New type of crushed sand to replace natural sand in concrete production

TEXT: Rolands Cepuritis

Issues related to aggregates prices, sales and technical issues of the production are usually among the topics of concern to coarse aggregate quarry managers. Another issue is the mass balance of production because, as part of a normal production process of crushed aggregates, up to 30 percent (rock dependant) of the material acquired from the bedrock is reduced to sizes smaller than 4 mm and thus cannot be used as coarse aggregates.

This co-generated material has long been the most unfavorable fraction in terms of assuring a profitable mass balance, since it can be hard to sell or even to get rid of at any price. As a result, in many places there are huge stockpiles of this “waste” that not only affects the profitability of the aggregate operation, but also causes environmental issues.

On the other hand, in the past few decades, the availability of suitable natural sand for concrete production near the point of consumption has been exhausted around many populated regions in the world. This has led to a search for a new replacement material; the aforementioned surplus fines from crushing operations are expected to be the next best alternative – due to both great availability and suitable physical properties.

Well-known differences between natural sand and surplus quarry fines

Early attempts to use this co-generated material as fine aggregate were mainly unsuccessful. However, today the important differences between natural sand and surplus quarry fines are well known by aggregate producers, and there are examples of the producer successfully overcoming the challenges after working in close cooperation with Metso.

With respect to particle shape, this includes optimizing the crushing process along with vertical shaft impact (VSI) crusher, rock-on-rock shaping at the end. This can result in a particle shape that closely resembles that of natural sand.

The issue of the high fines content can be resolved with either wet or dry processing, i.e. different washing techniques or air classification. Finally, if the sand particles are sieved and classified into reasonably narrow particle sizes of a good shape, they can be successfully used as high-quality manufactured sand already today.

The challenging task of crushed sand

Developing new approaches in the field of crushed sand production and then increasing its markets share is not a simple task, one of the reasons being that it involves many parties. This can be described as a sort of “chain”, starting with rock blasting in a hard rock quarry, advancing to rock crushing and screening, then continuing to a ready-mix producer, and finally ending up at the construction site, where the crushed sand concrete is to be evaluated by the end user, i.e. the contractor.

In order to bring all the involved parties together to solve the challenges, COIN, Concrete Innovation Centre (www.coinweb.no), was founded in 2006. Since 2009, Metso has been an active and key participant in the COIN project “High-quality manufactured sand for concrete”. This focus area has served as a source of funding and a hub for facilitating the crucial networking between professionals from the different industries involved and universities and research institutions. The purpose has been to create a better crushed sand solution for the future.

A new trend in crushed sand production

There are two scenarios for a new approach to crushed sand production. One includes choosing the best available geological resources and then trying to copy “mother nature” by putting extreme effort into shaping of the aggregates and the design of grading curves.

Another approach involves finding ways to make crushed sand perform as well or better than natural sand by utilizing its intrinsic properties. This also includes developing a new concrete mix design philosophy that is adapted to crushed sands with specially engineered properties that are different from those of natural sands. The latter is what has been chosen as the philosophy of the new approach of crushed sand production currently under development within the COIN project.

It is also worth mentioning here that crushed fine aggregates normally outperform natural sand when the most essential hardened concrete properties, such as compressive and tensile strength, are compared. At the same time, the problems of achieving economical mixes with suitable workability (fresh state properties of concrete) are normally reported as the main problem that can render the use of crushed sand non-favorable in many cases.

Findings on the performance of crushed sand

The current trend in the concrete construction industry is an attempt to increase the market share of self-compacting concrete. This is a special type of very flowable concrete that was developed two decades ago in Japan and is special due to its ability to compact and level itself into a mold without any external vibration.

In the previous work carried out by the author within the COIN project, it was recognized that crushed sand with its naturally high fines content is perfectly suited for this type of concrete. This is because of the high flowability and the fact that self-compacting concrete needs a much higher content of very fine particles (≤ 0.125 mm or 120 mesh) to assure good cohesion between the water and particles in the mix to avoid segregation and also to prevent interlocking of the coarse aggregate grains.

Research carried out elsewhere has also proved that increased fine particle content can in many cases be desirable compared to natural sand concrete. This is because a high fines content in natural sand usually indicates increased silt, clay or shale concentration, which is detrimental to both fresh and hardened concrete properties.

In crushed sand, the presence of these contaminants is rare (at least in Scandinavia) and some level of fines would normally increase the content of filler-modified cement paste, thus helping with lubrication between the coarser aggregates particles (≥ 0.125 mm or 120 mesh). When the optimum level of fines content is exceeded, they start reducing the flowability of the cement paste itself and thus the effect of lubrication with respect to coarse particles can then be overruled.

Further trials within the COIN project also proved that the type of the fines mattered as well. This means that not only is the total percentage of fine particles below the sieves 0.250 (60 mesh), 0.125 (120 mesh) or 0.063 (230 mesh) mm important, as normally interpreted in the past, but so are the characteristics they possess; these characteristics can vary widely for different crushed fine aggregates. The most important properties of the fines are believed to be their particle size distribution (specific surface), mineralogical composition, particle shape and surface texture.

For example, if the grading of two crushed fines (≤ 0.125 mm) coming from the same deposit is altered by classification (washing, in this case) and this is the only variable between two concrete mixes (i.e. the total fines content is kept constant), this can have a tremendous effect on the concrete slump-flow value (see below).

From a concrete technology point of view, those findings can be explained by the adsorption of the free water on a larger total surface area of a finer grading, thus leaving less water available for the lubrication of the cement paste itself. The recent findings have also indicated that varying the properties (flakiness) of coarser crushed sand fractions (0.125/2 mm and 2/4 mm) has much less relative impact on fresh concrete properties, and that the other fine particle (≤ 0.125 mm) characteristics, such as shape, surface texture and mineralogy, could have a similar effect as grading (specific surface).

Further improvement in concrete properties

The current state-of-the-art for crushed sand production is VSI shaping and wet or dry classification to reduce the total fines content. However, it is our belief – and our findings indicate – that further important improvement in concrete properties can be achieved if the properties of the very fine part of the crushed sand (≤ 0.125 mm) are modified (engineered).

Two tasks then have to be solved, in order to see if our hypothesis is valid also when applied to industrial scale production. The first task includes the concrete technology side, which would require a complete understanding of how and which fine particle properties affect fresh concrete properties. The second task involves finding an industrial solution that would render a controlled modification and optimization of the filler part properties possible also at aggregates quarries.

Otherwise, our findings would be only of scientific importance with limited practical relevance. What seemed to be a difficult task at the beginning turned out to be possible with the equipment readily offered by Metso. This is because if the fine (≤ 0.125 mm) part of the crushed sand is separated into different fractions by our static air-classifiers and stored into silos, we just need to find the best way of combining them afterwards.

Moreover, two-stage dry air-classification is a concept that is already used today, providing in total four dry filler fractions to be mixed back either before or during the concrete production. The static air-classifiers are perfectly suited for aggregate operations, since they do not have any moving parts; the inside of their chambers is lined with ceramic lining, assuring very low wear costs even with highly abrasive feeds.

At the same time, they can be designed to accommodate the throughput necessary for most aggregate operations. In addition, they do not have the problems usually associated with the wet classification process (washing), i.e. space and environmental issues from de-watering ponds and operation problems in places where negative temperatures are reached in the winter time.

Metso Outotec's role in developing a future sand solution

In order to proceed with the experiments to further elaborate the proposed hypo-thesis, great effort and care were taken in the preparation of a set of model materials (crushed fillers) that represent the complete range of local geological variety in Scandinavia. This involved collecting 4/22 mm crushed rock samples from 10 different quarries. Further processing included another step of Barmac VSI crushing to generate fines and screening of 0/4 mm crushed sand. Special care was taken to ensure that all the fines were generated only in this strictly controlled way.

Previous research findings within the COIN project and elsewhere had proved that the geometrical properties (such as shape) of the fine particles ≤ 0.125 mm can be affected by the crushing procedures applied. For example, an increase in VSI tip speed demonstrated an improvement in the properties of particles all the way down to and including the filler (≤ 0.125 mm or 120 mesh) sizes; moreover, those improvements proved the introduction of measurable changes in the fresh concrete properties.

After VSI crushing, the 0/4 mm crushed sand material was further split in two fractions – 2/4 mm and 0/2 mm. Almost 9 tons of the 0/2 mm fractions were then sent to Metso's air-classification laboratory in Lebanon, Pennsylvania, USA, for controlled removal of filler and alteration of filler grading. This means that as much material below 0.125 mm as possible was removed and further split into fractions 0.063/0.125, 0.020/0.063 and 0/0.020 mm.

Industrially, this could be achieved by connecting a gravitational-inertial (GI) air-classifier in a row with two centrifugal (C) classifiers. This is illustrated in the flow chart to the right. As mentioned, already today there are aggregate operations that have two of Metso static air-classifiers (GI and C) connected in a row to produce multiple different fine filler fractions.

The air-classification experiments with the 10 different materials are now finished. The results demonstrated that the classification process of the equipment can be successfully adapted to a narrow desired result by adjusting the classification process parameters, i.e. the air-flow, which is regulated by the main air inlet valve and adjusted by the secondary air inlet valve (primary and secondary air ratio).

The success of the experiments can be illustrated (figure above) by the initial gradings of filler from all 10 crushed sands after the first stage of classification, an example of classification results (when the filler part is distributed into separate fractions), and the possibilities of recombining those fractions if one of the initial gradings is chosen as the reference.

The acquired filler fractions have now been delivered to the Norwegian University of Science and Technology where work within the COIN project will continue with the further elaboration of the knowledge on how the different filler characteristics affect the properties of fresh concrete.

Crushed sand application on the aggregates market

So why look for new and even more sophisticated solutions when “good enough” technology for manufactured sand is already available? The answer is simple. With the exception of natural sand, in some cases, the aggregate prices in many places do not even keep up with inflation.

There are two reasons for the sometimes high price for natural sand compared to crushed aggregate: First, a good quality sand can have a huge impact on the cement consumption (the highest share of concrete self-price) needed to produce concrete with desired properties; second, the availability of good natural resources is almost exhausted in many parts of the world. This turns the spotlight on the two possible approaches in the crushed aggregate business (which can, in fact, be applied to any industry).

The first approach is perhaps the most common: producing coarse aggregates that are virtually the same as that of other suppliers and trying to compete on the market with price. This usually involves having only a marginal profit from every sold ton and an unavoidable need to produce and sell huge volumes in order to render the operation profitable. This has been difficult for many huge aggregate quarries – especially after the last financial crisis, and, as a result, a lot of the so-called “mammoth” quarries, taking the first approach in their operations have turned out to have overcapacities. Increasing the price when everyone else also has an overcapacity and is selling virtually the same product is, of course, a difficult task.

The second approach would be selling high added-value (profit part of the price) crushed aggregate products at volumes that today’s market can readily consume.

The basic commercial idea under future solutions for crushed sand from the aggregate producer’s point of view lies in developing a technology to have a different kind of aggregate production approach. This involves not only trying to compete with price, but actually producing a high added-value product that can even be brand named on the market to emphasize that it is unique; this has not been typical for natural sand or aggregates in general.

This interesting trend has recently been gaining ground. The producers who have invested into considerably improving the quality of their crushed sand want to emphasize that it is different from the material that is co-generated in the coarse aggregate production process. To the author’s knowledge, currently there are two brand-named crushed sands: the Advanta® engineered concrete sand produced by Luck Stone (www.luckstone.com) in Virginia, United States, and the RoboSand™ produced by Robo Silicon (www.robo.co.in) in India. In fact, Metso static air-classification solutions are being used in the production of Advanta® crushed sand.

Choosing the best sand production process

Proving that high-quality crushed sand is worth a higher price (even if true!) is not a simple and easy task. There are a couple of reasons for this: the aforementioned first historical unsuccessful attempts of using co-generated crushed fine material in concrete, and the fact that legislative requirements in many parts of the world can be discriminating towards the use of crushed sand if it has been tailor-made for special performance in concrete and not a replica of “ideal” natural sand.

Still, every coarse aggregate production company has a wide range of choices of what to do with their surplus fines – dump in huge stock piles, try to get rid of it at a very low or no price, or try to turn it into business through the different available options of crushed sand production.

The best choice in each case depends on the local market situation and on how much effort in the long run the producer is willing to put forward in developing its product, not only on the technical side, but also actively working with technical support, sales and promotion. In other words, crushed sand cannot be sold in the same way as crushed coarse aggregate, i.e. by sending a price offer and description of properties to a potential customer. In the author’s perception, crushed sand needs a much more technical kind of marketing and sales in order to make it a success story.

*This article was written prior to the merger of Metso and Outotec on July 1, 2020 and thus the author refers to the old company name.