Producing vaterite by CO₂ sequestration in the waste solution of chemical treatment of recycled concrete aggregates

Highlights

• A new method is proposed to produce vaterite from the waste solution of RCAs treatment.

• pH swing method is modified by an ionization suppressing technique.

• 1 kg RCA can produce 76.6 g vaterite and store 33.7 g CO₂.

Abstract

A recent study showed that the properties of recycled concrete aggregates (RCAs) can be significantly improved by acetic acid treatment. This study demonstrates that a useful product, vaterite, a metastable polymorph of calcium carbonate, can be produced from the waste solution of this treatment through an improved pH swing method. The salient feature of this improved method is that ethanol is added into the waste solution to not only prohibit the phase transformation of vaterite, but also to suppress the ionization of the regenerated acetic acid. This method allows not only more vaterite to be produced, but also more acetic acid to be regenerated from the waste solution through a CO₂ bubbling method. Meanwhile, CO₂ can be permanently stored in the produced vaterite to achieve sustainable CO₂ sequestration. Experimental results confirm that near-pure vaterite can be produced from the waste solution by adding 20% ethanol into the waste solution. 1 kg RCAs can be used to produce about 76.6 g vaterite and permanently store 33.7 g CO₂ by using this new method. The potential application of the produced vaterite as an internal curing agent for high performance concrete has also been demonstrated.

Introduction

Construction and demolition (C&D) waste is one of the largest waste flows in the world (Shi et al., 2015). Many technologies have been developed to recycle these C&D wastes. Among them, recycling waste/demolished concrete as aggregates for new concrete is one of the most promising. However, recycled concrete aggregates (RCAs) have inferior qualities compared with natural aggregates (NAs) because a layer of residual cement mortar is coated on the surface of the gravel in the RCAs. The porous nature of this residual mortar layer introduces many undesirable properties to the RCAs (Behera et al., 2014). The quality of the RCAs can be improved by removing this porous layer through soaking the RCAs in hydrochloric acid (Ismail and Ramli, 2013) or sulfuric acid solution (Tam et al., 2007) in which some hydration products of the cement can be dissolved. Some loose and cracked mortar can be removed from the RCAs by this method. However, strong acids are required, making this treatment unsafe. Moreover, these acids can introduce detrimental ions such as Cl⁻ and SO₄²⁻ to the RCAs, which can cause durability issues in concrete with RCAs. The waste solution generated by this process is also hazardous. Disposal of the waste treatment solution will create new environmental problems.

To address these drawbacks of the existing method, an environmentally friendly and cost-effective method was proposed to treat RCAs in our previous study (Wang et al., 2017). As shown in Fig. 1, in this new method, RCAs are first soaked in diluted acetic acid, which reacts with cement hydration products in the cement mortar attached to the surface of the RCAs, as described by:

$${\text{Ca}\left(\text{OH}\right)}_2+{2\text{CH}}_3{\text{COOH}}_{\left(\text{l}\right)}\to {{\text{Ca}}^{2+}}_{\left(\text{aq}\right)}{+2\text{CH}}_3{{\text{COO}}^{-}}_{\left(\text{aq}\right)}+{2\text{H}}_2{\text{O}}_{\left(\text{aq}\right)}\text{,}$$

$${\text{CaCO}}_3+{\text{CH}}_3{\text{COOH}}_{\left(\text{l}\right)}\to {{\text{Ca}}^{2+}}_{\left(\text{aq}\right)}+{\text{CH}}_3{{\text{COO}}^{-}}_{\left(\text{aq}\right)}+{\text{CO}}_{2\left(\text{g}\right)}+{\text{H}}_2{\text{O}}_{\left(\text{aq}\right)}\text{,}$$

$${\text{CaO}-\text{nSiO}}_2{-\text{mH}}_2\text{O}+{2\text{CH}}_3{\text{COOH}}_{\left(\text{l}\right)}\to {{\text{Ca}}^{2+}}_{\left(\text{aq}\right)}+{2\text{CH}}_3{{\text{COO}}^{-}}_{\left(\text{aq}\right)}+{(\text{m}+\text{1})\text{H}}_2{\text{O}}_{\left(\text{aq}\right)}+{\text{nSiO}}_{2\left(\text{s}\right)}\text{,}$$

In the above equations, Ca(OH)₂ and CaO-nSiO₂-mH₂O (calcium silicate hydrate (C-S-H)) are major hydration products of cement in the RCAs, while CaCO₃ was introduced into the RCAs by carbonation of Ca(OH)₂ in the RCAs after long-time exposure to air. This process removes unbound particles from the RCAs and weakens the attached mortar, making it possible to remove it from the RCAs through mechanical rubbing. This treatment can significantly improve the quality of the RCAs. Once used as aggregate in new concrete, these RCAs can enhance the compressive strength of the concrete at 28 days up to 25% (Wang et al., 2017).

More importantly, a useful product, vaterite – a metastable polymorph of CaCO₃ as explained later – can be produced from the waste solution of this treatment through a simple CO₂ bubbling method. As shown in Eqs. (1), (2), (3) and the authors’ previous study (Wang et al., 2017), the major chemical composition in the waste solution of this treatment is calcium acetate. By bubbling CO₂ to this waste solution, following reaction will occur:

$${{\text{Ca}}^{2+}}_{\left(\text{aq}\right)}+{2\text{CH}}_3{{\text{COO}}^{-}}_{\left(\text{aq}\right)}+{\text{CO}}_{2\left(\text{g}\right)}+{\text{H}}_2{\text{O}}_{\left(\text{aq}\right)}\to {\text{CaCO}}_{3\left(\text{s}\right)}+{2\text{CH}}_3{\text{COOH}}_{\left(\text{l}\right)}\text{.}$$

This reaction produces precipitated calcium carbonate (PCC), which not only partially compensates the cost of the treatment, but also produces multiple environmental and economic benefits, including: 1) no waste material is produced, eliminating the environmental issues created by the disposal of waste chemical solutions as in the existing methods; 2) CO₂ is permanently stored in the produced PCC, contributing to the mitigation of global climate change due to greenhouse gas emissions; and 3) acetic acid can be regenerated, further reducing the cost of the treatment.

Accelerated carbonation of minerals containing alkaline earth oxides has been used to permanently store CO₂ in insoluble carbonates. Although natural minerals rich in calcium and magnesium are widely available for CO₂ sequestration, industrial solid residues rich in calcium and magnesium are also examined because they are cheaper; available in industrial areas; and require no mining activities, crushing, or grinding. As one of largest industrial solids rich in calcium, cement-based materials have already been studied extensively for CO₂ storage through accelerated carbonation (Galan et al., 2010). However, existing studies show that directly carbonating minerals is not efficient due to thermodynamics (Sun et al., 2011). Therefore, a two-step carbonation technique (Azdarpour et al., 2015) is adopted in this study. In this technique, calcium ions are first extracted from the RCAs using acetic acid, as shown in Fig. 1 and Eqs. (1), (2), (3), (4) and then carbonated in solution through CO₂ bubbling. The efficiency of carbonation can be drastically improved since the carbonation of calcium in liquid is much faster than direct carbonation. However, a large amount of destabilized solid residue is generated in existing two-step carbonation techniques (Chiang et al., 2014). This is not an issue in the proposed method since the solid residue generated by this carbon sequestration method is the RCAs with improved quality.

CaCO₃ has several different schemes of atomic arrangements or polymorphs, which are vaterite, aragonite, and calcite. These are listed in order by thermodynamic stability of the individual form (Trushina et al., 2014). In the ambient environment, vaterite and aragonite are metastable and only calcite is thermodynamically stable; therefore, calcite is the predominantly polymorph in the produced PCC in existing studies. A previous study demonstrated that the polymorphs of CaCO₃ can be well manipulated in ethanol/water mixed solution by tuning the composition of this binary solution (Chen et al., 2006).

Compared with calcite, vaterite has many salient features such as high solubility, biocompatibility, colorlessness, low density, large surface area and porous structure (Udrea et al., 2012). It has found many highly useful applications in drug delivery, paper making, paints, reaction catalyzing, pollution adsorbing, plastics, and rubber manufacturing (Udrea et al., 2012). In particular, vaterite can transform into stable calcite through a Dissolution-recrystallization process. In this process, vaterite first dissolves releasing calcium and carbonate into the solution, which then re-precipitates on the surface of the growing calcite crystals (Juan Diego et al., 2011). Due to this phase transformation process, vaterite exhibits some “reactivity” in water, and therefore can be used as a binder to cement aggregates together, as demonstrated in the field of biomaterials (Combes et al., 2006) and recent applications in concrete (Chen et al., 2013). This provides a way to close the cycle of calcium in RCAs as a binder. In RCAs, calcium mainly comes from the hydration products of cement. In existing methods, calcium can only be recycled as fillers through the carbonation if no extra energy input such as calcination is used.

Eq. (4) suggests that acetic acid is regenerated the proposed process, which can dissolve the produced PCC so that very limited PCC can be produced in Eq. (4). To overcome this problem, a pH swing process (Azdarpour et al., 2015) is commonly used, in which the pH value of the solution is increased to favor the formation of PCC. In this study, this pH swing method is modified by an ionization suppressing technique (Liu et al., 2015) to maximize the yield of PCC. As a weak acid, acetic acid reacts with the water in a reversible fashion to form hydronium and acetate ions as shown in Eq. (5):

$${\text{CH}}_3\text{COOH}\rightleftarrows {\text{H}}^{+}+{\text{CH}}_3{\text{COO}}^{-}$$

The equilibrium constant for this reaction is referred to as the acid dissociation constant, K_a = [H⁺][CH₃COO⁻]/[CH₃COOH]. Eq. (5) suggests that less H⁺ should be produced in order to produce more PCC, which is equivalent to reducing the dissociation constant K_a. This can be done by adding an organic solvent such as ethanol to the waste solution, as the dissociation constant of acetic acid can be significantly reduced by the presence of an organic solvent (Grunwald and Berkowitz, 1951). More importantly, ethanol is needed to prohibit the phase transformation of the produced calcium carbonate so that the majority of the polymorphs present in the PCC are vaterite.

The focus of this paper is to demonstrate that vaterite can be produced from the waste solution of RCAs treatment using acetic acid through the improved pH swing method. As a preliminary study, commercially available pure CO₂ was used as the feeding gas to synthesize the vaterite. In the future, CO₂ containing industrial waste gas will be used. A series of characterization tests using Scanning Electron Microscopy (SEM), X-ray Powder Diffraction (XRD), and Field Emission Transmission Electron Microscopy (FETEM) were carried out to examine the morphology and phase of the produced vaterite, and the fusion growth of vaterite particles.

It should be pointed out that most chemicals used in this treatment – including acetic acid, ammonia, and ethanol – can be recycled in a way similar to that illustrated by Kakizawa et al. (2001). This will substantially reduce the cost of the proposed treatment.

As a value-added product, the produced vaterite can find many applications in concrete too. An existing study (Chen et al., 2013) shows that vaterite can be used as a binder for concrete. In this study, a new potential application of the produced vaterite as a water reservoir, or internal curing agent, for internally curing concrete was examined.

Autogenous shrinkage is an important phenomenon in early-age concrete, in which all the available water can be rapidly drawn into the hydration process and the demand for more water creates very fine capillaries. The surface tension within the capillaries causes autogenous shrinkage which can lead to premature cracks, making the concrete more vulnerable to the ingress of potentially aggressive species and thus severely reducing the durability of the concrete. Early age cracking due to autogeneous shrinkage is particular serious for high-performance concrete (HPC) because of its low water to cement (w/c) ratio (Bentz and Weiss, 2011). To remedy this problem, the internal curing method has been developed in which water-filled internal curing agents are added to the concrete mixture. These internal curing agents are used as water reservoirs which can gradually release water to maintain a high relative humidity inside the concrete pores to prevent self-desiccation in concrete (Bentz and Weiss, 2011). As a result, the autogenous shrinkage of the concrete can be reduced or eliminated, ultimately minimizing cracking and significantly promoting the durability of the concrete.