“We can reduce CO2 emissions from cement by a factor of 10”

Cli­mate change, caused by the estim­ated 43 bil­lion tonnes of car­bon diox­ide (CO2) emit­ted by human activ­ity per year, means that we urgently need low-car­bon solu­tions in all areas. The con­struc­tion industry is no excep­tion. Indeed, cement, which is the main com­pon­ent of con­crete, is respons­ible for approx­im­ately 7% of glob­al CO2 emissions.

The goal of the new Matéri­aux cimentaires éco-efficaces (MC²E) labor­at­ory, a col­lab­or­a­tion between the CNRS, l’ENS Par­is Saclay et Eco­cem, is to devel­op altern­at­ive low-car­bon-impact cements.  The col­lab­or­a­tion has already suc­ceeded in devel­op­ing a new (pat­en­ted) cement, based on waste residues from the steel industry, whose car­bon impact is reduced by about 80% com­pared to con­ven­tion­al cement.

Since it was inven­ted two cen­tur­ies ago, con­ven­tion­al, “Port­land”, cement, has been the main com­pon­ent of build­ing mater­i­als. This cement is made by mix­ing lime­stone and clay in a ratio of approx­im­ately 80% and 20% respect­ively. The mix­ture is heated to high tem­per­at­ures and under­goes a chem­ic­al reac­tion known as cal­cin­a­tion. Most of the CO2 emis­sions from the man­u­fac­ture of cement res­ult from the break­down of the lime­stone through cal­cin­a­tion (into lime and CO2). Over­all, one tonne of cement pro­duces between 800kg and one tonne of CO2.

One way to reduce these emis­sions would be to simply reduce the amount of cement used in con­struc­tion mater­i­als such as con­crete. This is feas­ible since cement is just a “filler” or “glue” and is mainly used to fill in the holes between aggreg­ates and sand in con­struc­tion mater­i­als to make them stick together. 

First of all, we man­aged to reduce the amount of cement in con­crete by a factor of five, start­ing with the idea that it is just a binder/glue and that the inter-gran­u­lar space can be filled with some­thing oth­er than cement. For con­ven­tion­al indus­tri­al adhes­ives, such as poly­mer-based glues, the start­ing point is a liquid sub­stance that is used to wet the two sur­faces to be bon­ded. This adhes­ive then hardens to give the struc­ture its mech­an­ic­al strength. Cement works in the same way: cement powder is mixed with water to pro­duce a liquid that can flow between the aggreg­ates and wet them. This liquid then hardens into a sol­id mater­i­al like concrete.

Our new cement men­tioned above is com­pletely dif­fer­ent since it does not con­tain con­ven­tion­al cement mater­i­als. It is made of ground slag, which is the waste residue pro­duced by the steel-mak­ing industry. We added a small amount of inert chem­ic­als or “addit­ives”) to this “finely ground gran­u­lated blast-fur­nace slag”, as it is known, to obtain a cement that can be used to make con­crete with a per­form­ance sim­il­ar to that of Port­land-cement-based concrete.

The pro­duc­tion of one tonne of this new cement res­ults in the emis­sion of approx­im­ately 90kg of CO2. Using this cement instead of Port­land cement could thus pro­duce high per­form­ance, long-last­ing low-car­bon con­crete. In addi­tion, its chem­ic­al char­ac­ter­ist­ics give it long-term mech­an­ic­al strength and improved res­ist­ance to sulph­ates and chlorides.

The cement we have pro­duced is already on the mar­ket and has been used on a large scale by the Vinci Group to build part of its headquar­ters in the La Défense dis­trict, just out­side Par­is. The new mater­i­al will also be used in the Par­is 2024 Olympic vil­lage and to con­struct the tun­nels of line 18 of the cap­it­al’s metro.

Until now, build­ers have not opted for envir­on­ment­ally friendly cement altern­at­ives because cement as a mater­i­al is so cheap. So much so that there is a lot of waste in the con­struc­tion sec­tor. In addi­tion, con­struc­tion com­pan­ies often use much more cement than neces­sary in their con­crete because they – wrongly – believe that the final product is mech­an­ic­ally safer if it con­tains more cement. The devel­op­ment of arti­fi­cial intel­li­gence tools, such as machine learn­ing, will sim­pli­fy the meth­ods for for­mu­lat­ing con­crete – in par­tic­u­lar when determ­in­ing the optim­al quant­it­ies of cement to use in con­struc­tion mater­i­als and thus reduce their impact in terms of CO2. Such an approach is new in the field, as, until now, engin­eers relied on com­puter sim­u­la­tions that require spe­cial­ist know­ledge. Machine learn­ing, on the oth­er hand, can be used by anyone.

We there­fore believe that it is pos­sible to achieve car­bon neut­ral­ity fairly quickly in this area without hav­ing to resort to so-called dis­rupt­ive tech­niques, that is, invent­ing com­pletely new ways of mak­ing cement. What could slow down this move­ment are inter­na­tion­al stand­ards, which are always slower to evolve than innov­a­tion itself.