Concrete is one of the most widely used construction materials worldwide. It’s essential for countless infrastructure projects, from buildings to bridges to streets.

Its manufacture is also a huge source of CO2 emissions — about 5 per cent of the total emissions worldwide — and requires large-scale mining operations to obtain the raw materials. Typically, concrete is made of Portland cement, water, and aggregates. Changes to the common mixture offer the potential to reduce the amount of cement and aggregate needed, and to offset the production of CO2.

Bio-crete and hempcrete

Bio-crete and hempcrete are both similar to traditional concrete, but replace the aggregates with plant fibre such as hemp fibre or rice husks, and use lime in place of Portland cement. The resulting material lacks concrete’s compressive strength and so requires structural framing.

Hemp is a versatile plant that can be made into hempcrete, sheet goods like wallboard, flooring, and fibrous insulation. Hemp’s advantages include insulating value of about about R-3.5 per 25 millimetres of thickness, moisture permeability, and fast rate of growth, sequestering carbon with low inputs of water and pesticides.

Klara Marosszeky has been active as a licensed hemp researcher, as a hemp grower, and with her company, the Australian Hemp Masonry Company, located in Lismore, New South Wales, who conducted a workshop showing hemp masonry construction.

A UK firm, Lime Technology, has developed methods to cast its hempcrete mixture on site around structural framing, as well as creating pre-cast panels. The company says building with its hempcrete mixture, which it calls Tradical Hemcrete, drastically reduces the amount of carbon dioxide emitted compared to the construction of a conventional building.

The company’s web site says the hempcrete mixture “locks up around 40kg of carbon dioxide for every m2 of wall, whereas a typical brick and block cavity wall will create in its manufacture around 100kg of CO2 per m2 of wall.”

Overall, the company says, the mixture can reduce the embodied CO2 emitted in a home’s construction by up to 40 per cent.

Another hemp advocate, Hempitecture, started a crowdfunding campaign aimed at raising $25,000 to build the only non-residential hemp structure in the US. Matthew Mead and Tyler Mauri, the executive team at Hempitecture, aim to build an educational facility, accessible to the public, that will demonstrate the strengths of hemp as a building material.

Their Kickstarter page states, “This material is non-toxic, carbon-negative, energy-efficient, adding up to what is quite possibly the most sustainable building material on the planet. This material is then cast around structural framing, creating a naturally breathable wall system.”

Greening concrete with CO2 and industrial waste

Manufacturing cement, the “glue” that typically holds concrete together, produces huge amounts of carbon dioxide, which ends up being released into the atmosphere. CarbonCure, a company based in Nova Scotia, Canada, has developed a process that sequesters CO2 by injecting it back into the curing concrete, converting the gaseous CO2 into limestone.

The process, the company says, actually increases the strength of the concrete itself, enabling the use of less concrete overall. In addition, use of their products gives users LEED credits for Recycled Content, Regional Materials, Innovation in Design, Regional Priority, and Carbon Impact.

The US Environmental Protection Agency recently approved fly ash for use in concrete. Fly ash, a waste product from coal-fired power plants, is normally sent to a landfill as waste. When used to replace some of the Portland cement in concrete, the EPA found that fly ash could be safe and environmentally sound, decreasing waste, decreasing use of virgin resources, and decreasing incidence of CO2 emissions. US Concrete, a concrete manufacturer, states that fly ash offers several benefits over standard cement mixes, including improved workability and pumpability.

Blast furnace slag, another industrial waste product, can also be added to concrete. Using slag to replace some cement reduces the overall embodied energy, as slag requires 90 per cent less energy to produce. Use of slag also offsets the CO2 emissions of cement production by 42 to 46 per cent.