ZHU Pinghua, LI Haichao, LIU Hui, YAN Xiancui, WANG Xinjie, CHEN Chunhong

(Department of Civil Engineering, Changzhou University, Changzhou 213164, China)

Abstract: The effects of carbon dioxide (CO2) curing conditions (temperature, relative humidity and CO2 curing time) on the physical properties of recycled coarse aggregate (RCA) with varying attached mortar (AM)contents were studied. Before and after CO2 curing, the physical properties in terms of the apparent density,water absorption and crushing value of RCA were tested and the quality of RCA was determined. Besides,scanning electron microscope was used to observe the microstructure of RCA. Results show that the physical properties variation of RCA with higher AM content are more significant, and the quality of RCA with lower AM content is easier to be upgraded during CO2 curing. The physical properties of RCA with 40.8% AM content are earlier stable than that with no less than 44.5% AM content during CO2 curing. The optimal temperature and relative humidity are 50 ℃ and 55% for CO2 curing, respectively. CO2 curing is incapable of upgrading the quality of RCA with AM no less than 50.6%. The quality of RCA with 44.5% AM content can be upgraded only under the optimum CO2 curing conditions. Under relative humidity higher than 40% and the CO2 curing time more than 12 h, CO2 curing upgrades the quality of RCA with 40.8% AM content.

Key words: recycled coarse aggregate; attached mortar content; carbon dioxide curing; curing time;relative humidity; temperature

With the rapid development of urbanization and infrastructure construction, a growing number of natural resources are consumed and then in shortage[1].Meanwhile, an increasing number of construction waste generates, which occupies land space and causes great damage to environment[2]. To deal with the problem of natural resource shortage and disposal of construction waste, recycled coarse aggregate (RCA) from crushing waste concrete is widely used in structural concrete, as a substitute for nature coarse aggregate[3]. It is of great economic and ecological significance to reuse the waste concrete as RCA.

However, when RCA was used to prepare recycled aggregate concrete (RAC), the mechanical properties and durability tended to be worse than that of natural aggregate concrete. Xiaoet al[4]found that the elastic modulus of RAC prepared by 100% RCA decreased by more than 30%. Kouet al[5]found that the incorporation of RCA reduced the resistance to chloride ion permeability of recycled concrete, significantly inferior to ordinary concrete. The higher water absorption of RCA resulted in the poor freezing resistance of RAC[6]. The worse mechanical and durability properties is owing to the old cement mortar attached to RCA.Due to the presence of attached mortar (AM), RCA has lower density and higher porosity as well as crushing value, compared with natural coarse aggregate[7]. More AM content leads to the decrease of RCA quality and the deterioration of RAC performance[8]. In order to enhance the utilization rate of RCA, it is of significance to enhance the properties of RCA to ensure them comparable to those of natural coarse aggregate.

To date, methods designed and applied for RCA modification are mainly divided into two routes: removing and strengthening[9]. The method of removal is to remove AM on the surface of RCA, mainly including ball milling[10], soaking[11]and heating[12].These treatment methods have a time-consuming and complex process. The methods for strengthening AM mainly include the treatments of polymer[13], biologicalprecipitation of carbonate[14]and multi-step mixing[15].Methods for strengthening are not only costly, but also result in environmental pollution[9]. It is necessary to develop a time-saving, environmentally friendly and economical method to improve the properties of RCA.

Table 1 Performance improvement in RCA after CO2 curing

In recent years, carbon dioxide (CO2) curing technology is widely used in the enhancement of RCA. The mechanism of CO2curing is that CO2reacts with calcium hydroxide (Ca(OH)2) and calcium silicate hydrate in AM to generate calcium carbonate (CaCO3) and silica gel[16]. The solid volume of CaCO3is approximately 12% higher than that of Ca(OH)2[17], resulting in a higher hardness of carbonated RCA (CRCA).Furthermore, the CO2curing products, mainly CaCO3,strengthen the interface transition zone (ITZ) in RCA and fill pores of AM[18]. CO2curing treatment can absorb CO2gas, which is an environmentally friendly modification method[19]. Table 1 lists the performance improvement in RCA after CO2curing from previous studies. Although CRCA obtained a higher apparent density and a lower crushing value and water absorption than untreated RCA, there was difference in experimental results of physical properties in varying studies, related to the CO2curing conditions applied and the amount of carbonatable compounds. Carbonatable compounds were mainly hydration products of cementitious material, mainly Ca(OH)2and calcium silicate hydrate. Therefore, it is necessary to investigate the influence of CO2curing on the properties of RCA with varying AM contents.

In this study, the effect of CO2curing factors(temperature, relative humidity and CO2curing time)on the properties of RCA with different AM contents was investigated. The properties (apparent density,water absorption and crushing value) of CRCA were tested. The grades of RCA before and after CO2curing were determined and compared. Besides, scanning electron microscope (SEM) was used to observe the microstructure of RCA before and after CO2curing.

2.1 Materials

RCA (5-20 mm) with different attached mortar contents were provided by the Jiangsu Lvhe Environmental Technology Co. Ltd. The gradation curves of RCA are shown in Fig.1. According to Chinese standard GB/T 25177-2010[26], all used RCA in this experiment meet the requirements of continuous gradation.The physical properties of RCA are listed in Table 2.According to the results in Table 2, the increase of AM content results in a decrease in apparent density and an increase in water absorption as well as crushing value.Consequently, the quality of RCA decreases with the increase of AM content.

Fig.1 Gradation curves of RCA

Table 2 Physical properties of RCA

2.2 Test methods

2.2.1 AM content

The AM content of RCA in this investigation was determined by thermal treatment[27]. First, dry RCA(initial mass,m1) was immersed in water for 2 h and then heated at 500 ℃ for 2 h. After that, RCA was immersed in water at room temperature. The remaining coarse aggregate was dried at 100 ℃ to a constant mass(final mass,m2). AM content of RCA specimen was calculated, as shown in Eq. (1):

2.2.2 CO2curing

RCA were placed in the CO2curing chamber.The specific CO2curing conditions (CO2curing time,relative humidity, temperature and CO2concentration)were set and shown in Table 3. According to the results from previous research[28], CO2concentration more than 20% has little effect on the performances of RCA during CO2curing process. Thus, CO2concentration in this investigation was controlled at 20% for CO2curing.

Table 3 CO2 curing condition

2.2.3 Measurements

According to the Chinese standard GB/T 25177-2010[26], the physical properties (apparent density,water absorption and crushing value) of CRCA were measured.

SEM (SUPRA55, Zeiss) was performed on the slices extracted from samples to examine the effect of CO2curing on the microstructure of RCA.

3.1 Effect of CO2 curing time on physical properties of RCA

Fig.2 shows the physical properties of CRCA after various time of CO2curing. It can be seen that the apparent density of CRCA increases and the water absorption and crushing value decreases as the CO2curing time increases up to 36 h. When CO2curing time further increases, the physical properties of CRCA change little. Specially, for the RCA4 with the lowest AM content (40.8%), its physical properties are stable in 24 h. The large porosity of AM provides a convenient access for CO2diffusion, and accelerates the CO2curing reaction between CO2and calcium ions. It results in the generation of CO2curing products CaCO3,filling cracks and pores in AM and narrowing the ITZ between RCA and its AM[29]. However, with the increase of CO2curing time, the porosity and pore size of RCA decrease, which hinders the further entrance of CO2. In addition, the calcium ion content in AM gradually decreases in the process of CO2curing, which also reduces the CO2curing reaction rate. Therefore,the variation of physical properties of CRCA gradually slow down and tend to be stable. For the RCA with AM content no less than 44.5% (RCA2, RCA3 and RCA4),36 h of CO2curing are suggested, while for the RCA with 40.8% AM content, 24 h of the CO2curing is appropriate.

Besides, it can be found in Fig.2 that as AM content of RCA increases progressively, the apparent density of RCA1, RCA2, RCA3 and RCA4 increases by 0.60%, 1.47%, 1.57% and 1.80% during CO2curing,respectively. The water absorption of RCA1, RCA2,RCA3 and RCA4 decreases by 8.8%, 10.0%, 11.4%and 12.8%, respectively; the crushing value reduces by 5.41%, 5.96%, 6.54% and 6.78%, respectively. For higher AM content of RCA, the improvement of physical properties is more significant during CO2curing,implying a higher CO2curing potential. On the one hand, high content of AM provides sufficient calcium ions for CO2curing. On the other hand, high AM content implies the smaller particle size of aggregate wrapped by mortar and contact area between alkaline cement hydration products in AM and CO2, which results in a higher CO2curing rate of RCA[30].

Fig.2 Physical properties of CRCA after different time: (a) Apparent density;(b) Water absorption;(c) Crushing value

3.2 Effect of relative humidity on the physical properties of RCA

The physical properties of CRCA under various relative humidity are illustrated in Fig.3. It can be observed that the apparent density of CRCA increases first and then decreases with the increase of relative humidity, while the water absorption and crushing value decrease first and then increase as the relative humidity from 40% to 70%. The optimal relative humidity in the CO2curing chamber is 55%. Too high or too low relative humidity has a negative effect on the physical properties of CRCA during CO2curing. Under the low relative humidity environment, the low CO2solubility is not conducive to improving the performance of RCA[31]. Increasing moisture can promote the dissolution of CO2and accelerate the CO2curing reaction rate, which improves the physical properties of RCA. However,too high relative humidity can capture and absorb CO2,which prevents CO2from penetrating into RCA and thus yields a slow rate of CO2curing reaction[30]. It is better to control the relative humidity at an appropriate value (55%) for CO2curing to achieve a better performance of CRCA.

Fig.3 Physical properties of CRCA under different relative humidity: (a) Apparent density;(b) Water absorption;(c) Crushing value

In Fig.3, compared with CO2curing under 40%relative humidity, the apparent density of RCA1,RCA2, RCA3 and RCA4 increases by 0.43%, 0.47%,0.56%, and 0.74% under 55% relative humidity, respectively. Compared with CO2curing under 40% relative humidity, the water absorption of RCA1, RCA2,RCA3 and RCA4 decreases by 4.49%, 6.81%, 7.74%,and 9.12% under 55% relative humidity, respectively;the crushing value of RCA1, RCA2, RCA3 and RCA4 decreases by 0.71%, 1.39%, 2.05%, and 2.37% under 55% relative humidity, respectively. For the RCA4 with the maximum AM, its physical properties improvement is the highest as relative humidity increases from 40%to 55% during CO2curing. This may be because higher AM content contains more calcium ions, and provides a larger contact surface for carbonation reaction between CO2gas and carbonatable compounds. In addition,RCA with less AM content has lower porosity, which may inhibit the permeability of CO2, thereby reducing the CO2curing degree.

3.3 Effect of CO2 curing temperature on the physical properties of RCA

Fig.4 Physical properties of CRCA at different temperature of CO2 curing:(a) Apparent density;(b) Water absorption;(c) Crushing value

Fig.4 displays the physical properties of CRCA at different CO2curing temperature. It can be seen that the apparent density of CRCA increases first and then decreases with the increase of temperature, while the water absorption and crushing value decrease and later increase. The optimal CO2curing temperature is 50 ℃to acquire a better performance of CRCA. Elevated temperature is beneficial to the diffusion and permeation of CO2and the precipitation of calcium ions from silicate[32]. Additionally, the hydration of cementitious materials in AM could also be accelerated to enhance RCA properties with a higher temperature. However,with the temperature further increases from 50 to 70℃,higher temperature decreases the solubility of CO2,causing the increase of pH value of pore solution in RCA and the decrease of dissolution rate of calcium ions[33].

In Fig.4, compared with 20 ℃ of CO2curing, the apparent density of RCA1, RCA2, RCA3 and RCA4 increases by 0.43%, 0.47%, 0.51%, and 0.52% with 50℃ of CO2curing temperature, respectively. Compared with 20 ℃ of CO2curing temperature, the water absorption of RCA1, RCA2, RCA3 and RCA4 decreases by 1.71%, 2.92%, 3.07%, and 3.47% with 50 ℃ of CO2curing, respectively; the crushing value of RCA1,RCA2, RCA3 and RCA4 decreases by 0.71%, 1.41%,2.10%, and 2.42% with 50 ℃ of CO2curing respectively. Relative high AM content tends to a remarkable improvement in properties of RCA with an appropriate increase in CO2curing temperature, which may also be due to the existence of more calcium ions.

3.4 Grades of RCA before and after CO2 curing

Fig.5 SEM images in the ITZ of CRCA after different CO2 curing time: (a) 0 h; (b) 12 h; (c) 48 h

Based on the requirements of apparent density,water absorption and crushing value in Chinese standard GB/T 25177-2010[26], all untreated RCA before CO2curing can only meet the requirements of gradeⅢ aggregate. As suggested previously, the optimum relative humidity and CO2curing temperature are 55%and 50 ℃ for the CO2curing of RCA, respectively. For the RCA with AM content no less than 50.6% (RCA3 and RCA4), CO2curing is incapable of upgrading the quality of RCA, although under the optimum CO2curing conditions. For RCA2 with 44.5% AM content,its quality can only be upgraded under the optimum CO2curing conditions (55% relative humidity and 50℃ CO2curing temperature). To upgrade the quality of RCA1 with 40.8% AM content, it is suggested to ensure that the relative humidity be larger than 40% and the CO2curing time be no less than 12 h. In addition,it should be noted that the quality of RCA1 with lower AM content is easier to be upgraded, although the physical properties improvement of RCA with higher AM content is more significant, as studied before.

3.5 Microscopic morphology

The microstructure variation of RCA sample after CO2curing process is the fundamental reason for the improvement of RCA properties[18]. SEM images of RCA before and after CO2curing are shown in Figs.5-7. The part outlined by the yellow dotted line is the ITZ. CaCO3are circled in white on the SEM images.For the RCA without CO2curing treatment, large micro-pores and numerous micro-cracks exist and acicular ettringite crystals appear in the ITZ area; the AM is loose porous and irregular. Because of high porosity,ITZ between coarse aggregate and AM may provide a path for crack propagation, considered to be weak[34]. Thus, RCA has higher water absorption and lower apparent density than natural coarse aggregate. Wide and loose ITZ reduces the bond between aggregate and mortar, resulting in higher crushing value of RCA.

In Fig.5, it is observed that the ITZ of CRCA,filled with CO2curing products, is narrower than that of RCA, indicating that CO2curing can improve the ITZ of RCA. Apparently, the number of pore and crack in CRCA is less than that in RCA. Moreover, numerous CaCO3crystals are observed in CRCA. AM in CRCA under 12 h of CO2curing is denser than that under 48 h of CO2curing from the result of Fig.5. It is evident that the number of CaCO3crystals produced by CO2curing for 48 h is more than that produced by CO2curing for 12 h. With the increase of CO2curing time, CO2enters into ITZ and reacts with the hydration products in RCA to produce CaCO3, which makes the microstructure of ITZ area denser.

Fig.6 SEM images in the ITZ of CRCA under varying relative humidity: (a) 40%; (b) 55%; (c) 70%

Fig.7 SEM images in the ITZ of CRCA at various temperature: (a) 20 ℃; (b) 50 ℃; (c) 70 ℃

As relative humidity in CO2curing chamber increases from 40% to 55%, ITZ in CRCA becomes denser and the quantity of CaCO3obviously increases,while the length of micro-cracks in AM reduces (Fig.6).When relative humidity further increases to 70%, the number of micro-cracks in CRCA increases. In addition, ITZ of CRCA under relative humidity of 70% is loose, compared with that under relative humidity of 55%. This result provides microscopic evidence for optimal relative humidity from the test results of CRCA performances in Fig.3. As can be seen from Fig.7, AM gradually becomes compacting when temperature increases from 20 to 50 ℃, fewer pores and cracks are observed in CRCA at 50 ℃. However, micro-pores and micro-cracks clearly appear in CRCA carbonated at 70 ℃. This explains from microscopic point that the performances of CRCA carbonated at 70 ℃ are worse than that at 50 ℃.

a) The physical properties of RCA with AM content no less than 44.5% were stable when the CO2curing time reached 36 h, while for the RCA4 with lower AM content (40.8%), the physical properties can be stable in 24 h. The optimal temperature and relative humidity were 50 ℃ and 55% for CO2curing, respectively. The physical properties variation of RCA with higher AM content were more significant after CO2curing.

b) For the RCA with AM no less than 50.6%, CO2curing was unable to upgrade the quality of RCA. The quality of RCA with 44.5% AM content can only be upgraded under the optimum temperature and relative humidity of CO2curing. To upgrade the quality of RCA with 40.8% AM content, it is suggested to ensure the relative humidity be larger than 40% and the CO2curing time be larger than 12 h. The quality of RCA with lower AM content was easier to be upgraded during CO2curing.

c) For the RCA without CO2curing, large micro-pores and numerous micro-cracks existed and the AM was loose porous and irregular. The ITZ in CRCA was narrower than that in RCA. More compact ITZ and denser AM were observed in CRCA under time of 48 h,temperature of 50 ℃ and relative humidity of 55%.