“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.