Concrete is not new. It has been around for thousands of years. Everyone knows that.
Not everyone knows that production of portland cement (the magic binder in concrete) accounts for about 5% of the worlds carbon dioxide emissions. That’s a lot of CO2!
There is research going on at the Massachusetts Institute of Technology is changing that. They are doing research at the atomic level. They are creating super concrete mixes that will reduce the environmental impact and increase performance, a win, win, situation!
So what can specifiers and engineers do now to create more sustainable concrete? They can specify coal fly ash or blast furnace slag as a replacement for the carbon dioxide producing portland cement. Depending on the application engineers typically replace up to 25% of the Portland cement. . That would equate to a 25% reduction in CO2 emissions. But poured in place structures can actually replace up to 55% of the portland cement and exceed the required design strengths. These mixes often have a reduced early strength but poured in place structures are often not required to be achieving maximum strength for well over 28 days. Concrete mixes that achieve high strength early are actually more susceptible to cracking allowing moisture to penetrate to the underlying steel reinforcement.
The fly ash or blast furnace slag alternatives actually make the concrete more durable. Slag is lighter than portland cement so by substituting a portion of the portland cement in the concrete mix, you are actually increasing the strength to weight ratio. You can specify recycled crushed aggregate instead of virgin limestone aggregate. You can specify aggregate screening as the fine aggregates instead of sand.
But wait there is more… Novacem has produced a new class of cement with a carbon negative footprint! This cement is based on magnesium oxide and hydrated magnesium carbonates. These compounds are produced at much lower temperatures thus reducing the carbon footprint and the production of the carbonates actually absorbs CO2.
And still more…The addition of heterogeneous photocatalysis to the surface of concrete which actually cleans the air around it. The pollutants are oxidized and removed from the surface by rain.
We live in amazing times with innovation all around us.
– Linda Smithe, RA, CSI, CDT, LEED AP BD & C,