Elsevier

Journal of Cleaner Production

Volume 158, 1 August 2017, Pages 285-295
Journal of Cleaner Production

Comparative assessment of the benefits associated with the absorption of CO2 with the use of RCA in structural concrete

https://doi.org/10.1016/j.jclepro.2017.03.230Get rights and content

Introduction

Ordinary Portland Cement (OPC) produces 1 MT of CO2 for every 1 MT of cement, while about 850 kg of CO2 is produced per 1 MT of blended cement (World Business Council for Sustainable Development, 2015). More than 50% of CO2 emitted from cement production is estimated to be from calcination of limestone; while some of this CO2 is reabsorbed through the reverse process of calcination, referred to as carbonation (Corinaldesi et al., 2010, Pade and Guimaraes, 2007). Literature states that CO2 emissions from the use of concrete is significantly overestimated (Corinaldesi et al., 2010), as the effect of carbonation of concrete on demolition and the processes that follow, including concrete crushing, recycling, etc., have not been taken into account. Therefore, it is important to correctly evaluate and incorporate the CO2 uptake associated with carbonation when estimating the total environmental impact associated with the use of concrete in the life cycle of a structural application. It is also noted that the application could have a primary and a secondary life, with the use of concrete and the re-use of recycled concrete, respectively.

Presently, concrete waste (CW) generated from demolished concrete (DC) is recycled to produce crushed concrete (CC), which is used as a road-base product (CC-RB) in Australia. The separated out coarse aggregate portion is referred to as recycled concrete aggregate (RCA), which has the potential to be used as an aggregate product in structural concrete replacing natural aggregate (NA). The resultant concrete product is referred to as recycled aggregate concrete (RAC). In considering the total environmental impact of replacing natural aggregate concrete (NAC) with RAC in structural applications, the CO2 uptake associated with CW, CC, RCA and RAC need to be accounted for.

During the process of carbonation, CO2 from the atmosphere diffuses into the concrete, lowering the pH and destabilizing the cement hydration products such as calcium silicate hydrate (C-S-H). Calcium silicates contribute significantly to concrete carbonation, with the formation of C-S-H and calcium hydroxide (Ca(OH)2) during mixing with water. These compounds are responsible for most of the CO2 uptake by producing recarbonation products and CaCO3. The carbonation reactions of concrete are observed to vary based on the different raw materials and additives used (Dayaram, 2010).

The amount of reabsorption of CO2 has been found to be dependent on a number of factors, as studied by Engelsen et al. (2005). The porosity of concrete and the humidity of its pores are found to improve the diffusion mechanism, while low environmental temperature reduces the rate of carbonation. The binder content is observed to have no material effect when the same water-cement ratio is adopted; and when the increased particle size results in a larger surface area, carbonation is found to increase. The partial pressure of CO2, the inclusion of pozzolanic material, and the outer environmental conditions are noticed to have an influence on the carbonation reactions (Engelsen et al., 2005). Collins (2013) states that concrete strength (higher the strength, higher the carbonation), area-to-volume ratio of crushed particles (higher the ratio, higher the carbonation), and the exposure environment (buried or submerged environment has less carbonation compared to open air), are the key variables affecting the process.

Ho and Lewis (1987) introduced a model to measure the depth of carbonation, x, relating it to the time and initial depth of carbonation. Incorporating the above with Fick's first law of diffusion results in the equation below, which can be used to estimate the CO2 uptake:CO2uptake=x*c*CaO*r*A*Mwhere x is the depth of carbonation, c is the quantity of OPC within the binder, CaO is the Calcium Oxide content within OPC, r is the proportion of CaO within fully carbonated OPC that converts to CaCO3, A is the surface area of the concrete (Collins, 2010, Lagerblad, 2006), and M is the dimensionless chemical molar fraction of molar weight of CO2/CaO.

A few researchers have quantified the extent of carbonation associated with CC and RCA. A study undertaken by Kikuchi and Kuroda concludes that the amount of CO2 uptake in one metric ton of recycled crusher-run (0–40 mm) is estimated to be approximately 11 kg. As the study has been conducted on eight recycling plants, conducting further surveys have been suggested to improve the accuracy of this estimation. Equally, the estimation of CO2 during the service period of the application has been suggested for a more accurate result (Kikuchi and Kuroda, 2011). Engelsen et al. have evaluated the effect of concrete carbonation on the overall CO2 emissions from cement and concrete production in Nordic countries (Engelsen et al., 2005). Lagerblad has conducted a comprehensive study on carbonation of concrete during service life and its secondary use. A methodology for quantification of carbonation is suggested for buildings, with the use of different parameters to account for different environmental conditions, exposure environment, and cement compositions (Lagerblad, 2006).

Several researchers have investigated the carbonation associated with RAC. The research by Etxeberria et al. observes a useful relationship between carbonation and the permeability of RCA and surrounding mortar (Etxeberria et al., 2007). When RCA is more permeable, the carbonation depths are expected to be higher; yet when the mortar is more permeable, the carbonation would not vary compared to NAC. The extra alkaline reserve of RAC due to the presence of the attached mortar is observed to contribute to decreasing carbonation. Collins analysed the CO2 captured during both the primary and secondary life of a structure into a life cycle assessment (LCA) model (Collins, 2010). The findings state that, while the carbon capture in the primary life of concrete is estimated to be relatively small due to limited exposure of a concrete application, if carbon capture is not considered in the secondary life, the emissions are overestimated by 13–48%. The results of the 2013 study by Collins found that the carbon capture during the secondary life is 41% of the CO2 emitted during manufacture of 100% Portland cement required for the concrete (Collins, 2013).

When comparing applications that would optimise carbon capture, Collins suggested that the use of RCA as gravel in roadbase was the best (Collins, 2013). This was based on the extent of crushing of concrete as a roadbase material, where the concrete is crushed to fine particles and subject to buried and moist exposure during the application. RCA crushed to 200 mm boulder sizes and serving as embankment protection is observed to absorb less CO2 due to their large size. Collins assumes that deterioration of the quality of RCA to be used as a constituent material in concrete is a result of crushing during recycling (Collins, 2010). As a result, the study has considered the life of the primary application to be 100 years and the secondary life to be 30 years, considering the latter to be a temporary application. However, this assumption is questionable as there is evidence that RAC can be effectively used in structural building applications and the durability of the applications can be managed by adopting alterations to mix design, preparation and mixing processes (Abbas et al., 2009, Jimenez et al., 2013, Jimenez et al., 2014, Limbachiya et al., 2012b, Tam and Tam, 2007, Tam and Tam, 2008, Tam et al., 2007).

Dayaram quantified the theoretical capacity of reabsorption of CO2 in concrete and concluded that cement in concrete had about 60% CO2 released in the calcination process which could be reabsorbed (Dayaram, 2010). The experimental investigations by Dayaram indicated that particles with sizes less than 10 mm get recarbonated completely during the first 21-day period (Dayaram, 2010). While higher strength concretes have been observed to take longer to reabsorb, smaller crushed sizes absorbed significant quantities within relatively shorter periods. Maximum CO2 uptake was observed for sizes less than 10 mm of the 20 MPa samples, which reabsorbed 420 kg CO2/MT of cement. This matches 83% of the calcination emissions released in the production of cement, while the largest 40 mm size fraction of the 40 MPa sample accounted for 70% reabsorption. Considering the net effect, this reduces the net CO2 released from 850 kg to 430 kg CO2/MT of cement. The experimental investigation further suggested that 80% of the total CO2 that was reabsorbed within the first 21 days (Dayaram, 2010). These findings question Collins (2010) hypothesis that the use of RCA as a roadbase material results in the highest reabsorption. This is because it is highly probable that RCA would have a significant residence time period associated with crushing, prior to use in an application during which a significant amount of carbonation would occur due to the much reduced particle size (Dayaram, 2010). This would involve the time in storage at the recycler, time in transit, and time in storage at the ready-mix concrete (RMC) manufacturing plant.

There are other related studies on concrete carbonation and CO2 emissions associated with the production, demolition and use of concrete in a structure. Pacheco et al. conducted a review on concrete carbonation, assessing carbonation depth measurement with the use of supplementary cementitious material and RCA (Pacheco Torgal et al., 2012). Guggemos and Horvath made a comparison of environmental effects of steel and concrete frame buildings and state that concrete structural frames have more associated energy use, CO2 and other emissions (Angela Acree Guggemos and Horvath, 2005). Flower and Sanjayan conducted a thorough analysis of the major contributing factors to CO2 emissions for commercially produced concrete mixes and concluded that cement was the most significant factor (Flower and Sanjayan, 2007).

Analysing previous literature, it is observed that, while quantitative evaluation of carbonation associated with DC, CC, RCA and RAC have been studied, those quantifications lack emphasis on the CO2 uptake considering the typical industrial supply chain for the production of RAC. On the other hand, there is a need to assess the CO2 uptake of DC, CC and RCA as well as RAC, as a part of assessing the total environmental impact associated with the use of RCA in structural concrete.

This paper draws knowledge from previous studies conducted on the CO2 uptake of DC, CC, RCA and RAC, and extends the knowledge base focusing on the evaluation of RCA as an alternative aggregate product to evaluate the monetised net benefit during the life cycle of a structural application. The outcome of this paper presents the environmental benefit as a monetised benefit, to enable easy comparison and integration with other relevant dimensions considered when comparing the two alternatives of using RAC against NAC in structural applications.

This paper aims to:

  • 1.

    evaluate the net incremental direct environmental benefit attached to RAC due to the CO2 uptake occurring with the carbonation of crushed concrete and RCA, prior to being embedded in concrete

  • 2.

    evaluate the incremental benefit associated with carbonation of concrete with the use of RAC in a structural application as opposed to natural aggregate concrete (NAC) using case studies, and to analyse the key factors contributing to the effect.

Section snippets

Methodology

For the purpose of this study, carbonation of CW, CC, RCA and RAC is analysed associated with two phases. They are:

  • 1.

    the phase in which the CW is in the form of large concrete pieces (LCP), CC and coarse RCA, up to the point where RCA is mixed with other constituent materials of concrete to produce RAC

  • 2.

    the phase in which the RAC is applied to a structural member/element and embedded in a structural application.

Results and analysis

This section presents the results, analysis and discussion of the outcomes of the paper.

Conclusions

The effect of carbonation of concrete waste and RCA prior to their use in concrete is considered significant compared to when it is embedded in a building structure in its secondary life. Experimental investigations state that 80% of the CO2 uptake associated with crushed concrete was observed to occur in the first 21 days following crushing, for most size fractions. This study evaluates the CO2 uptake associated with CW in two phases, where: 1) CW is in the form of LCP in a demolition site and

Acknowledgments

The authors would like to thank and acknowledge the support from: 1. Ward and Geoff Petherbridge, Waster Convertors Recycling, 2. Tony Leen, Berni Leen Contractors, 3. Jason Walsh and Walter Roemer, Alex Fraser Group, 4. Sally Hirst, Hirst Projects, 5. Valerie Francis, Department of Architecture, Building and Planning, 6. Luke Wade, Brookfield Multiplex.

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