Monday, September 25, 2017

Cement & Concrete

Issues and options

 

DR JD BAPAT offers his perspective on sustainability of cement and construction industry

Cement and concrete, to survive as building materials in the long term, must satisfy sustainability criteria. The conservation of materials and energy and the minimisation of emission of greenhouse gases and pollutants at all stages, starting from quarrying of raw materials for cement manufacture, aggregate, through transportation, materials handling and unit operations of size reduction, blending, pyro-processing, packaging and distribution, till cement or concrete reaches the user site, are key to ensuring sustainability. Here we briefly focus on the following related issues and possible options:

Conservation of limestone as raw material

Alternatives for fuel in cement production

Construction with strength and durability

 

Conservation of Limestone as Raw Material

The cement industry in India is set on the path of growth and modernisation. One of the biggest challenges before the industry in the coming years is the conservation of limestone, the principal raw material for cement production. As per the report of the Working Group on Cement Industry for XII Five Year Plan (2012-17), prepared by the Government of India, Ministry of Commerce and Industry, the investment in infrastructure is estimated at $1 trillion during that period. The cement demand for infrastructure projects such as the dedicated freight corridors, upgraded and new airports and ports, housing and road is likely to increase substantially. The additional annual installed manufacturing capacity requirement during the next 15 years (up to 2027) would be about 1035 million tonnes. As per the estimates of the Indian Bureau of Mines (IBM), the total cement grade limestone reserves available to meet industry requirements is around 89,862 million tonnes. Based on the expected growth and consumption pattern, the current available limestone reserves are expected to last only for another 35 - 41 years.

All the available limestone reserves cannot be exploited for the following reasons:

(a) Deposits in inaccessible areas.

(b) Lack of infrastructural facilities like rail, road network, power supply, water availability etc.

(c) Limestone deposits located near villages, towns, cultivated lands, historical monuments, important civil structures like dams, forest etc are blocked due to safety regulations and are not available for mining for cement manufacture.

(d) Limestone mining from a simple deposit is cost effective as compared to intricate and complex deposits, where the fluctuations in the grade often lead to the problems in the cement production or require beneficiation before utilisation and also require improved fuel with low ash contents.

(e) The availability of potential limestone deposits has also been restricted due to environmental constraints, as many of these deposits are located in reserve forests, bio-zones and environmentally sensitive areas, near tourist centres/hill-stations or under thickly populated or cultivated fields.

There is a need to conserve limestone used in cement (Ordinary Portland Cement or OPC) production, on the one hand and optimally use cement in the construction, on the other.

The following measures are suggested:

(i) Improving the utilisation of Low/Marginal Grade Limestone in cement production: This requires efforts in different areas, such as (a) using petcoke as fuel in cement production, (b) mine planning to blend high-grade with low-grade limestone, (c) enrichment of CaCO3 in limestone, through different beneficiation techniques.

(ii) Using Lesser Lime in Cement Production: Low lime cements such as belite and sulfoaluminate (C4A3S) type have been reported on commercial scale particularly in countries such as China, Japan and Russia. The cements are manufactured in smaller batch type process kilns and are found suitable for applications in coastal areas owing to their sulphate resistance. These have potential for utilization of low grade limestone and industrial wastes

(iii) Minimising the Clinker Production: Maximizing substitution of clinker with mineral admixtures, such as fly ash, blast furnace slag, in the production of blended cement. The relevant changes in Indian Standards may be initiated to permit the manufacture of composite cement with more than one mineral admixture, high volume fly ash cement and limestone cement. The Standards in European Countries and US allow such cements.

(iv) Using Lesser Cement in Concrete: Maximising the substitution of cement with mineral admixtures, such as fly ash, ground granulated blast furnace slag, metakaolin, rice husk ash and limestone powder. Appropriate application of high performance concrete such as self compacting concrete, roller compacted concrete, high volume fly ash concrete may be promoted, considering the requirements of strength and durability.

 

Alternatives for Fuel in Cement Production

India today boasts of modern state-of-the-art large capacity cement plants. The quality of Indian cement is at par with the best produced anywhere in the world. India is expected to overtake developed countries like USA, UK and Canada in terms of per capita cement consumption by 2025. The energy consumption per unit mass of production, both thermal and electrical, has been brought down considerably through modernization and productivity enhancement efforts. The thermal and electrical energy consumption achieved in the modern Indian cement plant is comparable with the best obtained globally. The decomposition of the raw material, limestone, creates most (about 60 per cent) of the cement industry’s direct CO2 emissions; the rest comes from coal burning and power generation. Whereas the cement installed capacity has increased from 168x106 t/a in 2006 to nearly 350x106 t/a in 2013, the CO2 emissions have also increased correspondingly but the rate of increase is lower. In fact, a study conducted by the World Business Council for Sustainable Development (WBCSD) indicates that the net CO2 emissions per tonne of cementitious, globally, have reduced by 17 per cent. It could be said that the cement industry in India has achieved a significant partial decoupling of economic growth, represented by the cement production and absolute CO2 emissions. Some of the Indian cement majors have signed a cooperation pact to support low-carbon investments in India. The pact was signed in Geneva with the member companies of WBCSD Cement Sustainability Initiative and International Finance Corporation (IFC). There are some negative factors that need to be tackled, some through technology upgradation and some through improved policy framework.

The electricity supply is unreliable in many areas of the country. Hence cement producers have installed their own captive power plants with high efficiency boilers and, more recently, waste heat recovery installations. Although the specific power consumption has been substantially reduced through modernisation and productivity enhancement measures, there are certain barriers to bring it down

further, namely high investment costs required for major retrofits and demand for high performance requires substantially high grinding energy for fine grinding of cement.

The fuel used in cement manufacture is mineral coal. In view of the poor railway transport linkage and the low quality and high cost of coal in the open market, many cement companies import coal, which is expensive. The alternate fuels in the kiln reduce dependence on coal. Some plants have substituted mineral coal with petcoke (solid carbonaceous residue produced by thermal decomposition of heavy petroleum fractions or cracked stocks, or both), partially or fully, for kiln burning. The alternative fuels currently used by the cement industry include domestic and industrial wastes (mainly solid). The cement kiln is particularly well-suited for such fuels for good reasons: the organic constituents (even toxic) are completely destroyed due to high temperature, long residence time and oxidizing condition in the kiln, the acidic gases get neutralized coming in contact with alkaline materials in the kiln, the energy component substitutes for fossil fuels and the inorganic components i.e., ashes, get integrated into the clinker product. These are effective substitutes with lower CO2 emissions than traditional solid fuels. The typical alternative fuels used by the cement industry are pre-treated industrial and municipal solid wastes (domestic waste), discarded tires, waste oil and solvents, plastics, textiles and paper residues, biomass: animal meal, logs, wood chips and residues, recycled wood and paper, agricultural residues like rice husk, sawdust, sewage sludge, biomass crops. These wastes may otherwise be burnt in incinerators, land-filled or improperly destroyed. The substitution of alternate fuels for cement production is about 10 per cent, globally. In India it is much less. In some European countries, the average substitution rate is over 50 per cent for the cement industry.

 

Construction with strength and durability

It is now an established fact that durable concrete means concrete with fewer micro-cracks (10–100 micron). Micro-cracks in concrete allow ingress of external deteriorating agents such as water, carbon dioxide, chlorides, sulfates, and so on, leading to the deterioration, distress, and destruction of the structure. They can be reduced by using pozzolanic or cementitious materials, collectively called mineral admixtures, to replace cement in concrete. The term includes all siliceous and aluminous materials, which in finely divided form and in the presence of water, react chemically with the calcium hydroxide generated during cement hydration to form additional compounds possessing cementitious properties [3]. They may be naturally occurring materials, industrial and agricultural wastes or by-products or materials that require less energy to manufacture. The action of mineral admixtures in concrete can be explained in a simplified manner as follows:

Cement + water à hydrated paste + Ca(OH)2 à hydrated paste to calcium-silicate-hydrate or C-S-H (primary hydration)

Mineral admixture + water à slurry

Mineral admixture + Ca(OH)2 from cement hydration + H2O àcalcium-silicate-hydrate or C-S-H (secondary hydration)

Number of other compounds are formed during hydration but C-S-H is the principal strength-giving compound in the hardened concrete. The formation of additional cementitious compounds during secondary hydration leads to a reduction in temperature rise and refinement of pore structure in the hardened concrete.

The replacement of Portland cement by mineral admixtures leads to sustainability as the mineral admixtures are mostly industrial and agricultural wastes and the volume of these wastes currently produced worldwide exceeds their utilization.

 

Strength

The compressive strength of concrete with mineral admixture ( fp(admixture)) can be represented by the combination of three distinct, additive, factors as follows:

fp(admixture) = fp(dilution) + fp(physical) + fpz(chemical)

where:

fp(dilution): represents the strength reduction proportional to the amount of cement in the mixture without considering any physical or chemical effect of mineral admixture.

fp(physical): represents the increase in strength due to the physical effect or heterogeneous nucleation due to mineral admixture.

fpz(chemical): represents the increase in strength related to the pozzolanic reaction

 

Durability

The strength and durability of concrete structure must go hand in hand. Durability is the ability of a structure to resist weathering action, chemical attack and abrasion, while maintaining minimum strength and other desired engineering properties, throughout its service life. In today’s context, designing structure for strength and durability is synonymous to designing for sustainability. The national standard codes of practice mostly follow prescriptive approach, defining the exposure conditions, design procedure, choice of building materials and their application during the construction, without assuring the expected minimum service life of the structure, when these specifications are followed. However, in 1996, CEB (Comité Européen du Béton or European Committee for Concrete) accepted performance-based approach with explicit attention for the durability-based service life, limit states and reliability in design. A similar change in approach is also reflected in the Indian Standard Code IS 456: 2000. However, much needs to be done in that area. The objective of the national structural design codes, as in Eurocode 2, should be to enable the design decision based on the life cycle cost. That will be a major step toward building sustainable structures and in order to do that the mechanisms and the mitigation of structural deterioration due to the attack of deleterious agents need to be understood.

One important factor common to world standards is the general acceptance of “permeability” as a criterion for durability. The RILEM (Reunion Internationale des Laboratoires et Experts des Materiaux, Systemes de Construction et Ouvrages (French): International Union of Laboratories and Experts in Construction Materials, Systems and Structures) technical committee, TC 116-PCD, was formed to evaluate the use of permeability as a criterion of concrete durability. The available test methods were evaluated and reported with regard to their suitability for routine testing of concrete transport parameters. The Roads and Transport Authority (RTA) of New South Wales, Australia, adopted the water sorptivity concept as an additional durability requirement for concrete to be used in bridge construction. The RTA simplified the test, for practicality, determining only the depth of water penetration after 24 h of wetting and referred to it as the sorptivity depth. The other two important factors, common to the standards, are the quality of construction work and the continuous maintenance of structures during use.

The Figure illustrates, typically in case of fly ash, how the strength of concrete increases with time and the permeability reduces (durability increases) with the replacement of cement with fly ash [4]. The interfacial transition zone (ITZ), between aggregate and cement paste, is considered as the weakest link in concrete. The reduction in width, porosity and calcium hydroxide content of the ITZ between the paste and the aggregate in concrete with mineral admixtures is reported to be responsible for the strength enhancement and the durability.




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