Restrictions imposed on extraction of natural sand have compelled the construction industry to look for alternative material. Manufactured sand (M-Sand) is one of the foremost alternatives, writes Suhas S Dhuri.
Sand is one of the ´active´ ingredients of concrete. It is ´active´ in the sense that it has a great bearing on some of the important characteristics of concrete, especially in the fresh state, which ultimately decides the overall performance of the concrete, though it is inert with respect to the various reactions that take place during the bonding of various constituents of concrete.
Traditionally, naturally available river sand has been the choice and generally, there are no issues with this practice. However, with continued use and with ever increasing construction activities, this resource is dwindling globally and restrictions put in place by various governmental agencies, with a view to preserve ecology and environment has brought in severe strains on the construction industry to look for alternate materials. The problem is acute in Mumbai with so much of construction activity around and a practically blanket ban on natural sand dredging.
One of the first alternate materials was crusher fines or crusher dust. This is not a suitable alternative, since the fine material content varies drastically. Its acicular particle shape and rough texture results harsh mixes, which are not cohesive. Added to this material is inconsistent in size distribution.
Manufactured sand on the other hand is a product derived from crushers like crusher fines, but from specially designed crushers and manufacturing process. When rock is crushed and sized in a quarry the main aim has generally been to produce coarse aggregates. This process has left over a proportion of excess fines of variable properties, generally finer than 5-mm size (stone dust/grit). The ready mixed concrete industry has for some time tried to find ways to utilise this material as a controlled replacement of natural sand. Manufactured sand is defined as a purpose-made crushed fine aggregate produced from a suitable source material. Production requires specially designed crushing equipment, screening and possibly washing. It is recognised from both local and overseas experience, that some quarry sources or some rock types within any particular quarry would not be suitable for use as manufactured sand in concrete.
Differences´ between M-sand and crusher dust
Aggregates crushing plants vary significantly in their degree of sophistication, in the range and extent of plant process controls and in the degree to which quarry raw feed is controlled to the plant. In simpler plant configurations the quarry raw feed, even when the raw feed is selected on some basis of quality, is processed through two, three or four stages of crushing and shaping and then is separated into a variety of sized aggregates. Usually all aggregate sizes are plus 5 mm and the fines removed at minus 5mm are stockpiled as crusher dust.
Most plants however direct the crushed fines to a single dust stockpile. Provided plant settings are not altered to any great extent and the quarry rock types do not vary excessively, the crusher dusts will settle to a relatively consistent product. Variation in the sizing of the product will result from screen and crusher wear, resetting, and the occasional errors such as a broken screen or screen overflow. Product quality will vary depending on the range and quantity of different type of rock. Manufactured sand is a further development in the use of crushed fine aggregate. The term ´Manufactured Sand´ has been developed in world-wide literature to refer to that crushed fine aggregate specifically developed for use as a fine aggregate in concrete. Like any other component of concrete, manufactured sand must be controlled by specification that is suited to the end performance required of the concrete
Natural sand and M-sand
Significant differences between natural sands and manufactured sands result from the geological processes of shaping and sorting that has occurred with most natural sands. The individual grains in natural sands tend to be rounded to sub rounded and have a smooth surface texture. If they are more-mature sediments (i.e., further from the source of erosion) they will tend to be better sorted size-wise, to the point with dune sands that they tend to approach single sized materials. Most natural sands have been abraded to the point that weaker minerals (clays and softer altered minerals) have been separated and removed from the deposits. Abrasion can continue to the point that natural sands become a single mineral material - quartz sands. Better shape and smoother surface texture of most natural sands reduces the inter-particle friction in the fine aggregate component of the concrete mix grading. The consequence is improved workability (usually measured as improved slump). Blending is effective in reducing the level of microfines in the fine aggregate used in the concrete mix compared with the microfines in the manufactured sand. Blending is in fact the simplest and probably the most cost effective means of minimising any adverse properties arising from the typical 10 to 20 per cent passing 150-micron fraction present in most manufactured sands. The microfines are the proportion of the grading most likely to include mineralogy that will increase water demand in the concrete mix.
However in the current situation, where natural sand is simply not available, total replacement of natural sand is imperative. Despite the improvements made to the properties of manufactured sand, cent percent use of manufactured sand has the risk of concrete segregation in high workability mixes. This problem can be taken care of by use of mineral admixtures like fly ash and designing mixes appropriately. Chemical admixture industry has been working on this issue as well and they have come out with products to address this problem. Thus today, it is possible to produce high strength pumpable concrete mixes replacing natural sand completely and this has been done in several projects.
It is even possible to replace natural sand totally in plaster work, with appropriate admixtures and proportions.
The author is CEO of e Cube Consultants.