Full length articleEvaluation of the reactivity of treated spent pot lining from primary aluminum production as cementitious materials
Graphical Abstract
Introduction
With a production of 4 billion tons in 2018, concrete is one of the most consumed materials in the world (Andrew, 2019). However, the concrete industry is a major consumer of natural resources and energy causing major greenhouse gas emissions (GHG). In particular, the production of one ton of Portland clinkers can produce up to 1 ton of CO2 of which 60% of comes from unavoidable decarbonation reactions (Strazza, Borghi Del, Gallo and Del Borghi, 2011). Such emissions have a major impact and represent every years 5-8% of global CO2 emissions (Le Quéré et al., 2015; Lehne and Preston, 2018). Various solutions are implemented to reduce GHG of cement industry. Optimizing manufacturing processes to make them more energy efficient and using supplementary cementitious materials (SCM) are ways to reach this goal (Kajaste and Hurme, 2016; Shanks et al., 2019; WBCSD and IEA, 2009). The partial replacement of cement with SCMs in concrete applications provide advantages from an economic and ecological point of view (Aïtcin and Flatt, 2015; Hewlett et al., 2004; Tokyay, 2016). These benefits pushed different industries to add supplementary treatments to optimize waste revalorization in cement (Zhang et al., 2013). Therefore, SCMs partially replace cement and are generally industrial by-products or wastes such as fly ash, silica fumes or blast furnace slags. SCM can be divided in three categories: latent hydraulic, pozzolans or inert materials (Hewlett et al., 2004; Tokyay, 2016). Besides the benefits on the carbon footprint of concrete, SCM allows a better valorization of non-conventional waste and reducing landfill (Agrawal et al., 2004).
For instance, the primary aluminum industry produces industrial wastes, such a spent pot lining (SPL) from electrolysis cells. Every ton of aluminum generated about 22 kg of SPL (Birry et al., 2016),which represents approximately 70,000 tons of SPL generated every year in Canada. SPL is a hazardous substance due to its leachable content in fluoride and cyanide, but also due to its hydro reactivity creating explosive gases (Kimmerle et al., 1993; Øye, 2017). SPL can be separated in two parts: the first cut is rich in carbonaceous materials coming from the cathode and the second cut consists in vitrified refractory. Developed in the 1990’s, the LCL&L (Low Caustic Leaching & Liming) process, through hydro metallurgic way, transforms SPL into an inert material. The second cut of SPL, when treated by LCL&L process, turns into an inert material called LCLL Ash (named ‘’LCLL’’ in this paper) studied in this article (Birry et al., 2016). LCLL ash is a grey powder, mainly composed of oxides of silicon and aluminum, with minor fractions of calcium, iron, sodium, and fluor oxides. This composition is comparable to a clay.
A similar by-product developed by a pyro metallurgic treatment of SPL allowed to obtain glass frit (GF) (Fares, 2008). This by-product was tested as a cementitious binder for incorporation into binary, ternary and quarterly mixtures. However, GF was studied for alkaline activation. The results demonstrate that GF has a similar reactivity to a latent hydraulic binder with a behavior similar to slag (Fares, 2008; Laldji and Tagnit-Hamou, 2016). The binary GF mixture exhibited slightly lower compressive strengths than the unblended mixture at early age of hydration, but greater compressive strength after 28 days. In addition, the GF exhibited a remarkable durability against freezing-thawing (resistance similar to the unblended mixture), lower chloride ion permeability, and improved resistance to the alkali silica reaction (Laldji and Tagnit-Hamou, 2016; Nova Pb inc., 2004).
In 2018, 2.47 million tons of primary aluminum were produced only in Quebec (Natural Resources Canada, 2019). The production of primary aluminum by the Hall-Heroult process generates hazardous wastes, then treated but currently unpreventable. The major advantage of SPL revalorization for this industry is to decrease the environmental impact of primary aluminum by avoiding the SPL landfilling. Using SPL without pretreatment in cement plants as a raw material is a common practice in Europe, Asia, Australia, Brazil and the Middle East (Broek and Øye, 2018; Jawi et al., 2020; Nunez, 2020; Personnet, 1999). For example, EGA in the United Arab Emirates promotes the use of SPL without hydro or pyro pretreatment, enforcing crushing and sieving. Thanks to the fluoride content, the addition of SPL in the raw meal decreases the clinkerisation temperature by 20°C to 100°C according to the SPL replacement percentage (Gomes et al., 2005; Jawi et al., 2020). This allows to reduce the fossil fuels consumption up to 4% and reduce the overall CO2 emissions by 1%. However, the high sodium content limits the use of SPL in cement plants from 0.2% to 0.75% to avoid durability concerns in concrete due to the alkalis silica reactions (Broek and Øye, 2018; Gomes et al., 2005; Jawi et al., 2020; Nunez, 2020). In North America, this practice in raw cement meal is not allowed due to the high percentage of fluoride in SPL.
For the concrete and construction industry, replacing a part of cement by treated SPL, can decrease the carbon footprint of concrete. The reuse of this local waste in Quebec can provide a new source of supplementary cementitious materials (SCM) and would limit the conventional SCM import from USA or Europe, such as fly ash or slag. This could further diminish the carbon footprint of concrete by limiting the transportation of conventional SCMs. The project aims to demonstrate the potentials of a circular economy, where the waste from one industry can feed in the other.
The purpose of this article is to study and understand the reactivity of the LCLL Ash in cement to avoid its landfilling. Moreover, the improvement of LCLL Ash reactivity was studied with an additional high temperature calcination treatment to increase the amorphous content. Hence, this article aims to answer the following questions:
- (i.)
Is the LCLL Ash reactive? What is its type of reactivity?
- (ii.)
Is it possible to increase the reactivity of LCLL Ash by calcination?
- (i.)
To answer these questions, this study evaluates the reactivity of LCLL Ash as well as calcined LCLL Ash according to three reactivity tests. Calcination is a common method to turn some mineral materials, such as clay or shale, into reactive SCMs (Ambroise et al., 1985; Mather, 1958; Murat, 1983). Replacing until 20 % of clinker in cement by calcined materials, allows to reach similar or better properties than unblended Portland cement (Li et al., 2015; Trümer and Ludwig, 2015). Currently, the most studied calcined materials are calcined clays, particularly clays with a high kaolinite content. A good example of calcined clay use is Limestone Calcined Clay Cement (LC3) which allows to make cementitious materials by replacing 50% of clinker with a mix of calcined clay and limestone (Cancio Díaz et al., 2017; Dhandapani et al., 2018; Favier and Scrivener, 2018; Scrivener et al., 2018). LC3s show similar or better properties than unblended Portland cements, particularly mechanical and durability properties (Dhandapani et al., 2018; Scrivener et al., 2018). Calcinating mineral materials to replace cement can seem counterintuitive, however, blended cement with calcined clay or calcined shale have a lower carbon footprint than unblended cement. This advantage stems from the uncalcined calcite and a lower calcination temperature of clay than clinker (Miller et al., 2018; Scrivener et al., 2018). Since LCLL Ash has a similar composition to clay, we can expect calcination to improve its reactivity in cement. For each test, LCLL Ash and calcined LCLL Ash was compared to others inert and reactive materials such as quartz powder, limestone filler and slag, fly ash, silica fume respectively. In the first analysis of mortar compressive strength, these materials were individually blended with cement to evaluate the mixes’ relative strength activity index. This observation was followed by Frattini tests to evaluate the pozzolanic activity of each material. To confirm the previous observations, the Rilem R3 tests (Li et al., 2018) were conducted to study the materials’ reactivity without the interference of cement. Quantitative X-ray diffraction tests followed to deepen the understanding of the reactivity of LCLL Ash and calcined LCLL Ash. The effect of the different SCMs replacement on the fresh properties was evaluated by measuring the yield stress and the plastic viscosity. Finally, the effect of SCMs on the microstructure was analyzed with mercury intrusion porosimetry (MIP) and scanning electron microscopy images (SEM).
Section snippets
Materials
In this study, the LCLL Ash comes from the Rio Tinto treatment plant based in Jonquière, QC, Canada. For the rest of the paper, LCLL will refer to LCLL Ash. A plain Portland cement (Type GU, Ciment Québec, QC, Canada) was used to prepare mortar and Frattini tests. The LCLL reactivity was compared to two fly ashes: FA-E (Type F fly ash, Ciment Québec, QC, Canada), and FA-PA (Type F fly ash, ProAsh, VA, USA). LCLL was also compared to a slag GGBS (Type S ground granulated blast furnace slag,
Mortar
The results of compressive strength and relative strengths Portland cement and Portland blended cements are respectively presented in Fig. 2, Fig. 3. For each specimen tested, the relative compressive strength (RCS) was calculated from Eq. 6.where and are respectively the compressive strength at i-days of the unblended cement reference and of blended cement with SCM.
As for a 20% replacement, a dilution effect appears and
LCLL reactivity
According to the previous mortar and Frattini tests, the reactivity of LCLL can be considered equivalent to an inert material in cement, similar to quartz. This result was expected since important quantities of low reactivity phases composed LCLL, such as corundum, quartz, albite and nepheline. The SEM observation of LCLL also confirms the low reactivity of LCLL grains in cement after 1 year. Moreover, the SEM observation shows LCLL monophasic grains with angular edges. The lower compressive
Conclusion
Based on the presented results, the following concluding remarks can be drawn:
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The valorization of treated SPL in cementitious materials, offers new opportunities for aluminum and cement industries to decrease the environmental impact of their materials by reusing a local waste and avoiding landfill.
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LCLL is not reactive in cement. Hydro reactivity and expansion due to gas generation can happen depending on the chemistry or the temperature of hydration of the binder. This hydro reactivity
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Credit author statement
Conceptualization: C.M.O.P., D.C., L.S.
Data curation: V.B., T.-H. T., C.M.O..P
Formal analysis: V.B.
Funding acquisition: C.M.O.P, D.C., L.S.
Investigation: C.M.O.P, D.C., L.S.
Methodology: V.B., C.M.O.P,
Project administration: C.M.O.P
Resources: C.M.O.P.
Software: V.B., C.M.O.P.
Supervision: C.M.O.P supervised V.B.; L.S. and D.C. supervised T.H.T.
Validation: V.B.
Visualization: V.B.
Writing - original draft: V.B.
Writing - review & editing: V.B. and C.M.O.P.
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.
Acknowledgments
The authors are grateful to NSERC CRD grant program (CRDPJ 515485 – 17), CRITM consortium, Rio Tinto and Ciment Québec Inc. for their financial support for this project.
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