The future of cement

The symposium marked the bicentenary of the discovery of the laws of hydraulicity by Louis Vicat, which have played a major role in the industrialisation of the cements used today. Working as an engineer in the early 19th century, Vicat’s research looked at the preparation and use of lime in submerged piles, as often used in river crossings. He found that not all limes produce very strong mortars and classed those that set and gained strength under water as ‘hydraulic’. He went on to demonstrate that the intimate blending of finely ground lime with a carefully-determined amount of clay or shale could produce an artificial hydraulic lime, which offered even greater strength when the two components were calcined together.

While others, such as Smeaton and Parker, had produced and used natural hydraulic limes before him, what set Vicat apart was his willingness to explain the results of his findings and make his discoveries freely available for others to exploit. This probably explains why the Vicat needle remains a key method for determining setting times even today.

Since Vicat’s time, global demand for cement has grown and its quality has become more consistent but is still based on the principles of raw mix proportioning and grinding that he pioneered. The world’s population has trebled over the past 65 years, whereas demand for cement has risen by a factor of 34. Today, global cement production is around 4100Mta, making it the second-most consumed substance on earth, after water.

Environmental responsibility

Not surprisingly, the global cement industry’s fuel consumption and contribution to CO2 emissions remains under the spotlight. The most established means of reducing both of these are through improved energy efficiency, clinker substitution and the use of alternative fuels (AFs). Cement producers logically improve efficiency as new equipment comes online, but making real progress when it comes to clinker substitution and AFs is often hindered by geographical or political constraints, or national standards. However, in Europe a 41 per cent AF substitution rate was recorded in 2014, but a rise to 60 per cent would mean 26Mta CO2 emissions being avoided, 16Mta of waste beneficially used and 11Mta less fossil fuels being consumed.

Encouragingly, electric power consumption, 45 per cent of which is used in cement grinding, has dropped by 10 per cent over the last 25 years. There is a greater appreciation today of the pros and cons of separate grinding and blending of the constituents of composite binders, enabling products to be optimised with regard to reactivity and grindability of different components. Monitoring particle size distribution (PSD), rather than surface area, it is possible to understand how advantages can be gained from achieving different narrow PSDs for different components, prior to intimate blending.

There has been considerable headway in reducing emissions. According to the European Environment Agency (EEA), between 1990 and 2014 EU emissions of sulphur oxides (SOx) fell by almost 90 per cent, non-methane volatile organic compounds (NMVOCs) by 60 per cent, nitrogen oxides (NOx) by 55 per cent, particulates (PM2.5) by close to 40 per cent, and ammonia (NH3) by over 20 per cent. Annual tonnages emitted have also fallen steadily, despite a peak in cement production in 2002-08.

The EU emission value for NOx was 1200mg/Nm3 in 1990, but by 2020 it will have dropped to 200mg/Nm3. Selective non-catalytic reduction (SNCR) installations, typically involving ammonia or urea injection, can operate below this level and results from initial industrial applications of selective catalytic reduction (SCR) are showing similar performance. Interestingly, SCR catalysts have also reduced emissions of volatile organic carbon by around 70 per cent without affecting CO concentrations.

Dust control technology has also advanced with new media for filters available in longer lengths and offering greater resistance to higher temperatures. One interesting development is the use of 3m-long ceramic candles instead of textile bags. Around 4000 such candles have operated for over 36 months at HeidelbergCement’s Rezzato plant in Italy, with the resulting stream of clean hot air being used elsewhere in the process or in symbiotic installations nearby.

Due to the increased production of clinker worldwide, cement manufacturing is now believed to contribute 7-9 per cent of global anthropogenic CO2 emissions, including its share of indirect emissions from the production of electric power consumed in factories. Despite its considerable expense, carbon capture and storage (CCS) is viewed as the eventual favoured control option for abatement.

Carbon capture technology

Carbon capture is expected to become the favoured control

option for carbon dioxide emission abatement

Collective studies by the cement sector have concentrated on the capture phase of this process. The European Cement Research Academy (ECRA) has produced a number of papers on this subject, jointly funded by the Cement Sustainability Initiative (CSI). The “CSI/ECRA Technology Papers 2017” hope to contribute towards aims to halve global CO2 emissions by 2050. ECRA has placed particular focus on the use of partial and total oxyfuel CO2 capture and is currently looking to trials of an industrial-scale oxyfuel kiln, as evidence suggests that this will be more efficient and more suited to retrofit to existing kilns than post-combustion techniques. The technology aims to create an exhaust gas, rich in CO2 via fuel combustion with pure oxygen in combination with the recycling of flue gas to moderate the temperature profile.  Parallel studies at a Norwegian cement plant led to CEMCAP, a SINTEF project, funded by the EU Horizon 2020 programme, to demonstrate different capture technologies in an industrially relevant environment.

As Giovanni Cinti, of Italcementi/HeidelbergCement, pointed out at the symposium: “Under the increasing demand of efficiency (low heat and energy consumption, high substitution rate of AFs and raw materials) as well as the progressive reduction of emission limits, which are now very close to the best achievable performance, old kiln lines are being replaced by new modern units, and will be more and more in the near future, perhaps opening the way to the use of carbon capture technologies.”

ECRA has placed particular focus on the partial and total oxyfuel CO2 capture

Meanwhile, the collaborative Nanocem project1 has identified the scope for a range of cement classes involving larger quantities of limestone, which can reduce CO2 emissions by up to 30 per cent. This new type of cement – the limestone-calcined-clay-cement concept, or LC3,2 requires abundantly available limestone and low-grade clay, offers cost-effective processing and requires no great modifications to existing cement plants.

Composite cements 

Over the past couple of decades, composite cements have taken a considerable share of the market formerly occupied by simple OPC. Development trends in the area of hydraulic binders have aimed to significantly reduce clinker content in favour of mineral additives. As a result, there is a pending amendment to the cement standard EN 197-1 introducing new ternary composite cement groups CEM II/C and CEM VI in the strength classes 32.5R and 42.5R. These are characterised by notably reduced amounts of clinker in combination with various permutations of natural pozzolana, siliceous fly ash, blastfurnace slag or limestone. While the new CEM II/C group contains up to 50 per cent mineral additives, the CEM VI group has less than 50 per cent clinker content with the main non-clinker components accounting for 50-65 per cent.

Due to the synergy effects of interactions between these additives, the compositions provide better or comparable mechanical and physical properties than cements with only one mineral additive. Concretes obtained with a water:cement ratio of 0.35 can be classified as ‘high strength’. Niche markets do exist for cements based on slag, calcium aluminate, calcium aluminosilicate, lime, phosphates, carbonate calcium silicates or alkali-activated materials – better known as geopolymers, and where OPC is rarely the best choice of activator. However, there is scope for increasing their usage by exploiting the performance of blends with OPC, eg, using aluminate cements to obtain various degrees of shrinkage compensation. Belite cement, based upon calcium hydrosilicates, is one example of a product that shows promise for the concrete products sector.

Composite cements, manufactured with GGBS and fly

ash, have made significant inroads in the market

share formerly occupied by simple OPC

When it comes to devising bulk cements to reduce concrete-related CO2, there are only four classes of alternative clinker systems that deserve serious attention. The first two are reactive belite-rich Portland cement clinker and belite ye’elimite-ferrite (BYF) clinker, both of which are hydraulic clinkers. The third is carbonate calcium silicate clinker, which hardens by reaction with CO2 gas, while the fourth is magnesium oxide clinker derived from magnesium silicates, which can also fall into the hydraulic category.

China has carried out much work to increase the reactivity of belite-rich clinkers, which not only conform to many existing standards but also offer a low heat of hydration, making them particularly suitable for large pours. Around 1Mt were produced in China in 2014. Its only technical barrier is its low rate of strength gain at early ages, whilst potential CO2 savings are around 10 per cent, relative to OPC, due to the lower kiln temperature required.

The ferrite phase in BYF cements appears to be quite reactive compared to that in OPC, despite its high average iron content. In one promising variant, minor components such as boron can be added to increase belite reactivity and/or decrease ye’elimite  reactivity. Independent research on this by Lafarge, HeidelbergCement and Vicat is now followed up in the EU-funded Ecocement project. The European Eco-binder project is also underway, which aims to provide proof of durability for any new alternative binder and show its ability to protect reinforcement steel from corrosion. This involves producing pre-fabricated components of large size and exposing them to aggressive environments.
Carbonatable clinkers require concretes to be cured in a CO2-rich atmosphere, thus restricting their use to factory-made concrete products. The approach used under the “Solidia” trademark currently offers CO2 savings of 30-40 per cent, but this could rise to 70 per cent if a circular CO2 economy develops. Clinker has been produced from plant trials and the resulting cement used by pre-cast customers, who found that compressive strengths typically achieved after one day are comparable with 28-day strengths for conventional concretes.

The embryonic magnesium oxide approach uses magnesium silicate raw materials, which contain no chemically-bound CO2, introducing the prospect of concretes with a negative carbon footprint, especially if using carbonation hardening.

Given the finite rate of production and dwindling reserves of good quality fly ash and granulated slag, these minerals cannot be relied upon as long-term components for composite cements. Therefore, the most probable new technologies for the future are currently BYF and LC3 binders which are compared in Table 1.

Embracing localism

The Paris event also raised the interesting standpoint that cement producers should stop concentrating on gaining acceptance of standards for ‘one size fits all’ products and instead embrace localism by using local materials to address local needs. By doing so, hydraulic binders and concretes would no longer be limited by product-based standards and codes, instead preferring performance-based standards, thereby taking full advantage of local resources.  

References

¹ www.nanocem.org
² www.lc3.ch