Concrete is widely used in all types of construction because of its compressive strength, durability, and the way it can be formed into a myriad of shapes. Concrete also provides thermal mass and, if finished right, can be both structure and an interior or exterior finished surface, avoiding the need to apply additional layers of finish.
Portland cement, which gives most concrete the ability to cure into a stone-like material, is energy-intensive and polluting to produce—up to a ton of carbon dioxide is released in producing a ton of cement. Strategies for reducing the environmental impact of concrete typically involve using less concrete or making the concrete with less cement.
Making concrete with less cement
Coal fly ash is a waste product of coal-fired power plants and can be used as a substitute for some of the portland cement in a concrete mixture. Up to 60% is common, but even higher levels of substitution are possible. It slows curing but ultimately makes the concrete stronger and gives it improved workability compared with a conventional mix. Trace heavy metals are found in fly ash, but they are also found in cement, and, once locked up concrete, are unlikely to represent a health hazard.
Other industrial and agricultural waste products, including ground blast-furnace slag and rice-hull ash, can also be used to replace some of the portland cement in concrete. For some applications, magnesium oxide cement and other alternatives to portland cement are being explored.
On large jobs, it’s possible to have the concrete mix designed specifically to reduce cement without compromising other properties. Some engineers and ready-mix plants offer proprietary technologies for optimizing a mix with minimal use of specialized admixtures.
Using less concrete
Smart design choices and specialized forming products can reduce the amount of concrete needed without compromising structural integrity. Precast concrete members use less concrete than site-formed elements with optimized geometries and aeration, which reduces the mass of concrete while improving its insulation value. Precast elements can also speed construction and reduce environmental damage on the construction site.
Stay-in-place insulating concrete forms (ICFs) made of polystyrene foam or a cementitious matrix of recycled foam or recycled wood fibers can reduce the amount of concrete used, avoid the impacts of temporary forms, and provide high levels of insulation.
Other green opportunities with concrete
A wide range of additives and admixtures are used to adjust the properties of concrete including accelerators, bonding agents, curing agents, densifiers, hardeners, plasticizers, retarders, sealers, and coloring agents. These can help make better concrete, but often come with their own environmental downsides, so seek out low-impact alternatives.
Forming can account for a significant portion of the total cost of poured concrete. Essentially, things get built twice: once in forms and again in concrete. Plywood has been the mainstay of concrete-forming companies for many years, though some companies have invested in reusable, durable forms—a more resource-efficient solution that is still relatively labor-intensive.
Forms also require form-release agents, most of which are petrochemical-based and off-gas large amounts of volatile organic compounds (VOCs). Vegetable-based form-release oils are available.
Recycled, lightweight, or thermally resistive aggregates offer reduced upstream impacts and/or improved properties in the finished concrete.
Walls and decks made of concrete do not provide much thermal insulation, but they can provide thermal mass which can help stabilize temperatures. To reduce thermal bridging, design walls, roofs, and foundations so that concrete does not extend all the way from interior to exterior.
Flowable cementitious mixes can also help with renovation of older buildings, by helping to level floors and correct roof slopes.