Sand is the most extracted solid material in the world, the second most used global resource behind water, and used in every construction project on earth. It is also one of the most critical ingredients in concrete production. The world uses around fifty billion tons of sand annually, twice the amount of sediment moved by rivers. Damming of rivers in the past century has dramatically decreased natural sand production. In 2012, 26 billion tons of sand went into making concrete alone. Due to sprawling urbanization around many cities in the United States and other countries, concrete is in high demand, and sand extracting occurs far more quickly than it renews.
Concrete production is not alone in significant sand usage. Glass manufacturing and oil well fracking use tremendous amounts of sand. The total U.S. sand production used by the fracking industry surged from only 5% in 2003 to 72% in 2014.
This massive increase in demand for concrete sand has led to price appreciation. Some reports have costs in 2022 skyrocketing to 185% higher than in 2021.
High demand has also led quality to decrease. Sand suppliers sometimes scrape the bottom of the barrel to get their product out to customers. Organic impurities in natural sand like silt and clay can wreak havoc on concrete’s air content and hardened properties that can cause unwanted cracking. Additional processing can significantly reduce impurities, but these extra steps raise costs and slow production.
Rising transportation costs also put pressure on concrete producers. Many producers pay more for the cost of delivery than the actual sand. In addition, sand pits in remote locations like dried-up riverbeds have higher transportation costs even before factoring in record-high fuel prices.
PROBLEMS WITH OTHER SOURCES
Desert sand: Particles subjected to wind erosion for countless millennia are much too round and often too delicate for concrete production. Sea sand: Needs washing, or the salt contained throughout will rust steel reinforcement.
A MORE VIABLE SOLUTION
Manufactured sand is more available than a desert or shoreline. It is often a byproduct of rock crushing, and some quarries make it a point to produce manufactured sand that meets ASTM C33.
A quick Google search on a middle Tennessee location yielded no less than 25 quarries within 100 mi (160 km). Only a fraction of those locations would need to produce manufactured sand for a concrete producer to have several viable options, providing a much better supply prospect than 1-2 options of costly, inconsistent, natural sand that could run out any day. THE
MANUFACTURED SAND STIGMA
Manufactured sand is not some secret that has disappeared for decades. On the contrary, many concrete producers know its availability and potential cost savings. However, often when manufactured sand comes up in conversation, eyebrows furrow, heads start shaking, and repressed memories of job site nightmares like blocked pumps and profanity-screaming finishers come surging back.
Manufactured sand is known for a couple of disadvantages.
- Workability Issues: Manufactured sand is angular and may become too angular due to specific rock-crushing processes. Desired process: Manufactured sand crushed with a jaw crusher is ideal because it is often more cubical and uniform, which is excellent for particle packing in concrete. These smoother angles make pumping and finishing easier with less cement paste needed.
- Less desired process: Manufactured sand crushed with a rotary or conical crusher tends to be irregular in shape and may produce more “shard-like” flaky, elongated particles. Concrete can use these aggregates, but the mix design needs modification to compensate for common performance issues like pumpability and finishability. Due to their added surface area, the odd-shaped particles require more cement paste in the concrete to pump. During 22 Shotcrete | 3rd Quarter 2022 www.shotcrete.org the finishing process on the concrete slab, those sharp particles “tear” the surface of the concrete when trowel finishing. Added cement paste will undoubtedly help but adding more of the most expensive material in the formula negates the cost-benefit of manufactured sand.
- Micro Fines: Micro fines classify as particles finer than a No. 200 sieve. Manufactured sand can have an abundance of these micro fines. Micro fines can increase the water demand in concrete, thus lowering its strength. They can also reduce workability and retention of workability even when a high-range water reducer (HRWR) is in the mix design. The added fines’ surface area requires more cement paste to maintain pumpability and finishability.
- Supply: Finding a quarry that can keep up with potential market demand could pose an issue for a concrete producer. Manufactured sand is often a byproduct of rock crushing, and sometimes a quarry will not produce enough to supply a concrete plant. For example, a cubic yard of 3000 psi (21 MPa) concrete mix could include around 1200 lb (540 kg) of sand. If that plant is doing 200 yards of that mix every day for five days a week, that is 600 tons (544 tonnes) of sand a week. It might take a substantial quarry to supply that much “by-product” to multiple concrete plants.
SOLUTIONS TO THE MANUFACTURED SAND STIGMA
Advancements in admixture technology have brought several solutions for manufactured sand issues.
- Increased Cement Paste: Some concrete producers have increased the percentage of manufactured sand in their formulas by increasing the cement paste. For these producers, the benefit of continuously supplying their customers outweighs the costs associated with added cement content.
- Chemical Solutions: Some admixtures on the market today catered to aggregate producers rather than concrete producers. They selectively react with clay contaminants to eliminate the negative effect of swelling clays in concrete. These admixtures and the aggregates blend at the quarry. Several chemical companies have modified conventional concrete admixtures like water reducers, air-entrainment, and viscosity modifying admixtures to help combat manufactured sand performance.
- Mineral Solutions: A drawback of chemical admixtures is that they break down over time during mixing and delivery. This issue has led some concrete producers to mineral admixtures and nano clays since a mineral admixture will not break down over time. It will continue to work as usual as the concrete sets.
Palygorskite mineral admixture is a natural suspension agent for almost any high-solids mixture and improves suspension and stabilization. As a result, paints, coatings, mining, fertilizer suspensions, oil & gas, and many other industrial applications have used the mineral for decades. The tiny rod-shaped particles have a typical length of two microns and a circumference of thirty nanometers.
At rest, electrically charged palygorskite particles form a 3-dimensional lattice structure that separates and suspends other particles preventing segregation and enabling a homogeneous mixture. In motion, the lattice structure quickly breaks, allowing the mixture to flow freely. This process results in lower viscosity and improved flowability and pumpability. The rate of ‘thixotropic recovery’ is instanta- neous. The lattice structure returns to a stable homogeneous suspension immediately after the shear stops.
In the past decade, palygorskite’s thixotropic properties have found their place in the concrete world. It is unaffected by freeze/thaw, is pH tolerant, and can cooperate with almost any admixture on the market. As a result, palygorskite is a reliable solution to increasing manufactured sand percentage in concrete mixes.
Palygorskite works in concrete by maintaining or improving the workability and flow of the high-solids mixture. In addition, the tiny particles create a lattice structure throughout- out the cement paste, making it more robust and creamier. This enhanced paste content advantage is also evident in high-percentage manufactured sand formulations.
Natural sand supply has no predicted increase any time soon. Therefore concrete producers need to get ahead of the curve by utilizing more manufactured sand today, or they risk scrambling to fill customer orders at a higher price.
Experience with manufactured sand has scarred many producers in the industry to the point that they will not even entertain the possibility of using it ever again. Nevertheless, growing concerns with natural sand availability and price have driven the concrete world to explore other avenues of fine aggregate.
Some have “fixed” their manufactured sand mixes the old-fashioned way by adding another sack of cement to the design to keep customers supplied. Others have turned to admixture companies for assistance with new and innovative chemical and mineral additives.
The industry must adopt these newer technologies to calm its fears of past experiences and keep up with growing demand.
Reprinted from Shotcrete magazine by kind permission of the American Shotcrete Association (shotcrete.org)