Prediction on CO₂ uptake of recycled aggregate concrete

Abstract

Carbonation of concrete is a process which absorbs carbon dioxide (CO₂). Recycled aggregate concrete (RAC) may own greater potential in CO₂ uptake due to the faster carbonation rate than natural aggregate concrete (NAC). A quantitative model was employed to predict the CO₂ uptake of RAC in this study. The carbonation of RAC and the specific surface area of recycled coarse aggregates (RCAs) were tested to verify accuracy of the quantitative model. Based on the verified model, results show that the CO₂ uptake capacity increases with the increase of RCA replacement percentage. The CO₂ uptake amount of 1 m³ C30 RAC within 50 years is 10.6, 13.8, 17.2, and 22.4 kg when the RCA replacement percentage is 30%, 50%, 70%, and 100%, respectively. The CO₂ uptake by RCAs is remarkable and reaches 35.8%–64.3% of the total CO₂ uptake by RAC when the RCA storage time being 30 days. Considering the fact that the amount of old hardened cement paste in RCAs is limited, there is an upper limit for the CO₂ uptake of RCAs.

Keywords

RAC / CO ₂ uptake / carbonation / specific surface area / RCA

Introduction

Concrete is the most widely used construction material today. It is estimated that approximately 25 billion tons of concrete are produced globally each year [¹], which definitely consumes a large amount of natural resources. For example, 42% of the aggregates produced are used for concrete production annually [²]. Besides, the cement manufacturing resulted in massive emission of carbon dioxide (CO₂), which accounts for about 7% of the total global CO₂ emission [³]. The disposal of concrete waste, as the main constituent in construction and demolition (C&D) waste, is another problem. Traditional disposal method by landfill has caused in serious environmental damages including occupation of land, soil and water pollution [⁴,⁵]. Therefore, from the viewpoint of sustainability, the concrete industry is expected to reduce the impact of C&D waste emission to the environment.

Utilizing the concrete waste to produce recycled coarse aggregates (RCAs) which is employed in the mix of recycled aggregate concrete (RAC), is considered as one of the effective methods to prompt the sustainability of concrete industry. The environmental benefits of RAC includes reducing landfill and saving natural coarse aggregates (NCAs) [⁶]. As a result, RAC as a structural material has drawn wide attention in academia and industry during the past decades. On the other hand, in order to estimate the environmental impact of RAC, several studies have been conducted to evaluate the CO₂ emission by applying the life cycle assessment (LCA) method [⁶–⁸]. However, these studies paid more attention to the CO₂ emission without considering the CO₂ uptake caused by the carbonation reaction. It is believed that, with only considering the CO₂ uptake process, the life cycle CO₂ assessment for RAC can be estimated correctly according to the cradle-to-gate principle [⁹].

CO₂ uptake measurement methods such as mass change, gamma densitometry, ignition, QXRD and coulometric to assess the carbon dioxide absorbed by the carbonated cement-based materials were already used in normal concrete [¹⁰,¹¹]. Nordic scholars conducted a series of studies on the CO₂ uptake of concrete and mentioned the effect of carbonation on the net CO₂ emission of concrete can be significant [¹²–¹⁴]. Shao et al. [¹⁵] used X-ray diffraction to identify the carbonation products. Kikuchi and Kuroda [¹⁶] analyzed the CO₂ uptake of demolished and crushed concrete by thermogravimetric analysis and differential thermal analysis (TG-DTA). Lee et al. [¹⁷] mentioned CO₂ ratio of absorption to emission is about 2.92% to 17.19% with the concrete service life for 20 to 60 years.

Up to now, several studies have also been conducted on the topic of accelerated carbonation processes of recycled concrete [¹⁸–²⁰], in order to achieve improved concrete mechanical properties. Owing to the quick formation of calcium carbonate in the microstructure [²¹], high early strength for the carbonated concrete can be obtained. It is stated from Fang et al. [²²] and Xuan et al. [²³] that for RCAs, the carbonated RCAs may result in an increase in density and strength while a decrease of water absorption. Corinaldesi et al. [²⁴] indicated that recycled aggregate no-fine concrete performed well in term of CO₂ uptake. Ueno et al. [²⁵] investigated the CO₂ uptake amount of demolished concrete and introduced several experimental methods to visualize the phenomenon.

In addition, models for concrete CO₂ uptake or carbonation have also been proposed. Based on the chemical compositions of cement, Steinour [²⁶] proposed a model to estimate the theoretical maximum CO₂ uptake while they also pointed out that the maximum value would not be achievable. Papadakis et al. [²⁷,²⁸] proposed models to predict the carbonation depth of concrete. The carbonation mechanism of a heap of RCA was investigated by Thiery et al. [²⁹]. They proposed a model which could concern the influence of the cementitious phase attached to the original aggregates on the CO₂ absorption rate. However, their models are not suitable for the prediction of RCAs carbonation because RCAs are small random-shaped particles with much larger surface areas. Fang et al. [²²] developed an empirical prediction mode to assess the potential CO₂ capture ability of RCAs subjected to accelerated carbonation, which was able to predict the CO₂ uptake in relation to relative humidity, particle size, carbonation duration, and cement content.

The abovementioned literature suggested that over the last decades, although research on concrete CO₂ uptake has attracted increasing attention, an in-depth understanding and evaluation of concrete CO₂ uptake for the RAC, especially those providing a method to easily predict the CO₂ uptake by RAC carbonation is still lack. Without considering CO₂ uptake capacity of RAC, the CO₂ uptake amount by RAC may be underestimated [³⁰]. This research aims to establish a rational method to predict the CO₂ uptake of RAC. Based on the analysis of the CO₂ uptake process in the life cycle of RAC, a quantitative CO₂ uptake model was proposed. In addition, the accuracy of this model was also evaluated by taking experiments. The findings of this study would fill the gap in this field, and also provide a reference for the life cycle CO₂ assessment on RAC, especially for analyzing the environmental benefits of RAC.

CO₂ uptake model for RAC

Theoretical background

The CO₂ uptake by RAC is a dynamic, slow and continuous process, occurring when the hydrated constituents of hardened cement paste are in contact with CO₂. Therefore, the CO₂ uptake amount should be a function of time. To establish a model to predict the CO₂ uptake of RAC, the calculation system boundary for CO₂ uptake of RAC needs to be defined first. This study considers the system boundary from raw material production to RAC demolition based on the cradle-to-grave principle. Due to the existence of hardened cement paste in both RCAs and new concrete, there are two main CO₂ uptake contributors of RAC in this system boundary.

1) CO₂ uptake from new hardened cement paste (NHCP) in RAC. NHCP is formed by the hydrated cement after new RAC casting. Due to the porosity and the pore water in RAC, the alkaline constituents during NHCP can react with CO₂ in the air gradually. The CO₂ uptake process of NHCP will continue in the entire service life after RAC casting. As NHCP continues to absorb CO₂, RAC is gradually divided into two parts, i.e., carbonated and non-carbonated zone, which are illustrated in Fig. 1.

2) CO₂ uptake from old hardened cement paste (OHCP) in RCAs. The surface of each single RCA made of concrete waste is covered by a layer of OHCP. Before the crushing of concrete waste, most RCAs are wrapped inside the concrete without the basic conditions for the carbonation reaction. That is to say, the carbonation process beginning from the concrete surface is extremely slow. Hence the OHCP of most RCAs can be considered non-carbonated at the time of production. However, after crushing, the OHCP of RCAs is exposed to the air. Each single RCA can be carbonated and absorb CO₂, respectively. Due to quite large exposed surface areas of RCAs, the absorption rate of RCAs becomes faster than the previous status.

It is worth noting that, the OHCP carbonation of RCAs in RAC has two different states, which are shown in Fig. 2. For RCAs in the carbonated zone, the OHCP is fully carbonated, with the carbonation effect of NHCP in RAC. However, for RCAs in the non-carbonated zone, the carbonation of OHCP occurs only in the storage time of RCAs. That is to say, the OHCP of RCAs in the non-carbonated zone is divided into carbonated and non-carbonated parts, which depends on the surface area and the storage time of RCAs.

In this research, the mix of RAC was designed to have the same compressive strength. 1 m³ RAC with carbonated of OHCP was assumed as the function unit during the following calculation. The CO₂ uptake amount of RAC during its life cycle is expressed as follows:

(1)CA=CAa+CAb,

where CA is the CO₂ uptake amount of RAC (kg), CAa is the CO₂ uptake amount of the NHCP in RAC (kg), CAb is the CO₂ uptake amount of the OHCP in RCAs (kg).

Figure 3 shows the diagram of the calculation procedures of the CO₂ uptake of RAC, which will be described in detail in the following.

The CO₂ uptake of NHCP in RAC

Yang et al. [³¹] mentioned that the CO₂ uptake amount of natural aggregate concrete (NAC) is mainly determined by the surface area of concrete, the carbonation depth and the amount of absorbable CO₂. Considering the process of carbonation, the CO₂ uptake of the NHCP in RAC is similar to that in NAC. In this research, the carbonated volume of RAC and the molar concentration of absorbable CO₂ of hardened cement are emphasized for analyzing the two main CO₂ uptake processes of RAC. The CO₂ uptake amount for the NHCP in RAC is calculated by Eq. (2).

(2)CAa=\mathrm{0.044}{m}_{\mathrm{0}}{V}_{\mathrm{c}}=\mathrm{0.044}{m}_{\mathrm{0}}{D}_{\mathrm{c}}{A}_{\mathrm{surface}},

where m₀ is the molar concentration of absorbable CO₂ of NHCP in 1 m³ concrete (mol/m³), V_c is the carbonated volume of RAC (m³), D_{\mathrm{c}} is the carbonation depth of RAC (m), A_{\mathrm{surface}} is the exposed surface area of RAC (m²).

There are three main parameters in Eq. (2), i.e., m₀, D_{\mathrm{c}}, and A_{\mathrm{surface}}. The exposed surface area A_{\mathrm{surface}} is not difficult to be determined for a certain RAC element or can be obtained by calculating the average surface area in the building coming to the function unit. As for the calculation of parameters m₀ and D_{\mathrm{c}}, more discussions will be presented in the following.

Calculation of parameter m₀

When the hardened cement paste is fully carbonated, the molar concentration of absorbable CO₂ is related to the amount of cementitious materials, degree of hydration and the mineral admixtures. Some studies [³¹,³²] reported that the parameter m₀ is significantly dependent on the amount of cement rather than water-cement ratio. However, the result is adapted to the ordinary Portland cement (OPC) and the effect of the type and amount of mineral admixtures on m₀ is ignored. Niu et al. [³³] have further studied the effect of mineral admixtures on m₀ and reported that the secondary hydration process, type and amount of mineral admixture are significant factors which will influence the value of m₀. Hence, a series of formulas to determine m₀ for different types of cement were proposed according to the Niu et al. [³³] (see Table 1).

Calculation of parameter D_c

Carbonation depth (D_c), a key parameter to evaluate the CO₂ uptake of RAC, determines the carbonated zone of RAC and reflects the rate of carbonation. Compared with NAC, the carbonation depth of RAC is affected by the RCA replacement percentage besides the conventional factors including water-cement ratio, temperature, service time, curing condition, etc. Bostanci et al. [³⁴] demonstrated that RAC could perform higher carbonation depth than NAC. It could be due to the higher water absorption of RCA compared with NCA, which results in the lower moisture content and more porous microstructure of RAC.

Xiao and Lei [³⁵] also pointed that the carbonation depth of RAC tends to increase with the increase of the RCA replacement percentage and proposed a formula to predict the carbonation depth of RAC.

(3)D_{\mathrm{c}}=\mathrm{839}{g}_{\mathrm{RC}}{(\mathrm{1}-R)}^{\mathrm{1.1}}\sqrt{{\displaystyle \frac{(W/({\mathit{\gamma }}_{\mathrm{c}}C)-\mathrm{0.34})}{{\mathit{\gamma }}_{\mathrm{HD}}{\mathit{\gamma }}_{\mathrm{c}}C}}{n}_{\mathrm{0}}t},

where D_{\mathrm{c}} is the carbonation depth (mm), R is the relative humidity (%). C is the amount of cement used in 1 m³ RAC (kg). W is the amount of water used in 1 m³ RAC (kg). {\mathit{\gamma }}_{\mathrm{c}} is the correction factor for the type of cement. When the cement is OPC, {\mathit{\gamma }}_{\mathrm{c}} = 1 or {\mathit{\gamma }}_{\mathrm{c}}=\mathrm{1}-\betacorresponding to other types of cement, where \beta is the mass percent of mineral admixtures (%). {\mathit{\gamma }}_{\mathrm{HD}} is the correction factor for the hydration degree of cement. {\mathit{\gamma }}_{\mathrm{HD}}=\mathrm{1} or {\mathit{\gamma }}_{\mathrm{HD}}=\mathrm{0.85} can be used for 28 days of standard curing. As for the curing time ranging from 28 to 90 days, the linear interpolation for {\mathit{\gamma }}_{\mathrm{HD}} is proposed. n_{\mathrm{0}} is the concentration of CO₂ by volume (%). t is the time of carbonation (days). {{\displaystyle g}}_{\mathrm{R}\mathrm{C}} is the influence factor for RCA. When the RCA replacement percentage is 0%, g_{\mathrm{RC}}=\mathrm{1} or g_{\mathrm{RC}}=\mathrm{1.5} for 100% RCA replacement percentage. As for the replacement percentage varying from 0% to 100%, the linear interpolation for {{\displaystyle g}}_{\mathrm{R}\mathrm{C}} is proposed.

CO₂ uptake of OHCP in RCAs

Due to the diversity of concrete waste, the RCAs produced by concrete waste must have different characters, especially in the composition of OHCP. The complexity of the composition of OHCP in RCAs makes it difficult to evaluate the CO₂ uptake. It is impossible and superfluous to analyze the mix proportion of each single RCA. Considering the feasibility of prediction, some assumptions to simplify the model of RCA were proposed as follows.

1) The mix proportion of the OHCP in RCAs is the same regardless of the source of RCAs.

2) RCAs are simplified as the spheres with the surfaces covered with a certain thickness of OHCP.

3) All RCAs are distributed uniformly in a function unit of RAC.

According to the two carbonation stages of OHCP, the CO₂ uptake amount of the OHCP in RCAs is calculated by Eq. (4).

(4)CAb=CA{b}_{\mathrm{1}}+CA{b}_{\mathrm{2}}\hspace{0.17em},

where CAb is the CO₂ uptake amount of the OHCP in RCAs (kg), CAb₁ is the CO₂ uptake amount of the OHCP in the carbonated zone (kg), CAb₂ is the CO₂ uptake amount of the OHCP in the non-carbonated zone (kg).

OHCP in the carbonated zone

In the carbonated zone of RAC, the NHCP has already been carbonated. Due to the carbonation of NHCP, the OHCP in RCAs belonging to the carbonated zone is considered as fully carbonated. The simplified model of RCA in the carbonated zone is shown in Fig. 4.

According to the simplified model of RCA, the CO₂ uptake amount of RCAs belonging to the carbonated zone depends on the amount of RCAs in the carbonated zone, the content of OHCP and the molar concentration of absorbable CO₂ of OHCP. The calculation method is expressed as follows.

(5)CA{b}_{\mathrm{1}}\mathrm{=0}\mathrm{.044}\lambda q{M}_{\mathrm{g}}{\displaystyle \frac{V_{\mathrm{c}}}{V_{\mathrm{0}}}}\mathrm{=0}\mathrm{.044}\lambda q{M}_{\mathrm{g}}{\displaystyle \frac{D_{\mathrm{c}}{A}_{\mathrm{surface}}}{V_{\mathrm{0}}}},

where CAb₁ is the CO₂ uptake amount of the OHCP in RCAs belonging to the carbonated zone (kg); \lambda is the content of OHCP in the RCAs (%), \lambda is an important parameter influenced by the properties of RCA, such as density, water absorption, and compressive strength. However, to determine \lambda in RCA is complex. Some complicated experimental methods including thermal treatment [³⁶], acid treatments [³⁷], Computer Tomography (CT) [³⁸] as well as image analysis [³⁹] were used to measure \lambda, and some formulas for predicating \lambda were also proposed in Ref. [⁴⁰]. In this paper, in order to simplify the evaluation method, \lambda is assumed be a constant value of 0.41 with statistical significance according to the experimental result reported by Liu et al. [³⁹]. M_g is the mass of all the RCAs in RAC. V₀ is the volume of the RAC. q is the molar concentration of absorbable CO₂ of per unit mass of OHCP at kg (mol/kg). When the mix proportion is assumed, q for all the RCAs keeps the same value. It can be determined by analyzing the mix proportion combining with the calculation formulas in Table 1. q in Eq. (5) can be formulated as

(6)q={\displaystyle \frac{{m^{\prime }}_{\mathrm{o}}}{M_{\mathrm{s}}}},

where {m^{\prime }}_{\mathrm{o}} is the amount of absorbable CO₂ according to the mix proportion assumed for OHCP (mol); it can be calculated referring to Table 1. M_s is the mass of OHCP corresponding to the mix proportion assumed (kg).

OHCP in the non-carbonated zone

In practical engineering, according to the different supply requirements, RCAs will be stored for different time in plants. The CO₂ uptake process of the OHCP in the non-carbonated zone mainly occurs in the storage time of RCAs. During storage time, RCAs will be exposed to the air and carbonated slowly. Yang et al. [³¹] have mentioned that the storage position of concrete lumps has little effect on carbonation rate and the innermost concrete lumps are also carbonated at a rate similar to the outermost ones. RCAs exposed in the air can be considered as concrete lumps with different sizes. Therefore, for the same storage time, the carbonation depths of all RCAs are supposed to be the same with the assumption of same mix proportion of OHCP.

The simplified model of RCA in the non-carbonated zone is shown in Fig. 5. According to the model, the OHCP of RCA in this zone can be divided into the outside carbonated part and the inner non-carbonated part. The carbonation depth of each single RCA (d_c) in the non-carbonated zone is controlled by the exposure time (t_{\mathrm{c}}) and the maximum thickness of OHCP (d_cmax). The time point when d_c is equal to the maximum thickness of OHCP d_cmax is defined as the critical exposure time (t_{\mathrm{cp}}). Considering different exposure time of RCAs, the calculation for CO₂ uptake of the OHCP in RCAs belonging to the non-carbonated zone should be divided into two situations as follows.

1) When t_{\mathrm{c}}\le t_{\mathrm{cp}}, a single RCA in the non-carbonated zone is similar to a concrete lump exposed to the air and absorbs CO₂ continuously. For a single RCA in the non-carbonated zone, its CO₂ uptake amount can be calculated as

(7)n_i=\mathrm{0.044}q{m}_{\mathrm{s}}=\mathrm{0.044}q{\rho }_{\mathrm{s}}{v}_{\mathrm{s}}^i,

where n_i is the CO₂ uptake amount of a single RCA in the non-carbonated zone (kg), q is the molar concentration of absorbable CO₂ of per unit mass of OHCP at kg (mol/kg), m_s is the mass of carbonated OHCP in the RCA (kg), {\rho }_{\mathrm{s}} is the density of OHCP (kg/m³), v_{\mathrm{s}}^i is the carbonated volume of OHCP in the RCA (m³). Considering the limit storage time of RCAs, the carbonation depth of the single RCA would be very small. By utilizing mathematical approximations, v_s^i in Eq. (7) can be expressed as

(8){{\displaystyle v}}_s^i={\displaystyle \frac{\mathrm{1}}{\mathrm{6}}}\mathrm{\pi }\left[{\left({{\displaystyle d}}_{\mathrm{g}}^{i\mathrm{3}}-\left({{\displaystyle d}}_{\mathrm{g}}^{i\mathrm{3}}-{{\displaystyle \mathrm{2}d}}_{\mathrm{c}}\right)\right)}^{\mathrm{3}}\right]\approx {\displaystyle \frac{\mathrm{1}}{\mathrm{6}}}\mathrm{\pi }{{\displaystyle d}}_{\mathrm{g}}^{i\mathrm{3}}\left[\left(\mathrm{1}-\left(\mathrm{1}-\mathrm{6}{\displaystyle \frac{{{\displaystyle d}}_{\mathrm{c}}}{{{\displaystyle d}}_{\mathrm{g}}^i}}\right)\right)\right]=\mathrm{\pi }{{\displaystyle d}}_{\mathrm{c}}{{\displaystyle d}}_{\mathrm{g}}^{i\mathrm{2}}={{\displaystyle d}}_{\mathrm{c}}{{\displaystyle d}}_{\mathrm{g}}^i,

where d_c is the carbonation depth of the RCA (m), a_{\mathrm{g}}^i is the diameter of the RCA (m), a_{\mathrm{g}}^i is the surface area of the RCA (m²).

For all RCAs in the non-carbonated zone, the CO₂ uptake amount is affected by the carbonation depth, the surface area and the quantity of all RCAs in the non-carbonated zone. Due to the same mix proportion assumed of RCAs, it can be calculated as Eq. (9).

(9)CA{b}_{\mathrm{2}}={\displaystyle \mathrm{\sum }_i{n}_i}=\mathrm{0.044}q{\rho }_{\mathrm{s}}{d}_{\mathrm{c}}{\displaystyle \mathrm{\sum }_i{a}_{\mathrm{c}}^i}=\mathrm{0.044}q{\rho }_{\mathrm{s}}{d}_{\mathrm{c}}{A}_{\mathrm{g}},

where A_{\mathrm{g}} is the surface area of all RCAs in the non-carbonated zone (m²). To calculate A_{\mathrm{g}}, the specific surface area of RCAs is introduced. Equation (9) can be expressed further as Eq. (10).

(10)CA{b}_{\mathrm{2}}=\mathrm{0.044}\eta q{\rho }_{\mathrm{s}}{d}_{\mathrm{c}}{M}_{\mathrm{g}}\left(\mathrm{1}-{\displaystyle \frac{D_{\mathrm{c}}{A}_{\mathrm{surface}}}{V_{\mathrm{0}}}}\right),

where \eta is the specific surface area of RCAs (m²/kg). The value of \eta is equal to the surface area of unit mass of RCAs at kg. M_g is the mass of all the RCAs in RAC. Since the carbonated volume of OHCP in a single RCA which is less than the max volume of OHCP, the control condition expressed in Eq. (11) should be considered before using Eq. (10).

(11)d_{\mathrm{c}}{a}_{\mathrm{g}}{\rho }_{\mathrm{s}}\le \lambda m_{\mathrm{g}},

where m_g is the mass of an RCA (kg) and a_g is the surface area of an RCA (kg). Hence by Eq. (11), the maximum thickness of OHCP d_cmax is obtained in Eq. (12). Besides, the exposure time t_cp can be quantified together with Eq. (12) and Eq. (3), which can be expressed by Eq. (13).

(12)d_{\mathrm{c}\max }={\displaystyle \frac{\lambda }{\eta {\rho }_{\mathrm{s}}}},

(13)t_{\mathrm{cp}}={\displaystyle \frac{\mathrm{1}}{n_{\mathrm{0}}}}{\left[{\displaystyle \frac{d_{\mathrm{c}\max }}{\mathrm{839}{g}_{\mathrm{RC}}{(\mathrm{1}-R)}^{\mathrm{1.1}}}}\right]}^{\mathrm{2}}{\mathit{\gamma }}_{\mathrm{HD}}{\mathit{\gamma }}_{\mathrm{c}}C/(W/({\mathit{\gamma }}_{\mathrm{c}}C)-\mathrm{0.34}).

2) When t>t_{\mathrm{cp}}, the OHCP of RCAs will be fully carbonated during the storage time. Hence the CO₂ uptake amount of RCAs in the non-carbonated zone will reach the maximum and it can be formulated as

(14)CA{b}_{\mathrm{2}}=\mathrm{0.044}\lambda q{M}_{\mathrm{g}}\left(\mathrm{1}-{\displaystyle \frac{D_{\mathrm{c}}{A}_{\mathrm{surface}}}{V_{\mathrm{0}}}}\right).

Especially, due to the application of mathematical approximation in Eq. (8), the accuracy of the formula is needed to be analyzed further. According to the approximation, the smaller the value of d_{\mathrm{c}}/d_{\mathrm{g}}^i is, the more accurate the result can be. Considering small carbonation depth of RCAs in storage time, Eq. (8) is appropriate for the RCAs with relatively large diameters. However, if the diameter of RCA is less than 5mm, applying Eq. (8) to calculate v_{\mathrm{s}}^i will cause some deviations. In this study, the CO₂ uptake by the RCAs which are less than 5 mm in diameter are not discussed.

Calculation of parameter \eta

The specific surface area of RCAs \eta is a key parameter in the analysis on CO₂ uptake of RCAs. The larger value of \eta means more surface area of RCAs exposed to the air. When all the RCAs are simplified as spheres, \eta can be calculated based on the average diameter. According to the average diameter theory, all the RCAs are equivalent to the spheres with the same average diameter on the basis of mass invariance and volume invariance. Here are the calculation steps:

1) Calculating the average diameter \overline{d} for RCAs according to the proportion after sieving.

(15){\displaystyle \frac{\mathrm{1}}{\mathrm{6}}}\mathrm{\pi }{{\displaystyle \overline{d}}}^{\mathrm{3}}={\displaystyle \mathrm{\sum }_i\mathrm{\pi }{{\displaystyle \overline{{{\displaystyle d}}_i}}}^{\mathrm{3}}{{\displaystyle \omega }}_i},

where {\overline{d}}_i is the average diameter of i size sieving hole, {\omega }_i is the retained percentage of residue for i size sieving hole. Equation (15) can be simplified as

(16)\overline{d}=\sqrt[\mathrm{3}]{\mathrm{\sum }_i{{\displaystyle \overline{d}}}_i^{\mathrm{3}}{{\displaystyle \omega }}_i}

2) Calculating the quantity of the spheres with average diameter according to the principle of mass invariance of RCAs

(17)n={\displaystyle \frac{\mathrm{6}{m}_{\mathrm{x}}}{\rho \mathrm{\pi }{\overline{d}}^{\mathrm{3}}}},

where n is the quantity of the spheres with an average diameter, m_x is the mass of RCAs to be measured, \rho is the density of the aggregate.

3) Calculating the specific surface area of RCAs \eta.

(18)\eta ={\displaystyle \frac{n\mathrm{\pi }{\overline{d}}^{\mathrm{2}}}{m_{\mathrm{x}}}}={\displaystyle \frac{\mathrm{6}}{\rho \overline{d}}}.

Experimental program

Experiments were conducted to evaluate the accuracy of the proposed prediction model. Considering the natural carbonation rate of concrete is very slow, accelerated carbonation technique was applied to investigate the carbonation of concrete. Under the condition of high CO₂ concentration, several experimental methods were also proposed to measure the CO₂ uptake by NAC or RAC, such as TG-DTA, X-ray diffraction and X-ray photoelectron spectroscopy [¹⁵,¹⁶,⁴⁰]. In this study, the phenolphthalein indicator was chosen to verify the accuracy of the CO₂ uptake model of RAC. By using a phenolphthalein pH indicator, carbonated and non-carbonated zone can be distinguished easily. In the non-carbonated zone of RAC, a purple-red color will appear, which indicates this zone is still highly alkaline. While in the carbonated zone of RAC, due to reduction of alkalinity, no coloration will appear. It is necessary to note that there exists the transition region, i.e., the partially carbonated zone between carbonated and non-carbonated zone. Due to the difficulty in evaluating the carbonation degree, the influence of the transition region is not considered in this investigation.

RAC carbonation experiment

In the CO₂ uptake model of RAC, the carbonation depth is one of the key parameters, which is significant to the CO₂ uptake amount of both NHCP and OHCP. The accelerated carbonation experiments for the RAC specimens with 28 days’ strength grade of C30 were designed and prepared in this experiment.

Experiment design

1) Materials

The mix proportion of RAC used for the experiment is listed in Table 2. Five RCA replacement percentages, i.e., 0%, 30%, 50%, 70%, and 100%, were used to prepare the specimens; the corresponding specimens were named as A0, B1, B2, B3, B4. To compensate the strength loss due to RCA incorporation, additional cement was employed in the mixes, which resulted in the change of the water-cement ratio, i.e., 0.45, 0.44, 0.43, 0.42, 0.41. Besides, due to the high water absorption of RCA, additional water was also necessary to be considered according to Xiao et al.’s [⁴¹] investigation. Additional water was used to assure target water-cement ratio of the specimens, which was calculated from the measured effective water absorption. The measured effective water absorption of the RCAs was 4.08% as shown in Table 2. The cement was OPC of 42.5 grade. RCAs and NCAs were from a local mix-plant in Shanghai, China. Some physical properties of RCAs were also measured and listed in Table 3. Before casting RAC specimens, the RCAs were cleaned up by water. Fine aggregates are river sand.

In this experiment, cube specimens in the size of 100 mm × 100 mm × 100 mm and prism specimens in 100 mm × 100 mm × 300 mm were cast and all the specimens were cured under the standard condition for 28 days. After curing, the compressive strength of the cube specimens was tested in a multifunctional structural test system. The results are listed in Table 4 and each data in the results is an average value of three identical specimens.

2) Experimental setup

The accelerated carbonation experiments were executed by following the Chinese standard GB/T 50082-2009 (2010) [⁴²]. The prism specimens mentioned above were used to conduct the accelerated carbonation experiment. Three RAC specimens of each concrete type, totally 15 concrete specimens, were used to carry out the accelerated carbonation experiment. All the concrete specimens were cured for 28 days under the standard condition and then were dried in oven under 60°C for 48 h. Before the carbonation of each specimen, the two surfaces to bear pressure were chosen as the carbonation surfaces, and the other surfaces were sealed by wax, shown in Fig. 6. Finally, the wax-sealed specimens were placed in carbonation test box. The conditions for accelerated carbonation test are as follows: temperature of 20 °C±2 °C, humidity of 60%±5% and CO₂ concentration of 20%±2%. The carbonation test box and the specimens to be carbonated are illustrated in Fig. 7.

At the carbonation age of 7, 14, 28 days, the specimens were removed from the carbonation test box. By using concrete press machine, 50 mm thickness of the specimens was split for each time. After using wax to seal the broken surface, the specimens retained were put back into the carbonation test box to be carbonated again. The removed part was used to measure carbonation depth by using phenolphthalein indicator [⁴³]. The phenolphthalein indicator used in the experiment is a phenolphthalein 1% ethanol solution. An example for the specimen identified by the phenolphthalein indicator is shown in Fig. 8.

Results on the carbonation

The measured values and predicted values of carbonation depth with different carbonation ages are shown in Table 5. The predicted values of carbonation depth were calculated by Eq. (3). By analyzing the measured values and predicted values of carbonation depth, some conclusions can be drawn as follows.

1) Both of the predicted values and the measured values reflect the same trend, i.e., the carbonation depth increases with the increase of carbonation age. For each carbonation age, the carbonation depth of RAC is larger than that of NAC. With the increase of the RCA replacement percentage, the carbonation depth of RAC will increase gradually.

2) Corresponding to the carbonation time of 7, 14, 28 days, the average ratio of the measured depth to the predicted value is 1.32, 1.28, 0.99, respectively. The predicted values are slightly lower than the measured values when the carbonation age is shorter, i.e., 7 and 14 days. When the carbonation age is 28 days, the two values are close to each other. It indicates that the prediction model of carbonation depth is more reliable for long carbonation time.

3) By comparing the predicted values with the measured values, the results show that the absolute value of the deviation is small and falls into an interval from 1.2 to 2.9 mm. Considering the discreteness of the experiment, it can be considered that the RAC carbonation depth prediction model formulated as Eq. (3) is applicable to the prediction of RAC with the same strength.

Specific surface area experiment of RCAs

Experiment objectives

The specific surface area of RCAs is another key parameter for calculating the CO₂ uptake of RCAs, which directly affects the accuracy of the calculation results. Based on the average particle diameter theory, the theoretical prediction for the specific surface area of RCAs is established, but its accuracy needs to be verified by experiment.

At present, there is no definite conclusion on how to measure the surface area of aggregates. Zhou [⁴⁴] obtained the surface area of NCAs by measuring the average thickness of wax covered on the surface and the amount of wax carried off by NCAs. Although the method can be used to determine the surface of RCAs, scraping the wax thickness artificially results in poor accuracy. Ji et al. [⁴⁵] designed an experiment to measure the surface area of NCAs by analogy theory, but the experimental method is not applicable to RCA due to the strong water absorption of RCA. In this study, a new experimental method for measuring the surface area of RCAs was proposed based on the above two methods. The experimental method avoid the influence of water absorption of RCA and it can also effectively reduce accidental error.

Experiment design

1) Materials

The density of the wax is 0.9 g/cm³ and its melting point is about 50°C. RCAs used in the test are from the same batch with RCAs used in the carbonation experiment with the size of 5–25 mm. The particle gradation of RCAs is shown in Fig. 9. Cube concrete in the size of 100 mm × 100 mm × 100 mm was prepared under 28 days’ standard curing. The surface area of the concrete block can be obtained by easy calculation, i.e., 0.06 m², which is used to measure the surface of RCAs through analogy analysis.

2) Experimental setup

The test steps for determining the specific surface area of RCAs are as follows.

a) Mix cement slurry with cement-water ratio of 1:2.

b) By using steel network with holes, 5 kg RCAs and the concrete block were put into the cement slurry. After stirring for 10 min, the surfaces of the RCAs and the concrete block were covered with a layer of cement slurry. Since the cement slurry was attached, the RCAs and concrete block got the same bond ability of the surface. Then the RCAs and concrete block were dried until the weight kept a constant. The mass of RCAs was g₁ and the mass of concrete block was g_2.

c) Heat wax to be melted and kept it at a constant temperature. By using a steel network with holes, the 5 kg RCAs and the concrete block were put into the melting wax to make them both covered with a layer of wax evenly. Then the RCAs and the concrete block were taken out from the melting wax to cool down. Now the mass of RCAs was {g^{\prime }}_{\mathrm{1}} and the mass of concrete block was {g^{\prime }}_{\mathrm{2}} finally.

d) The surface area of concrete block can be obtained by calculation, i.e., 0.06 m². Due to the same surface bond ability of the RCAs and concrete block, the amount of wax attached to the surfaces can reflect the difference in surface area between the RCAs and concrete block. According to the mass change of the RCAs and the concrete block, the surface area of RCAs A_g can be calculated as

(19)A_{\mathrm{g}}\mathrm{=0}\mathrm{.06}{\displaystyle \frac{{g^{\prime }}_{\mathrm{1}}-g_{\mathrm{1}}}{{g^{\prime }}_{\mathrm{2}}-g_{\mathrm{2}}}}.

e) The specific surface area of RCAs can be formulated as \eta =A_{\mathrm{g}}/\mathrm{5}. And the average value of three tests was considered as the specific surface area of RCAs.

Results on the specific surface area

The results of the specific surface area experiment for RCAs are shown in Table 6. According to the results of three tests, the average measured value of the specific surface area of the RCAs was 0.1089 m²/kg and the coefficient of variation was 7.88%. Based on the average diameter theory, the predicted specific surface area of the RCAs is 0.1396 m²/kg.

The measured-to-predicted value ratio is 0.78. It indicates that there is some deviation contrasting the measured value to the predicted value. The deviation may be derived from the simplification of the RCA model, i.e., ignoring the needle shape of the aggregate. Although the predicted value is partially deviated from the measured value, they follow the same trend. Taking into account the complexity of the experimental method, the use of the average diameter theory to predict the specific surface area of RCAs is significant for applying the CO₂ uptake model of RAC.

Case study

To analyze the CO₂ uptake amount of RAC, a case study of C30 RAC was conducted using the CO₂ uptake model of RAC. A life cycle of 50 years was adopted for calculating the CO₂ uptake by the system boundary from the cradle to grave, including approximately 50 years’ service life of RAC and 30 days’ storage time of RCAs.

Basic information

To calculate the CO₂ uptake of RAC, some basic information needs to be determined. According to the CO₂ uptake model, a certain mix proportion of RCAs needs to be proposed first. For simplifying the calculation, the mix proportion of NHCP in RAC and the OHCP in RCAs were assumed to be the same, which is listed in Table 2. The basic physical properties of RCAs are listed in the Table 3 and the particle gradation is shown in Fig. 9. With five RCA replacement percentages, i.e., 0%, 30%, 50%, 70%, and 100%, it was convenient to analyze the influence from RCA. The NHCP and OHCP were assumed to be cured for 28 days under the standard condition and their densities were both specified as 2000 kg/m³. On the other side, by referring to the situations of Shanghai, some environmental parameters were determined. The relative humidity is 76% and CO₂ concentration is 0.034%. As for the surface area of 1 m³ RAC, the average value of a building, i.e., 5.68 m² was proposed according to the Lee et al. [¹⁷].

Prediction results

According to the CO₂ uptake model, the predictions of CO₂ uptake for C30 RAC are shown in Table 7. In the table, CAa is the CO₂ uptake amount of NHCP in RAC; CAb₁ is the CO₂ uptake amount of OHCP in the carbonated zone and CAb₂ is the CO₂ uptake amount of OHCP i in the non-carbonated zone. CA is the sum of CAa, CAb₁, CAb₂, which reflects the total CO₂ uptake amount of C30 RAC in the life cycle.

The CO₂ uptake of RAC with different RCA replacement percentages is illustrated in Fig. 10. It is indicated that the CO₂ uptake of 1 m³ C30 RAC within 50 years’ life cycle increases with the increase of the RCA replacement percentage. The CO₂ uptake amount is 10.6, 13.8, 17.2, 22.4 kg when the RCA replacement percentage is 30%, 50%, 70%, 100%, respectively. About 0.75 tonnes of CO₂ will be emitted for producing 1 ton cement [⁴⁶]. Considering the data, the CO₂ uptake by C30 RAC with different RCA replacement percentages is equivalent to 3.84%–7.72% of the CO₂ emission in the cement production process. Hence it indicates that the effect of RCA on the CO₂ uptake is remarkable. On the other hand, compared with NAC, the CO₂ uptake of RAC is higher. When the RCA replacement percentage is 30%, 50%, 70%, and 100%, the CO₂ uptake of RAC is 1.71, 2.23, 2.77, 3.61 times that of NCA, respectively.

Considering the composition of the CO₂ uptake by RAC carbonation, when only calculating the CO₂ uptake of NHCP in RAC without considering the CO₂ uptake of RCAs, the CO₂ uptake amount will be much smaller. According to the results in this study, the account of RCAs CO₂ uptake is very considerable. When the RCA replacement percentage is 30%, 50%, 70%, and 100%, the CO₂ absorbed by RCAs is 3.8, 6.6, 9.6, and 14.4 kg, respectively, and the value is 0.56, 0.92, 1.26, and 1.80 times the CO₂ uptake by the NHCP in RAC, occupying 35.8%–64.3% of the total CO₂ uptake amount by RAC.

Obviously, due to fully carbonation of the OHCP, a single RCA in the carbonated zone will absorb more CO₂ than that in the non-carbonated zone. However, considering the total CO₂ uptake amount by RCAs, the contribution of RCAs in the non-carbonated zone is much greater than that in the carbonated zone, accounting for 81.9% of the total CO₂ uptake amount by RCAs. The reason is that most RCAs are distributed in the non-carbonated zone and the carbonated zone is relatively small. Considering that the CO₂ uptake process of RCA in non-carbonated zone occurs during the storage time of RCA mainly, the storage time of RCAs before using greatly influences the CO₂ uptake of RCAs.

Influence of storage time of RCAs

Kikuchi and Kuroda [¹⁶] mentioned the storage time of RCAs is normally about 30 to 90 days. Therefore, the CO₂ uptake of RCAs with longer storage time needs to be further analyzed. The RAC with 100% RCA replacement percentage is used as an example to conduct the analysis. The relationship between the storage time of RCAs and the CO₂ uptake of RAC with 100% RCA replacement is illustrated in Fig. 11. It can be found that, with the increase in storage time, both the total CO₂ uptake of RCAs and RAC increase gradually. When the storage time of RCAs is 90 days, the CO₂ uptake of RCAs is 23.1 kg occupying 74.3% of the total CO₂ uptake of RAC. In this case, the CO₂ uptake of RCAs is the main part of the CO₂ uptake of RAC.

Especially, it is important to note there is an upper limit on the CO₂ uptake of RCA. When the storage time of RCAs is 90 days, the carbonation depth of the OHCP is about 0.8 mm, which is less than the maximum thickness of OHCP. If the storage time extending to more than the critical exposure time of 300 days, the OHCP in RCAs can be fully carbonated and the CO₂ uptake of RCAs will reach the maximum with no increase anymore. The maximum CO₂ uptake of RCAs is about 39.3 kg. Under the storage time for 30 to 90 days, the CO₂ uptake of RCAs during storage time can reach about 30.2%–55.2% of the maximum amount.

Conclusions

This study has proposed a quantitative model to evaluate the CO₂ uptake for RAC, with particular attention to the effects of both NHCP in RAC and OHCP in RCAs. Based on the experiments on the RAC carbonation and the specific surface area of RCAs, the accuracy of the model was discussed. According to the CO₂ uptake model for RAC and the case study, the following conclusions can be drawn.

1) There are two main contributors of CO₂ uptake for RAC: the CO₂ uptake of NHCP in RAC and the CO₂ uptake of OHCP in RCAs. For the CO₂ uptake process of NHCP, it mainly occurs after the new concrete cast, while the CO₂ uptake process of OHCP in RCA starts from the storage time and continues entire life cycle of RAC.

2) Compared with the CO₂ uptake of the NHCP in RAC, the amount of CO₂ uptake of OHCP in RCAs is remarkable. The CO₂ uptake of RCAs is significantly affected by the storage time of RCAs. When the storage time is 30 days, the CO₂ uptake of RCAs is about 35.8% – 64.3% of the total CO₂ uptake amount by RAC for different RCA replacement percentages according to the case study.

3) The amount of CO₂ uptake for RAC increases with the increase of the RCA replacement percentage and the CO₂ uptake amount of RAC is more than that of NAC. According to the case study, the CO₂ uptake amount of 1 m³ C30 RAC within 50 years’ life cycle is 10.6, 13.8, 17.2, 22.4 kg when the RCA replacement percentage is 30%, 50%, 70%, and 100%, respectively, which is equivalent to 3.84%–7.72% of the CO₂ emission in the cement production process.

4) There is an upper limit for the CO₂ uptake amount of the RCAs considering that the OHCP in RCAs is limited. If the storage time exceeds the critical exposure time, the OHCP in RCAs would be fully carbonated and thus the CO₂ uptake of RCAs reaches the maximum value.

5) The interfacial area between the carbonated and non-carbonated parts were not well considered, it could be taken into account in the future work to obtain more accurate results for further investigation and validation.

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