Riccardo Stoppa, Mike Sumner
GCP Applied Technologies
Cement is the world’s most prevalent man-made material, exceeding 4 billion tonnes annually. Manufacture on such a large scale risks significant impact to the environment and the industry has an excellent record of continuous improvement, with specific attention and success in areas such as energy efficiency, replacement of fossil fuels and reduction of emissions, such as SOx, NOx, dust, etc. Sustainable development is high on the agenda with wide support for the WBCSD Cement Sustainability Initiative (CSI).
Given such high volumes, cement manufacture accounts for a significant part of man-made CO2 emissions. This principally arises during clinker production, due to calcination of calcium carbonate, fossil fuel burning and electricity consumption (mostly for grinding). Approximately, each tonne of Portland cement clinker requires 1.25 tonne of limestone, which accounts for 0.5 tonne of CO2 release during calcination. Primary energy, when mostly supplied from fossil fuels releases 0.2 to 0.5 tonnes of CO2 (depending on fuel and process type and efficiency). Thus each tonne of clinker has an associated 0.7 to 1.0 tonnes of CO2 (the CSI CO2 protocol adopts a default value of 0.862 t CO2 per tonne of purchased clinker). Electricity consumption adds another 0.05 – 0.15 tonnes of CO2 (depending on total kWh/t and the fuel source for electricity generation).
Manufacturers have made public commitments to significantly reduce net CO2 emissions, with reduction protocols established and legislation in place worldwide, including CO taxes and 2 emission trading schemes. There are a number of approaches to reduce the carbon footprint for cement and some are already highly implemented, such as improved energy efficiency and the utilisation of alternative fuels. Reducing the clinker factor in cement, by utilising higher amounts of supplementary cementitious materials (SCM’s), is an additional approach and this can be facilitated to a significant degree by the appropriate use of cement additives.
CO2 Emission Reduction
Reducing energy consumption has been a continuous improvement focus for decades [2], with benefits to both costs and environmental impact. Clinker process optimisation has resulted in significantly reduced GJ/t and consequent reduced CO2 emission. The industry has also embraced the widespread use of alternative/ waste fuels that are zero carbon rated. De-carbonated raw materials, mineralization [3] and reduced tri- calcium silicate content also have had a part to play. While many efforts have concerned the kiln and fuels, optimization of the grinding process has also had a role, albeit with lesser impact, in reducing CO2 [4]. Specific energy consumption can be reduced by using improved mill system design, optimization of existing equipment, selection of cement grade, and improving clinker grindability. There has also been some attention to offsetting CO2 (e.g. forest plantation/management) and even CO2 capture and storage (CCS).
However, an important tool for reduction of CO2 emissions is to lower the clinker factor by using an increased amount of supplementary cementitious material (SCM) in cement.
Lower Cement Clinker Factor
Not surprisingly cement clinker content largely correlates with the CO2 emitted per tonne. Using a base CO2 factor of 862kg CO2/clinker tonne, each 1% drop in clinker factor can reduce emitted CO2 by 8 – 9 kg/t cement. The utilization of materials such as blast-furnace slag, fly ash, pozzolan and limestone in cement, has been practised for many years on the basis of market needs and economic attractiveness. Interest in SCMs is further increasing as an economic means to reduce CO2 emissions.
However, increasing SCM level to lower the clinker factor usually results in somewhat poorer cement performance. For example, although slower early strength development is most commonly the limiting factor, each 1% increase in SCM can reduce 28-day mortar strength by some 0.2 – 0.8MPa (depending on many factors, such as SCM type, fineness, mill system, clinker characteristics, etc.).
Increasing the level of clinker replacement and still maintaining market acceptance can be significantly assisted by an increase in cement performance, for example by increased clinker quality, increased cement fineness or by appropriate application of a chemical additive.
Increasing clinker quality requires full consideration of the effects of all production factors, including chemical and physical properties and process operation. Raising LSF (hence C3S) is often practised, but must be balanced against impacts on burning regime and microstructure. Mineralization has facilitated raised C3S levels with some success. A strength increase of 1MPa can be expected to allow a 1- 2% reduction in clinker factor for equal performance, and thus provide a reduction of 8 – 17kg CO2/cement tonne. A ready alternative to changing base clinker quality is to use a cement performance additive (or Quality Improver) [5].
Utilisation of Cement Additives
These are chemical compounds that, when integrated into the cement manufacturing process, allow the cement producers to increase the output and efficiency of grinding and significantly improve the performance, quality and handling of the finished cement, thereby allowing an increase in cement productivity and profitability. Whilst the principal advantage of such additives is to create economic gain from lower cement compositional costs, increased output and increased cement volume per tonne of clinker, they can also provide lower CO2 emission per cement tonne.
Used as a “grinding aid” there is an immediate ability to increase output, lower kWh/t, and hence reduce CO2 emission, but a more significant impact on CO2 is attained as a result of improved cement hydration and performance via the use of “quality improvers”, which create an opportunity for lower fineness or, more significantly, for increased replacement of clinker with SCM’s.
In broad terms a 10m2/kg reduction in Blaine (i.e., Blaine equivalent for CEM I, allowing for the different influences of SCM, gypsum, etc.) can reduce energy consumption by some 1 – 2 kWh/t and hence reduce 0.5 – 1 kg CO2/cement tonne. Using a cement additive with a modest strength enhancing effect of 2MPa above target at 28d could allow a 40m2/kg reduction in Blaine and hence a reduction of 4 – 8 kWh/t and 2 – 4kg CO2/cement tonne.
More typically, instead of simply reducing fineness, cement additives have been used to reduce the clinker factor. These formulated additives have been used for more than 80 years and have been demonstrated to provide strength gains in the range 2 – 10MPa (both early and at 28-days). Thus it is quite common that cement additives can lower clinker factor (increasing SCM levels) by some 3 – 10%, with an associated reduction of CO2/cement tonne.
The selection of the appropriate additive largely depends on how the SCM influences cement performance. For example, slag and fly ash have most influence on early strength and setting time, so that additive formulations are used that can shorten setting time and increase early strength. Limestone cements require an additive that can increase 28d strength, whilst some SCM’s may require an additive that reduces water demand.
The economic benefit arising from lower clinker factor is not discussed here, but largely depends on the cost differential between SCM and clinker. Reducing clinker factor does, however, provide a direct reduction in emissions of CO2/cement tonne, by some 25 – 90kg.
Thus the quality improving effect of additives and the ability to reduce clinker factor is the more significant lever to reduce emissions of CO2/cement tonne.
Cement Additives and CO : Guidelines
The guidelines to assess the2beneficial impact of cement additives in the reduction of CO are reported in table 1. Ad-hoc adjustments shall be 2 made to account the conditions and settings of specific systems.
Case Study
A case study with an Italian CEM II / A-LL 42.5 R cement is reported in tables 2 – 5. A field trial was completed over a period of 2 days, to compare an untreated cement (“Reference” from here onwards) with the same cement treated with a GCP Applied Technologies customised quality improver (“Additive”). Thanks to the combined effect of process optimization and strengths enhancement by means of the additive, it was possible to produce a cement with 5% additional clinker replacement with limestone, at constant cement performance (strengths, setting time, rheology). The mill output was also significantly increased by means of the grinding aid contribution of the Additive, allowing a decrease of the specific energy consumption. Field trials results are reported in Table 2.
The effect of clinker replacement and of specific energy consumption on CO2 emissions may be calculated according to the guidelines of Table 1, and is reported in Table 3. The CO2 emissions by the clinkerisation process may be reduced by roughly 6% (or 43 kg of CO2 per ton of cement), while those of the grinding system may be reduced by 15% (or 3 kg of CO2 per ton of cement). The total CO2 emissions per cement ton are reduced by some 6% by means of the use of a GCP Applied Technology’s customised quality improver.
A summary of the field trial results is reported in table 5. It shall be noted that a cement of equivalent (or superior) strengths was produced by the use of the additive, while increasing total production capacity (+17%), decreasing manufacturing costs of cement (-5%), and –last but not least– decreasing the CO2 emissions per cement ton significantly (-6%).
Conclusions
Cement manufacture accounts for 5% of man-made CO2 emissions and an important tool for reduction of CO2 emissions is to lower the clinker factor (increase SCM’s) in cement. However, increasing SCM level usually results in a somewhat poorer cement performance. Chemical additives, in addition to helping reduce specific energy consumption, are also able to partially offset the undesired effects of lower clinker factor, therefore allowing a decrease of CO2 emissions per cement tonne.
A set of guidelines is provided (Table 1), showing how, in typical operational conditions, a customised additive reduces CO2 emissions. The percentage reduction is a function of the specificities of individual cases, related to SCM type and additive capability.
An example is also reported (Tables 2 – 5), showing the practical benefits of the application of a GCP customised additive in a Portland-limestone cement. In addition to significant economic savings (4 – 5% on variable costs), a 6,3% reduction of CO2 emissions was also achieved, principally from lowering the clinker factor, and to a lesser extent by optimizing the grinding system (-15% on specific energy consumption).
Cement Additives, including GCP Applied Technologies customised solutions, are therefore a viable and ready applied means to help reduce CO2 emissions in the production of cement.
Refeences: [1] Cement Sustainability Initiative, www.wbcsdcement.org [2] C.A. Hendriks, E Worrell, D. de Jager, K. Blok, ve P. Riemer. Emission Reduction of Greenhouse Gases from the Cement Industry Vancouver, Canada (2004). [3] G. K. Moir. Improvement in the early strength properties of Portland Cement Phil. Trans. Roy. Soc. London vA310, -138 (1983) sf. 127. [4] Reducing CO2 emissions through cement grinding optimization. ICR Cement Plant Environmental Handbook, 2nd Edition, November 2014 [5] M. Sumner. Ensuring Additive Rewards. ICR, no. 9, pp. 71- 74 2010.