By Alla Linetsky, P.Eng. and Concrete Connections Member
Concrete Elegance Inc., Toronto, Ontario, Canada
(416) 567-5529
www.concreteelegance.ca
Concrete is already one of the less environmentally harmful materials that can be used as countertops. Its ingredients are plentiful, it does not off-gas anything harmful, and its production is not as energy-intensive as that of resins or natural stone. Still, it can be made with considerably less environmental impact than the traditional combination of natural sand, gravel and cement.
There are two ways to reduce concrete’s environmental footprint:
- Replace the inert aggregate with waste material(s)
- Replace some of the cement with waste products
Each of these steps not only makes concrete “greener”, but also creates additional benefits for the product and/or its makers.
1. Waste Aggregate
Ordinary sand and gravel operations do not harm the environment to a great extent; however, they still require excavating machinery, they can disrupt environmentally sensitive wildlife habitats, and they leave large open pits which may fill with water, but do not support healthy ecosystems. There are three viable substitutes for sand and gravel: crushed concrete, foundry slag and a type of “slag” produced in glass recycling facilities. All of these substitutes have a downside: concrete made with them is not as strong as with ordinary sand and gravel, and requires additional reinforcement.
Crushed concrete is available from concrete recyclers. If you take your concrete waste to an appropriate recycling site, you can probably fill up there with crushed concrete. If you’re lucky, the recycler will grade it and sell you the size you need. There is no additional benefit from using crushed concrete aggregate as it weighs the same as ordinary concrete.
Foundry slag can be thought of as “industrial lava rock”, as it is produced in a very similar manner. It is simply the melted and re-solidified stone that is left after the metal is extracted from the ore. Slag is very porous and lightweight, and very weak. Usually, only a portion of the aggregate is replaced with slag; however, it is possible to replace all of the aggregate provided that the reactive component of the concrete is very high performance. This means it is well superplasticized, with a water/cement ratio below 30%. A densifier is essential, and a shrinkage reducer is very helpful. A pozzolanic densifier can also be a waste product – more on that later.
Because slag is very porous, it absorbs far more water than ordinary aggregate. The absorbed water is not considered part of the mix water in water/cement ratio calculations, and most slag users pre-soak it prior to loading it into the mixer. The soaked slag then helps cure the concrete from the inside out as it slowly gives up its water.
Concrete made with 100% slag aggregate is about 30% lighter than ordinary concrete, although the full weight savings are not realized until the concrete has dried out. Slag for concrete producers is graded and sold in bulk by the ton. At Concrete Elegance, we buy a blended grade of slag, from very fine up to 3/8” diameter.
Another lightweight porous material is formed during smelting of recycled glass. It is fibrous and spongy, and has little compressive strength. It is possible that different glass recyclers produce different types of such waste material; contact your local recycling facility to find out whether or not you can use their waste products.
2. Supplementary Cementitious Materials
Concrete makes an excellent repository for inert waste products that can add bulk while at the same time replacing a resource-intensive aggregate. To really score environmental points, however, one needs to find a waste product substitute for cement, an energy-guzzling raw material.
Fortunately, most supplementary cementitious materials (SCMs) also improve the strength of concrete. These pozzolanic densifiers are waste products that react with both cement and water and can replace part of cement:
- fly ash, produced by burning of coal at electric generating stations,
- granulated ground blastfurnace slag (ggbs), which is the same slag as used for lightweight aggregate, but ground to a particle size that is one-tenth of cement particle size or smaller,
- silica fume, a byproduct of computer chip manufacturing,
- vitreous calcium aluminosilicate (VCAS), a recycled glass product that has the added advantage of being white in colour (the rest are grey or brown). It is the only proprietory product and can be purchased through www.vitrominerals.com
Each SCM has an optimal replacement rate for the greatest increase in concrete strength, as well as the maximum replacement rate, above which concrete strength actually decreases. The generally accepted construction industry standard is that 20% of cement can be safely replaced with a SCM without harming the concrete. However, other admixtures may affect the way the SCMs react, and sometimes, two or more SCMs are used in the same mix, provided that their combined proportion does not exceed 20%. Much research is being conducted in this area, and the papers that are published tend to examine only one aspect of SCM addition at a time.
With fly ash, by far the most common and cheapest SCM, the replacement rate can greatly exceed 20%, especially when the fly ash contains a lot of calcium (type C fly ash). However, other additives are generally needed to increase its reactivity in water and promote bond formation. Because fly ash differs so greatly in composition, depending on the origin of coal, interested readers are encouraged to experiment with different substitution rates and test their samples for compressive strength and shrinkage.
Fly ash and silica fume increase water (or superplasticizer) demand, ggbs is neutral, and VCAS actually decreases it. Again, you will need to experiment to adjust your mix to the new ingredients.
3. What’s next?
Environmental awareness is driving research towards portland-free cements, which are not only environmentally friendlier, but also produce stronger and more durable concrete. This class of cements is part of the geopolymer family, which may well revolutionize the construction industry. In the meantime, those of us in the concrete countertop field, unencumbered by approvals processes, are in an excellent position to adopt these new materials and make our customers feel better about buying our creations.