Performance of eco-friendly concrete: a safe direction to sustainable cities development

Constructing sustainable cities for the future usually encounters some challenges such as reducing the environmental footprint through using eco-friendly materials. Coarse recycled aggregate retrieved from demolished concrete structures is considered one of the eco-friendly building materials. This study aims to investigate the impact of strengthened recycled aggregate with pozzolan slurry on the different properties of concrete. Three various groups of pozzolan slurries; silica fume with fly ash, cement with fly ash, and nano-silica are used to strengthen the inferior properties of recycled aggregate. Findings showed that the proposed treatment method efficiently improved the quality of recycled aggregate. Also, this method achieved eco-friendly concrete with preferable mechanical behavior and greater resistivity against chloride diffusion. The wastage of compressive strength was 10–20%, flexural strength was 5–16%, and the elastic modulus was 13–30% of the recycled aggregate concrete in comparison with normal concrete at 28 days. By considering the application of recycled aggregate in the manufacture of new concrete, this study's results can serve as principles for achieving sustainable concrete infrastructure in the smart cities of the future.


Introduction
Nowadays, the fast-track development in urban areas results in massive-scale new construction, particularly in some countries with emerging economies.Such construction works demand vast amounts of exhaustion and production of natural aggregate (NA) which leads to the exhaustion of natural resources and difficulty in achieving sustainable evolution for smart cities [1,2].Incorporating recycled aggregate (RA) in concrete basically decreases a large amount of construction and demolition wastes and the need for NA in a sustainable way [3].The different properties of RA have been studied in detail worldwide, and the results indicate that integrating RA adversely affects the properties of concrete [4][5][6].Based on the undesirable influence of RA in concrete, several enhancement techniques were applied to improve its quality.In general, eliminating and improving the old mortar are the two effective approaches to enhance the RA properties [7].The first technique of eliminating old mortar from RA includes mechanical treatment [8], pre-soaking in some acid solutions [9][10][11], microwave and thermal treatment [12].The second technique for improving the old mortar on RA involves polymer treatment [13,14], sodium silicate solution [15,16], and carbonation treatment [17,18].However, such enhancement approaches of RA usually have a high energy depletion, cost, and time [7].Thus, a further eco-friendly enhancement approach is urgently needed to refine the RA quality.
Integrating pozzolanic materials to refine the RA quality has recently become popular [19].Bui et al. [20] suggested a reinforced method via soaking RA in sodium silicate solution, then covering it thru silica fume to optimize the mechanical features of concrete.Through applying this method, the RA efficiency was greatly optimized and the mechanical properties of concrete were improved as well.Wang et al. [21] used different types of pozzolanic materials (e.g., fly ash, metakaolin, silica fume, and slag) as cement replacement materials to produce sustainable concrete.Results revealed that using such materials significantly improved the mechanical properties of concrete.Based on the aforementioned studies, it is vital to investigate the slurry immersion technology via merging two treated materials to refine the RA quality and produce eco-friendly concrete.In addition, creating some relationships among the different mechanical characteristics and durability of the resulting concrete still requires more investigation.
This study suggests an eco-friendly technology to reinforce the RA quality and enhance concrete performance.Immersion of RA in various slurries of pozzolan, i.e., silica fume with fly ash, cement with fly ash, and nano-silica is the adopted approach to strengthen its quality.This approach is suitable to produce sustainable concrete comprising 100% of RA with the same procedures as the conventional style.The properties of RA before and after treatment are investigated and their impacts on the resulting concrete are detected.Based on the experimental findings, relationships among different characteristics of concrete are established as well as the cost and the environmental side are analyzed for the different concrete samples.The organization of this research is in the following manner: In Section 2, the used materials, and the proposed treatment technique are briefly presented.Section 3 displays the different properties of RA before and after treatment as well the different properties of concrete.Section 4 presents the discussion for highlighting the aforementioned prospects.The conclusion of this study is displayed in Section 5.

Materials
In this study, the RA is retrieved from the destruction of concrete structures and provided by the Zhongtian Group, Zhejiang province.The RA consists of RA with adhered mortar, crushed brick, adhered mortar, pebbles, and other materials, as shown in Fig. 1.Two kinds of coarse RA are utilized; untreated and treated RA.The suggested technique that is clarified in the subsequent section is applied to produce treated RA.For comparison, crushed granite (NA) is utilized for the preparation of controlled specimens.Fine aggregate (sand) is used in this study.The grain size distribution for the fine aggregate, NA, and RA are displayed in Fig. 2 according to BS 812-103.1.Table 1 displays the chemical compositions of cement and the used pozzolanic materials.

Proposed treatment technique for RA
Three various pozzolan slurries: silica fume with fly ash (SF&FA), cement with fly ash (C&FA), and nano-silica (NS) are utilized to treat and reinforce the RA surface.For SF&FA and C&FA slurries, the materials are evenly employed with an amount ratio of 40% of the full weight of RA.Whereas NS is utilized with a 3% substitution ratio of C&FA slurry to fulfill the economic aspect.The binder to water ratio is fixed at 1:5 for the three slurries.The dosage proportions of the slurries are the results of the author's preceding research work [22].To prepare the slurries, the water (twice the weight of RA) is mixed with the pozzolanic materials for 2-3 min.Then, RA is immersed in the slurries for 4 h.After that, RA is taken out and dried for 3 days at room temperature.Finally, the extremely loose materials are eliminated from the hardened RA by the sieving procedure.The treated RA by the suggested method is displayed in Fig. 3. Table 2 presents the material proportions and mix design for all the concrete mixtures.All the mixtures are mixed according to BS1881-125.The properties of treated and untreated RA are measured in accordance with BS 812 part 2. The compressive, flexural strengths and modulus of elasticity are specified in accordance with BS 1881-116, BS 1881-118, and ASTM C469, respectively.The chloride diffusion test is performed based on NT BUILD 492.and the oven-dried density of untreated RA reduced by 10.31%, 20.42%, and 26.74%, respectively, compared with NA.Furthermore, the water absorption and porosity percentages of RA are found to be 8.98% and 18.95%, respectively.These values are much higher than that of NA, of which water absorption and porosity have only about 0.5% and 0.92%, respectively.This is attributed to the weak adhered mortar that is a vital component of RA has very low particle densities and high water absorption and the presence of crushed brick that distinguishes it with porous structure.After treating the RA, the quite fine particles of the pozzolanic materials efficiently cover the pores and thus excess the different densities of RA.Treated RA by SF&FA, C&FA, and NS slurries increased the different densities by 4%, 8%, and 6%, successively, and decreased the water absorption by 24%, 27%, and 31%, successively, in comparison with untreated RA.

Compressive strength (f c )
Figure 4 displays the f c developments at 3, 7, and 28 days of the different specimens.It could be observed that the f c of untreated RAC is inferior to NAC.The RAC compressive strength is 40% smaller than that of NAC at 28 days.The presence of weak mortar in RA and the absorbent structure of the crushed bricks adversely influences the strength characteristics of concrete.In the treated RAC, the weak mortar was reinforced and the aggregate surface was optimized.The interaction among treated RA and new cement mortar at the ITZ becomes reinforced via a pozzolanic reaction [23].Thus, the f c of treated RAC thru the proposed technique was enhanced by 25-35% in comparison with uncured RAC at 28 days, based on the slurry kind.
The strength development was higher in reinforced RAC with SF&FA and NS slurries than in RAC cured by C&FA slurry.The presence of pozzolanic materials in the treated RA react with CH the major component of adhered mortar and establish extra C-S-H gel that enhances the adhesion amongst the RA and the cement mortar [24,25].At 28 days of curing age, the compressive strengths of the concrete cured with SF&FA, C&FA, and NS were less than by 10%, 20%, and 15%, respectively, compared with NAC.Results concluded that the integration of SF and FA is more convenient for pozzolan slurry to realize the higher f c of RAC.

Flexural strength (f f )
The f f developments at 3, 7, and 28 days of all concrete mixtures are presented in Fig. 5.The flexural strength of treated RAC is considerably increased by 19-31% compared to the uncured RAC at 28 days.The flexural strength of the treated concrete samples becomes comparable with that of NAC at various ages.The RAC strengthened by SF&FA, C&FA, and NS experienced around 1.4%, 16%, and 9%, respectively, reductions in flexural strength in comparison with NAC at 28 days.
The RAC strengthened by SF&FA slurry achieved maximum development in flexural strength than the others at 28 days.A reason for the greater strength is attributed to the higher pozzolanic nature of SF.This higher reactivity enhanced aggregate matrix bond related to the construction of a fewer porous transition phase and a good interaction among the mortar and the aggregate.Besides, the addition of SF in treating RA acts as reinforcing padding in RA, producing a reduction of pores and voids size in the weak mortar and the crushed bricks.In general, the bond strength among concrete matrix and aggregate highly shows a significant role in flexural strength development [26].This bond is proved when the cured RA was inserted into RAC which leads to improving the flexural strength.

Elastic modulus
The statically elastic modulus of the concrete mixtures is observed in Fig. 6.It should be observed from this figure that all elastic modulus of untreated and treated RAC are less than that of the control concrete because of the reduced stiffness and bulk density of RA.For untreated RAC, the reduction of elastic modulus was 37% of the control concrete.After treating the RA, the elastic  modulus for RAC remarkably increased.The modulus of strengthened RAC increased by 20-30% at 28 days based on the type of slurry.Thus, these experimental results set the significance of strengthening RA with the suggested approach in enhancing the mechanical characteristics of RAC.

Chloride migration coefficient
Figure 7 shows the coefficients of migration for the various concrete specimens at 28 days.The impedance of uncured RAC to chloride migration is lower by 44% compared with NAC.After treating the aggregate, the resistance of chloride diffusion of RAC significantly increased by 13-38% compared with untreated RAC.Reinforced RA by using the pozzolan slurries improved the new ITZ in concrete, which reduces the penetration of chloride.
As can be shown, the diffusion coefficients decreased to a level that is comparable with that of NAC.The surface treatment of RA with SF&FA slurry is efficient in improving the impedance of RAC to chloride diffusion.This is attributed to the pozzolanic materials such as SF enhancing the impedance to the chloride diffusion of RAC due to its chloride bending capability.

Relations between the various characteristics of reinforced RAC
Based on the experimental values, the treating method for RA efficiently improved the durability and mechanical characteristics of RAC.To feasible use of treated RAC in the practices, the effective correlations among the durability and mechanical characteristics of reinforced RAC are established.Figure 8 (a) displays the relation among the f f and the f c of RAC for various curing ages.The f f was adapted to have a good linear correlation with R 2 = 0.88. Figure 8 (b) displays the correlation among the modulus of elasticity E c and f c of RAC at 28 days.For the given compressive strength value, the higher the elastic modulus was, the greater the compressive strength was.The E c was adapted to have a clear linear correlation with R 2 = 0.97 of RAC specimens.The correlation among the compressive strength f c and the diffusion coefficient D C of RAC is established.The R 2 = 0.84 is calculated from the linear regression analysis as presented in Fig. 8 (c).As can be shown, there is a good linear correlation among f c and D C for RAC specimens at 28 days.

Discussion
The use of RA as a substitute for the NA in producing concrete is worth considering the depletion of NA, reducing landfills, saving the environment, and realizing sustainable concrete infrastructure in future smart cities.This study proposed an eco-friendly technique to enhance the RA properties.Strengthening RA with pozzolanic materials via a simple method helps to decrease the cost of treated materials and apparatus besides producing concrete containing 100% coarse RA.To study the economic aspect of the suggested technique, a simple cost estimation of 1 m 3 is performed for the various concrete mixes.Table 4 summarizes the cost for the various concrete components.As can be seen in Table 5, the treated RAC with SF&FA and C&FA slurries are similar to NAC in whence of cost.The strengthened RAC with NS slurry appears the greatest expensive treatment due to the great cost of NS.To analyze the environmental effect of the concrete mixtures, the CO 2 release of each concrete mixture is calculated [27].The CO 2 emission factor (EF) for the various concrete constituents was via literature review (see Table 4) and estimated based on the following equation [27]: where, w refers to the weight of concrete compo- nent (kg), EF refers to the factor of CO 2 emission for each component of concrete (kg-CO 2 /ton); EF mixer refers to the factor of CO 2 emission of concrete mixer (kg-CO 2 /m 3 ).The CO 2 emission for 1 m 3 concrete mixes is displayed in Table 5.As can be seen, the RAC shows the lowest environmental impact.The CO 2 emission for the treated RAC with SF&FA slurry is less than NAC.Whereas the treated RAC with NS and C&FA slurries are similar to NAC.In conclusion, the appropriate utilization of treating materials in strengthening RAC can gain evident practical advantages in the performance of RAC.Therefore, the construction sector can efficiently decrease the costs and the CO 2 emissions as well as help to achieve sustainable cities in the future by using this technique.

Conclusions
According to the experimental findings of the mechanical properties of RAC adapted with reinforced RA by various kinds of pozzolan slurry, the following conclusions could be observed: (1) Accordingly, to NA, the untreated RA has higher water absorption and porosity, and lower particle densities, due to the various attached mortar and the presence of crushed brick and the various undesired materials.
(2) The compressive and flexural strengths of RAC are similar and even exceed the NAC.They all have an affinity to rise with the treated RA by pozzolan slurries that enhance the bonding strength between aggregate and cement mortar.
(3) The modulus of elasticity for RAC has been less than the elastic modulus of NAC normally because of the lower stiffness and bulk density of RA.The chloride ion migration coefficient of treated RAC was higher than that of untreated RAC due to improving the new ITZ in concrete by the pozzolan slurries, which reduces the chloride ion penetration.(4) According to the laboratory tests, formulas for the relationships among the compressive strength and elastic modulus, flexural strength, and chloride diffusion coefficient were suggested for further research and applications.(5) Based on the economic and environmental analysis, the proposed technique can efficiently reduce the costs and the CO 2 emissions as well as enable the use of 100% coarse RA in concrete.

Fig. 1
Fig.1The composition of the used RA

Fig. 8
Fig. 8 Relations between the characteristics of treated RAC

Table 3
displays the different properties of NA, untreated RA, and treated RA with the three different slurries.The apparent density, the saturated-surface-dry density, Grain size distributions for different aggregates

Table 1
Percentages of chemical compositions for the treated materials Fig. 3 Treated RA with different slurries

Table 2
Mixture proportions of the specimens (kg/m 3 )

Table 3
Different properties of the aggregates

Table 4
Unit cost and CO 2 emission data for the various concrete constituents

Table 5
Cost analysis and CO 2 emission