The cement industry worldwide is facing growing challenges in the context of saving material and energy resources as well as reducing its CO2 emissions. The International Energy Agency highlighted in its 'Road Map for the Cement Industry' that the main levers for the cement producers are the use of alternative materials, be it as fuel or raw material and in addition the reduction of the clinker/cement factor by utilisation of well tried and proven materials like slag, fly ash, pozzolanas or limestone fines. This underlines that in the years to come cement will depend on OPC clinker to a high degree. New cements will therefore most certainly first take into account higher amounts of main constituents besides clinker which show pozzolanic or latent hydraulic properties.
Artificial materials that originate from natural or industrial resources but require additional thermal treatment and/or activation may also have a role to play. It is not clear at present to what extent cements based on magnesia can play a role. On the other hand, sulphoaluminate cements may have a significant role to play. Unfortunately, due to their specific raw materials as well as their performance in concrete they will most probably not be able to substitute relevant parts of today's cement markets.
Cement production has undergone a tremendous development from its beginnings some 2000 years ago until the present day. While the use of cement in concrete has a very long history, the industrial production of cements started in the middle of the 19th century, at first with shaft kilns. These were later on replaced by rotary kilns as standard equipment worldwide. Annual global cement production has reached some 2.8Bnt/yr and is expected to increase still further to around 4Bnt/yr. Major growth is forecast in countries like China and India as well as the Middle East and Africa (MENA), (Figure 1). At the same time, the cement industry is facing challenges such as increased energy costs, requirements to reduce CO2 emissions and problems of sourcing raw material of sufficient quality and quantity.
The World Business Council for Sustainable Development and its Cement Sustainablity Initiative (CSI), comprising global cement producers, has initiated a project called 'Getting the Numbers Right,' which for the first time provides a good database for most of the global cement industry with respect to CO2 and energy performance. Figure 2 shows the energy performance of global cement productions covered by the CSI members.
Development
While cement production has traditionally been focused on OPC, composite and blast furnace slag cements have been developed and are now a central part of the cement-type portfolio of producers. At the same time Portland limestone and Portland pozzolanic cements have gained importance, especially in regions where slag or fly-ash are not available. In the global context of cost reduction and CO2 constraints, cement producers strive to lower the clinker content in their cements. Limits are given by cement performance, so that product quality of the final concrete may not be impaired. Figure 3 shows the different cements types and their calcium oxide, silicon dioxide and aluminium/iron oxide content.
The reduction of clinker levels in cement predominantly takes into account well-tried and tested main constituents. While the global availability of latent hydraulic and pozzolanic materials of industrial origin is certainly limited, a special focus is on cements with high limestone content.
This is basically an extension of the current cement standards as they have been developed worldwide and certainly provide opportunities for the future. As an example, research is performed in the context of the European standard with the main focus on strength development and durability of the concrete produced.
Figure 4 shows the range of current cement types standardised in Europe today and the extension that is currently under research. In any case, the production of cements with extended use of well tried and tested constituents certainly requires excellent quality assurance mechanisms as they have been successfully implemented in the cement industry. In addition, the inherent characteristics of cement production guarantee large volume flows and good homogenisation resulting in constant product quality.
Natural composition?
All future cements will certainly have to be based on materials which are globally available in sufficient amounts. Figure 5 shows that CaO, SiO2 as well as Fe2O3 and Al2O3 are certainly key materials in this respect. Magnesium oxide seems to be available, even if the deposits might not be as close to the surface as those currently exploited by the cement industry. Based on calcium, silicon, aluminium and iron, new cements could be foreseen, starting from Portland cement all the way to pure aluminosilicates that certainly contain no lime at all. From a CO2 point of view this could be a beneficial approach. However, alumo-silicates or geopolymers, as they are also known, require activators in order to initiate the hardening process. An alkaline activation is a potential choice. Nevertheless the energy requirements to produce these activators are certainly significant.
Requirements for new technologies
All cements have to fulfil the requirements on durability, strength development, early strength development, workability, cost and environment. Depending on the cement composition, these criteria can be fulfilled to different degrees. It is in the hands of the producer to optimise the different cement types with respect to these categories. The consumer will choose the appropriate cement type for the dedicated construction. Especially for new cements to be developed in the future, durability is one of the essential requirements. The question of carbonation resistance, resistance against chloride penetration, only to give a few examples, must be complied with. In temperate climates durability aspects to a high degree are also determined by frost-thaw resistance.
Among constituents that might not have developed to their full potential as cement constituents, calcined clays could play an important role. It is known that these materials exhibit pozzolanic properties. However, the calcination process is pretty much determined by the origin and the composition of these clays. Typically, calcination temperatures are in the range of 700-850°C. Availability on a global scale is good, although in some countries clays are not available at all.
(Figure 6 shows a thermogravimetric pattern for a clay sample).
Future pozzolans
On the other hand, pozzolanic materials or latent hydraulic materials of the future might be derived from waste materials.
It is known that granulated slags from pig-iron production exhibit extremely good latent hydraulic properties. In the same context, vitrified waste materials that exhibit adequate composition could be promising latent hydraulic materials. Some of these wastes, such as lignite ashes, have sufficient amounts of calcium in some countries. Others with a low calcium content might be useful for producing other pozzolans.
Future cements
In the literature, quite a few reports are given with respect to new types of cements on a research scale. Celitement, for example, is based on calcium silicate hybrid phases. Production is foreseen by hydro-thermal synthesis and by reactive milling of lime in a silicon component. The Ca/Si ratio is lower than OPC clinker, consequently CO2 emissions and energy requirements might be lower. However, it is currently much too early to give any estimation about the future potential of this binder with respect to durability, production cost or even the technical potential for relevant substitution of current cements.
Novacem has reported a cement based on magnesium oxide and hydrated magnesium carbonates. According to Novacem, the raw material is based on magnesium silicates which are digested and subsequently carbonated at elevated temperature and pressure. While magnesia-based cements have been known for a long time, it is an open question whether in the end Novacem will provide sufficient durability to substitute relevant amounts of today's cement. Novacem indicates that significant research has to be done, but has made significant progress to date.
Summary
For good reasons, today's world is relying to a high degree on OPC. Future developments will certainly take into account well-tried and proven main constituents and will consequently result in a lower clinker factor than today.
Recent developments have generally not passed the research scale yet. The cement industry and the scientific community will certainly follow these developments with great interest. From today's point of view, it is an open question as to whether these new materials will be able to substitute today's cement to a meaningful amount.
