Dr. JD Bapat sheds light on some of the emerging green alternatives in cement manufacturing and the benefits they offer.
The cement industry faces a number of challenges that include depleting fossil fuel reserves, scarcity of raw materials, perpetually increasing demand for cement and concretes and growing environmental concerns linked to climate change. Every tonne of Ordinary Portland Cement (OPC) that is produced releases on average a similar amount of carbon dioxide (CO2) into the atmosphere or in total roughly 5 per cent of all anthropogenic carbon emissions. The cement production capacity in India is estimated to rise from the current (2014) 366x106 t/a to 550x106 t/a in 2020. Along with the rise in production, there will also be corresponding increase in the industry´s absolute energy use and the CO2 emissions. Figure 1 gives a rather alarming picture of global cement industry CO2 emissions by 2050. All efforts must be made to mitigate the likely grim situation.
Improved production methods and formulations that reduce CO2 emissions from the cement manufacturing process are thus high on the agenda. Emission reduction is also needed to counter the impacts on product cost of new regulations and escalating fuel prices. In this regard, locally available minerals, recycled materials and waste (industry, agriculture and domestic) may be suitable for blending with OPC as partial replacement. Fly ash (FA), blast furnace slag (BFS) and silica fume (SF) are three well known examples of cement replacement materials that are in use today.
Alternative fuels for kiln and power from renewable sources
While the Indian cement industry has achieved significant achievements in terms of improvement in energy efficiency, use of alternate fuels for kiln has not really taken off. At present, the Thermal Substitution Rate (TSR) of Indian cement industry is just 0.5-1 per cent, while in some developed countries, this figure is as high as 60 per cent. The use of alternative fuels for meeting energy requirement is a sustainable initiative which helps save fossil fuel and mitigates GHG emissions and also facilitates the difficult task of waste disposal in an environmentally sound manner.
The Cement Manufacturers´ Association (CMA) has identified hazardous wastes from industry; refuse derived fuel (RDF) from municipal solid waste (MSW), used tyres, biomass and plastic waste as the promising alternative fuels for Indian cement industry. However the long-term and sustained availability of these alternative fuels, in the vicinity of cement plant, is an important consideration for their use, as the plant will have to invest in making necessary modifications in the kiln section, including pre-heater, burner, etc. Biomass is one of the promising alternative fuels. The cement companies have an option of captive energy-crop plantation, to ensure continued supply of biomass for kiln burning. The government also needs to bring in appropriate changes in the emission standards and policies to promote the use of alternate fuels.
The biomass-based alternative fuels are considered carbon neutral as biomass growth fixes the carbon emitted during its combustion. The availability of biomass is also quite large among the alternative fuels and as per the estimate it can replace 10.38x106 t/a of coal used in the cement industry. That is equivalent to TSR potential of 36 per cent and the CO2 reduction potential of 17.6x106 t/a, on the national scale.
Among the renewable energy (RE) alternatives, solar PV and wind power hold promise for cement plants. A modern 1 MTPA capacity cement plant requires about 15 MW installed capacity of electrical energy for its operation. The use of renewable energy is site-specific and may be possible only in some cases. The experience shows that higher renewable energy rates can be achieved if older, smaller capacity wind turbines are replaced by modern higher capacity wind turbines and erecting solar PV systems on the same site, to gain from both wind and solar power in a single system. The wind and solar power have the advantage of ´banking´ and ´wheeling´ (feeding power into the national electricity grid at one location (banking), to use it later at different location (wheeling)), thereby yielding higher benefits. Four cement plants in the South have already installed wind power units generating about 80 MW power and fed into the national power grid. One of the major reasons for very few solar PV installations is the price constraint and the requirement of large area. At present, solar PV power is nearly three times costlier in comparison with the coal based power. The cement plant can install solar PV/wind power station in the used mine areas also.
Innovative size reduction technology
The new grinding technologies that lead to higher replacement of cement by mineral admixtures, in the blended cement, with improved performance, should be welcome. Presently the Indian Standards allow maximum 35 per cent fly ash (FA) and 70 per cent blast furnace slag (BFS), as the replacement of cement. The increased use of FA (>25per cent) or BFS (>50 per cent) results in a final product that is slow to develop early compressive strength (1, 3, 7 day). Kumar et. al.[5,6] report, higher replacement without compromising on early strength development is possible through mechanical activation of FA and BFS. The activation is done through size reduction in vibratory or attrition mills. It was found that up to 65 per cent of the clinker in blended cement could be replaced with such activated FA. The strength of the resulting product was comparable to that of commercial cement containing only 20 to 25 per cent FA.
The increased reactivity along with reduced water requirement of vibratory and attrition-milled FA are attributed to the fact that, with the new technique, the small (<1 micron) particles of the fly ash retain their original spherical shape. Because the spherical shape of particles remains intact in mechanically-activated FA, the resulting hydrated cement demonstrated lower porosity and improved strength compared to a product made with ball-milled FA. The mechanically activated ground granulated blast furnace slag (GGBS) could replace 50 to 95 per cent of the clinker in Portland slag cement (PSC). The test results showed that Portland slag cement containing 80 to 85 per cent mechanically activated GGBS was much stronger than typical commercial Portland slag cement, which contained 35 per cent GGBS. Both 1-day and 28-day strength were found to increase. The EMC Cement Company, near Jewett, Texas, USA, produces energetically modified cement (EMC) and pozzolana using a commercialized technology based on this mechanical activation concept. The plant began operations in September, 2004 with an initial production capacity of about 1,50,000 t/a, which can be increased to meet demand. Waste fly ash from a power plant is conveyed directly to the EMC production facility. The electrical energy used for mechanical activation ranges from 30-50 kWh/t product (EMC Cement 2012) and should be viewed in comparison with the thermal and electrical energy savings accrued to the plant due to higher FA or slag replacement. The block diagram to manufacture PPC with mechanically activated FA is shown in Figure 2.
Portland limestone cement
The Portland limestone cement or PLC is a slightly modified version of Portland cement that improves the environmental footprint and gives comparable performance. The Indian Standards IS269-2013 permits addition of limestone (LS) up to 5 per cent as performance improver. As per the European Standard EN 197-1 (2000), 5 per cent LS addition is permitted in all 27 approved types, as minor additional constituent (MAC). Out of that, 4 are PLC, permitting higher additions of LS, in addition to 5 per cent as MAC. The CEM II/A-L and CEM II/A-LL allow 6 to 20 per cent LS addition and CEM II/B-L and CEM II/B-LL allow 21-35 per cent LS addition. PLC is described in ASTM C595 & AASHTO M240 specifications and may contain 5-15 per cent LS.
It can be made at any Portland cement manufacturing plant. The crushed and dried LS is weigh-fed to the finish grinding mill along with clinker and gypsum. The LS, being relatively softer, is more easily ground than the clinker (which is harder) and gets concentrated in the fine particle fraction. The overall fineness must be higher (for equivalent performance). In order to bring the fineness of the clinker fraction similar to OPC, production rate is slowed and some additional grinding energy is required. However it is more than offset by lower clinker content and related kiln fuel savings. The addition of grinding aids is found to increase the mill performance. The grinding improves the particle size distribution and the hydration is enhanced by physical and chemical interaction. The addition of LS up to 15 per cent has significance, as may be seen in Figure 3.
As may be seen in the figure, there is no adverse impact on the compressive strength and the porosity of cement up to 15 per cent LS addition.
In fresh concrete, PLC improves workability and reduces ´bleeding´ as compared to OPC. A wide range of data show the average strength of mortars and concretes with and without LS are the same. However, the optimum amount of LS needs to be determined for each combination of LS and clinker. Permeability is somewhat reduced with the use of LS, probably due more to the reduction in the connectivity of the pores rather than to their volume. Freeze/thaw resistance is equivalent in OPC and PLC, when the amount of entrained air or more specifically, the air-void system is controlled. The sulphate resistance is primarily a function of tricalcium aluminate (C3A) content of cement and the water/cement ratio. The performance of PLC is equivalent to that of the OPC, under sulphate attack, for extended test periods. The use of up to 5 per cent LS in cement does not increase the susceptibility of mortars to alkali-silica reaction (ASR) . More research is required as to how the properties of clinker and LS could be best utilised, to obtain optimum benefits during production and the application of PLC in concrete.