Concrete can be made a sustainable material right from raw material production to demolition of buildings [2, 3]. Concrete is a composite material consisting of binding materials, aggregates, water and admixtures. Since sand comprises about 35% of the total aggregate volume in concrete, the search for an eco-friendly and sustainable alternate has become necessary [3, 4].
Prof. S. K. Singh, Head, PPC Division, Ms S. K. Kirthika, PhD Student, AcSIR CSIR-Central Building Research Institute, Roorkee, Uttarakhand, India
The use of natural crushed rock sand (CRS) in concrete has significantly increased over the last decade, especially in areas where natural sands are scarce [5, 11]. Cortes et al.  found that a larger volume of paste of cement mortar will be required to attain adequate flowability and strength when angular CRS is used, instead of natural round aggregates of the same grain size distribution. But the cost of CRS is relatively more than natural sand. It can be used as an alternative to manufactured self-compacting concrete (SCC) with natural sand with 25-60 MPa compressive strength .
Construction and Demolition (C&D) waste is one of the biggest waste streams in most countries. India was among the top 10 countries in the world to generate the highest amount of C&D waste. The bigger challenge is that 70-75% of this waste remains untreated . The use of C&D as coarse and fine aggregates in concrete production is a logical step with both economic and environmental benefits [13-16].
Many researchers have found that 30% replacement of river sand (RS) with recycled fine aggregate (RFA) is more suitable [15, 16]. However, utilizing RFA produces low density concrete and RFA increases mechanical properties of the concrete with appropriate proportion of superplasticizer. Kou et al.  found that 10% replacement of silica fumes with cement in the RFA concrete increases both mechanical and durability properties of concrete. In addition, the incorporation of recycled concrete fine aggregate significantly increases the shrinkage and creep deformation [15, 16].
The non-availability of land for landfills and increase in cost of the landfill for various industries presents the option of using waste products as an alternative source. Initial studies have reported utilization of industrial by-products such as blast furnace slag, waste foundry sand (WFS), coal bottom ash (CBA), cement kiln dust (CKD) and wood ash (WA) as fine aggregate in concrete [8, 17-18].
Very few applications of manufactured sand can be seen in the construction industry due to lack of knowledge about the effectiveness of the different types of m-sand in concrete and how it makes the concrete sustainable. This paper envisages use of m-sand as a replacement of river sand in concrete, and its advantages, disadvantages, applications and barriers on its use in India.
Need for Manufactured Sands
Natural sands such as river sand, marine sand, quarry dust and dune sand have their own drawbacks. For instance, marine sand has a high percentage of salt [19-21]; quarry sand has a large amount of fines/dust; and because of the loosely packed soil particles, dune sand is unfit to be used in concrete. Despite the shortcomings, these sands are still used widely, leading to a huge depletion of natural sand, and causing environmental imbalance (Fig 1).
In addition, the available natural sand is becoming costlier . Various steps are being taken by different organizations to restrict the exploitation of these natural resources. The National Green Tribunal (NGT) has reported a huge loss of about Rs. 1,611 crores due to rampant illegal sand mining . The MoEF and the Supreme Court of India have also given various directives to ban sand mining. These issues have forced the construction industry to use alternative materials like manufactured sand as fine aggregate in concrete, without compromising the quality of construction.
Alternative Sand / Manufactured Sand
Alternative sand is mainly categorised as crushed rock sand (CRS), industrial by-product sand and recycled fine aggregate (RFA). IS 383  has suggested that any fine aggregates produced by any means of mechanical separations, crushing etc. can be considered as manufactured sand. It also considers RFA from C&D waste as manufactured sand. Many international researchers consider that CRS, produced from virgin rocks as manufactured sand [23-28]. Researchers are exploring use of CRS, recycled aggregate and some industrial by-products as fine aggregate in construction, either to be used as full or partial replacement of natural sand . These sands are more cost-effective and eco-friendly.
Crushed Rock Sand
CRS, produced by crushing virgin rocks, is generally more angular and has a rougher surface texture than naturally weathered sand particles . Diorite, metamorphic siltstone, granite, limestone, sandstone, feldspathic quartzite etc. are some of the parent rocks used for production of CRS [23-27]. The properties of CRS depend on their lithological character, composition and production process .
CRS is produced by a variety of crushing equipment, including cone crushers, impact crushers, roll crushers and rod mills . The comminution of rock materials into finer grained particle sizes by means of different kind of crushing is the key process in the making of crushed rock aggregates . The adaptation of the crushing process to the actual rock type and to the intended end use is crucial for the final result to be achieved . This implies that choice of crushers and their combinations, the number of crushing stages, the feeding, gap setting and operation of the individual crushers, and, of course, the maintenance also affects the properties of CRS [31, 32].
Crushing equipment is mainly classified as compression crushers and impact crushers. Of late, the vertical shaft impact (VSI) is gaining importance . Sand can be screened in various conditions: wet, moist or dry, of which air screening and hydrocyclone process are being used. . Screening helps in removal of dirt, dust and high fines from the sand. A typical manufacturing process of CRS is given in Fig. 2. Presence of high fines (size <75 µm) is highly harmful as it leads to shrinkage in concrete. However, the CRS fines contribute to an increase in paste volume, which is useful for the development of self-compacting concrete (SCC) . It has been found that 0.40-0.55 w/c ratio is more suitable for concrete containing CRS and about 0.6% to 1.5% superplasticizer dosage helps in achieving appropriate workability [5, 33]. Shen et al.  found that utilizing CRS in concrete makes it dense, and fine hydration products such as calcium sulfoaluminate hydrates / ettringite (AFt), calcium hydroxide (CH) and calcium aluminate hydrates mono sulfates (AFm) and Calcium silica hydrate (C-S-H) gel help in production of ultra-high strength concrete.
The durability parameters like chloride ion penetration, porosity, carbonation, freeze-thaw etc. at constant w/c ratio rises with increase in percentage replacement of CRS in concrete [34, 35]. Increase in volume change and carbonation rate are observed for small additions of fines, despite exhibiting improvement in compressive strength . Vijayalakshmi et al.  reported that as per ASTM C 1202 specifications, chloride permeability of granite dust concrete is higher than control concrete at ages of 180 and 365 days. Beixing et al.  compared durability properties of low and high strength crushed limestone concrete and observed that low strength CRS concrete improves its resistance to chloride ion penetration, freeze thaw more than high strength CRS concrete.
Industrial by-products are produced from iron mills, steel mills, thermal power plants, oil-fuel industries etc. A typical production of industrial by-product is shown in Fig 3. It is reported that only 15% of industrial waste is used as resource material and the rest is dumped as landfill . Non-availability of land for landfills and increase in cost of landfill is leading to use of by-products from industries in construction. Industrial by-products include blast furnace slag, waste foundry sand (WFS), coal bottom ash (CBA), cement kiln dust (CKD) and wood ash (WA) that can be used as fine aggregate in concrete [1, 8]. Indian standards have also allowed copper slag, iron slag and steel slag .
The problem in industrial by-product is the presence of high fines, which make concrete more susceptible to shrinkage and the workability of concrete also gets affected. Various steps such as air screening, Hydrocyclone process etc are taken to remove the ultra-fine particles. Concrete with WFS has 15-25% increased compressive strength [8, 38]. Use of steel slag up to 30% as sand in concrete mixes shows better compressive and tensile strength and better acid resistance than control concrete . According to Singh and Siddique , the reduced shrinkage strain exhibited by bottom ash concrete mixes probably due to lower free water cement ratio. During the mixing procedure the porous particles of dry coal bottom ash are retained as part of water internally. Devi et al  carried out an acid resistance experiment of concrete in both sulphuric and hydrochloric acid and observed that the weight reduction is less for 40% substitution of normal sand by steel slag when compared to control concrete. Al-Jabri et al.  stated that up to 40% substitution of sand by copper slag shows a diminishing in the surface water absorption, and after that the water absorption quickly increases.
Therefore, utilization of industrial by-products in concrete is not only a new way to reduce waste but also helps in production of value-added products. However, since each of the industrial by product is different in physical and chemical characteristics, their behaviour in concrete has to be well established before being used as fine aggregate.
Recycled Fine Aggregate (RFA)
Recycled aggregates are produced from the re-processing of mineral waste materials with the largest source from C&D waste [15, 42]. The trend of replacing natural sand with recycled fine aggregate (RFA) is gaining importance. Large-scale recycling of demolished concrete will contribute not only to the solution for a growing waste disposal problem but will also help in conserving natural sand. Recycled concrete fine aggregate, recycled brick fine aggregate, recycled glass fine aggregate, recycled bitumen aggregate etc. are some of the major recycled fine aggregates being used in the construction industry .
Researchers have reported that shape and texture of RFA aggregates depend mainly on the crusher type [43, 44]. They also noted that the rotational speed of the VSI crusher had no effect on the particle shape or particle size distribution of RFA, but they do affect other properties of concrete . A schematic production process of recycled fine aggregate is given in Fig. 4. The type of recycled fine aggregate to be used varies based on applications, for example, recycled bitumen aggregates are used in making pavements etc. Experimental investigations on RFA found that it requires a higher range of water reducer admixtures than RS because of the high fineness. RFA has particle sizes finer than 4 mm and up to 75 µm or below is suitable for mortar or concrete.
Reports have stated that use of RFA in concrete at higher percentage leads to less workable and durable concrete. This is due to presence of the old adhering mortar present in them. There are very few reports on the solution to water absorption viz. pre-soaking, triple mixing batching and mixing by centrifugal method . Evanglesita et al  stated that the non-steady chloride migration coefficient increases linearly with the replacement ratio of fine aggregate, reaching in the present case, and increase of 34% for concrete with total replacement of RFA compared to control concrete with RFA. Nevertheless, Zhao et al  suggested that pre-wetting of RFA can help in reducing the problem of drying shrinkage to a greater extent.
Benefits of Alternative Sands in Construction
- Wide availability of sources gives flexibility of manufacturing in nearby construction sites, thus driving down transportation costs and assuring timely supply to meet demand.
- Size of particles can be controlled in CRS and RFA.
- All alternative sands are eco-friendly, with low carbon-dioxide emission.
- Do not contain organic and soluble compounds that affect the setting time and properties of cement, so the required strength of concrete is maintained.
- Presence of impurities such as clay, dust and silt etc.is almost negligible.
- More cost-effective than natural sand.
- Help in reducing dumping of waste in landfills.
- Improve the mechanical and durability properties of concrete more than natural sands.
- Alternative sands have two grades for concreting and plastering which saves work on filtering the sand.
- There is no exploitation of river beds, thus preventing environmental catastrophes like water scarcity, ground water depletion etc.
- Workability issues: Manufactured sand can be of a coarser and angular texture than natural sand, which is smooth and rounded due to natural gradation. This can lead to more water and cement requirement to achieve the expected workability, leading to increased costs.
- Larger proportion of micro fines: Manufactured sand can contain larger amounts of micro fine particles than natural sand, owing to its production process. This again can affect the strength and workability of the screed or concrete.
- Low moisture content: Naturally available river sand has moisture trapped between its particles which is required for good concreting. On the other hand, only water washed M-Sand retains moisture.
- Due to its high demand, there are instances of alternative sands being adulterated with extraneous materials.
Many suppliers from places like Bengaluru, Chennai, Coimbatore, Hyderabad, Erode, Pune and Madurai are importing manufactured sand from countries like Malaysia, Thailand, Singapore, China etc. M-sand has become more expensive due to transportation from these countries. Hence, producing M-sand from crushing rocks and quarry stones to the stipulated 4.75 mm has become prevalent in a few southern states. Karnataka, which faces a significant shortage, has intensified M-sand production by 164 production units with aggregate capacity of 20-million tons. Telangana produces 7.2-million tons a year, Tamil Nadu 3.24-million tons a year; and Andhra Pradesh and Gujarat one-million tons a year.
The Pune-Mumbai expressway was completely built using manufactured sand, as was the runway of Thiruvananthapuram International Airport. Companies like CDE Asia, Puzzolana, Propel, POABS Group, Thriveni etc are serving the Indian market using wet processing equipment for production of M-sand from quarries, mines and recycling operations. CDE Asia has established a hydrocyclone process to remove the fines (<75 microns). Thriveni in Karnataka produces premium concrete sand and plaster sand produced and has established three plants with each producing over 7000 MT of fine and coarse aggregates every day.
|Table 1: Code and specifications|
|S. No.||Year||Codes and specifications|
|1||1986||Recycled aggregate & recycled aggregate concrete –Development report from 1945 to1985 RILEM Technical Committee 37-DRC|
|2||2002||Japanese standardization of recycling materials to concrete|
|3||2008||Guide to use manufactured sand concrete, CCA T60, Australia|
|4||2012||Standard specifications for roads & structures, NCDOT, Raleigh NC|
|5||2013||Aggregate for Concrete, BS EN 1260|
|6||2013||Aggregate for Concrete, CS3-2013, Hong Kong|
|7||2016||Standard specifications for concrete aggregate. Annual Book of ASTM Standards ASTM C33-2016|
IL&FS Environment has set up India’s first operational large-scale Construction & Demolition Waste Recycling facility for North Delhi Municipal Corporation on a PPP framework. Its plant at Burari eases the burden of 5000 tons of C&D waste that Delhi generates per day, by recycling it into construction-grade aggregates. Using advanced technology, the plant recovers 95% of C&D waste, and uses recycled sewage water for processing the waste. The wet-processing technology minimizes dust and noise pollution.
Although several parts of India are producing premium quality of M-sand, the actual usage in construction is still limited. Sand mafia, illegal sand mining, adulteration and poor quality of sand are still prevailing in India, and there are no standards in the utilization of M-sand. Hence, people still lack confidence. Though users consider crushed rock sand as M-sand and have started to use it but use of RFA, coal bottom ash, iron slag, steel slag, red mud, copper slag, foundry sand etc. is not prevalent at all, which is leading to landfill problems and environmental pollution. So, exploring other alternative sands is also important to meet the demand and supply gap.
The world is devouring sand in great quantities, though there has been greater concern on conserving the natural resources, leading to awareness of alternatives, especially after the World War II, in countries like Germany, USA, Japan, UK etc. . From the year 1918, the ACI committee had initiated utilization of CRS and quarry dust, but there were no proper experimental investigations on their properties. Some of the codes and guidelines followed all over the World are elaborated in Table 1. In addition, there are a few European, American and Australian codes available to assess the characteristics of CRS and other sands [48-53].
However, river sand is still being used for construction because all the codes and specifications consider only CRS as manufactured sand and the only permissible percentage fineness is prescribed. No other specifications based on their properties, types of alternative sand and their usages in construction are present. This is probably due to lack of knowledge about different types and their corresponding properties of manufactured sand.
Hurdles in Using Alternative Sands
Manufactured sand is being used in India, but is still not popular due to the following reasons:
Lack of standard: RILEM and a few other countries have certain specifications regarding RFA and CRS, but no detailed specifications/ provisions are mentioned, which would give assurance to users. In India too there are no details and specifications, especially regarding gradation, properties like void content, sand equivalent, limits of deleterious materials, and on proper mix proportions.
No government support: Besides the lack of government support, there are malpractices in every stage of construction, and lack of an appropriate policy to formulate and implement proper management of waste and use of alternatives. This is because even the government buildings are being built with river sand. Although, there are guidelines for sustainable mining, illegal sand mining continues to be prevalent.
Limited awareness: Lack of awareness of the possibilities of production and use of alternative sand in construction is a major barrier. There is a need to generate experimental data on different types of alternative sands and on their properties so that they can be recognised by the government and private construction sectors. The mass media partakes only in publicizing the criminal activities of illegal mining but does not provide information on the alarming depletion of river sand or on alternative sands available in the market.
Absence of suitable technology: Very few commercially viable and accepted technologies for production of alternative sands are present. Knowledge to tackle feedstock storage and transportation for proper quality of the sands is still lacking. There are no proper testing equipment and lab facilities to ensure the quality of alternative sands and the presence of quarry dust and inspection of production units is not undertaken.
Hesitancy of people: Limited knowledge has made people reluctant towards using alternative sands. This reluctance is more amongst the government officials and builders regarding the sands’ effect on quality.
There is a pressing need to create awareness of the problems related to environmental degradation, waste management and use of alternative sands. This can be brought about through the media, encouragement from different organisations, through seminars and conferences, which could generate extensive experimental data, which, in turn, would jhelp in creating guidelines and codes.
Government organizations should set an example of using alternative sand in buildings, bridges, highways, skyscrapers, etc. to bring confidence in the public at large. The construction industry can articulate guidelines to inspire use of alternative sand in projects and create a market mandate. An association of builders in consultation with experts and scientists can start work to promote its usage. Likewise, contractors and builders can endorse the sands in their client buildings. Research and academic institutions can play a vital role in the development of evaluation methods and specifications for use of alternative sands in construction. The media can sensitize the public by highlighting the importance of conserving our natural resources.
Increasing dependence on natural sand has led to river ecosystem disruption and environmental imbalance. Thus, finding an alternative to natural sand has become mandatory. Ove the past few decades, developed countries have been maintaining their global position in production of alternative sand and its applications in construction. Alternative sand is being used in India, particularly in the southern region.
CRS, industrial by-product, and RFA are found to be striking alternatives to natural sand. Re-using C&D waste and industrial by-products not only reduces environmental problems, but also helps in manufacturing sustainable materials and buildings/infrastructure like bridges etc. These alternatives are economical and eco-friendly. There is a huge lack of awareness and an aversion to using the sands. Government bodies, construction industries, research and academic institutions and the media should promote use of alternative sands in construction.
This paper has been submitted with the kind permission of Director, CSIR-CBRI, Roorkee. The financial assistance for this project is provided by NBCC (India) Limited, New Delhi.
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