The cement industry: the challenges of the future

Thanks to an unique combination of high mechanical performance, durability, ease of use, wide availability of raw materials, good possibility of properties modification and relatively low cost, concrete is the most common construction material in the world and the second most used after water. The active ingredient of concrete is Portland cement, and the active ingredient of Portland cement is Portland clinker: an artificial blend of calcium silicates and silico-aluminates that can react with water and, thanks to this hydraulic reaction, hardens and transforms the fluid mix of cement and water into a solid mass, binding together fine and coarse aggregates.

The typical manufacturing process of Portland cement (see figure 1) starts with the extraction of raw materials, mainly limestone and clay, that are quarried and properly blended and ground to prepare the so-called raw mix. This is fired in a special kiln in a high temperature process where silica and lime (with alumina/iron oxides added to raw mix to improve process efficiency) react to form the calcium silicates and aluminates that compose the Portland clinker.

Clinker is then finely ground together with gypsum and secondary mineral additions (such as limestone, fly ash, granulated blast furnace slag, natural or artificial pozzolans) to obtain the well-known grey powder usually referred to as Portland cement, used by millions of construction workers as hydraulic binder in concrete.

Reducing greenhouse gases to make production more sustainable

Cement manufacturing is a typical heavy industry process characterized by a high energy demand due to both fuels needed to reach the high temperature required and the electrical energy that drives the grinding mills and all the machinery of a modern cement plant. Moreover, during clinker production limestone (natural calcium carbonate) is decomposed and releases in the atmosphere relevant quantities of carbon dioxide. The result is that cement industry is reported to be responsible for 5%-8% of total anthropogenic greenhouse gas emissions, mainly associated to clinker production and grinding. The table no 1 summarizes modern data about CO₂ emissions in cement and concrete production, while the table no 2 summarizes the main strategies that can be implemented for the reduction of emissions are briefly described and commented. The graph reports a forecast of the global cement production until 2050, that is supposed to reach 5 billion tons.

Having this in mind, it appears that the reduction or elimination of CO₂ emissions is a serious challenge for the cement industry, and the more promising strategy is the reduction of the quantity of clinker contained in cement. 

Blended cements (cements where clinker is partially substituted with other materials) are not a novelty: their production and use have been a common industrial practice for a long time and the use of secondary cementitious materials such as limestone, fly ash, slag, natural or artificial pozzolans is well known and described in technical standards. On the other hand, the reduction of the clinker factor that is now required is far beyond any level the building industry has previously been accustomed to.

To face this challenge, new technical standards have recently been released (for example the European standards EN 197-5 and EN 197-6), describing the production of new cement types (named CEM II/C and CEM VI) with very low clinker content. Moreover, new types of secondary cementitious materials and their combination with traditional ones are being investigated and developed, and in some cases, they are already available on the market. A typical example is represented by calcined clays and calcined clays/limestone combination: this will probably have the highest potential for significant clinker reduction.

Low clinker cements present however some issues, mainly reduced early strength and increased water demand.

First of all, the reduction in the active ingredient (the clinker) limits the mechanical performance that can be reached. Second, some cementitious materials often absorb a significant amount of water, increasing the initial viscosity of fresh concrete. It is then mandatory to correct strength and water demand using suitable cement additives: these are chemical products that influence the cement hydration accelerating the strength increase and improving the viscosity that the cement will have once used in concrete. Moreover, these additives also work as grinding aids, increasing the output of grinding mills in cement plants, and reducing the specific energy consumption.

Cement additives can be tailor made according to the required targets and to the type of cement/clinker, considering its chemistry and mineralogy. They are commonly used as process additives, usually added to cement during grinding: this usually turns into indirect CO₂ savings, that can be more or less evident depending on the energy mix used to generate electricity. The cement produced, thanks to the improved early hydration and the reduced water demand guaranteed by the presence of additive, can have higher clinker substitution, with remarkable reduction of greenhouse gas emissions.

The global cement industry is going to be subjected to a change that never happened before, but with the correct approach to innovation there are good possibilities to succeed.

Cement additives: Frequently Asked Questions

What are cement additives?

Cement additives, also known as grinding aids, are chemical products used during manufacturing process of modern Portland cements. They are usually added directly in the mill during the grinding of clinker, gypsum and secondary cementitious materials.

Why grinding aids are used?

Grinding aids are mainly used to increase the efficacy of the production process. This means to produce a higher amount of cement with the same energy consumption, or to produce a finer and more reactive cement.

Moreover, modern cement additives also play a role in performance enhancement from a chemical point of view: during cement hydration (the complex series of chemical reactions that take place when cement is mixed with water, bringing to hardening and mechanical strength development) the presence of cement additives modifies the reactivity and allows reaching higher strength, or better control of hardening kinetics, or reduced water demand.

Grinding aid and cement additive are synonyms?

Basically yes, because modern products available in the market acts both as grinding efficiency improvers and as cement performance enhancers.

What are typical dosages of cement additives?

Typical dosages lie in the range 200-300 grams to 2-3 kilograms per ton of cement.

Which is their mechanism of action?

Cement grinding (as it happens in many other grinding operations) is a low efficiency process: only a minor part of the energy used (measured in kilowatt-hour – kWh) is actually converted to cement fineness increase. A relevant part of this energy is wasted in form of heat. This happens because as fineness increases, there are agglomeration phenomena of fines particles that reduce the overall efficiency of the process. Cement additives allow to control and reduce this agglomeration. This pushes the hourly mill production and the fineness that can be reached, with the same energy consumption.

What product lines are available?

For more than twenty years, Mapei has been placing on the market two lines of cement additives: MA.G.A. (Mapei Grinding Aids) and MA.P.E. (Mapei Performance Enhancer). These products are often formulated according to the requirement of specific cement plants.

Can we calculate the reduction in the CO₂ emissions obtained using cement additives?

It can be estimated that for each ton of cement produced with the use of cement additive, there is a 20 kg CO₂ reduction with respect to the same cement produced without additive. This calculation considers an average additive dosage of 350 g/t, a 25% mill production increase, and a 2% clinker reduction in cement composition. It is also based on a 0.57 kg CO₂/kWh energy mix and on 862 kg CO₂ per ton of clinker produced. For a medium size cement plant, this corresponds to a reduction in the range of some tens of million kg CO₂ every year.