the difference between cement & concrete and their opportunities for sustainability

the MIT concrete sustainability hub has published an article explaining the difference between cement and concrete, and the opportunities each material offers for sustainability. ‘concrete is the world’s most consumed construction material,’ says the article. ‘yet there’s a lot the public doesn’t know about it or its environmental impact.’

often confused with cement, concrete is one of the world’s most-used material after water. the difference between the two is that concrete is a composite made from different materials and one of them is cement. the process goes as follows: cement is produced with limestone being quarried and then mixed with a silica source like slag or fly ash. the cement plants then grind the clinker down to a fine powder and mix with it with additives resulting in cement.

zaha hadid’s fire station at the vitra design museum
image by victor garcia 

‘cement is then brought to sites where it is mixed with water, where it becomes cement paste,’ explains professor franz-josef ulm, faculty director of the MIT concrete sustainability hub (CSHub). ‘if you add sand to that paste it becomes mortar. and if you add to the mortar large aggregates — stones of a diameter of up to an inch — it becomes concrete.’

one of the most attractive characteristics of concrete is its solidity and durability. this is due to a chemical reaction that occurs when cement and water mix in a process known as hydration. ‘hydration occurs when cement and water react,’ continues ulm. ‘during hydration, the clinker dissolves into the calcium and recombines with water and silica to form calcium silica hydrates.’ the calcium silica hydrates create strong bonds that give its strength to the material while making it incredibly porous. in spite of is porosity, cement still possesses great strength and binding properties. by decreasing the porosity, a denser and even stronger final product is created.

created by engineers in the 1980s, high-performance concrete (HPC) does exactly that. ‘high-performance concrete developed in the 1980s when people realized that the capillary pores can be reduced in part by reducing the water-to-cement ratio,’ says ulm. ‘with the addition of certain ingredients as well, this created more CSH and reduced the water that remained after hydration. essentially, it reduced the larger pores filled with water and increased the strength of the material.’

until here, we’re good. the thing is that reducing the water-to-cement ratio for HPC requires more cement, and depending on how it is produced, it can increase its environmental impact. this is because when calcium carbonate is fired in a kiln to create cement, it produces carbon dioxide (CO2). emissions are also produced from heating cement kilns as it is done using fossil fuels because of the high temperatures it requires.

skate parks all over the world use concrete
image by lucie delavay 

overall, cement contributes around 8% of global emissions, a number quite high, considering for example the 2% the aviation industry produces. this is due mostly because big part of our built environment is made of concrete. CSHub’s executive director jeremy gregory, however, sees concrete’s scale as an opportunity to mitigate climate change. ‘concrete is the most-used building material in the world. and because we use so much of it, any reductions we make in its footprint will have a big impact on global emissions.’

‘when it comes to reducing the emissions of cement, we can increase the efficiency of cement kilns by increasing our use of waste materials as energy sources rather than fossil fuels,’ explains gregory. ‘we can also use blended cements that have less clinker, such as portland limestone cement, which mixes unheated limestone in the final grinding step of cement production. the last thing we can do is capture and store or utilize the carbon emitted during cement production.’

image by rory gardiner

by capturing, utilizing and storing carbon, a significant potential to reduce cement and concrete’s environmental impact is created while large market opportunities arise. according to the center for climate and energy solutions, this utilization of carbon in concrete will have a $400 billion global market by 2030. companies like solida cement and carbon cure are already working on this, designing cement and concrete that uses and sequesters CO2 during the production.

‘what’s clear, though,’ says gregory, ’is that low-carbon concrete mixtures will have to use many of these strategies. this means we need to rethink how we design our concrete mixtures.’ here gregory proposes that each project should have its own specific mix depending on performance rather than ingredients. to do this, architects, engineers and specifiers will have to help to design the optimal mix for their project instead of relying on a pre-designed mix.

‘change doesn’t have to happen based on just technology,’ he concludes. ‘it can also happen by how we work together toward common objectives.’