃-anhydrous geopolymer. Finally, the effect of reducing the water content (i.e. w/b ratio) using SPs on the properties of Ca(OH)₂/Na₂SO₄ geopolymer is also evaluated. The outcome of this study will be helpful with the aim of understanding the behavior of the “one-part” geopolymer and supporting its in-situ applications.