Elsevier

Cement and Concrete Research

Volume 54, December 2013, Pages 143-150
Cement and Concrete Research

The role of alumina on performance of alkali-activated slag paste exposed to 50 °C

https://doi.org/10.1016/j.cemconres.2013.09.009Get rights and content

Highlights

  • Strength of alkali-activated slag (AAS) pastes after exposure to 50 °C is studied.

  • AAS pastes with high alumina content lose strength after the exposure.

  • C4AH13 and CAH10 form in these AAS pastes.

  • Conversion of these calcium alumina hydrates is associated with the strength loss.

  • AAS pastes with low alumina content maintain its strength after the exposure.

Abstract

The strength and microstructural evolution of two alkali-activated slags, with distinct alumina content, exposed to 50 °C have been investigated. These two slags are ground-granulated blast furnace slag (containing 13% (wt.) alumina) and phosphorous slag (containing 3% (wt.) alumina). They were hydrated in the presence of a combination of sodium hydroxide and sodium silicate solution at different ratios. The microstructure of the resultant slag pastes was assessed by X-ray diffraction, differential thermogravimetric analysis, and scanning electron microscopy. The results obtained from these techniques reveal the presence of hexagonal hydrates: CAH10 and C4AH13 in all alkali-activated ground-granulated blast-furnace slag pastes (AAGBS). These hydrates are not observed in pastes formed by alkali-activated ground phosphorous slag (AAGPS). Upon exposure to 50 °C, the aforementioned hydration products of AAGBS pastes convert to C3AH6, leading to a rapid deterioration in the strength of the paste. In contrast, no strength loss was detected in AAGPS pastes following exposure to 50 °C.

Introduction

The iron and steel industries produce large quantities of ground granulated blast-furnace slag, a non-metallic by-product of which calcium silicates are the primary constituent. When activated by alkali compounds, the ground granulated blast-furnace slag will possess cementitious properties, and thereby act as a binder which is often referred to as alkali-activated granulated blast-furnace slag (AAGBS). AAGBS can be used as a substitute for ordinary Portland cement (OPC), and because an industrial waste is being replaced with a purpose-built compound, AAGBS is regarded as an environmentally friendlier alternative. Besides environmental benefits and cost savings, the AAGBS has been reported to have advantages of higher strength [1], [2], [3], superior fire resistance [4] and better overall durability [5], [6] when compared to OPC.

Because of these advantages, AAGBS concrete has been put into use in the building and construction industry. For example, in 1989, the former USSR used 3 million m3 of AAGBS to construct high-rise buildings, dams, water ponds, irrigation canals, and railway sleepers [7]. Similarly, the rapid-hardening properties of AAGBS concrete have been taken advantage of in China for airport projects, and given its resistance to chemically aggressive environments, AAGBS concrete has replaced OPC for use in seaport projects also [8]. There is also some evidence for the long-term durability of AAGBS concretes as such materials have been used for 15–25 years in roads, pavements and highways in countries including Finland, Romania and Bulgaria without problems [9].

The properties of alkali-activated slag (AAS) concrete are found to be affected by the chemical compositions of its starting material, ground granulated slag. The slag may include granulated blast-furnace slag (GBS), electrothermal phosphorus slag and other metallurgical slags. Of these, GBS is most commonly used. The major compounds that constitute GBS are CaO, SiO2, Al2O3 and MgO [10]. Glukhovsky and Raksha [11] have studied AAGBS pastes made with 29 starting materials, in order to evaluate the effect of chemical compositions of slag on its strength. They have found that the AAS paste made with slags containing between 15 and 20% Al2O3 and between 40 and 50% CaO shows the highest strength, regardless of the curing conditions and the nature of activator. As the contents of these compounds decrease, so does the strength of the corresponding pastes. The CaO content plays an important role in determining the strength of AAS paste because the main hydration product formed is calcium silicate hydrate (C-S-H) [12]. However, at present, the structure of the C-S-H gel is imperfectly understood and a lot of ambiguities still remain. The main reaction product generally cited for AAS is C-S-H gel, similar to that found in Portland cement but with lower Ca/Si ratios [10], [12]. Some authors have also found the incorporation of Al in C-S-H gel [13], [14]. The aluminum replaces the silicon in bridging positions so the gel is named as C-A-S-H [15]. With increases of Al2O3 content in slags, some calcium aluminate hydrates such as CAH10, C2AH8 and C4AH13 are also observed in AAS pastes [12], [13], [16], [17]. According to Regourd [18], C4AH13 helps form crystalline bridges between the slag grains and therefore contributes to strength of the AAS paste.

It is noted that CAH10, C2AH8 and C4AH13 are also the major strength giving phases of aluminate cement pastes. These hydrates are all metastable hexagonal plate-type phases that have excellent binding properties. However, they convert with time to cubic hydrogarnet C3AH6. These are well-known conversion reactions [19]:3CAH10  C3AH6 + 2AH3 + 18H3C2AH8  2C3AH6 + AH3 + 9HC4AH13  2C3AH6 + 9H.

The conversion process is accompanied by a change in crystal structure of the hydrates (CAH10, C2AH8 and C4AH13 hexagonal; C3AH6 cubic). The density of the cubic phase is greater than that of the hexagonal phases and so, conversion results in a significant reduction in the volume of the solids. A large increase in porosity takes place upon conversion and results in the marked deterioration in the strength of the paste. Under such conditions, the conversion may lead to the failure of structures built with alumina cement.

The Al2O3 content of slags varies with the type of ore used and furnace operation, and may vary within a wide range. Osborn et al. [20] have conducted a large-scale study on chemical compositions of GBS from 9 countries, and concluded that these slags have very similar SiO2 and CaO contents, but have an obvious difference in Al2O3 contents (from 7.1% to 13.9%). It is noted that the variation of Al2O3 content in cement has significant influence on strength of cementitious materials due to conversion (discussed above). Although this issue has been extensively studied for alumina cement pastes, the effects of Al2O3 content on the strength of alkali-activated slag based materials have received less attention.

Therefore, the aim of this paper is to investigate the effect of Al2O3 in the slag on the compressive strength of alkali-activated slag pastes exposed to 50 °C. The high temperature exposure is selected in the current study because the conversion in general is slow at room temperature and below, and its effects may not be appreciable after many years. However, at higher temperatures, it can occur extremely rapidly, within a few weeks or even days. In order to investigate the effects of Al2O3 content on the strength performance during the exposure, the samples were prepared by using two slags with distinct different Al2O3 content.

Section snippets

Materials

The ground phosphorus slag (GPS) has a vitreous structure similar to that of the GBS. The Al2O3 content of the GPS is lower than that of the GBS. The GPS was obtained from Kaiyang Cement Company (China) while the GBS was obtained from Cement Australia. X-ray fluorescence (XRF) was used to determine the chemical composition of slags and results are summarized in Table 1.

Two alkali activators were used: 1) industrial sodium silicate liquid (grade D) comprising 29.4% SiO2, 14.7% Na2O, and 55.9% H2

Workability

Slump results for AAGBS and AAGPS are shown in Table 3. The slag activated by NaOH + Na2SiO3 displays less workability than the slag activated by NaOH. However, the difference is negligible as the w/c ratio was kept constant for all mixes.

Compressive strength

The qualitative observations of AAGBS and AAGPS (before and after thermal exposure) are presented in Fig. 1. Visually, after exposure to 50 °C, parts of the AAGBS pastes lost cohesion and separated into 7–10 mm pieces (Fig. 1(a) and (b)). In contrast, the AAGPS

Conclusions

In AAGBS pastes, the presence of the metastable compounds, CAH10 and C4AH13, is observed. The generation of these compounds is independent of the alkali activator used in this investigation. When AAGBS pastes were exposed to 50 °C, the conversion reaction ensues rapidly, causing the metastable compounds to crystallize into the compound, C3AH6. The formation of C3AH6 seriously affects the microstructure of the paste. SEM confirmed the change in microstructure; the hexagonally-shaped metastable

Acknowledgments

The authors acknowledge financial support from the Australia Research Council and the use of the facilities within the Monash Centre for Electron Microscopy.

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