Concrete is the most-used building material in world. It also contributes up to 8% of global CO2 emissions with over 3 trillion liters of water used in production.
The CO2 & Water Problem
About CO2 in Cement
CO2 from cement production is emitted in three ways – 50% emitted is through process emissions due to the chemical reaction, 40% is emitted through the burning of fossil fuels to heat the kilns in achieving the chemical reaction and 10% is from fuels required to mine and transport the materials.
Approximately 1 ton of CO2 is produced for every 1 ton of cement produced.
The largest issue we face with CO2 emissions in cement production is that it is mostly created via chemical reaction, thus improving efficiency of the process will not greatly reduce CO2.
There have been a few improvements made in CO2 emissions in three main areas:
1) Use of more efficient kilns
2) Use of alternative fuels
3) Reduction of the proportion of Portland clinker in cement
However, even with the above changes, overall emissions have been unchanged.
About Water in Concrete
A typical mix of concrete has approximately 20% water content and a 0.45 to 0.67 water-cementitious materials ratio (w/cm). Because water is used at various stages of concrete production, it is estimated that a tenth of the world’s industrial water is used in concrete alone.
The SCMs Problem
About Fly Ash & Slag
Fly ash and slag are the two most common SCMs used in concrete.
Fly ash is produced as a by-product of waste from power generating plants and slag is a by-product of blast furnaces in iron-making. Both fly ash and slag are extremely harmful wastes to us and our environment.
While using these by-products in concrete has proven to be beneficial as a secondary-use of a waste product, there are still varying degrees of issues that we are faced with by using these materials.
Previously, about 1 billion tons of coal were mined in the United States, which has produced 90 million tons of by-product waste, with 61% of that fly ash and 22% bottom ash. Only 7.5 million tons of these waste by-products are actually used in cement and concrete production leaving the rest to be landfilled.
Due mostly to increases in environmental regulations with the EPA, the fly ash production has slowed greatly, and in some areas of the country, have been completely halted. U.S. coal production hit a 42-year low in 2019 when production fell 16%. In 2020, coal production fell even more, at another 29% decrease in production in the U.S. alone.
According to Yale University’s School of the Environment, new coal plant construction fell by 84% between 2015 and 2018. This is again, due to the fact that coal plants are generating around 30% of our global energy-related CO2 emissions.
Steel production is also down, meaning that slag is not as abundant, which had previously been used in place of fly ash.