A scientific rationale for the enhanced sequestration of CO2 in concrete

. Oxford Economics forecasts that concrete construction will grow by 85% to USD 8 trillion worldwide by 2030 and will lead to significant usage of Portland cement (PC). Every 1 kg of PC production generates ≈0.8 kg of CO 2 , which is about 1.5 Gt of CO 2 emission per year for PC production. One of the ways to reduce the carbon footprint of concrete is by sequestrating CO 2 using of the following approaches: (i) pre-carbonation; (ii) wet-carbonation; or (iii) CO 2 curing of concrete in various types of carbonation chambers. The efficiency of these methods is measured by calculating CO 2 Sequestration Factor (CSF). It is reported that the CSF of carbonation curing approach is 10 to 15%. However, it was found that the method used for calculating CSF does not consider the un-carbonated parts of the specimens, hence it does not represent the actual efficiency of the CO 2 sequestration methods. Therefore, modification for calculating the CSF is proposed in this paper. Using the modified method, it was found that the CSF of carbonation curing method is less than 2% and wet carbonation is the most efficient method (≈30%). Further, a way forward is proposed to enhance the CO 2 sequestration in concrete, which will not compromise fresh or hardened properties of concrete and would significantly contribute to the net zero carbon agenda compared to existing approaches


Introduction
At the Paris Climate Agreement in 2015, the conference of parties aimed to limit global warming to less than 2 ºC. The construction sector is one of the fastest-growing and energy-intensive industries -contributing about 11% of global energy-related carbon emissions. Ribeirinho et al. [1] reported that about 60% of the anticipated constructions by 2030 is yet to be builtindicating that the world is witnessing and foresees the usage of a significant amount of concrete. Therefore, Portland cement (PC) production is expected to significantly grow from the current value of ≈4.7 BT to ≈6 BT in 2050 [2]. Each kg of cement production emits about 0.8 kg of CO2 and this is estimated to be 85% of CO2 emissions by the building materials sector [3]. Therefore, reduction in the use of PC is one of the ways to reduce CO2 emissions by the building materials sector. In the past several decades, the partial replacement of PC with supplementary cementitious materials (SCMs) has gained wide acceptance to increase the quality of concrete and reduce CO2 emissions [4]. In the Netherlands, up to 85% of slag was used to replace PC in one of the projects during 90's. Such large amount of PC replacement is possible for special projects with a particular exposure condition. Recent developments in Limestone Calcined Clay (LC3) [5][6][7] and Geopolymers (and other activated cementitious systems) [8] could partly help address the * Corresponding author: d.k.kamde@leeds.ac.uk CO2 challenge, but currently, PC is the only promising material capable of meeting the construction demandsindicating that the PC production cannot be significantly reduced. Therefore, the replacement of PC with SCMs alone is not sufficient to meet the CO2 target expected from the construction sector. To reduce the carbon footprint of concrete and concrete products, CO2 sequestration has been successfully trialled by storing CO2 in cementitious materials, industrial minerals, concrete etc. [9,10]. Towards this, CO2 is injected into the mineral component used to manufacture concrete or freshly manufactured concrete products are exposed to CO2 enriched environment. The efficiency of these methods can be measured by calculating the CO2 sequestration factor (CSF), which is the percentage of the mass of CO2 absorbed in the mass of the binder. It was observed that the CSF is determined only by analysing the CO2 captured in the carbonated region (i.e., up to a depth of a few mm from the surface of concrete) -which can be a false positive representation of the efficiency of some of the CO2 sequestration methods. This paper presents a new approach to calculate the true potential of CO2 sequestration methods and ways to enhance the CO2 sequestration in concrete

COsequestration of cementitious systems
The following methods are generally used for CO2 sequestration in concrete: (i) carbonation of mineral admixtures and aggregates; (ii) carbonation of cementitious systems during mixing with gaseous CO2 (pre-carbonation); (iii) addition of carbonated water during mixing (wet-carbonation); or (iv) carbonation of concrete in flow through-, concentrated-and pressurised-carbonation chambers (CO2 curing). The carbonation of mineral admixtures and aggregates is effective, but it does not contribute to modifications of the microstructure of concrete, hence its mechanical or durability properties. Concretes produced with such procedure may have limited applications. Another way of CO2 sequestration is to expose cementitious system to concentrated CO2 during mixing using: gaseous CO2 [pre-carbonation, see Fig. 1(a)] or CO2 mixed in carbonated water [wet-carbonation, see Fig. 1 During these processes, the un-hydrated cement compounds, such as C3A, C3S and C2S, react to CO2 to form C-S-H, calcite and calcium hydroxide (see Equations 1 to 3). These reactions can help enhance mechanical and durability characteristics and store CO2 in cementitious systems for their lifetime. For example, injection of 0.05% gaseous CO2 can increase compressive strength by 10-14% [11]. Suescum-Morales et al. [12] reported that the mixing of carbonated water (with 14.1-14.4 mg/L CO2) during concrete mixing could increase compressive strength by 10-20%, potentially reducing the required cement content to manufacture a similar strength uncarbonated concrete by 5-8%. Note that C3A has a higher solubility in water than C3S. Therefore, it reacts with CO2 faster than C3S -leading to reduction in initial setting time of concrete by 40%, which makes it difficult to be used for concretes with the requirement of high workability retention time [11]. Fig. 1(c), (d) and (e) show schematic representations highlighting methods of CO2 curing of concrete. During this, CO2 reacts with hydration products such as C-S-H and calcium hydroxide to form calcite (see Equations 4 and 5). The carbonation of concrete in flow-through chambers ( Fig. 1(c)) is very similar to natural carbonation of concrete but with higher CO2 concentration. The exposure of blocks of concrete to 10% CO2 concentration from power and/or cement plants can increase compressive strength by 30% [13]. However, sulfuric or nitric acid in exhaust gases can reduce compressive strength by about 50% [13]. Therefore, the industrial exhaust gases need to be processed before using them for concrete curing, which can be an energy-intensive process. Another approach for CO2 sequestration is by placing concrete blocks in carbonation chambers with concentrated levels of CO2 ( Fig. 1(d)). The 2-hour and 7-day compressive strengths of concrete cured in a CO2 enriched carbonation chamber were found to be about 300% and 20% greater than the equivalent air-cured specimens, respectively [14] -indicating that the initial hydration reactions are rapid, but overall heat of hydration is approximately the same as the normal concrete. To enhance carbonation in such systems, 20% CO2 concentration with 50-70% RH was suggested by Pu et al. and Vu et al. [15,16]. Excessive RH can restrict the entry of CO2 due to pores filled with water and higher CO2 concentration is of no importance after carbonating a few mm of concrete from the surface. To further enhance carbonation, Zhang and Shao [17] reported the carbonation in pressurised conditions ( Fig. 1(e)) with a pressure of 2 bars for 12 h can increase in the absorbed CO2 in concrete by 15% than the absorbed CO2 in concentrated carbonation without pressure. However, this process is highly energy intensive. ( The use concentrated CO2 and high pressure in these carbonation approaches require highly trained staff, high capital expenditure and additional health and safety precautions. Also, the carbonation of cover concrete using these methods could hinder carbonation of the concrete core, hence the total quantity of CO2 sequestration in concrete during the lifetime of a concrete structural element. Therefore, these methods are not effective in exploiting the maximum potential of CO2 sequestration in cementitious systems. Towards this, Fig. 1(f) shows the schematic representation of the proposed method in a preliminary work [18] with mixing of cementitious system using carbonated water under 40 bar CO2 pressure and mixing in gaseous CO2 atmosphere for 25 min (for more details, see Fig. 3(b)). It was demonstrated that higher quantities of CO2 could be sequestrated by injecting CO2 into the wet mix whilst manufacturing concrete. To overcome the energyintensive process of application of pressure, Suescum-Morales et al. [12] reported that mixing of only carbonated water (see Fig. 1(b)) could double CO2 sequestration capacity than CO2 curing methods. However, the effect of addition of carbonated water on early hydration and the resulting loss in workability were not investigated. These were the reported challenges by the authors [18]

Efficiency of existing methods of carbon sequestration
To investigate the efficiency of these methods, data from various publications collected by Meng et al. [19] were used to compare the CSF. Ideally, CSF is the ratio of the degree of carbonation in the carbonated concrete volume to the total volume of the cementitious binders in the whole of the structural element. Fig. 2(a) shows that the average of reported CSF for various cementitious systems using pre-, flow through-, concentrated-, pressurised-and wet-carbonation is 0.6, 16, 11, 15 and 33%, respectively. The reported CSFs for CO2 curing methods using flow-through, concentrated-, and pressurised-carbonation chambers are higher than expected. The data indicated that the CSF for concrete with w/b of 0.3, 0.4, and 0.55 were 7, 13, and 20%, respectively (see red, green and blue boxes in Fig. 2(a)) [19]. The high CSF for concrete with high w/b can be attributed to the high permeability of concrete, which would facilitate the CO2 penetration. However, structural concrete is designed to have low permeability -indicating that even pressured carbonation methodology may not be able to help achieving high CO2 sequestration in structural concrete. The reported CSF in Fig. 2(a) is determined from the carbonated portion of the specimen (≈ 2 mm from the surface of the specimen), which does not represent the true potential of the sequestration methods where the core of the specimen is carbonated. To calculate the true potential of CO2 curing methods, it is important to calculate the average CSF of the whole specimen. For simplicity of calculation, the following assumptions are made: (i) the specimens are 50 mm cubes, (ii) 2 mm of cover of the cube is carbonated, and (iii) there is zero carbonation of the cementitious system beyond 2 mm cover. Fig. 2(b) shows that the average CSF for various methods except wet mixing is less than 2%. Note that the calculations are done by considering the specimen size of 50 mm. If the concrete element size is bigger, the CSF for concrete with CO2 curing will be lesser than the calculated average CSF. As expected, the wet carbonation method could achieve CSF close to the theoretical CSF of PC, 45-50%. Therefore, more efforts are required to refine the method presented in [12,18] to overcome the flash set of cementitious systems.

Fig. 2: Effect of sequestration method on CO2 Sequestration Factor (CSF)
3 Way forward to enhance CO2 sequestration in concrete Fig. 3(a) shows the schematics of three possible modes of CO2 sequestration in cementitious systems. First schematic representation shows that when cementitious system is exposed to CO2 after it is set, only the surface of cementitious systems captures the CO2 and hinders the further carbonation of the core cementitious system. These methods capture less than 2% CO2 %by volume of cement (%bvoc). However, these methods are promoted with a promise to achieve early required strength for elements to be moved from the curing chamber to the yard or lowering the cement content to achieve similar strength. Whereas concrete structures during their service life can absorb about 15-30% CO2 (a) Pre-carbonation (b) Wet carbonation [12] (c) Flow throughcarbonation (d) Concentrated carbonation (e) Pressurised-carbonation (f) Preliminary study [18] bvoc [20], which is the optimal potential for sequestrating CO2 in concrete. If such large CO2 sequestration can be achieved, the required curing time or PC content can be significantly reduced. Therefore, any method which leads to CO2 sequestration of less than 15% bvoc is inefficient from a net zero carbon agenda aspect. The second schematic representation shows that if CO2 and water are mixed at the same time, it can lead to a flash set due to the reaction of CO2 with aluminates [18] (see Fig. 3(b)). Similar procedure could capture CO2 greater than 20% bvoc [12]. The results indicated that this method has significant potential for large quantity of CO2 sequestration in concrete, but the flash set was the challenge [18]. The third schematic representation shows the desired sequestration without compromising the characteristics of cementitious systems. The authors are currently trying to address these limitations by further exploring their previous work [18].
(a) Schematics of approaches to enhance CO2 sequestration (b) Schematic of setup used in preliminary work [18]

Conclusions
The various pieces of scientific literature report the advantages of the following methods for CO2 sequestration: (i) pre-carbonation, (ii) flow through-, (iii) concentrated-, (iv) pressurised-, and (v) wet-carbonation on mechanical and durability characteristics of concrete. It is claimed that the concretes produced using these methods can capture a significant amount of CO2, which is reported by calculating the percentage of the mass of CO2 absorbed in the mass of binder (CO2 sequestration factor (CSF)). The CSF reported in literature was obtained by evaluating the amount of CO2 captured in carbonated portion (a few mm from surface) of the specimen, without considering that the core of the concrete specimen may not be carbonated. Therefore, a new approach is proposed to calculate CSF by considering the CO2 captured in cover averaged with the total volume of the specimen. It shows that the corrected CSF for carbonation curing methods (<2%) was 10 times less than the reported CSF (15-20%). The wet carbonation approach (similar to that proposed by authors in a preliminary study) shows CSF ≈30%, which is closer to the theoretical CSF of PC (i.e., 50%). However, flash set of cementitious systems is one of the major drawbacks of concrete produced using wet carbonation method. Therefore, the authors recommend that research should focus on modifying the wet carbonation approach to overcome the flash set of cementitious systems and to enhance the CO2 sequestration in cementitious systems.