Treatment Methods for the Quality Improvement of Recycled Concrete Aggregate (RCA) – A Review

DING Yahong, WU Jun, XU Ping, ZHANG Xianggang, FAN Yuhui

(Department of Civil Engineering, Henan Polytechnic University, Jiaozuo 454003, China)

Abstract: For the purpose of providing references for further research and practical application about the quality improvement of RCA, in this paper, various treatment methods were firstly classified into four categories: removing old mortar (OM), strengthening OM, multi-stage mixing methods, and combination methods. Thereafter, the improvement mechanisms and important conclusions of various treatment methods were elucidated and summarised respectively. In the section of discussion, the improved effects as well as advantages and disadvantages of various treatment methods were compared and discussed respectively, and recommendations for the selection of treatment methods were proposed. Finally, the further research directions were pointed out, and an integrative programme on the quality improvement of RCA was recommended.

Key words: RCA; treatment methods; removing OM; strengthening OM; interfacial transition zone (ITZ)

1 Introduction

With the continuous development of urbanization and the construction of infrastructures, the consumption of concrete has been increasing constantly, thus rendering the depletion of natural resources due to the overexploitation of natural aggregate (NA). On the other hand, a large amount of construction and demolition waste (C&DW) is generated every year, and most of them are usually piled in the suburbs or landfills,which not only occupies plenty of land, but also causes serious environmental pollution. In order to solve the problem of natural resources depletion and alleviate the environment pressure imposed by the increasing C&DW, so far recycled aggregate concrete (RAC) has attracted considerable attention, which is made out of RCA derived from C&DW through partially or totally replacing NA.

Usually, C&DW contains many impurities, such as steels, plastics, glass, woods, asphalt, ceramics, red bricks, which can be removed manually or by near-infrared sorting technology[1]. After crushing and screening, the RCA with particle diameter more than 4.75 mm is called coarse RCA (CRCA), while the other one with particle diameter less than 4.75 mm is called fine RCA (FRCA). Since FRCA significantly worsen the properties of RAC or recycled aggregate mortar (RAM),the current focus is mostly on the quality improvement of CRCA[2]. In this paper, RCA is considered as CRCA.

Fig.1 The compositional structures of NAC and RAC[5]

Noticeably, the compositional structures of natural aggregate concrete (NAC) and RAC are different. As shown in Fig.1, NAC is a three-phase heterogeneous composite, including NA, new mortar (NM), and ITZ-1 located between NA and NM. By contrast, RAC is a six-phase heterogeneous composite[3,4], including NA,OM, NM, ITZ-1, ITZ-2 located between NA and OM,and ITZ-3 located between OM and NM.

Apparently, RAC has more phases than NAC,such as OM, ITZ-2, and ITZ-3, which are responsible for the degradation of workability, mechanical properties, and durability. Firstly, due to OM, ITZ-2, and micro-cracks introduced in crushing process, RCA is inferior to NA, showing higher porosity and water absorption as well as lower density and crushing value[6-9]. Consequently, with the same mix proportion, the effective water-binder ratio of NM in RAC decreases because of the higher water absorption of RCA, thus making the workability of RAC worse than NAC[10,11].Moreover, owing to the wall effect of the aggregate,RCA with a higher water absorption will release some water to the interface between OM and NM, which causes a higher local water-binder ratio and forms weaker ITZ-3[12,13]. In fact, ITZ is composed of three phases: a water film, a layer of calcium hydroxide (CH)crystals, and a porous paste matrix layer[13], and it is the weakest phase in RAC, which plays a crucial role in affecting the mechanical properties and durability of RAC[5,14-16]. For example, by Vickers hardness tester, it was found that the compressive strength of RAC depends on the relative microhardness values of ITZ-3 and ITZ-2. When the water-binder ratio is low, it is determined by ITZ-2; when the water-binder ratio is high, it is determined by ITZ-3[16-18]. By nano-indentation technology, it was found that the elastic modulus of ITZ-2 is lower than that of OM, and that of ITZ-3 is also lower than that of NM[19]. Besides, the width of ITZ also has remarkable effects on the mechanical properties and durability of RAC[5,20,21].

Admittedly, OM and ITZ play a vital role in influencing the workability, mechanical properties, and durability of RAC. However, many methods can be adopted to improve the quality of RCA[22,23], thus the adverse influences imposed by OM and ITZ on RAC can be alleviated. For instance, Tam[24]used HCl, H2SO4, and HNO3solutions to treat RCA, and found that the OM can be effectively removed. Moreover, the ITZ-3 of the resulting RAC is enhanced, thus the mechanical properties of the resulting RAC are improved. Yue[3]found that after RAC is subjected to carbonation, the microhardness values of ITZ and mortar are increased obviously, and the widths of ITZ decrease. Li[25]reported that compared to normal mixing method (NMM), the amount of CH in ITZ-3 is declined and the micro-structure of ITZ-3 is more homogeneous when RAC is prepared by two-stage mixing approach (TSMA, also called double mixing method, DMM), which indicates that TSMA has a beneficial influence on the mechanical properties of RAC. Xuan[26,27]reported that the mechanical properties and durability of RAC, which are prepared with carbonated RCA by DMM, are superior to those of RAC which are prepared with non-carbonatedRCA by DMM, because the microhardness values of ITZ-3 are improved by carbonation.

Table 1 The classification of treatment methods for the quality improvement of RCA

In conclusion, treating RCA by suitable treatment methods can improve its quality and the properties of the resulting RAC, thus promoting its large-scale application in engineering field. Therefore, for the purpose of providing reference for further research and practical application about the quality improvement of RCA,in this paper, the various treatment methods are firstly classified into four categories in section 1, then the improvement mechanisms and important conclusions are elucidated and summarised respectively in section 2-5.In section 6, the improved effects as well as advantages and disadvantages are compared and discussed respectively, and recommendations for the selection of treatment methods are proposed. Finally, conclusions and recommendations for future studies are listed in section 7 and 8.

As shown in Table 1, the treatment methods for the quality improvement of RCA are classified into four types: removing OM, strengthening OM, multi-stage mixing methods, and combination methods.

2 Removing OM

By reducing the content of OM in RCA, removing OM can not only improve the quality of RCA, but also can decrease the amount of ITZ-2 and ITZ-3 in RAC.In this section, the improvement mechanisms and important conclusions of various treatment methods are elucidated and summarised, respectively.

2.1 Ultrasonic cleaning

By immersing RCA in water and imposing ultrasonic, the dust, crumbs and loosening OM in RCA can be removed. However, the improved effects of ultrasonic cleaning are not obvious because the stronger OM attached on RCA cannot be removed. Katz[28]found that the 28 d compressive strength of RAC is improved by only 7%.

2.2 Heavy media separation process

Due to the density difference between RCA and the heavy media suspension, the density of RCA with a lower OM content is high and it will sink to the bottom of heavy media suspension, while the density of RCA with a higher OM content is low and it will float to the top of heavy media suspension. Kang[29]found that the higher the density of heavy media suspension is, the better the properties of the sinking RCA and the resulting RAC are, but the recovery ratio of sinking RCA will decrease.

2.3 Multi-stage crushing process

Different types of crushers are utilised in different crushing stages, so as to reduce the OM content and modify the particle shape of RCA. Generally, jaw crusher is used in the first stage, while the others such as screw crusher[30], cone crusher[31], impact crusher[32],and hammer mill crusher[33]are used in the following stages.

The research results showed that when the size of RCA increases, the OM content of RCA will decrease,and when the maximum size of RCA is approximately equal to that of NA used in parent concrete, the OM content of RCA may attain the lowest value[34]. Moreover, the improved effects of RCA treated by impact crusher and jaw crusher in the second stage of multistage crushing process have no obvious difference[35].

2.4 Mechanical grinding

Fig.2 Eccentric rotor mill[36]

Fig.3 Ball mill[37]

Mechanical grinding machines generally include eccentric rotor mill[36], ball mill[37], and screw mill[38],which can remove the OM attached on the surface of RCA and modify the particle shape of RCA, as shown in Fig.2, Fig.3, and Fig.4, respectively. Eccentric rotor mill imposes friction and squeeze on RCA that is between eccentric rotor and external cylinder by the rotating eccentric rotor. Ball mill uses steel balls to impose impact and abrasion on RCA in the rotating drum.Screw mill uses the rotating screw blade to impose friction and squeeze on RCA.

Fig.4 Screw mill[38]

The research results showed that ball mill makes the particle shape of RCA round and regular, while eccentric rotor mill makes the particle shape of RCA round but maintains the original surface regularity. Furthermore, after two times of mechanical grinding, the improved effects will not further increase[37].

2.5 Acid solution soaking

Acid solutions can react with the hydration products of cement paste, thus the OM of RCA can be weakened and removed. The commonly used acid solutions include HCl, H2SO4, HNO3, CH3COOH, and the chemical reaction equations are as follows[24,39]：

Tam[24]reported that the reaction products between H3PO4solution and CaO or Fe2O3are unstable,hence most OM of RCA cannot be removed. Moreover,Saravanakumar[40]reported that H2SO4is better than HCl in terms of the improved effects on RCA, while Kim[41]found the opposed result, because the gypsum formed by sulphate ions and calcium ions can react with tricalcium aluminate to form ettringite, which increases more voids.

Moreover, acid solutions with low concentrations can make the surface of RCA cleaner and more uniform, but those with high concentrations can damage the surface of RCA, making it fragile and porous.Therefore, HCl solution with a concentration of 0.1 mol is enough[42-44], while CH3COOH solution with a concentration of 0.3 mol is enough[39]. Noticeably, acid solution soaking will not disturb the alkaline environment of RCA and RAC[40], and the contents of chloride and sulphide will also remain within the ranges required by standards[24,45-46].

In addition, Wang[39]found that when the concentration of CH3COOH solution is 1%, the soaking time of 24 h is enough. Interestingly, the 28 d compressive strength of RAC increases by 13.8% when using the waste CH3COOH solution to replace mixing water and prepare RAC.

3 Strengthening OM

By filling the pores, voids, and micro-cracks in OM, strengthening OM can not only improve the quality of RCA, but also can enhance the ITZ-2 and ITZ-3 in RAC. In this section, the improvement mechanisms and important conclusions of various treatment methods are elucidated and summarised, respectively.

3.1 Silicate solution impregnating

At present, Na2SiO3and CaSiO3are commonly used to improve the properties of RCA. By impregnating RCA in silicate solutions, the pores, voids and micro-cracks of RCA will be filled. Furthermore, silicate particles can be regarded as nucleation sites and accelerate the hydration of cement, which can form much denser calcium silicate hydrate (C-S-H) and strength ITZ-3 of RAC[45,47]. In addition, Na2SiO3reacts with CH in RCA to form C-S-H, whose reaction formula is as follows[48]:

With regard to reducing the water absorption of RCA, Na2SiO3is inferior to polymers, because it has no hydrophobic effect. However, when treating RCA with 30% Na2SiO3solution and then followed by 40%silane solution, the combination effect is worse than their individual treatment. By contrast, when 30%Na2SiO3solution is followed by 30% siloxane solution,the combination effect is remarkably enhanced[49,50].

3.2 Polymer solution impregnating

When RCA is impregnated with polymer solutions, the polymer particles will diffuse into the pores,voids and micro-cracks of RCA and then polymerize,forming a polymer film on the surface of RCA, which will reduce the water absorption of RCA. Polymers commonly used to improve the properties of RCA include polyvinyl alcohol (PVA), silane, siloxane[49,51].

Some research results showed that PVA is a water-soluble polymer, which can dissolve in concrete mixing process and change the flocculation of cement.At the same time, PVA film on the surface of RCA can reduce the local water-binder ratio of ITZ-3, thus reducing the thickness of ITZ-3 and increasing the microhardness of ITZ-3[52]. However, PVA film can also delay the hydration process of cement, consequently,the initial and final setting time of cement will be delayed and the compressive strength of RAC at early age will be declined[53]. Nevertheless, as time goes by, PVA can gradually dissolve in alkaline environment, and its influence on the later strength development will gradually decrease[54]. Therefore, the concentration of PVA solution to treat RCA should not be too high, with the optimum value about 10%[51].

Compared to PVA, silane and siloxane are more effective with regard to reducing the water absorption of RCA. The reasons can be explained as follows: the silane particles of 1-2 nm and siloxane particles of 5-10 nm can effectively penetrate into the pores of RCA[50],then silane and siloxane hydrolyse and dealcoholize and react with alkoxyl groups to form silanol. After that, silanol reacts with hydroxyl groups of silicate contained in cement to bond to RAC. Further, after water evaporation, the silanol reacts with another silanol to form siloxane cross-link, which makes RCA hydrophobic[11,30].

Admittedly, polymer is effective to reduce the water absorption of RCA. However, many researchers have verified that the polymer can hinder the hydration of cement, thus reducing the compressive strength of RAC. For example, Yaowarat[53]and Ho[55]found that PVA reduces the compressive strength of RAC. Moreover, Zhu[11]found that although silane can improve the durability of RAC, the 28 d compressive strength of RAC incorporated with 0.5% silane decreases by 38%. Tsujino[30]also confirmed this viewpoint. Besides,Hwang[56]found that the compressive strength of RAM decreases with the incorporation of styrene–butadiene rubber (SBR) latex or polyacrylic ester (PAE) emulsion.

3.3 Modifier solution impregnating

Using modifier solutions to impregnate RCA can fill the pores, voids and micro-cracks of RCA and form a waterproof film on the surface of RCA, thus reducing the water absorption of RAC. The commonly used modifiers include oil-based modifier, paraffin, and concrete additives such as lyophobic active agent, polycarboxylate dispersant.

The research results showed that using polycarboxylate dispersant to treat RCA can improve the mechanical properties and durability of the resulting RAC[57]. However, using lyophobic active agent[54], oilbased modifier[30], and paraffin[58]to treat RCA will worsen the mechanical properties of the resulting RAC,although they can effectively reduce the water absorption of RCA.

3.4 Pozzolanic material slurry impregnating

Because pozzolanic materials have morphological effect, filling effect, pozzolanic effect, and nucleation effect[54,55,59], utilising pozzolanic material slurries to impregnate RCA can fill the pores, voids and micro-cracks of RCA, thus improving the properties of RCA and strengthening the ITZ-3 of RAC. The commonly used pozzolanic materials include cement, silica fume, fly ash, slag, geopolymer, and metakaolin.

Impregnating RCA with pozzolanic material slurries can effectively improve the compressive strength of RAC, but with regard to the water absorption of RCA, some references[59,60]found that it decreases after treatment, while other references[61,62]found that it increases after treatment.

3.5 Nano-material slurry impregnating

Impregnating RCA with nano-material slurries mainly uses their filling effect to fill the pores, voids and micro-cracks of RCA, thus strengthening RCA and ITZ-3. Besides, nano-materials also have pozzolanic effect, accelerating cement hydration effect, and nucleation effect of hydration products. The commonly used nano-materials include nano-silicon and nano-calcium carbonate[63,64].

The nano-indentation test showed that the surface of OM and partial ITZ-3 adjacent to OM surface are strengthened by impregnating RCA with nano-material slurries, but the other part of ITZ-3 far from OM and ITZ-2 are not strengthened[63]. Moreover, in respect of improving the properties of RCA and RAC, impregnating RCA with nano-silicon slurry is superior to incorporating nano-silicon into RAC directly in the mixing process[65].

3.6 Carbonation

The carbonation mechanisms are that the completely hydrated cement paste contains about 70%C-S-H, 20% CH, 10% ettringite (AFt) and monosulphate (AFm). After carbonation, its total solid volume will increase by about 13%[66], thus the pores, voids,and micro-cracks of RCA are filled and the ITZ-2 and ITZ-3 become denser than before. The reaction equations are as follows: after carbonation, the volumes in Eqs.(6) and (7) increase by 11.5%, and 23.1%, respectively, while those in Eqs.(8) and (9) decrease by 44.9%, and 31.8%, respectively. Besides, the unhydrated tricalcium silicate (C3S) and dicalcium silicate (C2S)can also react with CO2, with the volumes increasing by 92.5%, and 108.7%, as shown in Eqs.(10) and (11),respectively[66,67]:

It has been reported that CH and C-S-H are the main reactants of carbonation, and CH is the first reactant, followed by C-S-H[68]. It is notable that carbonation strengthens not only OM and ITZ-3[26], but also ITZ-2 of RCA[69], as shown in Fig.5 and Fig.6, respectively. Therefore, the compressive strength of RAM cured by carbonation for 2 h is comparable to that of RAM cured by normal curing for 28 d[70,71], and after carbonation for 24 h, it reaches 1.5 times as high as that of RAM cured by steam[72].

The size of RCA and the quality of RCA, such as the strength grades, water-binder ratios, and storage time of parent concrete, affect the carbonation degree of RCA. For instance, the smaller the size of RCA is,the larger the specific surface area is, thus leading to a higher carbonation degree[73]. Moreover, the lower the strength grade of parent concrete is[74]or the higher the water-binder ratio of parent concrete is[69], the higher the carbonation degree is. The reason is that the above-mentioned two factors can cause a higher porosity of OM, which facilitates the diffusion of CO2.In addition, RCA with longer storage time has been already carbonized with CO2in the air, hence its carbonation degree is lower than that of RCA with shorter storage time when subjected to carbonation[26,27]. However, lime water can be used to impregnate RCA before carbonation, so as to improve the carbonation degree of RCA[68,75].

The carbonation conditions, such as CO2concentrations, pressure, temperatures, relative humidity,and treatment time, also affect the carbonation degree of RCA. For example, the carbonation degree firstly increases and then decreases with CO2concentration increasing, and reaches the biggest value when CO2concentration is about 70%[75]. Furthermore, the higher pressure will inhibit carbonation degree[71,76], and 0.1 bar is suitable[26,27,73], while the temperature range of 20-80℃ has little effect on the carbonation degree[71].Moreover, relative humidity affects carbonation degree significantly[70,73], and the optimum values range from 50% to 65%[71]. In addition, the change trend of temperature curve showed that carbonation reacts fastest in the first 2 h, after that, the curve begins to decline,which indicates that the reaction rate slows down[71-73].After 24 h of carbonation, the carbonation degree almost no longer changes[67,77].

Fig.5 Microhardness of ITZ-3 in carbonated and non-carbonated NRCA[26]

Fig.6 Microhardness of ITZ-2 in carbonated and non-carbonated MRCA[69]

Besides, mercury intrusion porosimetry (MIP)tests showed that the CaCO3produced by carbonation mainly fills the medium pores of RCA with the size of 0.1-1 μm, because the size of CaCO3is larger than 0.1 μm[68]. X-ray diffraction (XRD) analyses showed that with the carbonation time increases, the peak value of CH decreases while that of CaCO3increases, but CH still exists after 72 h carbonation[67]. The reason is that the CaCO3produced by the reaction between CH and CO2covers the surface of CH, which hinders the further carbonation reaction[68].

3.7 Microbially induced carbonate precipitation (MICP)

MICP is an environmentally friendly treatment method, which was firstly used to repair cracks in decorative stones and limestone monuments[78]. At present,it is widely used in self-healing concrete[79,80]. There are two different metabolic pathways, autotrophic type and heterotrophic type[79], and the latter is achieved by sulphur cycle and nitrogen cycle. In the field of nitrogen cycle, carbonate precipitation induced by urea hydrolysis bacteria is widely used[81], and the main bacteria used are from genus Bacillus, such as Bacillus subtilis[82], Bacillus pasteurii[83], Bacillus sphaericus[84].Nowadays, Bacillus pasteurii is widely used, because it can produce more urease[85], and it is more active[86]as well as lacks pathogenicity[87]than the other bacteria in extreme environments.

The mechanisms of MICP caused by urea hydrolysis bacteria are that urease is firstly produced by the metabolism of urea hydrolysis bacteria, then urea is catalysed by urease to hydrolyse to carbonic acid and ammonia, which cause the concentration of carbonic acid and pH value increasing in the bacterial environment.After that, carbonic acid and ammonia further hydrolyse to carbonate ion and ammonium ion, respectively[79]. The cell walls of bacteria, which are nucleation sites, are negatively charged[84], hence calcium cations are attracted to the surfaces of cell walls and then react with carbonate ions to form CaCO3precipitation, which covers the surface and fill the pores of RCA, as shown in Fig.7. Therefore, the water absorption of RCA is reduced[84,88]. The reaction equations are as follows[79-81,86]:

The research results showed that the smaller the size of RCA particle is, the larger the specific surface area is and the larger the amount of CaCO3precipitation is[83,84].Moreover, the pore size distribution of OM also affects the effect of MICP. In fact, the size of a bacterium is larger than 1 μm, so CaCO3precipitation mainly occurs in the pores larger than 1 μm[78,85]. However, because the pores larger than 1 μm are limited in cement-based materials, MICP is considered to be a surface treatment method, as shown in Fig.7.

Fig.7 CaCO3 precipitate on the surface and in the pores of RCA[84]

Furthermore, the diversity of CaCO3mineralization and various saturation levels lead to different polymorphs, including three anhydrous forms (calcite,aragonite and vaterite), two hydrated crystalline phases(monohydrate calcite and ikaite), and various amorphous CaCO3phases[80]. Among which, calcite, aragonite and vaterite are non-hydrated crystals, as shown in Fig.8. Thermodynamics research results showed that rhombohedral calcite is the most stable polymorph, followed by acicular aragonite which is metastable, while spherical aragonite is the most unstable polymorph,and when exposed to water, it rapidly transforms into aragonite at a higher temperature and calcite at a lower temperature[85].

Fig.8 SEM micrographs of anhydrous forms of CaCO3 polymorphs: (a) calcite; (b) aragonite; (c)vaterite[80]

In addition, the types and yields of polymorphs produced by MICP are influenced by bacteria species and concentrations, components and concentrations of culture medium, pH values and temperatures[84,86,89]. For example, a high bacteria concentration significantly increases the hydrolysis rate of urea, while high urea concentration and calcium ion concentration do not increase the precipitation of CaCO3. Moreover, forming calcite or vaterite is controlled by bacteria activity,initial calcium ion concentrations, and the types of calcium sources. Calcite is formed at a relatively low super-saturation condition, while vaterite is formed at a relatively high super-saturation condition. In terms of calcium sources, when organic acid, more precisely, calcium lactate is used, calcite will be produced,while when calcium chloride is used, vaterite will be produced[78,80,85]. Besides, the yield of CaCO3increases with the increase of pH value and temperature, but the activity of bacteria will be inhibited when they exceed the threshold values, thus resulting in a decrease in the yield of CaCO3[89].

4 Multi-stage mixing methods

Multi-stage mixing methods include double mixing method (DMM, also called two-stage mixing approach,TSMA)[17,90]and triple mixing method (TMM)[91].The improvement mechanisms of them are that the mixing water is divided into two parts and mixed in different stages, so as to form a pozzolanic material coating with a low water-binder ratio on the surface of RCA. The coating can not only fill the pores, voids, and micro-cracks of RCA, but also can decrease the thickness and increase the microhardness of ITZ-3, thus improving the properties of RCA as well as the mechanical properties and durability of RAC. The mixing processes of DMM, TMM, and normal mixing method(NMM)[17]are showed in Fig.9.

The research results showed that TMM is superior to DMM about the property improvement of RAC, because the particle sizes of pozzolanic materials such as fly ash or silica fume are smaller than that of cement, so the filling effect of pozzolanic materials is more obvious than that of cement. Furthermore, the pozzolanic effect of pozzolanic materials can consume a large amount of CH in OM and ITZ-3[91,93,94]. However, Urban[95]adopted TMM to prepare RAC and NAC,and found that the 28 d compressive strength of RAC and NAC whose coarse aggregate were all coated with cement are similar, and they are higher than those of RAC and NAC whose coarse aggregate were coated with fly ash and recycled concrete powder.

Fig.9 The mixing processes of different mixing methods[17,90-92]

In addition, Tam[90,96,97]found that the improved percentages of the mechanical properties of RAC prepared by DMM fluctuate with the replacement ratio of RCA. However, Liu[98]found that DMM has little effect on the freeze-thaw resistance of RAC. Besides,Liang[92]found that the 28 d compressive strength of RAC, which were prepared by mortar mixing approach(MMA) and sand enveloped mixing approach (SEMA)with a 100% replacement ratio of RCA, are 28.3% and 6.4% higher than that of RAC prepared by DMM, respectively. The mixing processes of MMA and SEMA are showed in Fig.9.

5 Combination methods

Combination methods combine two or more kinds of individual methods together for treating RCA more than once, so as to enhance the improved effects on the properties of RCA and RAC. The individual methods come from the categories of removing OM, strengthening OM, or multi-stage mixing methods. At present,the commonly used combination methods are showed in Table 1 in Section 1. Apart from the heating methods in thermal-mechanical grinding and thermal-acid solution soaking, the other individual methods to form the combination methods have been introduced in Section 2, Section 3, and Section 4. Therefore, in this section,the improvement mechanisms and important conclusions of heating methods are only elucidated and summarised, respectively.

Fig.10 The scheme of electric pulse heating device[102,106]

The heating methods include conventional heating, microwave heating and electric pulse heating.Conventional heating uses the difference of thermal expansion among the components of RCA to introduce temperature stress, especially at ITZ, thus making OM brittle and easy to remove[99,100]. Microwave heating produces high temperature gradients by using the difference of thermodynamic property, dielectric property,and tensile strength among the components of RCA,which results in high stress gradients, especially at ITZ.Moreover, rapid evaporation of water in RCA produces significant pore pressure. Consequently, the high stress gradients and pore pressure make the OM brittle and easy to remove[101-105]. Electric pulse heating uses a high voltage electric field to polarize different components of RCA, and the intensity and location of polarization depend on the electrical properties of different components. The charge imbalance at ITZ leads to the formation of local plasma current discharge channel,which causes thermal expansion and radial shock wave,and makes OM brittle and fragmented[101,102,106,107]. The scheme of electric pulse heating device is showed in Fig.10.

Fig.11 The liberation degrees of RCA treated by microwave heating and electric pulse heating

The liberation degrees of RCA treated by microwave heating and electric pulse heating from different references are plotted in Fig.11. The references 1-6 in Fig.11 correspond to Refs.[104], [103], [102], [106],[101], [107] respectively.

In respect of microwave heating and electric pulse heating, the larger the size of NA is, the thicker the ITZ of concrete is, hence the fracture energy of RCA decreases and the liberation degree increases. Moreover,the mineral properties of NA have little effect on the liberation degree of RCA. Furthermore, even a low energy level is effective on the liberation degree of RCA,while a high energy level increases the fragmentation degree of concrete[103,106]. In addition, applying microwave pre-heating before mechanical grinding can obviously improve the liberation degree of RCA, but it has no effect on that of RCA treated by electric pulse heating[101]. Besides, with the increase of concrete strength,the fragmentation degree of concrete treated by electric pulse heating increases firstly and then decreases, and it is more effective for the cement with low CH content[107].

For conventional heating and microwave heating,immersing RCA in cold water before and after heating can facilitate the removal of OM because of pore pressure generated by the rapid evaporation of water during heating process and shrinkage stress generated by thermal expansion and contraction[67,105,108].

Thermogravimetric analyses showed that the free water of RCA will evaporate when the temperature is less than 105 ℃; the combined water of C-S-H with poorer hydration will dehydrate at 105-200 ℃, while the combined water of C-S-H with better hydration will dehydrate at 200-420 ℃; CH will decompose at 420-550 ℃; CaCO3with poorer crystallization will decompose at 550-720 ℃, while CaCO3with better crystallization will decompose at 720-950 ℃[44,66,68,104].Therefore, the optimum temperature range for conventional heating is about 300-600 ℃. In addition, fine powder less than 4 mm can regain bonding activity at 600 ℃, hence they can be used as pozzolanic material to replace cement[38,99].

In addition, in order to improve the strength of NAC and the liberation degree of RCA treated by microwave heating, Tsujino[109,110]and Noguchi[38]used epoxy resin to bond pozzolanic material as well as ferrous oxide and coated NA before fabricating concrete, while Choi[111,112]used cement slurry containing iron oxide to coat NA. After applying microwave heating and Los Angeles abrasion, they all found that the recovery ratio of NA can be over 90%. This technology includes strengthening technology of concrete and recycling technology of RCA, which solves the trade-offs between NAC strength and NA recovery ratio as well as energy consumption and RCA quality.

6 Discussion

For the purpose of determining which ones are more effective and feasible among the various treatment methods, not only should their improvement effects on the properties of RCA and RAC be considered,but also their advantages and disadvantages should be compared in terms of treatment equipment, energy consumption, treatment costs, production cycles, process complexity, and recovery ratios of RCA.

6.1 The improved effects of various treatment methods

The improved percentages of water absorption of RCA and 28 d compressive strength of RAC are illustrated in Fig.12. As can be seen from Fig.12, the treatment methods, whose improved percentages of water absorption of RCA are over 60%, include those whose co-ordinates correspond to 5, 6, 7, 13, 14, and 15, respectively. Similarly, the treatment methods, whose improved percentages of 28 d compressive strength of RAC are over 25%, include those whose co-ordinates correspond to 3, 8, 11, 12, 14, 15, and 17, respectively.These phenomena indicate that the above-mentioned treatment methods are relatively effective to improve these two indexes of RCA and RAC. Besides, carbonation-multi-stage mixing method[27,113]and pozzolanic material slurry impregnating-multi-stage mixing method[92]are also very effective to improve the mechanical properties and durability of RAC.

Fig.12 The improved percentages of water absorption of RCA and 28 d compressive strength of RAC (The treatment methods corresponding to co-ordinates 1-17 are as follows:1. Ultrasonic cleaning[28]; 2. Multi-stage crushing process[30,32]; 3. Mechanical grinding[100,114]; 4. Acid solution soaking[10,24,39,40,42,114,115]; 5. Silicate solution impregnating[42,50,94,116-118]; 6. Polymer solution impregnating[30,50,51];7. Modifier solution impregnating[30,58]; 8. Pozzolanic material slurry impregnating[28,59,61]; 9. Nano-material slurry impregnating[63-65,118,119]; 10. Carbonation[26,67,73,77,120,121];11. Microbially induced carbonate precipitation[83,84,88,122];12. Multi-stage mixing methods[16,17,90,91,96,97]; 13. Thermal-mechanical grinding[105]; 14. Thermal-acid solution soaking[123,124]; 15. Acid solution soaking-mechanical grinding[39,125-127]; 16. Acid solution soaking-silicate solution impregnating[45,47,116]; 17. Pozzolanic material slurry impregnating-silicate solution impregnating[117])

6.2 The advantages and disadvantages of various treatment methods

With regard to removing OM, ultrasonic cleaning will consume plenty of energy and water, and the improved effects are not obvious. Heavy media separation process consists of several complex systems, including heavy media separation system, heavy media recovery system, and waste water recycling system. Acid solution soaking, such as HCl and H2SO4solutions, will introduce chloride ions and sulphate ions, which will accelerate cement setting[41]. Moreover, chloride ions will erode steel bars and sulphate ions will cause expansion of RAC, thus deteriorating the durability of RAC.However, CH3COOH, a kind of weak acid, is relatively safer, cleaner, and cheaper to treat RCA than strong acid. Particularly, using the waste CH3COOH solution after treating RCA to prepare RAC can improve their compressive strength[39,44]. Therefore, CH3COOH is recommended to treat RCA. Besides, multi-stage crushing process and mechanical grinding are also recommended, because they can effectively reduce the OM content and modify the particle shape of RCA. However,excessive treatment should be avoided, because it not only consumes plenty of energy and induces damage to RCA, but also increases the amount of fine power and decreases the recovery ratio of RCA.

In respect of strengthening OM, polymers and modifiers can effectively reduce the water absorption of RCA because hydrophobic film is formed on the surface of RCA, but meanwhile hydrophobic film can also hinder the bond between RCA and NM, thus the mechanical properties of RAC deteriorate. Pozzolanic materials and nano-materials can improve the mechanical properties and durability of RAC to a certain extent,but the former may lead to the increase of the water absorption of RCA, which will make the workability of RAC worse, and the costs of the latter are relatively high. MICP is an environmentally friendly surface treatment method, however, several factors restrict its large-scale commercial application. For instance, when CaCO3precipitation is over-saturated, bacteria will be wrapped up and nutrient transport will be restricted,thus resulting in bacteria death[85]. Moreover, the factors affecting the type and yield of CaCO3are complex.Furthermore, when calcium chloride is used as the calcium source in culture medium, the residual chloride ions in RCA will erode steel bars in future, hence calcium acetate or calcium lactate are recommended. In addition, the costs of culture medium make up as high as 60% of the whole operation costs of MICP, which impose huge economic limitation on the application of MICP. Besides, the CaCO3produced by MICP mainly precipitate on the surface of RCA, but precipitate in the pores of RCA is relatively limited[81]. Therefore,Na2SiO3is recommended to treat RCA, because it can react with CH to form C-S-H, but meanwhile the reaction product NaOH may cause alkali-aggregate reaction, hence pozzolanic materials should be used to consume NaOH. Furthermore, carbonation is also recommended to treat RCA, because it can not only strengthen OM and ITZ-3, but also ITZ-2 of RCA.Noticeably, impregnating RCA with lime water before carbonation can improve its carbonation degree.

As for multi-stage mixing methods, they are promising treatment methods for improving the quality of RCA and RAC, because compared with removing OM and strengthening OM, they do not require other processing equipment and special materials, nor additional processing time and costs. Furthermore, the recovery ratio of RCA is higher than removing OM.

Table 2 The selection of treatment methods

In terms of combination methods, microwave heating-mechanical grinding is recommended. The reasons are as follows: Firstly, conventional heating has no selectivity for different components of RCA,usually heating for several hours, which heats the different components of RCA to a same temperature and consumes plenty of energy. By contrast, microwave heating and electric pulse heating have selectivity and heat the different components of RCA to different temperatures. Moreover, only a few minutes and milliseconds are needed for the heating process, respectively, thus little energy is consumed[105]. Secondly, RCA treated by electric pulse heating need to be placed in water, which increases some additional work, such as draining water, filtering and drying RCA[102]. Besides,weak acid solution soaking-mechanical grinding, carbonation-multi-stage mixing method, and pozzolanic material slurry impregnating-silicate solution impregnating are also recommended.

6.3 The selection of treatment methods

Based on the factors such as improvement effects, treatment costs, energy consumption, production cycles, process complexity, and recovery ratios of RCA, recommendations for the selection of treatment methods are proposed, as shown in Table 2. The improvement percentages for 28 d compressive strength of RAC are the average values of the data in Fig.12.Moreover, in order to enhance the improvement effects of RCA, two or more kinds of individual methods could be combined together to treat RCA more than once.

Usually, the strength grade of concrete varies greatly with the origin of C&DW. Therefore, treatment methods could be selected according to the strength grade of concrete. When the strength grade is low,treatment methods of removing OM could be selected to obtain high quality RCA, because the OM can be removed easily due to the inferior ITZ-2. When the strength grade is high, treatment methods of strengthening OM could be selected to obtain a high recovery ratio of RCA, because the OM cannot be removed easily due to the superior ITZ-2.

7 Conclusions

a) Removing OM: CH3COOH is better than HCl and H2SO4to treat RCA, because it will not introduce chloride ions and sulphate ions. Particularly, using the waste CH3COOH solution after treating RCA to prepare RAC can improve their compressive strength.Moreover, multi-stage crushing process and mechanical grinding can effectively reduce the OM content and modify the particle shape of RCA. However, the improved effects of ultrasonic cleaning are not obvious,and heavy media separation process needs several complex systems.

b) Strengthening OM: Na2SiO3should combine with pozzolanic materials to eliminate the possible alkali-aggregate reaction. Carbonation can not only strengthen OM and ITZ-3, but also ITZ-2 of RCA.Moreover, impregnating RCA with lime water before carbonation can improve its carbonation degree. By contrast, polymers and modifiers can effectively reduce the water absorption of RCA, but the hydrophobic film formed on the surface of RCA will make deterioration to the mechanical properties of RAC. Besides,nano-materials and MICP are effective to improve the proprieties of RCA, but their costs are relatively high.In addition, MICP is considered to be a surface treatment method, and requires strict control conditions to acquire the desirable CaCO3polymorph.

c) Multi-stage mixing methods: DMM and TMM are promising treatment methods, because their improved effects are effective and only the mixing processes need to be modified.

d) Combination methods: They are more effective than their corresponding individual treatment methods.As for thermal-mechanical grinding, microwave heating has selectivity for different components of RCA and consumes little energy and time, while conventional heating goes the opposite. Moreover, the treatment process of microwave heating is simpler than that of electric pulse heating, because the latter needs place RCA in water. Besides, immersing RCA in cold water before and after heating can facilitate the removal of OM.

8 Recommendations for future studies

a) At present, the coating materials of RCA used in TMM are limited to pozzolanic materials. Therefore,applying other promising coating materials to TMM,such as silicate and nano-materials, needs to conduct further research.

b) The combination methods should be further investigated, therefore, an integrative programme on the quality improvement of RCA is recommended, in which multi-stage crushing process could be firstly adopted to crush RCA, then carbonation or combination methods such as microwave heating-mechanical grinding, weak acid soaking-mechanical grinding, and pozzolanic material slurry impregnating-silicate solution impregnating could be adopted to improve the quality of RCA, and eventually multi-stage mixing methods could be utilised to prepare RAC.