Concrete is the most widely-used building material. The world’s consumption of concrete per capita has tripled in the last fifty years, due to the rapid development of countries such as China and India, as well as the continued demand to replace, repair and retrofit existing structures. It is estimated that 7-8% of global CO2 emissions result from cement production and curing.

A typical concrete mix consists of 10 to 15 percent cement, 60 to 75 percent fine and course aggregate and 15 to 20 percent water. In some mixes, 5 to 8 percent of the concrete will consist of entrained air. The ratio of water to cement in the mix is of critical importance because it affects the strength and workability of the concrete. Structural concrete typically has a w/c ratio ranging between 0.35-0.45. In green design, the cement is commonly replaced by fly ash or blast furnace slag. Concrete mixes exceeding a 40% cement replacement are commonly referred to as High Volume Fly Ash (HVFA) concrete. Replacing cement with fly ash and/or blast furnace slag can greatly reduce the environmental impact of concrete construction. However, if no other modifications are made to the mix design, the substitution of fly ash or slag will result in delayed strength gain. However, recent studies have shown that a reduction in the w/c ratio can produce 28-day strength gains in a 50% HVFA replacement mix comparable to a typical 15% replacement concrete mix design. The addition of fly ash and slag provides a better workability in the concrete which partly offsets the lower water content of this type of mix, however a plasticizer add mixture may need to be specified with these reduced w/c ratios.

In addition to utilizing cement replacement in concrete mixes, there are many emerging sustainable alternatives to concrete aggregates that utilize recycled or natural materials. Ongoing developments in improved batch plant efficiencies and alternative concrete products provide new avenues for achieving sustainable concrete design.

Refer to the links provided below for more information on sustainable concrete design:

Concrete Carbonation- article by SEI Sustainability Committee members on carbonation of concrete (ability of concrete to absorb CO2) Nov 2009

Article: “Analysis of High Replacement Fly Ash in Masonry Grout”

Slag – what it is, how to calculate it for LEED and how to incorporate it into your specifications

Link to descriptions of various cement replacement materials