Making sand

With some countries beginning to run out of natural sand and extreme shortfalls of production being experienced, more quarry operators are looking to the advantages of manufactured sand. When done right, manufactured sand can outperform natural sands on costs, quality and performance.

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By  Colin Foreman Published  December 4, 2004

Making Sand|~|50 Products Body.jpg|~||~|Every country has its own specifications for aggregates, regardless of size or use. These specifications have been developed from trial and error or experimental data. This applies particularly to sand where the specification is method-based. Unless method-based specifications are continually updated, to take into account new technologies and construction techniques, advancement in concrete technology will remain stifled. This effectively ensures construction practices established many years ago will continue, retarding innovation. Using performance-based specifications introduces the use of new materials and techniques that would otherwise not be considered. This ensures cheaper and, in many cases, higher quality products. Many fine aggregates worldwide are based on natural sand specifications. These are not ideal when it comes to specifying manufactured sands. Manufactured sand, at times, does not meet the specified requirements, and has therefore has been ruled out as a possible alternative. Experience has shown however that with the advancement in Metso Minerals Barmac VSI manufactured sand technology, a product is available which outperforms natural sand on cost and in the properties of concrete, in both its plastic and hardened states. The most important consideration when using manufactured sand is that it should not be treated in the same manner as natural sand. This implies that the specified gradations for natural sands may not be appropriate for manufactured sand. The quarry operator is primarily interested in maximising his return on the range of saleable products produced. This includes the production of sand. Where a quarry is not set up to make high quality products, it results in the production of poor quality crusher fines. In some markets, however, the potential for the operator to produce a manufactured sand superior in performance to that of conventional sands has been identified and specifications have changed to meet this need. The ability of the Barmac VSI to process an all-in feed gradation allows for the processing of crusher fines that would otherwise be scalped as a poor quality sand product, with little or no saleable value. The practice of wasting these fines can put a strain on a quarry’s resources and cash flow. The cost of transporting poorly shaped sand to waste stockpiles; the realisation that 100% of the quarry’s throughput will not be a quality saleable product; and the cost of removal or reprocessing of a large waste stockpile at the end of the quarry’s life; contribute to the strain. Economically, this practice is not sustainable. Processing all material, even crusher fines, through a Barmac VSI will mitigate these costs. All feed to the VSI, whether fine or coarse, is turned into cubical aggregate, fit for use as concrete, asphalt and roadbase aggregates. When used to produce high quality manufactured sand, the Barmac VSI is generally installed in a closed circuit arrangement to produce a screened product ranging from 3 to 8 mm. The production of cubical shaped aggregate is extremely important for many of the applications in which Barmac VSIs have been applied. This includes shaping of quarry aggregates, providing shaped industrial mineral products or reducing and shaping recyclable materials. The improved shape achieved increases the saleability of the final product, adding value to a quarry operation. To allow consistent feed rates and limit circuit surges, a surge bin is usually placed before the VSI. This allows maximum control of the crusher gradation, percentage reporting to cascade and improved screening accuracy. To ensure effective shaping of all feed a post screen closed circuit arrangement is installed. The fresh feed requirements and the re-circulating load govern sizing of the VSI for this duty. The shape of the aggregate particles is dependent on the shape of the feed material, crushability of the rock and tip speed of the rotor. Shape improvement will be seen across all product size fractions, but the greatest shape improvement will be seen with materials that have a flaky and/or elongated nature. When processing mature alluvial gravel, shape improvements in the coarse fractions will be minimal with high quality manufactured sand being produced. When specifying a crusher to produce a high quality manufactured sand the benefits of each type must be evaluated. A Barmac VSI is able to process a wide range of feed gradations as well as wet material. Also, when using the VSI crusher, shape improvement will be seen over all the particle fractions. A cone crusher will provide greater F(80)/P(80) reduction along with a gradation which is able to meet ASTM C33. Everyone understands that smooth, rounded particles, in addition to easily rolling and sliding over each other, can form a far more homogenous matrix than flat particles when they achieve their final packing position. From a concrete aggregate classification point of view, flat particles are classified as flaky and are rejected as unsuitable for use, (apart from the fact that they do not pack to form a dense matrix). Great care should be given to the sand manufacturing process, as the incorrect selection of crusher and crushing circuit will result in particles with unacceptable shape characteristics. The strength of concrete, with all other factors being constant, is directly proportional to its density. At maximum density the compressive strength of the mix is at an absolute maximum value. Aggregates with cubical shape, such as those produced by a Barmac VSI, allow maximum densities to be achieved. The flexible strength of concrete is increased by manufactured sand. Due to its production process the surface texture is rougher than that of natural sand, which improves the adhesion of the cement paste to the sand particles. To maximise the flexural strength, it is therefore imperative the shape of the particles be rounded or cubical without having a polished surface. Experience in the field has proven direct shape measurements on fine aggregates consistently show lower air voids, which denotes improved particle packing. By measuring flow time when performing the uncompacted voids test an additional parameter is determined. Superior shaped particles will have lower flow times than misshapen particles. Independent of other properties of a material, this test gives a good indication as to how the sand will perform in a dynamic situation such as workability and pumpability. From laboratory testing, Barmac VSI aggregates exhibit lower flow times than products from other types of crushers and in some cases lower than that for naturally occurring sands. When using the Barmac VSI produced manufactured sand, instead of natural sand, there is a consistent requirement to reduce the minus 75 micron fraction in the sand, in order for the sand to comply with the specifications. Why is this? When evaluating the use of clay-sized particles in concrete, their origin and structure requires understanding. To an engineer, clay is a material with high water demand coupled with low structural strength. To a soil scientist, however, clay is a mineral with a defined structure that has the potential to store nutrients and absorb water. Some of these naturally occurring clays have the potential to absorb up to 200 times their own weight in water. Based on this behaviour of clays, natural sand specifications correctly prohibit their use because of the detrimental affect on the finished concrete. Natural sands also contain organic materials that have a similar negative effect on concrete. The minus 75 micron product of Barmac VSI aggregates does not contain the same structure and water absorption characteristics as that of naturally occurring clays. These micro-fines are like coarse and fine aggregates, solid rock particles with set dimensions that have a minimal capacity to absorb water. It has been field proven that they have a positive effect on concrete during and after placement. Qualities, such as strength, (both flexural and compressive), workability, pumpability and permeability are all improved by the presence of this size fraction. Recent research into unwashed Barmac VSI manufactured sands in the United States of America has shown that up to 20% higher flexural strengths are achieved by incorporating high micro-fines into concrete. Since flexural strength of concrete usually determines the thickness of a concrete slab or road pavement, all steps should be taken to maximise the flexural strength of the concrete. By the use of manufactured sand, a 20% flexural strength increase in a concrete slab can be achieved, resulting in the slab being 10% thinner, assuming other factors remain constant. By using concrete made with high quality, well-shaped sand, with an elevated amount of micro-fines, a lower permeability concrete can be reduced. This ensures increased protection of the reinforcement, which allows the cover to the reinforcement to be reduced (the weight of the structure may be reduced by as much as 20%). Alternatively, the load carrying capacity can be increased by as much as 30% without increasing the weight of the structure or the crushing strength of the concrete.||**||

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