Introduction

Sand is a core ingredient in making cement concrete and mortar. Traditionally river sand, which is formed by natural weathering of rocks over many years, is preferred as a fine aggregate. The economic development fuelling the growth of infrastructure and housing generates huge demand for building materials like sand. But the indiscriminate mining of sand from riverbeds is posing a serious threat to environment such as erosion of riverbed and banks, triggering landslides, loss of vegetation on the bank of rivers, lowering the underground water table etc. Hence, sand mining from riverbeds is being restricted or banned by the authorities.

Manufactured sand, which is obtained by crushing the rock, is emerging as a viable alternative to river sand. This material is in use for quite some time in developed countries. The use of this sand (also called artificial sand, M- Sand, Robo Sand etc.,) is picking up in India in major cities. Use of scientifically produced Manufactured Sand as an alternative to river sand is the need of the hour and will provide a long term solution to Indian Construction Industry.

Manufactured Sand

Manufactured sand is crushed fine aggregate produced from a source material and designed for use in concrete or for other specific products. Only source materials with suitable strength, durability and shape characteristics should be used. Production generally involves crushing, screening and possibly washing. Separation into discrete fractions, recombining and blending may be necessary.

Manufactured Sand is produced by feeding hard stones of varying sizes to primary and secondary crushers (Jaw crusher and Cone crusher), for size reduction and these crushed stones are further crushed in Vertical Shaft Impact (VSI) crusher to reduce the particle size to that of sand.

During the production processes, it is ensured that sand stockpiles are not contaminated with weathered/highly altered rock or with clay and other contaminants. Crushing of multiple source rocks into a single sand stockpile is also not be permitted unless it can be demonstrated that such a process is under blending control and produces a consistent product.

The fine particles obtained, as a by-product during crushing of rocks to produce coarse aggregates (by jaw crusher and/or cone crusher) is known as Crusher Dust/Quarry Dust. This often contains higher percentage of dusty, flaky particles and particle sizes are un-controlled and hence not suitable for construction.

Screening and Washing with built-in process of different stages of screening, Manufactured Sand plants ensure proper grading for better particle size distribution.
By washing, the percentage of micro fines (passing 75 micron) is controlled below 15% by weight. The washing facility also provides keeps the Manufactured Sand in wet or partially wet condition. This will help to reduce the water absorption rate by Manufactured Sand during concrete manufacturing and hence better workability and workability retention.

Suitability of Manufactured Sand as fine aggregates for Concrete & Mortars – A Case Study

A study was conducted at Indian Institute of Science, Bangalore, to ascertain the suitability of Manufactured Sand as fine aggregate in Concrete (grades of M20, M30) and Cement mortars (ratio 1:4 and 1:6).

Properties River Sand Manufactured Sand Advantages of Manufactured Sand
Shape Spherical particle Cubical particle Higher Cohesion & Compressive Strength
Gradation Cannot be Controlled Can be Controlled Reduction in Voids & Higher Cohesion
Particle Passing 75micron Up to 3%(IS:383-1970) Up to 15%(IS:383-1970) Refer Note below
Clay & Organic impurities Likely to be present(retard the setting & comp. strength) Absent Better Concrete Quality
Grading zone (IS-383) Mostly conforms to Zone II & III Manufactured to conform to Zone II Zone II ideal for Concrete

Tests were conducted to study fresh and hardened properties of the Concrete and Mortars made with river and Manufactured Sand.

1. Grading limits of Manufactured Sand fell within the grading Zone-II of fine aggregates as specified in IS 383.
2. Shape of the Manufactured Sand particles resembled the shape of river sand particles.
3. Bulk density and specific gravity of Manufactured Sand were comparable to those of river sand.

Resources

http://www.thehindu.com/news/national/kerala/all-in-a-grain-of-msand/article7644375.ece
https://www.masterbuilder.co.in/river-sand-substitutes-overview/

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