With access to natural sand limited either by natural attrition or legislative design, the quarrying industry worldwide is having to experiment with manufactured sand from conventional aggregate sources. Georg Diem and Francis Lacote explain why the answer is crushed sand.
The demand for economical, efficient ways to produce crushed sand is growing, as natural sand deposits located near growth centres are depleted and environmental regulations get stricter.
What makes the change even more significant is the rapid pace of urbanisation. According to the United Nations, the number of people living in cities could double by 2050 to 6.5 billion. To accommodate growing populations, more houses, parks, roads, and underground utilities, among others, sand is required in one form or another as a basic building component.
Many aggregates producers are now studying the possibility of producing crushed sand, and at the same time looking at how to address the growing piles of quarrying waste, which may contain a fair proportion of fine aggregates that could be used in construction projects.
Is there a solution that would benefit all parties involved? Georg Diem, a senior process expert on the use of crushed sand in concrete production, and Francis Lacote, a sales support manager at Metso, delve into the topic.
A natural replacement?
Sand – whether extracted from natural gravel or sand deposits or made by crushing bedrock – is used in many applications because of its mineral and physical qualities. The most typical application for sand is concrete, where it accounts for about 30 to 40 per cent of the material volume.
“Concrete is the most common man- made material,” Lacote said. “You can find it everywhere. It is an essential element in the construction of infrastructure and in the building industry. Most of the interstate road network in the US is made of durable concrete, which can withstand heavy traffic and changing weather conditions.
Dams – like the Three Gorges Dam in China, where Metso’s crushing equipment has been used to produce aggregates – are the biggest structures made of concrete.
“Asphalt, which is common in road and highway construction, is the second most important application.
There are also the ‘functional’ aggregates, which are used, for example, for water purification in water treatment plants, golf courses and bunkers, children’s playgrounds, and horse-riding arenas, as well as on roads and pavements in icy climates to prevent sliding and slipping. In all of these applications, it is possible to replace natural sand with crushed sand, and even improve the properties of the end product.”
A diminishing resource
The use of natural sand is regulated by law in many places and totally forbidden in some places, like Japan. Some countries, including Sweden, have called for crushed sand to
be used in place of natural sand whenever possible. In Australia, the constraints relating to obtaining approvals are increasing, too.
Illegal sand mining has become an issue in some developing countries where demand is unbridled and the regulatory consequences are minimal. Selling sand is considered a quick way to make money with very little investment; all that is needed is a truck, a driver and a place from which to extract the sand. This is one of the reasons why India, among others, has established a sand policy to encourage the creation of new manufactured sand units by favouring them in the permit process for new quarries.
“I would say there are three categories of countries,” Diem said. “Take Sweden, where you have a special natural gravel tax to regulate sand mining. Some other European countries restrict the opening of new pits by other means. Then there are countries, like India, Singapore, and Malaysia, that have huge problems with non-existent or quickly diminishing natural sand resources. In these countries, sand is mined from open pits, beaches, and in-land dunes, and dredged from ocean and riverbeds. Legislation is not always in place or is just not effective [if it is].”
Crushed sand in concrete
Diem has a clear view of why crushed sand works well in concrete production.
“Concrete producers must meet certain minimum safety margins for the compressive strength of their products,” he said. “The higher the variability of the raw materials, the higher the cement content needed to maintain a certain level of margins. Thus, at the end of the day, the cost of raw materials of a lower quality and lower variability is comparable to the cost of better quality materials, but of a variable quality.
“With crushed sand produced in a controlled, engineered process, it is possible to achieve stable quality. It is much more difficult to control the variability of natural sand, especially if the sand is not washed,” he explained.
Washing, however, is not always feasible due to problems like lack of space, availability of water resources, environmental issues from dewatering ponds, and operational problems in places that experience freezing temperatures, eg in the Nordic countries.
“There are also other direct technical benefits from the use of the crushed sand,” Diem said. “For example, natural sand grains are rounded as a result of weathering, whereas crushed sand particles are typically angular and have a rougher surface texture, enabling better bonding with the cement paste in concrete, and thus providing improved strength properties at the same cement consumption (a constant water to cement ratio).”
When you are developing a concrete product with natural sand, opportunities for improvement are scarce. However, with crushed sand, Diem said, there are more opportunities and a wider range, and you can find the best fit for different types of concrete.
A totally different hurdle is the acceptance of the use of crushed sand in the mix design. Queensland still requires 40 per cent of the fine aggregate used in a concrete mix design to be made up of natural sand. This will need to change in the future to reflect the declining resource availability and the improved availability of high quality crushed sand products.
A business case?
Legislation and availability of natural sand dictate the price of sand – and thus profitability. There are also costs associated with obtaining the approvals, which can result in it not being economically viable to extract the resource.
“Transportation costs from sand quarries contribute significantly to the overall cost of the product to the end user,” Lacote explained.
“If the end product is of higher value, like silica sand for glass manufacturing, higher transportation costs can be allowed. Higher volume sand products with lower margins are a different case. The sand production site needs to be close to the location where it will be used.
“It’s a lot about the economies of scale. If you have a good sandpit near the place of use, you just screen the sand to the right size. With crushed sand, you need to consider many other aspects. When crushing bedrock, production costs are higher and there are additional costs involved. Between 30 per cent and 40 per cent – and sometimes as much as 50 per cent – of the crushed rock may end up in the quarry waste pile,” Lacote clarified.
“The smaller the particle size and the softer the material quarried, the more waste is generated. To improve profitability, you either need to reduce the amount of waste, or figure out new uses for the fines.
“Particles less than 4mm are often considered waste, that is, not good enough for any purpose. The material is often piled and, increasingly, producers have to pay waste fees for their piles. It would benefit all parties if the fines could be used and sold for other purposes. In Finland, this type of quarry waste can partly be used for surfacing driveways and yards.”
According to Lacote, making a business case is largely dependent on how the industry can develop and tailor the technical properties of crushed sand for different applications.
It also requires educating and convincing stakeholders about the good qualities of crushed sand so that it becomes acceptable and widely adopted. Naturally, pricing needs to be competitive.
“It is possible to use most of the rock for concrete production. A certain level of rock hardness is needed,” Lacote said. “One of the biggest enemies is too much mica, which is difficult and expensive to remove. The question is: what does the market look like? Is there a lot of competition? In other words, is there a lot of sand available at a reasonable price?
“The quality of natural sand varies more than that of sand produced in an industrial process. Consistent quality is a typical feature of crushed sand. Concrete manufacturers benefit from the use of sand that is of a stable quality, because it allows them to use less cement, typically between five per cent and 20 per cent less cement. The decreased use of cement also benefits the environment through reduced carbon dioxide emissions,” Lacote continued.
Diem approached the question with an illustrative case example. “Let’s take a case from Norway,” he said. “In this case, the transportation distance is short, less than 40km for both manufactured and natural sand. The price for high quality natural sand is NOK80 to 85 ($AUD12.77 to $AUD13.62) per tonne, and the price for incomparable crushed sand as a by-product from coarse aggregate production is NOK30 to 35 ($4.79 to $5.59). There is room to spend money on improving the quality.
“To add VSI crushing, you’d have to add NOK10 ($1.60) to the production costs – and for fines classification, another NOK10. This would allow for a product with a higher profit margin. However, it’s not so straightforward, as you also have to be able to invest considerable resources in the development of the whole process. The producer needs to work together with the customer and understand their needs – and do technical sales on a totally new level that is not typical of the aggregate market in general. It’s both a challenge and an opportunity.”
Choosing the right technology
“Producing crushed sand is usually a much more complex process than simply extracting natural sand,” Diem explained. “Standalone crushed sand plants are rare, and production is usually integrated with the production of coarse crushed aggregate.
“The exact layout of a crushing plant can vary. The production process usually involves several size reduction stages, which include crushing, transport, size control and classification equipment.”
“In terms of the equipment, all the necessary technology is already there,” Lacote added. “Metso has lots of knowledge in this area and can provide valuable insight for customers considering the manufacturing of crushed sand.
“Further development is always underway. The biggest gap is in the material technology for concrete. Until now, all textbooks have been written with the default assumption of using natural sand. Practical concrete technologists don’t understand the new crushed material or how to optimise its use. A lot of research has been done during recent years, but it hasn’t been applied much.”
“If you want to develop a crushed sand product at a quarry, you have to have a good understanding of the end material, that is, concrete,” Diem said. “You cannot just give
a sample to your end customers – they will test it with current knowledge and, in many instances, fail. Instead, a much more direct technical sales approach is needed, educating customers in the possibilities of the new material and conducting lab trials together – pretty much in the same way that concrete admixtures, for example, are typically sold.”
Meeting the challenge
Metso has years of know-how in different solutions for sand processing and a good portfolio of related equipment.
Shape is a major issue. This is where the role of crushing equipment is critical to ensure an acceptable shape and size. Cone crushers like the Metso MX, Metso Nordberg HP Series and GP Series enable the production of finer products, together with coarser aggregates.
They also lower the initial capital investment and save energy, due to having fewer crushing stages. Metso’s HRC high pressure grinding roll technology lends itself particularly well to
the production of sand. The inter-particle comminution method makes the cubical/ angularproductshapeunderthecorrect operating conditions.
“Studies show that the best end product for various purposes can be achieved with high velocity impact crushing,” Lacote said. “The Metso Barmac VSI vertical shaft autogenous impactor improves the soundness and shape of the material and produces a quality that is very close to natural sand.
“Most other VSI crushers use metallic parts to crush rock but the Barmac VSI uses a rock-on-rock crushing method. The better the grain shape of the end product, the better its performance in concrete, asphalt and base mixtures. The rock-on-rock crushing and grinding action also provides the lowest cost per tonne of any impact crushing method.”
Classifiers are then used to remove the needed amount of fine particles and dust. Metso’s static air classifiers, which are environmentally friendly and economical to use, produce consistent sand quality. strength.
Good, workable fresh concrete is easy to lay on a floor or cast in a form without too much physical effort. And the fresh concrete mass needs to stay uniform, without aggregate segregation or water bleeding. For asphalt and functional aggregates, there are grading specifications that must be met.”
Fulfilling these demands requires the crusher product to be screened/classified. The sand top size, normally 2mm to 4mm, is screened with vibrating screens. Rock dust/filler can be classified best by wet processing or with air classifiers. Air classification is suitable for both concrete and asphalt production.
In asphalt production, dry asphalt sand produced with air classification is an economical solution; it eliminates the need to evaporate water, which is an energy-intensive process in a hot mix asphalt plant.
Concrete sand can be tailored using two-stage air classification and mixing the classified filler back into the sand, if needed. In a basic case, one-stage air classification is adequate.