Concrete production: an ideal option for foundry sand?

University lab research points to a viable avenue for used foundry sand--giving it to concrete producers as a valuable aggregate.

At some point in the near future, landfills across North America will staunchly post "No Foundry Sand" signs. And when that happens, foundries not reclaiming 100% are going to be literally buried under their own sand, with perhaps nothing to do but build castles.

Feared by foundrymen for years, environmentalists are calling for the reduction of foundry waste disposed in landfills, and reuse has reached a new, urgent stage. Continuing research sponsored in part by six Wisconsin foundries and a supplier, however, may result in a new "road" for foundries to follow when dealing with waste problems.

In lieu of this issue, the University of Wisconsin-Milwaukee's Center for By-Products Utilization (CBU) is finding practical, commercial uses for spent foundry sand. CBU's research is sponsored by the Wisconsin Dept. of Natural Resources, Badger Mining Corp., Falk Corp., Maynard Steel Casting Co. and other Wisconsin-based foundries.

"When concrete producers buy sand, it's washed and uniformly sized," said Tarun Naik, CBU director. "Foundry sand is beautiful, hard sand when clean. And it's all practically one size."

During the study, CBU collected, evaluated and analyzed data on foundry processes, materials, waste generation and potential uses of spent foundry sand. Samples of used foundry sand and air-cooled slag from different foundries were evaluated as potential construction products. In addition, a clean/new foundry sand from Badger Mining Corp. was used for control experiments.

All samples were tested for their physical properties to establish baseline data for future quality control use. The used sands showed variation in their physical properties based on equipment, casting size and shape, and type and amount of binders used.

All used foundry sand samples showed lower unit weight, lower fineness modulus, higher water absorption, lower specific gravity, and higher loss on wetting and drying in sodium sulfate solution than the respective properties of traditional concrete sand.

In order to design optimum cost-effective solutions, it was decided to use as-received foundry sand--without processing or cleaning--for the study.

Three different products to best utilize used foundry sand were prepared at the CBU. They were portland cement concrete, masonry blocks and paving stones. All products were tested and evaluated for performance based on applicable standards.

Concrete

To use as much of the foundry sand as possible, replacement levels of 25% and 35% were selected. Regular concrete sand was replaced with used foundry sands, and one concrete mix was prepared without any foundry sand. Additional mixes were prepared with used foundry sand, replacing 25% and 35% of regular sand with used foundry sand. Similar mixes were prepared using clean/new foundry sand.

All mixes were designed as a high-performance concrete, readily acceptable for high-rise construction, as well as everyday construction. The performance of concrete was tested for compressive strength, tensile strength and modulus of elasticity.

Results proved foundry sand is an excellent material for making concrete. Concrete produced with foundry sand could be marketed for most needs without significant additional fundamental research.

"There are concrete plants everywhere--it's a big advantage," Naik said of the market for used foundry sand. "Concrete plants are already located between two points--if not in all cases, nearer to foundries than landfills."

Masonry Blocks

Concrete masonry block mixture proportions were developed for bench-scale production. The size of the masonry block prepared in the lab was smaller than the normal size available in the market, yet proportional to a full-size block produced by a local block manufacturer.

All masonry block mixtures were tested for compressive strength, bulk density and water absorption. Masonry blocks passed all the ASTM requirements for strength, water absorption and bulk density. The control mix, the mix with clean/new foundry sand, and other mixes with used foundry sand, reached about the same strength at the specified age (28 days).

Based on these test results, large-scale block manufacturing could be started. The level of used foundry sand can be fine-tuned based upon overall performance requirements of a given market.

Paving Stones

Paving stone mixes were also developed. One control mix, one mix with replacement of regular sand with clean/new foundry sand and additional mixes with regular aggregates replaced with used foundry sand were designed and evaluated. The size of the paving stone prepared in the laboratory was the same size available in the market.

Paving stones were tested for compressive strength, water absorption, resistance to abrasion and freezing/thawing and achieved acceptable results. Strength results of paving stones produced with foundry sand were lower than desirable, but so were those created without foundry sand.

With proper doses of admixtures, paving stones will reach ASTM strength requirements.

Market Potential

The results of the physical properties revealed foundry sand probably cannot replace regular concrete sand in its entirety. However, concrete, masonry blocks and paving stones showed very promising possibilities for foundry by-products.

In general, the specified strength of normally used structural grade concrete is 400-6000 PSI. These strength levels were achieved with even 35% replacement of regular sand with used foundry sand. Modulus of elasticity (a property crucial to long-term serviceability) for concrete made with used foundry sand showed similar results as the control mix without foundry sand.

Naik said the center discovered 1/4-1/3 of sand can be replaced with used foundry sand in concrete applications. The CBU is varying the recipe further to determine if more than that can be replaced.

"In Wisconsin, if just 40% of all concrete producers blend foundry sand at 1/4, we'd run out of foundry sand," Naik said. "We are developing it carefully for long-term durability and other factors. There's no need to build a bridge if it falls down."

Noting that it costs a Milwaukee foundry about $15/ton to dispose of foundry sand, he said, "Foundries can save $15-20/ton by giving it to concrete producers. Or foundries could go and give it to them for $20/ton. And concrete makers don't have to buy it from the supplier."

Additionally funded new products design and testing are continuing. More partners for research and development phases are being sought to bring these products to market.

The next step, Naik said, will be to evaluate the long-term performance not only for concrete and cast concrete products, but also for "manufactured dirt" to be used for excavation and landfills. "We're two years away from market analysis," he said. "The next phase is implementation, when we'll tell engineers how to use it."

For further information, call AFS at 708/824-0181 to order a full report on alternate utilization of foundry waste sand (#RR9206).