A review on different treatment methods for enhancing the properties of recycled aggregates for sustainable construction materials

Construction and Building Materials 233 (2020) 117894

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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Review

A review on different treatment methods for enhancing the properties of recycled aggregates for sustainable construction materials Abhijit Mistri a,b, Sriman Kumar Bhattacharyya a, Navdeep Dhami b, Abhijit Mukherjee b, Sudhirkumar V. Barai a,⇑ a b

Department of Civil Engineering, Indian Institute of Technology, Kharagpur, West Bengal 721302, India School of Civil and Mechanical Engineering, Curtin University, Bentley, WA 6102, Australia

h i g h l i g h t s
Clear methodology for effective use of C&D waste in concrete is warranted.
Treatment methods for mitigation of weaknesses within RCA and RAC has discussed.
Performance enhancement methods on RCA considering sustainability has analysed.
Present review recommends for the strengthening of attached mortar not the removal.

a r t i c l e

i n f o

Article history: Received 7 May 2019 Received in revised form 11 October 2019 Accepted 17 December 2019

Keywords: Recycled aggregate Concrete Construction and demolition wastes Treatment methods Sustainable construction materials Attached mortar

a b s t r a c t With global increase in construction and demolition, recycling of construction debris as an aggregate can be a vital step towards achieving sustainability in concrete construction. However, a clear methodology for reuse of construction and demolition (C&D) waste in concrete is warranted for its use in practice. This paper reviews the challenges revealed hitherto such as weak interfacial transition zone, high water absorption and presence of micro cracks in the use of C&D wastes as the recycled aggregate (RA). Methods of mitigation of these weaknesses through various treatments have been reported. This review has a special focus on India, a country that generates one of the world’s highest quantity of C&D waste. After analysing all the treatment methods, the authors summarize that the strengthening of attached mortar (AM) technique is better than removing of AM, which is also cost-effective, eco-friendly and sustainable. Use of nano-materials and pozzolana along with different mixing methods and application of bio-cement is found to be superior and environmental friendly approach for improving the properties of recycled aggregates. Ó 2019 Elsevier Ltd. All rights reserved.

Contents 1. 2. 3. 4. 5.

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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 The significance of using recycled aggregates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Indian statistics of RA with respect to the world . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Recycling process of RCA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Different treatment methods for enhancement of RA properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 5.1. Treatment of RA for better performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 5.1.1. Removal of attached mortar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 5.1.2. Strengthening of attached mortar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 5.1.3. Modification of the mixing process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Overview of treatment methods for RCA (review papers on treatment methods for RCA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

⇑ Corresponding author. E-mail addresses: [email protected], [email protected] (S.V. Barai). https://doi.org/10.1016/j.conbuildmat.2019.117894 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

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A. Mistri et al. / Construction and Building Materials 233 (2020) 117894

7.

Summary and recommendation . . Declaration of Competing Interest Acknowledgement . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . .

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guidelines in this context have also been addressed. This review further provides a comparative analysis of different techniques utilized for treatment of RA taking into consideration their advantages and disadvantages. The researchers have also made an attempt to explore the impact of each treatment on durability, state of fresh/hardened concrete and their environmental impact.

1. Introduction Rapid growth in population and economy in both developed and developing countries has led to a huge demand for infrastructure development. Extensive increase in the rate of construction creates immense depletion of natural resources [1]. Moreover, the progression of industrialization and urbanization demands massive renewal of old built facilities which generate huge amounts of construction and demolition (C&D) waste. This pressing issue of C&D waste needs safe disposal. Current construction industry faces the dual problem of diminishing natural resources and burgeoning C&D wastes. Recycling of C&D wastes into new construction can alleviate both the problems simultaneously. In the C&D debris, where the waste concrete percentage is very high, the reuse of old concrete as a source of aggregate has emerged as an attractive alternative to natural aggregates (NA) in concrete [2]. Demolished building wastes, concrete road bases, rejected precast concrete members, unused concrete in concrete mixing plants and tested specimens from different laboratories are the sources of waste concrete. Recycled aggregate (RA) is the aggregate generated by crushing of waste parent concrete. This RA consists of both recycled coarse aggregate (RCA) and recycled fine aggregate (RFA). RCA has some weaknesses over NA due to the attached mortar on its surface. It is more porous than NA, due to the presence of a higher percentage of capillary pores and microcracks on the surface (and sometimes in the interfacial transition zone (ITZ)) [3]. The porosity of RCA mainly depends on two parameters: (a) original composition of the mixture and (b) process of making RA [4]. Several researchers have found that the physical and mechanical properties of RA are inferior to NA [2,3]. When RA is used for making concrete, it has been found to reduce workability, compressive strength, split tensile, flexural strength along with other engineering properties compared to NA [2,5–8]. Therefore, RA needs preprocessing prior to use in concrete. Modification of engineering properties of RA or recycled aggregate concrete (RAC) can be done fundamentally by the following three methods: (i) treatment of RA for better performance (ii) improvement of binder especially for RA and (iii) combination of these two methods, as reported in the previously published literature [2,5,9–11]. This paper discusses different treatments/enhancement methods that have been established in the earlier studies along with critical analysis of the methodologies, their applications and limitations. The statistics of cement and concrete production in India, C&D waste generation, recycling processes and government

2. The significance of using recycled aggregates Massive expansion in the construction industry has been recorded in 20th and 21st century [2]. According to global carbon dioxide emission database, construction industry stands second in global carbon dioxide generation [12]. Globally, building project consumes about 40% energy in both developed and developing countries [13]. According to Zhang and Wang’s study of China’s building constructions [14], it was found that 72.89% of the total energy consumption arises from manufacturing of raw materials. In European Union countries, buildings are responsible for about 50% of total energy consumption [15,16]. Suzuki and Oka, [17] discussed input/output table based on the energy consumption and CO2 emission of different construction industries. The operational energy in their study has been found out to be 82% while the construction stage energy requirement has been reported to be 15%. Such energy emissions data from different studies is tabulated in Table 1. In all these studies, energy requirement for different activities has been directly related to amount of CO2 emission. It has also been noticed in this study that the energy requirement for demolition stage is different from that of the construction stage. Only 3% of total energy is required for demolition stage [17]. Another study by Peng [13] concluded that during the demolition stage, about 2% of the total carbon emissions are generated. Table 2 lists the details of different construction materials, their unit energy consumption and CO2 emission generation. Aggregate occupies about 70–80% of the total volume of concrete [18] and has a great influence on the environment and sustainability. The reuse of C&D waste generated by construction sector has a great influence on the environment as demolition processes requires 2–3% of total energy requirement and can lead to the preservation of natural resources as well. Rapid growth in the construction industry and urbanization has led to a significant increase in the amount of waste in the last decade. Conventional practices allowed utilization of this waste in landfills and as a base filling material for pavements. With the rise in demand of virgin aggregate coupled with the use of natural resources for their

Table 1 Life cycle CO2 emissions from various studies. Reference

[17] [21] [21] [22] [23] [13]

Level of building

Developed Developed Developed Developed Developed Developing

Country

Japan U.S.A Europe U.S.A Australia China

10 10 10 10

LCCO2 emissions Operational stage

Construction stage

Demolition stage

82% 90% 88% 99.6% 85.4% 85.4%

15% 10% 12% 0.4% 14.8% 12.6%

3% – – – – 2%

Note: The data is from office buildings, LCCO2 = Life cycle CO2 emissions of buildings.

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A. Mistri et al. / Construction and Building Materials 233 (2020) 117894 Table 2 Unit energy consumption and CO2 emissions generated during the production of different building materials [13].

Table 3 Indian standard recommendation for the application of recycled concrete aggregate into concrete [44].

Main building materials

Unit energy consumption (kJ/unit)

Unit CO2 emissions (kg/unit)

Type of works

Maximum percentage of replacement

Grade of concrete

Concrete Brick Cement Lime Mortar Gravel Stone Steel Ceramic tiles Paint Glass Wood Organic material Aluminum Copper

1247.74 2000 4464 4644 3972 23.8 12,943 33,906 15,400 5837 16,000 1800 90,353 12,964 23,579

0.242 0.2 0.894 1.2 0.792 0.002 2.33 2.208 1.4 0.89 1.4 0.2 17.07 1.407 1.01

Lean concrete Plain concrete Reinforced concrete

100% with RA 25% with RA 20% with RA

Up to M15 – Up to M25

preservation, RA has been developed as a suitable substitute to virgin aggregate. This practice of utilizing RA can also be a promising and sustainable option for long-term usage. As RA individually has poor performance compared to NA, attempts have been made to improve their performance by mixing with some other pozzolanic materials. According to Meyer [19], the carbon footprint can be improved by using RA along with other industrialised wastes including silica fume, fly ash, blast furnace slag etc. As per the World Energy Council, ‘‘sustainable growth is no longer an option, it is a necessity” [20]. Therefore, recycling of aggregate and use of green construction materials is a must for development of sustainable construction industry. 3. Indian statistics of RA with respect to the world Researchers from different developed and developing countries have reported their findings on quantity of C&D waste, its characterization as aggregate, practical applications for its usage [2,4,5,24]. Bohmer et al. [25] in their studies from Europe found out that on an average about 850 million tonnes of C&D waste is generated per year. In another study by Tam [26], country-wise statistics of C&D waste generation and percentage recycled out of the total waste (municipal wastes, construction wastes, and others) has been made available. In that study it was reported that the percentage of C&D waste in Spain is at 70%, United Kingdom is over 50%, Australia is at 44%, Denmark is at 25–50% making their percentage much more than the cumulative waste. It has also been reported that the recycling percentage of C&D waste from Denmark (80%), Netherlands (75%), Japan (65%), and Germany (40–60%) is much more than other countries [26]. Again from the statistics of Environmental Protection Department (EPD) [27], about 2.36 million tonnes of municipal solid waste was generated in 2015 in Hong Kong out of which C&D waste was around 40.15%. As per authors’ review of literature on RA statistics from India, very little information is available. This section is devoted to address the scenario of cement, concrete, C&D waste and government guidelines on RA from India. Cement industry of India is well known throughout the world as it is the second largest cement producing country after China [28,29]. This industry has played a vital role in the industrial development and economic growth of the country [28]. As per 2015-16 statistics, cement production capacity of India was about 421.10 million tonnes with a net production of cement at about 283.45 million tonnes [28]. There is no specific data for concrete production in India, however, standard database of cement production is made available every year which can be used to predict concrete production. The mix proportion of

concrete is assumed to comprise of 370 kg/m3 cement, 190 L/m3 water, 725 kg/m3 sand and 1150 kg/m3 of coarse aggregate for making standard grade concrete [30]. The effective use of cement for the year 2015-16 was found to be approximately 250 million tonnes [31,32]. From this calculation, about 16.55 billion tonnes of concrete was produced in the year 2015-16 and the amount of natural coarse aggregate was about 7.77 billion tonnes. As per the statistics from 2016, C&D waste generation in India has been found to be about 23 million tonnes annually which was expected to double by the next year [33]. About 5% of the C&D waste is recycled in the form of floor filling materials in new construction, road and pavement application. Although RAC for structural application has not been reported in India, several research groups have been focusing on the effective utilization of RA [1,2,5,7,34–43]. Recently, Indian standard IS383:2016 [44] has proposed that the utilization of RA can be done with similar applications as NA. The manufacturing process of RA from C&D waste is as follows: 1) inspection of the documentation of C&D waste plant 2) weighing of waste 3) mechanical and manual segregation (basic batching of various types of wastes like, brick, concrete block, steel, stone, tiles, wood, etc.) and resizing as per application 4) dry and wet processing for cleaning. The Indian standard recommendation for application of RA into fresh concrete is tabulated in Table 3. The first C&D recycled plant in India was set up in the year 2009 in Delhi by IL&FS Environment [45]. Presently, the plant recycles

Demolition of old concrete source

Material separation

Material homogenization and batching

Size reduction as per application

Cleaning process for better quality

Characterization of aggregates

Treatment on RCA (if any)

Application into concrete Fig. 1. Flowchart for the concrete aggregate recycling process.

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A. Mistri et al. / Construction and Building Materials 233 (2020) 117894

Old ITZ

Recycled aggregate with minimum AM on

Recycled aggregate with AM on the

Multiple small sized aggregates within

Only AM

Fig. 2. Different types of recycled aggregate (AM = attached mortar).

about 2000 tonnes per day of C&D waste into aggregate which is used for commercial cement brick, ready-mix concrete, pavement blocks, hollow bricks, etc. About 95% of incoming C&D waste is recycled using wet-processing (minimizing the dust and noise pollution) and the water consumed for the process is used recycled sewage water [45].

4. Recycling process of RCA Recycling of concrete involves several steps to generate usable RCA. Screening and sorting of demolished concrete from C&D debris is the first step of recycling process. Demolished concrete goes through different crushing processes to acquire desirable grading of recycled aggregate. Impact crusher, jaw crusher, cone crusher or sometimes manual crushing by hammer are preferred during primary and secondary crushing stage of parent concrete to produce RA. Based on the available literature step by step flowchart for recycling of aggregate is represented in Fig. 1. Some researchers have also developed methods like autogenous cleaning process [46], pre-soaking treatment in water [47], chemical treatment, thermal treatment [48], microwave heating method [49] and mechanical grinding method for removing adhered mortar to obtain high quality of RA. Depending upon the amount of attached mortar, recycled aggregate has been classified into different categories as shown in Fig. 2.

5. Different treatment methods for enhancement of RA properties As mentioned earlier, the enhancement of RA properties can be done fundamentally by three methods: Treatment of RA for better performance, improvement of binder especially for RA and combination of these two methods. The primary objective of different treatment methods is to utilize RA effectively into fresh concrete. Therefore, this document has tried to investigate all the possible published literature available on these three methods.

5.1. Treatment of RA for better performance Many methods are used to treat RA to ensure its effective application in fresh concrete, increase its durability and other engineering properties. This treatment has been reported to aid in mitigation of all the drawbacks of untreated RA in fresh or hardened concrete. These methods can be subdivided into two groups: removal of attached mortar and strengthening of attached mortar [10], discussed separately for detailed investigation below.

5.1.1. Removal of attached mortar Effective techniques for removal of attached mortar are: (a) presoaking in water (b) autogenous cleaning (c) mechanical grinding (d) heat grinding (e) acid treatment and (f) thermo-chemical treatment. Sometimes the combination of these methods is also possible. The most important parameter required to select the treatment method is information about the source and composition of RA as this will affect the efficacy of the treatment method. 5.1.1.1. Pre-soaking in water. Water-washing of RCA or pre-soaking in water is the first and foremost treatment method for removal of attached mortar part. Pre-soaking in water has been proposed by Katz [47]. In this method, RCA is cleaned with water washing to remove dust particles, loosely attached mortar and other impurities. The paper also reported that ultrasonic bath treatment has the potential to improve engineering properties of RCA (increase in compressive strength by 7%). This method has two basic limitations: (i) can remove only a small percentage of loosely attached mortar or dust particles and (ii) water utilized for washing of aggregate needs suitable recycling after water is contaminated/mixed with cement dust and other impurities. In terms of durability properties, this method does not have a significant influence, as attached mortar is still present with old RCA. 5.1.1.2. Autogenous cleaning process. The primary interest of autogenous cleaning process is to eliminate the attached mortar layer from RA surface to enhance its quality [4]. RCAs are placed in a rotating mill drum, having a diameter of 300 mm with a depth of 500 mm, in order to collide against surrounding particles. During operation, mill drum is filled with one-third of its volume and rotated at about 60 rpm. The autogenous cleaning requires water wash of RCA for removal of all the loose particle and dust from its surface. The post-processing of autogenous cleaning is almost similar to the pre-soaking in water suggested by Katz [47]. High efficiency of autogenous cleaning process has been found to be with 10–15 min of mill drum rotation. This method has an advantage over the presoaking treatment as this is able to remove more percentage of attached mortar but also comes with the limitation that only a small percentage of attached mortar can be separated. 5.1.1.3. Mechanical grinding. Another effective method for minimizing the attached mortar in RA is by mechanical grinding using crushing and ball-milling [50]. From the basic theory, the ball milling can separate more attached mortar than autogenous cleaning. The process is similar to the Los Angeles abrasion test of aggregates where a high speed rotating eccentric gear is used in a grinding mill. Dimitriou et al. suggested another treatment method, a combination of RCA soaked in water and mechanical grinding (rotation at 10 rpm for five hours) to remove weak AM [51]. Although the mechanical grinding process can remove a sig-

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A. Mistri et al. / Construction and Building Materials 233 (2020) 117894 Table 4 Removing of attached mortar treatment and their properties. Treatment methods

Aggregate type

WA (%)

SP

Abr (%)

MC (%)

Pre-soaking in water [47] Mechanical grinding + Soaking [51]

RCA NA RCA Treated

5.5 2.5 7.2 3.7

2.41 2.52 2.28 2.49

29 32 15

0.5–0.6 2.7–2.9 2.6–2.8

Autogenous cleaning [46]

NA RCA Treated (10 min)

1.28 4.94 4.01

2.634 2.268 2.358

Acid solution (3 days 0.1 M HCl) treatment [56]

NA RCA Treated

0.6 4.44 3.58

2.6 2.33 2.39

Thermo chemical treatment [43]

RFA RFA(300 °C) RFA(500 °C) RFA(600 °C)

Thermal, chemical, acid [60]

NA RCA Treated Treated Treated Treated

(HCl) (350 °C) (750 °C) (C2H4O2)

CR (%)

24.32 29.15 27.39

IM (%)

Sound (%)

MRC (%)

30 41 14

0 24 9

13.98 21.78 20.27 – 1.41 1.58 1.78

5.91 5.66 5.35 6.73 5.79

2.295 2.305 2.334 2.302 2.299

23.57 25.31 27.89 50.01 23.51

27.42 – – – –

3.02 4.56 3.98 41.02 3.92

Note: CR = Crushing value, IM = Impact value, Abr = Abrasion value, WA = Water absorption, MC = Moisture content, SP = Specific gravity, MRC = Mortar content/Loss, Sound = Soundness value.

nificant amount of AM, this process is not capable of complete removal. The energy requirement for this process is compelling but cost analysis for the treatment is cheaper compared to NA [51]. Another disadvantage of mechanical grinding is the creation of meso-micro cracks and fissures on the modified RCA surface which can minimize the mechanical and durability properties of RAC [52]. This treatment method needs further investigation. 5.1.1.4. Heat grinding. Both heat grinding [53] and selective heat grinding [54] methods can be applied for enhancement of the RCA properties. However, there is a difference between these two methods. Heat grinding methods weaken the attached mortar at around 300 °C before mechanical grinding. The concept behind this method is similar to the one with effect of temperature on concrete. However, the properties of RCA are expected to be degraded at high temperatures (like 500–550 °C). This is due to the phase transformation of minerals in aggregates, cement mortar, interface and thermal micro-cracking. After complete treatment of RCA, if any AM is present within the aggregate surface, this may act as a weak fracture-processing zone in RAC. On the other hand, microwave is used for selective heat grinding in order to heat up and weaken the old ITZs of RCA. Selective heat grinding method claimed the production of high-quality RCA but at a tiny scale level (400–450 gm concrete sample was tested by Bru et al. [54]). This process may not be useful in large-scale treated RCA production as the total energy requirement for the treatment is significantly higher. 5.1.1.5. Acid treatment. The removal of attached cement mortar from the RCA surface is a very difficult task as the cement hydration is a chemical reaction between the cement constituents with water for a long period of time, thus imparting the concrete’s strong and stiff structure. However, there are different acids (strong to medium concentration) which are used to remove the attached mortar. The hydration products of cement are dissolved in this acid solution [10]. A number of studies have shown the applications of different acid solutions in removing attached mortar from RCA [9,43,55–59]. Most commonly used acids in this category have been sulfuric acid (H2SO4), hydrochloric acid (HCl), phosphoric acid (H3PO4) and acetic acid (CH3COOH) [10].

Ismail and Ramli [56] used HCl (of different molarities like 0.1, 0.5 and 0.8 M) for treating RCA for 1, 3 and 7 days. The treatment container was periodically shaken in order to accelerate the dissolution reactions between bonding of old parent aggregate with acid. The properties of acid treated RCA can be found in Table 4. The main difference was seen in the percentage loss of mortar, wherein higher acid concentration turned higher mortar loss. For example, one day 0.8 M acid immersion showed 5.11% mortar loss [56]. Acid treatment demands safety precautions while handling the acids and follow final washing by fresh water to remove chemical impurities from RCA. This process requires a huge amount of water. This chemically contaminated (most of the time sulfate and chloride) water needs suitable recycling to protect the environment. In the current scenario where clean water is not sufficient to meet World’s drinking water requirements, there is a big question of adopting this type of chemical treatment method for RA.

5.1.1.6. Thermo-chemical treatment. Thermo-chemical treatment on RA by Kumar and Minocha [43] is a combination of both thermal and chemical treatment. Firstly, RA was soaked in water for 24 h followed by heating of the samples in a muffle furnace at a constant thermal load for 120 ± 5 min. Thermal loads at 300, 400, 500, and 600 °C were used to remove partially or weak attached mortar. Along with temperature treatment, three acid molarities of 0.1, 0.4, and 0.7 M HCl were used. The study compared the convention thermal and chemical treatment separately and combined (thermochemical) treatment methods. It was concluded that the thermochemical treatment removes the cement paste to a greater extent. This study was conducted on recycled fine aggregate wherein the result showed 300 °C thermal load along with chemical treatment is the most efficient way to remove cement paste compared to other possible combinations. The strong chemical (like 0.7 M of HCl) treatment requires a large amount of water to wash all the harmful Cl- from aggregates. Drawbacks of these thermochemical methods include both the above-mentioned issues separately (thermal and chemical) for the treatment of RA. Al-Bayati et al. [60] experimented on different treatment methods and their effectiveness on attached mortar removal using different techniques as mechanical, thermal and chemical

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treatments on a particular RA coming from a ready-mix plant. The study was quite interesting because different treatments have been applied to a single source of RA and characterised (physical and mechanical) accordingly. For thermal treatment on RA, a thermal load of 250, 350, 500 and 750 °C was applied. For acid treatment, HCl (concentration about 37%) and acetic acid (C2H4O2) (concentration about 99.7%) were used. The details of the properties of RA before and after treatment can be found in Table 4. The advanced charecterization was carried out using Energy Dispersive X-ray Analyzer (EDAX) and Scanning Electron Microscopy (SEM) analysis. For thermal load at 750 °C, a drastic change in mass loss (41.02%) from RA was observed. The advanced characterization showed that the surface morphology of untreated RA is rough, porous and heterogeneous, whereas the treated surface is comparatively homogenous with less attached mortar (completely depending on treatment type). Although the treatment methods were effective for improving the physical properties, the study did not recommend any thorough relationship between physical and morphological properties of RA as well as RAC [60]. 5.1.2. Strengthening of attached mortar The very basic idea behind the strengthening is to fill the micro (sometimes meso) pores of the attached mortar in RCA surface. With respect to micro-pore, strengthening has some common objectives such as efficient filling of micro-cracks, surface coating on attached mortar in compact form and being water-repellent. According to Shi et al. 2016 [10] developing strong ITZs by means of filling the weak zones of AM are the primary objectives of any treatment for strengthening. There are several methods on treatment of RCA by means of strengthening the AM: (a) pozzolana slurry (b) emulsion polymer (c) bio-deposition of calcium carbonate (d) carbonation (e) cement slurry coating (d) sodium silicate solution and (f) different mixing methods. Each of the methods has been discussed separately in the next subsection for better understanding. 5.1.2.1. Pozzolana slurry. Mineral admixture is an excellent, efficient, sustainable by-product (coming from coal, iron, etc., industries) to mitigate a major percentage of negative properties of RA, especially from a durability point of view. The suitable modification in design mix by incorporating mineral admixture (like fly

ash, silica fume, GGBS, etc.) can lead to enhancement of concrete properties [2,5,10,24]. The basic science of using mineral admixture (sometimes pozzolana) is the pore refinement of AM and secondary hydration between added mineral admixture and byproduct of cement hydration (primarily, Ca(OH)2 or CH). The strength of concrete (broadly, engineering properties) is directly dependent on the pores and internal cracks present in the composite [61]. ITZ is the weakest zone in concrete and composed of large size of crystals (initial hydration product of cement) which make micro-pores within normal concrete [61,62]. Fig. 3 is a representation of this ITZ formed structure of normal concrete. This detailed depiction of ITZ helps to understand what treatment method/material is suitable for its improvement in case a similar normal concrete becomes RA. The macro-micro pores need to be filled with a fine particle which will not only act as inert filler but also help in the hydration process (secondary hydration). Therefore, a comparatively better material (here RA) can be obtained using a coating of pozzolana which is the primary factor for creation of superior quality RA. Kou and Poon, [24] studied the durability properties of RAC using fly ash as partial replacement of cement and reported its positive impact. Li et al. [63] investigated the effect of coating of RA by pozzolanic powder (fly ash, silica fume, and GGBS). Different mixing of pozzolana was used and the combination of Portland cement along with fly ash and silica fume is found to be more effective for high strength RAC with better packing density and denser ITZ. This could be because of the better packing in terms of pore filling. Fig. 4 shows the microstructure of new and old ITZs without nano-silica (NS) and with NS by Li et al. [64]. Strengthening of AM using pozzolana actually helps to make a better quality RA or RAC compared to the previously described techniques of removing attached mortar. The workability may be reduced as fine particle is present in a given mixture, but this issue can be solved using proper design mix of concrete and use of appropriate admixture. The different mixing approaches that were developed for property enhancement of RA have been discussed in detail in the subsequent sections.

5.1.2.2. Emulsion polymer. Emulsion polymer is a colloidal dispersion of discrete polymer particles. The most common polymer emulsion for treatment of RA is the polyvinyl alcohol (PVA) emul-

Fig. 3. Aggregate, interfacial transition zone and the bulk cement paste in concrete [61].

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Fig. 4. Microstructure of new and old ITZs a) without nano-silica b) with nano silica [64].

Table 5 Strengthening of attached mortar treatment and their properties. Treatment methods

Aggregate type

WA (%)

SP

Pozzolana slurry [24]

NA (10 mm) NA (20 mm) RCA (10 mm) RCA (20 mm)

1.12 1.11 4.26 3.52

2.62 2.62 2.49 2.58

CR (%)

Polymer emulsion (20 mm aggregate) [65]

NA RCA Treated (Oven) Treated (Air)

0.68 6.23 2.39 1.62

2.662 2.423 2.466 2.472

Carbonation [93]

RA (5–10 mm) RA (10–20 mm) Treated (5–10 mm) Treated (10–20 mm)

6.22 6.19 5.31 4.81

2.611 2.583 2.621 2.604

– 27.8 – 21.9

Nano silica Bacteria (ureolatic) Bacteria (non-ureolatic) [34]

NA RCA Treated (3% NS) Treated (UR-Bact) Treated (NR-Bact)

0.65 1.4 0.9 0.5 0.5

2.62 2.2 2.7 2.87 2.89

20.06 26.51

IM (%)

Porosity (%) 1.62 1.62 8.69 8.69

20.32 26.89

Note: WA = Water absorption, SP = Specific gravity, CR = Crushing value, IM = Impact value.

sion and some saline-based polymers. The water-repellent properties of this polymer emulsion makes it suitable for the reduction of water absorption in porous materials (here RA). Treatment of RA with polymer emulsion can fill the micro-pores and micro-cracks of AM surface, thus resulting in significantly lower water absorption of RA. Many of the review papers have reported that RA can be treated with 6–12% PVA (10% is the recommended value) to enhance the physical and mechanical properties of RA. However, there is a long procedure of this treatment as per the report by Kou and Poon [65]. The aggregate is placed in desiccators with a vacuum pump, which is operated at a pressure of about 920 mbar for 6 h. Then the solution is made as per suitable PVA percentage (6–12% with water). The solution is then applied to the RA and allowed to soak for 24 h with the vacuum pump on. After the presoaking, the aggregates are removed from the vacuum desiccators and treated aggregates are ready for testing. The details of the procedure can be found in the published literature [65]. It was observed that the water absorption improved significantly with this treatment (refer Table 5). Mansur et al. observed that PVA can improve the bond strength of cement paste and aggregate phase [66]. The study also suggested that the application of PVA could reduce the effective water/cement ratio of ITZ (which further

results the reduction of the porous transition zone thickness). This fact is very common in nature as described in literature [61,62] that a thin layer of water is formed on the surface of aggregate after compacting fresh concrete. This layer of water increases the effective water-cement ratio around aggregate compared to the bulk cement paste. Now, if the water absorption of RA is restrained by PVA treatment, comparatively less water is available on the surface of RA. All the published literature [67–72] of surface treatment using PVA is done with the prime objective ‘‘reduction of water absorption of RA”. Although the polymer treatment can reduce water absorption, improve workability, can fill some pores, and act as a water repellent agent; the mechanical properties of treated RA perform similar to untreated RA [10]. This treatment method has a few drawbacks, as it cannot improve the strength of AM, requires special desiccators, pump with some specific pressure, time and energy consumption. Overall, economic consideration and large quantity production aspects also limit its usage. That is why this method may not be suggestible and much less common in practice. 5.1.2.3. Bio-deposition of calcium carbonate. Microbial carbonate precipitation (MCP) has recently been explored as a promising

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Fig. 5. FESEM micrograph of (a) untreated RCA, and (b) bacteria-treated RCA [85].

technology for improving the engineering properties of different construction materials [73]. This technology is based on a natural process wherein bacterial cells produce inorganic minerals as part of their basic metabolic activities [73,74]. In the process of urea hydrolysis, via metabolic activity of bacteria, urea is converted into ammonia (NH4) and carbon dioxide (CO2). This ammonia (NH4) reacts with water (H2O) within the microenvironment to produce ammonium (NH+4) and hydroxyl ions (OH) increasing the pH of the microenvironment. At high pH, carbon dioxide (CO2) reacts with hydroxyl ions (OH) to form bicarbonate (HCO 3 ). Finally, in the presence of calcium source (for example, calcium chloride (CaCl2)), bicarbonate (HCO–3) reacts with hydroxyl ions (OH–) to precipitate calcium carbonate (CaCO3). The whole process of calcium carbonate precipitation is represented in Eqs. (1) to (4).

NH2  CO  NH2 þ H2 O ! 2NH3 þ CO2

ð1Þ

2NH3 þ 2H2 O ! 2NHþ4 þ 2OH

ð2Þ

CO2 þ 2OH ! HCO3

ð3Þ

Ca2þ þ HCO3 þ OH ! CaCO3 þ H2 O

ð4Þ

This natural bio-cementation process has now been established widely for remediation, restoration and durability enhancement of a large number of construction materials [75,76]. Bio-deposition has also been found to have a significant effect on improving the surface properties of concrete via reducing the permeability [73,76–83]. In case of RCA, the improvement in the surface properties can lead to better performance by reducing the porosity and moisture absorption inside. The basic advantage of MCP is that bacteria are small in size and can enter into the micro pores where cement or fly ash cannot enter. Grabiec et al. [84] confirmed the feasibility of applying calcifying bacteria on RCA in order to decrease water absorption while improving other engineering properties. Qiu et al. [85] experimented on surface treatment of RCA using microbial carbonate precipitation. The study confirmed that MCP is able to precipitate CaCO3 crystals within the RCA pores and surface, reducing its water absorption. Fig. 5 shows the microstructure of RA before and after treatment with calcifying bacteria. Surface deposition of CaCO3 crystals has been reported to cause 20–30% decrease in the water absorption in different studies [11,85]. The whole mechanism of deposition depends on the type of bacteria, applied materials and methods, and aggregate porosity [86]. This MCP method can be applied to both normal weight aggregate and lightweight aggregate. The constructive healing and

consolidation of pores within the aggregates with MCP is the prime reason for lowering in RA water absorption. Wang et al. [11] conducted an experiment applying bio-deposition treatment with two different representative recycled aggregates. The study concludes that bio-deposition treatment can decrease water absorption of RCAs effectively by filling large pores with higher porosity. The biogenic CaCO3 reinforcement effect on the surface of aggregates can lead to improvement of fragmentation resistance. Bacteria such as Bacillus pasteurii, B. megaterium, and others are commonly used as a crack healer in microbial concrete, while B. sphaericus is used widely for surface treatment [87]. A very recent review paper on microbial healing in concrete cracks tabulated the detailed information of applications of different microorganisms in concrete with ureolytic bacteria [81]. Another recent study by Singh et al. [34] investigated the surface treatment of RA using nano-silica and bacteria (both ureolytic and non-ureolytic) induced mineral precipitation. Water absorption and specific gravity improved significantly by this treatment (refer to Table 5). FESEM and EDX analysis found in literature for the investigation of effect of bacterial treatment on RA, reported that bacterial calcite can build comparatively densified ITZ [34]. Therefore, the enhancement of micro–macro properties of RA can be achieved. The feasibility and efficiency of microbial treatment have already been established and application on RA for property enhancement has also been validated by several researchers. However, the research is in laboratory scale and field applications are expected in the near future although a few field trials on application of bio-deposition have been successfully done [88]. Easier mode of application of bacteria culture, economical nutrients for bacterial growth and cementation reagents need more studies. 5.1.2.4. Carbonation. Carbonation is very common in nature and especially suitable for concrete. The main hydrated product of cement is calcium silicate hydrate (C-S-H) gel and Ca(OH)2. When atmospheric CO2 enters into the concrete through micro-pore and reacts with Ca(OH)2 to form CaCO3, this phenomenon is known as surface carbonation [61]. This process (refer Eqs. (5), and (6)) increases the volume of concrete by about 11% and 23% for Ca (OH)2 and C-S-H respectively [10]. For normal concrete structure, this process can cause carbonation shrinkage, which is not desirable. However, in case of RA, this process helps to get better quality RA as AM are strengthened. AM is composed of several micropores (depending upon the parent source of concrete) which can fill up partially through this process. But there are some limitations for natural carbonation process, dependence on the presence of Ca (OH)2, environmental conditions, surface characterisation of RA,

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time of process etc. Researchers have accelerated the process for RA by applying CO2 and pressure externally [89–91].

CaðOHÞ2 þ CO2 ! CaCO3 þ H2 O

ð5Þ

C  S  H þ CO2 ! CaCO3 þ SiO2 :nH2 O

ð6Þ

The rate of carbonation for Ca(OH)2 is higher initially than that of C-S-H [92]. After carbonation starts within the concrete, the pores start to fill and continue until the blockage of all connected pores (also depends on the favorable condition). Ca(OH)2 cannot react completely with CO2 and some percentage still remains within the concrete [89,92]. The carbonation of C-S-H starts with decalcification, where the Ca2+ within the interlayer reacts with CO2– 3 . The hydration products are different and for that reason the rate of carbonation varies [58,91], amongst that, Ca(OH)2 reacts in the first phase of carbonation. The accelerated carbonation process instruments include a compact steel container of specific volume, CO2 storage tank and air pump [93]. Before carbonation treatment, RA needs to be kept at 25 ± 3 °C with a relative humidity of 50 ± 5%. The chamber is vacuumed first with 0.6 bar air pressure, RA is put into the chamber, and CO2 is injected at a constant level up to 5 bar and kept (with 100% concentration of CO2) for 7 days. The detailed procedure can be found in the literature [93,94]. The physical properties of RA and treated RA are presented in Table 5. There are several studies on the same carbonation treatment [10] to enhance the properties of RA; however, the same effect on compressive strength is more complex [95]. The Ca(OH)2 content is the prime factor of this type of treatment which varies for a different type of cement and concrete (like OPC, PPC, and PSC). There is an uncertainty of this carbonation treatment for lower calcium hydroxide content concrete and pore distribution within concrete. 5.1.2.5. Cement slurry coating. Martirena et al. studied cement coating on RA surface and examined the properties of treated aggregate as well as the use of treated aggregate into concrete [96]. The coated RA (0.16–0.23 mm cement slurry layer on the outer surface) shows a significant reduction in porosity (55% reduction) which results in lower water absorption. The thin layer of coating results in the improvement of physical, mechanical and durability properties of concrete when treated RA is used [96]. This treatment needs to be explored more as the cement slurry is more porous in nature compared to attached mortar of RA. 5.1.2.6. Sodium silicate solution. Sodium silicate solution treatment is almost similar to the polymer emulsion treatment. This treatment is also known as waterglass treatment or pore blocking surface treatment [97]. The sodium silicate treatment is one of the inorganic treatments of RA in which silicate-based solutions and

9

fluosilicate have been substantiated to be active in order to block the capillary pores within concrete surfaces [98]. However, there is an evidence that sodium silicate treatment on concrete is not able to improve chloride ions penetration as well as water penetration [99,100]. The reaction between sodium silicate and calcium hydroxide can lead to higher alkali content in the concrete (ref. Eq. (7)) which may further cause an alkali-silica reaction [10,97]. Therefore, the drawback of the sodium silicate treatment is that it can damage durability property.

Na2 O  SiO2 þ CaðOHÞ2 ! CaO  nSiO2  xH2 OðGelsÞ þ 2NaOH

ð7Þ

5.1.3. Modification of the mixing process There are some new approaches in literature which have been carried out during the mixing process to enhance the performance of RAC. It has been observed that some of the shortcomings of RA properties as well as RAC can be overcome by using improved mixing techniques. Different modified mixing processes are a) double mixing method, b) two-stage mixing approach, and c) triple mixing method. By using double mixing method the compressive strength is found to be more and the long-term properties (like, chloride penetration and carbonation depth) are improved significantly as compared to the concrete produced by normal mixing method [101]. The second improved mixing approach for RCA is the Twostage mixing approach (TSMA), where the mixing process is dived into two parts and completely dependent on addition of water in the concrete mix at different time. Half of the calculated water for is added with aggregates first and then binder materials are applied for denser ITZ, remaining water is then added at altered times [102]. The compressive strength of RAC was found to increase up to 21%, and the claim for that was development of an effective and strong ITZ by filling of voids and cracks on the surface of RCA [102]. Another claim about TSMA is its effecacy for enhancement of durability. This is probably the improvement in the interface of aggregate surface. The modification over TSMA has also been carried out by Tam and Tam [103] and this is also known as modified TSMA. The third modified mixing approach is the triple mixing approach in which the RCA surface and ITZ can be improved further by surface coating of pozzolanic materials. It was reported that this mixing approach has a significant influence on improving different fresh and hardened engineering properties of concrete compared to double mixing method [104]. Fig. 6 shows the different mixing approaches that were developed in order to improve the RA properties by means of surface coating. The picture clearly explains how different mixing methods help to develop a better and densified microstructure. Normal mixing approach makes the loosest ITZ, wherein the TSMA with silica fume (or silica fume and cement gel) can improve

Fig. 6. RA structure after adopting different mixing approaches (i) NMA, (ii) TSMAs and (iii) TSMAsc [103].

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this towards a denser ITZ. It can also be noticed that modified TSMA [103] has more advantages and benefits than TSMA [102]. The reason behind this was reported to be modified TSMA is capable of filling micropores and microcracks by silica fume (or silica fume and cement gel) in addition to the effect of secondary reaction between Ca(OH)2 and silica fume [103]. There are no such drawbacks on modification of mixing (modified TSMA) process [103] as the method helps to achieve better concrete with same design mix proportions (or materials). Different mixing method can be implemented in the batching plant; however, the time of mixing may increase.

6. Overview of treatment methods for RCA (review papers on treatment methods for RCA) There are limited published review papers on performance enhancement of RCA or treatment method on RCA. The suggestion from the experts are varying and that is probably based on the research methodology. Kisku et al. [2] reviewed more than 200 journal papers and reported all possible problems associated with RCA. The review paper has also reported that there are many methods for RAC making procedure but the established methods have not been confirmed yet. It was also conveyed that RA properties could be improved using proper surface treatment by adding a suitable percentage of mineral admixture. Several researchers have emphasized the TSMA method. Performance enhancement of RCA by Shi et al. [10] described all the possible treatment methods for RCA based on attached mortar removal and strengthening of attached mortar. The final summary of their study suggested carbon dioxide pretreatment of RCA as an efficient and eco-friendly method for improvement of RCA properties. However, some questions come into account that there should be sufficient hydrated product (like, calcium hydroxide) to react with applied CO2. Carbonation is a very common phenomenon in nature that can occur in an open environment. Now, normal concrete has a tendency of carbonation on concrete surface and when RA is made and kept in open environment for a certain period of time, aggregate surface is open for carbonation as well as other degradation. Therefore, carbonation treatment may not be as suitable as expected for all the cases. Another important factor of carbonation is the presence of Ca(OH)2 within the attached mortar of RA. This Ca(OH)2 content percentage is related to the old parent concrete source history. It can be assumed that the concrete which is made of OPC cement, Ca(OH)2 content may be higher compared to the concrete made of PPC, PSC, and OPC with mineral admixture. Safiuddin et al. [8] reviewed the physical, mechanical and durability properties of RA and along with property enhancement or treatment methods on RA. Their paper summarized the problems associated with engineering properties of RA and emphasized on the durability of RA that needs to be solved. Appropriate material selection (basically, the old parent RA of good quality) can lead to high strength or high-performance concrete. The performance of RA or RAC can be enhanced after adopting suitable methods like adjusting the water-cement ratio, new mixing techniques, incorporating pozzolana and extended curing [8]. Pan et al. [97] summarized several RA surface enhancement techniques. The existing concrete surface (fresh or already exposed to environment) can be improved to protect by surface coating techniques like hydrophobic impregnation, pore blocking surface treatment. The content of Ca(OH)2 within the attached mortar or hardened cement paste will affect such kind of surface treatment. Although there are several treatment methods available in literature, their applicability and suitable guidelines need to be established. Luo et al. [105] reviewed the applications of nanotechnology on RAC. The review found that although there are major research outcomes that have

already been conducted on RA and RAC; there exists a great challenge while producing a satisfactory and stable performance of RAC for practical applications. Recent treatment methods on RA based on nanotechnology application and research on the application of nanotechnology on RA or RAC are required to mitigate the drawbacks. 7. Summary and recommendation At present, the research on RA is not a new field of interest, significant research on RA has been conducted since the last 20– 25 years. However, any proper solution for effective use of RA in structural and non-structural applications has not been confirmed yet. In the meantime, quantity of RA is increasing day-by-day and needs immediate disposal for clean environment, protection of soil and avoiding water contamination from C&D waste along with sustainability issue. The present study is an attempt to review all the possible treatment methods on RA with an aim to analyze them carefully. This review paper will help the researchers and technologists to understand the state of art on different treatment methods on RA as well as the ability to think critically about the methods to be adopted for practice. Several researchers suggest carbonation treatment but natural carbonation and property degradation of RA with time needs to be considered. If old parent concrete is made of pozzolana/slag cement, the scope of carbonation may decrease. With the review of all the treatment methods and considering sustainable environment and construction, it can be summarized that strengthening of attached mortar is a better technique. Within this scope, the use of pozzolana and nano-silica coating using the TSMA (modified TSMA) is a superior established method. The second possible solution on treatment could be bio-deposition of calcium carbonate, in which bacteria can heal the micro-pore and micro-cracks of AM as well as ITZs. This results in the strengthening of the RA along with attached mortar. Due to some lagging of field application, this method is not much used in practice, but in the near future, this might be possible. From the basic mechanical properties and durability point of view, the above-discussed two methods can be considered for effective use of RA for sustainable construction. The present recommendation is to strengthen the attached mortar and also suggest to make RA from C&D waste when it is needed for basic test and application, otherwise do not crush C&D waste concrete boulders too early of application as it may deteriorate the properties. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement The first author wishes to acknowledge the joint PhD program between Indian Institute of Technology Kharagpur, India and Curtin University, Australia. References [1] B.B. Mukharjee, S.V. Barai, Influence of Nano-Silica on the properties of recycled aggregate concrete, Constr. Build. Mater. 55 (2014) 29–37, https:// doi.org/10.1016/j.conbuildmat.2014.01.003. [2] N. Kisku, H. Joshi, M. Ansari, S.K. Panda, S. Nayak, S.C. Dutta, A critical review and assessment for usage of recycled aggregate as sustainable construction material, Constr. Build. Mater. 131 (2017) 721–740, https://doi.org/10.1016/ j.conbuildmat.2016.11.029. [3] A. Mistri, S.K. Bhattacharyya, N. Dhami, A. Mukherjee, S.V. Barai, Petrographic investigation on recycled coarse aggregate and identification the reason

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