Effects of mineral admixtures and lime on disintegration of alkali-activated slag exposed to 50 °C
Introduction
Alkali-activated slag (AAS) is a clinker-free binder made of ground granulated blast-furnace slag (GGBS), and is activated by alkaline activators. Commonly used alkaline activators are sodium hydroxide and sodium silicate solutions. As slag is waste material from other manufacturing processes, the use of AAS in construction has many environmental advantages, such as reduction of costs for industrial waste disposal, low energy cost and low greenhouse gas emission [1]. Furthermore, AAS has been reported to exhibit mechanical and durability characteristics similar or superior to those of ordinary Portland cement (OPC) binder [2], [3], [4], [5], [6]. Consequently, AAS has been established as a method for producing sustainable binders for construction.
However, safety concerns about the structural use of AAS concrete have been raised recently because AAS pastes can totally disintegrate when exposed to oven drying at 40–50 °C [7]. This disintegration has been found to be correlated with the alumina content in slags [8]. The authors attributed the disintegration of AAS pastes at 50 °C to crystalline conversion from the hexagonal hydrates CAH10 and C4AH13 to the cubic hydrate C3AH6 with a lower specific weight. In essence, this conversion causes a remarkable increase in porosity which is responsible for the decrease in strength, because of the transformation of the voluminous hexagonal phases into the denser cubic phase.
Although the occurrence of conversion in the AAS system has been identified only recently, the detrimental effects of conversion on the strength of high alumina cement (HAC) have been known since the 1970s. Since then, prevention of conversion has been of great concern to civil engineers and has resulted in extensive study of HAC systems. Collepardi et al. [9] investigated the effects of mineral admixtures including fly ash (FA) and silica fume (SF) on preventing the conversion of HAC pastes. They found that SF performed better than FA in reducing the conversion of hexagonal aluminate hydrates (CAH10 and C4AH13) into the cubic hydrate (C3AH6) which is responsible for the strength loss of HAC pastes at higher temperature (>20 °C). Majumdar and Singh [10] also agreed that the conversion could be avoided by the addition of SF in HAC pastes. They reported that the compressive strength of HAC/SF pastes (HAC/SF mass ratio = 60/40) remained almost unchanged up to one year when the pastes were water-cured at 40 °C. However, the strength of the HAC/SF pastes was significantly lower than that of plain HAC pastes. The low strength is attributed to the presence of large amounts of unhydrated SF on the hydrates’ surface, which can serve as defect sites in HAC systems. To promote the dissolution of silica, sodium sulphate has been added into HAC/SF systems. It was found by Ding et al. [11] that the presence of sodium ions could significantly reduce the amounts of unhydrated SF. The increase in dissolved silica had beneficial effects not only in enhancing strength, but also in reducing the risk of conversion, as dissolved silica can react with CAH10 or C4AH13 to form stratlingite (C2ASH8). The authors reported that a 28 days’ strength of 40 MPa was developed in HAC/SF/Na2SO4 pastes and the strength of the samples was improved to 50 MPa when water-cured at 38 °C up to 90 days. Under the same curing condition, plain HAC pastes experienced significant strength loss.
It is noted that the use of both FA and SF in AAS concrete is recommended for different purposes. Fly ashes and slags are by-products that are commonly used to produce blended cements and concretes. However, rates of slag utilization are much higher than those of FA [12]. Attempts have been made, therefore, to replace partial blast furnace slag with FA in AAS pastes to increase the utilization of fly ashes [13], [14]. Initial studies [15], [16] of slag/FA blends have indicated an optimum slag/FA mass ratio of 50/50. These slag/FA blends exhibit engineering properties comparable to their plain AAS counterparts [15] but superior to those of OPC counterparts [16]. Unlike FA, which is used to replace blast furnace slag, SF is often used as an additive (the replacement rate generally being less than 15%) to improve the properties of AAS concrete. The replacement of slag with SF has been found to improve the strength [17] and workability [18] of AAS pastes and to increase their resistance to exposure to high temperatures [19].
Besides SF and FA, lime is also added to AAS concrete to improve its properties. Lime has been found to be a suitable retarder to control the setting time of AAS concrete [20]. Collins and Sanjayan [21] studied the effects of lime on the strength development and found that the presence of lime could significantly increase the strength of AAS pastes. In a recent study, Yang et al. [22] even attempted to develop an AAS system using lime as the main activator. At a water to binder ratio of 0.3, 7.5% Ca(OH)2 and 1% Na2SiO3 activated mortars developed a compressive strength of 32 MPa at the age of 28 days. When Ca(OH)2 is served as an activator for slags, C4AH13 has been identified as one of the main hydration products in AAS pastes. It is noted that C4AH13 is metastable phase that converts with time to the more stable and denser hydrogarnet C3AH6. This conversion reaction produces an increase in porosity and results in lower strength. This phenomenon has been observed in HAC systems [23].
From the existing research concerning the use of additives to augment the properties of cementitious materials, it can be concluded that (1) FA, SF and lime have been proposed for use in making AAS concretes; (2) these additives have significant influences on the strength of HAC systems due to conversion reactions. Although conversion reactions also take place in AAS pastes, the effects of the three additives on the strength of AAS pastes under conditions favouring conversion have never been investigated. Thus the aim of this paper is to investigate the effects of FA, SF and lime on the strength of AAS pastes when exposed to 50 °C.
Section snippets
Materials
The primary raw materials used in this study were provided by local suppliers. The chemical compositions and properties given by the manufacturers are summarized in Table 1. The alkaline activators used were liquid sodium silicate (grade D) comprising 29.4% SiO2, 14.7% Na2O, and 55.9% H2O; wt. ratio: SiO2/Na2O = 2 and sodium hydroxide solution (8 M). The 8 M NaOH solution was prepared by dissolving 98% purity NaOH flakes in distilled water and then cooling the solution for 24 h. The activators were
Effect of mineral admixtures on hydration products of AAS
X-ray diffraction was used to study the changes in crystallinity of the AAS pastes with different mineral admixtures cured under different conditions (Fig. 1). It should be noted that the reactants used and the binders produced in this study contained a very high percentage of amorphous to semi-crystalline phases. It was therefore difficult to identify all the crystalline phases according to Joint Committee on Powder Diffraction Standards (JCPDS). X-ray diffraction analysis was conducted over
Conclusions
- (1)
CAH10 and C4AH13 did not appear in SF50 and SF50H samples which retained the same strength after exposure to 50 °C. In contrast, these calcium aluminate hydrates were identified in other samples which experienced significant strength loss after the same exposure. The evidence is consistent with that the transformation of hexagonal calcium aluminate hydrates (CAH10 and C4AH13) into the cubic hydrate (C3AH6) is the reason for the strength loss of AAS pastes during thermal exposure.
- (2)
The strength
Acknowledgments
The authors are grateful for the financial support provided by the Australian Research Council to conduct this study. Dr. He thanks Deakin University for funding her Alfred Deakin Fellowship.
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