Elsevier

Journal of Cleaner Production

Volume 283, 10 February 2021, 124612
Journal of Cleaner Production

Utilization of barium slag to improve chloride-binding ability of cement-based material

https://doi.org/10.1016/j.jclepro.2020.124612Get rights and content

Highlights

  • The utilization of 10–30% BS as SCM in cement pastes could increase the chloride binding capacity in cement-based materials.

  • BS could impede chlorine migration by the refined and complexed pore structure.

  • BS could increase the chemical immobilizing of chlorine ions by increasing the amount of Friedel’s salt.

Abstract

In this study, barium slag (BS) was utilized as a supplementary cementitious material (SCM) to improve the chloride-binding ability of cement pastes. Hardened paste specimens with 0–30% BS content were prepared, and the chloride binding rate (CBR) was evaluated. The compressive strength was tested, and the hydration heat was assessed. The mechanism behind was investigated by XRD, TGA, NMR, EDS, and MIP analyses. The results showed that 10–30% BS greatly promoted the CBR of the cement-BS system. The reasons were demonstrated in three angles: a) the physical resistance of ion-migration was enhanced because of the optimized pore structures; b) the chemically combined chloride was significantly strengthened because the formation of Friedel’s salt in hardened pastes was facilitated; c) the physically adsorbed chloride was reduced because less amount of C–S–H gel was generated. These inferences provided one innovative way of utilizing BS in the Portland cement system and might provide useful experience in promoting the chloride-binding ability of cementitious materials.

Introduction

Marine construction was increasingly increased in recent years, and most of the construction materials were gained from the mainland; high cost and long-distance transportation showed great disadvantages. The development of alternatives for mainland resources attracted more and more attention in the field of marine construction. The application of sea resources, such as marine sand, coarse aggregate, and seawater, not only reduced the cost of the project, but also alleviated the shortage of mainland resources (Xiao et al., 2017; Younis et al., 2020). However, steel bar erosion aroused by chloride ions from sea resources was recognized as one of the main factors damaging the structure which should be carefully considered in reinforced concrete (Bao et al., 2020; Florea and Brouwers, 2012; Garcés et al., 2008; Zhang et al., 2017a). These chloride ions brought by raw materials existed in hardened concrete in three forms: (1) Free chloride ions, existing in pore solution; and they could freely immigrate and exhibited great potential risk of steel bar corrosion (Elakneswaran et al., 2009; Ma et al., 2018b). (2) Chemically combined chloride ions, presenting in hydration products, such as Friedel’s salt (FS, 3CaO·Al2O3·CaCl2·10H2O) and Kuzel’s salt (KS, 3CaO·Al2O3·0.5CaCl2·0.5CaSO4·10H2O), and this kind of chloride ions seemed no contribution to steel bar corrosion; if this kind of chemical binding was promoted, the risk of steel bar corrosion would be significantly declined (Chen et al., 2020; Liu, X.H. et al., 2019). (3) Physically adsorbed chloride ions, which were mainly absorbed by hydration products (C–S–H gel); the adsorption was reversible and depended on the pore structure and microstructure of C–S–H gel (Cao et al., 2020; Chen and Gao, 2019). Among these three, chemical binding acted as the most efficient role in combining chloride ions. The main mechanism behind the chemical binding was the transformation of the AFm-like phase. Generally, if sulfate ions in AFm-like phase were substituted by chloride ions, the transformation to FS and KS would happen and the chloride ions would be chemically combined in the system (Florea and Brouwers, 2012).

Nowadays, sustainable development has been the global concern, and it may make sense to meet the idea of environmental friendliness and resource conservation for applying the industrial waste in building materials (Gu et al., 2020; Li et al., 2015; Li et al., 2016; Wang et al., 2021; Wang et al., 2020; Zhang et al., 2021). Barium slag (BS) is solid waste discharged during the refining process of barite, and it’s a kind of toxic hazardous waste containing soluble barium salt. In China, the annual discharge of BS is about 1 million tons, and the most popular way for disposal of the emitted BS is landfilling or ash dam. This not only caused pollution of groundwater and air, but also encroached on land and wasted natural resources (Gu et al., 2019; Jiang, 2007; Wei et al., 2020). At present, many progresses have been made in the utilization of BS. In the construction and building material field, BS was used as a mineralizer in the calcining process of cement manufacture: if an appropriate amount of BS was added in the process of cement calcination, the dissolved barium ions would change the viscosity of the mesophase at high temperature by participating in the reaction, thus promoting the crystallization growth, reducing the firing temperature, and shortening the firing time of cement clinker (Chen et al., 2016; Wang et al., 2012; Yang et al., 2019a). Another related report said BS presented the potential cementitious property, and the utilization of BS as SCM was proposed in the literature (Jiang, 2007). There are several clinker minerals in BS; if BS was ground to a certain fineness similar to cement, these minerals would have a good cementitious performance. Furthermore, a small amount of soluble barium ions in BS could react with sulfate ions from gypsum, which meant that barium ions and aluminate salt could competitively react with sulfate ions. The consumption of sulfate by barium ions decreased the amount of sulfate ions involved in the formation of ettringite (AFt), and relatively, the transformation of AFt to AFm would be advanced. These reactions might influence the production of AFt and the setting process of the cement system at the early stage. However, at the later stage, the release of a small amount of soluble barium ions might facilitate the transformation of AFm to FS or KS by reacting with sulfate ions in AFm, thereby increasing the chemically combined chloride. In this case, the harmful ions, not only chloride ions but also barium ions in BS, would be immobilized, with great environmental benefits (Hou et al., 2020; Niu et al., 2019, 2020; Xue et al., 2020).

On the basis of the hypothesis above, in this study, an attempt to use the dissolved barium ions in BS for improving the chloride binding ability of cement-based building materials was made. The binders composed of cement and BS were designed, and the chloride-binding rate (CBR) of the pastes with 0–30% BS content was measured at 3 d, 7 d, 28 d, and 60 d. Hydration products of these hardened paste samples were characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), solid-state nuclear magnetic resonance (NMR), and energy disperse spectroscopy (EDS). The chemical binding ability was obtained by the analysis of the hydration process and hydration products. The physical adsorption capacity was evaluated based on compressive strength and hydration heat analyses. The physical blocking of migration was assessed by pore structure analysis. The microstructure of BS was observed by scanning electron microscope (SEM) and barium ion concentration in BS was analyzed by an inductively coupled plasma optical emission spectrometry (ICP-OES). The mechanisms of chemical binding, physical adsorption, and physical blocking of migration were investigated. The conclusion was inferred and hoping for providing significative guidance for increasing the immobilization of chloride ions, and also offering useful experience for the utilization of BS.

Section snippets

Raw materials

The Portland cement (P. I type cement, according to Chinese standard GB175-2007) and barium slag (BS, supplied by a large barium salt factory in Hubei province, China) were used in this study. BS was ground in a ball mill, and the BS powders passed through a 200-mesh sieve were prepared for the next experiments. The chemical compositions of Portland cement and BS are presented in Table 1.

The XRD pattern of BS is presented in Fig. 1. According to the XRD pattern, it was found that BS was

Chloride binding ability

CBR values of cement-BS pastes at 3 d, 7 d, 28 d, and 60 d are presented in Fig. 4. In this figure, the CBR of the control samples was marked as 100%, and the CBR of the ones with 10–30% BS content corresponding to each curing age was calculated. It could be observed from Fig. 4 that incorporating BS increased the CBR in the system. With the dosage of 10% BS, CBR of the samples at 3 d–60 d age could have a growth rate of about 10%. And the more content of BS was added, the larger increase of

Conclusion

The utilization of 10–30% BS as SCM in cement pastes impeded the migration of chloride ions by the optimized and complexed pore structures, increased the chemically combined chloride ions by facilitating the formation of Friedel’s salt, and reduced the physically adsorbed chloride ions by cutting down the formation of C–S–H gel. Among these three aspects, the chemical binding and migration resistance should be dominated, and thus, the chloride-binding ability of cement-based materials was

CRediT authorship contribution statement

Pian Chen: Conceived and designed the research, Investigation, Experiments, Formal analysis, Formal analysis, Writing - original draft, Writing - review & editing. Baoguo Ma: Methodology, Formal analysis, Writing - original draft, Supervision, Funding acquisition. Hongbo Tan: Methodology, Writing - original draft, Writing - review & editing, Validation, Funding acquisition. Xiaohai Liu: Methodology, Experiments, Formal analysis. Ting Zhang: Formal analysis, Writing - review & editing. Chunbao

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

Funding came from the projects of the National Key R&D Program of China (2016YFC0701003-5, and 2016YFC0700904).

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