An Eco-Friendly Option
The Romans started making concrete more than 2,000 years ago, but it wasn't quite like today's concrete. They had a different formula, which resulted in a substance that was not as strong as the modern product. 'Roman concrete is . . . considerably weaker than modern concretes. It's approximately ten times weaker,' says Renato Perucchio, a mechanical engineer at the University of Rochester in New York. 'What this material is assumed to have is phenomenal resistance over time.' That resistance, or durability against the elements, may be due to one of the concrete's key ingredients: volcanic ash. Modern concrete is a mix of a lime-based cement, water, sand and so-called aggregates such as fine gravel.
Surkhi, was the commonest pozzolanic materials used in India resembles volcanic ash. It has been used along with lime in many of our old structures, before modern Portland cement has taken its roots in India. Even after Portland cement made its appearance in the field of construction, Surkhi was used as an admixture to remedy some of the shortcomings of cement concrete. Surkhi was one of the main constituents in waterproofing treatments in conjunction with lime and sometimes even with cement for extending valuable pozzolanic action to make the treatment impervious. Surkhi is an artificial pozzolana made by powdering bricks or burnt clay balls. In some major works, for large scale production of Surkhi, clay balls are specially burnt for this purpose and then powdered. By its nature, it is a very complex material differing widely in its qualities and performances. Bhakra Nangal Dam is a standing ovation to the use of Surkhi in the construction.
The Indian cement sector had been under strict government control for almost the whole of the period 1969 to 1982. Government intervention took place both directly and indirectly. Direct intervention happened in the form of government control over production, capacity and distribution of cement, while indirect intervention took the form of price control. During this period, many companies and their plants started off but still growth was not seen at the desired rate. In 1977, higher prices were allowed for cement produced by new plants or major expansions of existing plants. Due to slow development, the uniform price imposed by the government, was substituted by a three-tier price system in 1979. Different prices were assigned to cement produced in low, medium and high cost plants.
Later on during the shortage of cement the producers were allowed to use Surkhi, burnt clay as a pozzolanic material to produce more quantity of cement and then the problem of quality started. Indiscriminate use of any sort of burnt clay material, without ensuring proper quality control norms created many issues with Pozzolana cement and gave it a bad name in late seventies and early eighties. Therefore initially when blended cements were reintroduced based on fly ash as a performance enhancer had to face users resistance. Now the blended cement available based with fly ash as a pozzolana is much different than the Surkhi-based Pozzolana cement.
As per the papers published by NCCBM, the production of ordinary Portland Cement in India has currently remained to about 25 per cent and that of Portland Pozzolana cement has steadily increased to about 67 per cent. The production of Portland slag cement is limited to 8 per cent. The production trend of different types of cement such as OPC, PPC, PSC and others in the last decade indicates that the trend is more towards blended cements. This is a favourable change in the product mix of Indian cement industry as PPC and PSC are more specialised types of cement with advantages in terms of better durability characteristics.
Many developed countries started using such blended cements in large quantities in construction of critical structures such as rocket launch pads, sea water jetties, huge dams, etc. Production of blended cements directly increases the plant capacity without any need for creating additional clinker making capacity. In an environment of growing competition, another development has been the introduction of higher grades of cement particularly for OPC. However, realisation is growing fast amongst the consumers at large, that the properties of durability are of greater importance than strength.
Improvement in construction technologies and the need for durable construction in different aggressive environments have enhanced the use of blended cements. Under such prevailing trend the need of stringent control on quality parameters related to raw materials and cement becomes essential. In the present paper, efforts have been made to present quality trends of OPC and PPC as their production is around 92 per cent of total. In order to conserve high grade limestone and fuels, thrust is given for the production of variety of cements using inferior grade and/or alternative raw materials by adopting state-of-art manufacturing technology. To maintain the quality of cements and to promote the use of different types of wastes/by-products or inferior grade raw materials in the cement manufacture, BIS (Bureau of Indian Standards) has incorporated amendments in the standard specifications and also revised some of the specifications. The salient features of the modifications in standards are:
- Increase in limit of pozzolana in PPC
- Increase in limit of granulated blast furnace slag in PSC
- Increase in limit of MgO in PSC
- Specifying the upper limit of compressive strength at 672 ¦ 4 hrs for OPC-33 & 43 grades
- Addition of performance improver up to 5 per cent in OPC
- Enhancing the limits of insoluble residue
- Increasing the limits of SO3 to 3.5 per cent Specifying requirements of cement for pre-cast concrete
Most of the cement plants in India have the state-of-the-art technology and are able to achieve their capacity utilisation as per the market demand. Cement industry is going ahead with a modification and upgradation of technology particularly in energy conservation. Cement industry's average energy consumption is about 725 kcal/kg clinker thermal energy and 80 kWh/t cement electrical energy.
Efforts are on relating to utilisation of alternate energy sources/fuels either partially or fully substitute coal in cement manufacture in the coming years, namely, pet coke, lignite, natural gas, and biomass wastes including fruit of Jatropha Carcus, Pongamia and Algae. Although the modern cement plants have incorporated the latest technology, yet there is scope for further improvement in the areas of coprocessing of waste derived/hazardous combustible wastes as fuel, cogeneration of power, bulk transport of cement, palletising and shrink wrapping for packing and dispatch.
The R&D activities and thrust to reduce CO2 emission associated with properties has resulted a trend towards the increased use of blended cement with the share of blended cements increasing to 75 per cent followed by Ordinary Portland Cement (OPC) (ÿ 25 per cent). Fly ash based Portland Pozzolana Cement (PPC) contributes the major share (67 per cent) of the total production with 8 per cent of Portland Slag Cement (PSC). In addition to conservation of natural resources, fuel and reduction of green house gases, there are other advantages of using blended cements.
The cement industry in India has also shown potential to utilise hazardous waste. The awareness on utilisation of wastes and environmental pollutants in the manufacture of cement have increased many folds in the last decade. The production of PPC and PSC of uniform quality by utilising more proportion of fly ash and/or slag could be possible by their assured consumer acceptance and research activities.
By and large (BIS) Indian specifications have kept pace with the specifications from developed countries and have been quite dynamic with times and technology. The amendments in the specification are expected to come in the shortest possible of times, which is rather difficult for such a large country. Recently BIS has come out with codes for composite cement which allows the use of two different SCMs like fly ash and slag together. It is one step forward. There is a need for standard specifications for composite cements, using variety of wastes and keeping in view the pragmatic approach of utilisation of cement in different types of construction activities has also been realised.
Various wastes like lead-zinc slag, copper slag, steel slag, marble dust, which were not allowed for utilisation earlier, have now been identified as suitable material for the use in cement and other building products. As availability of coal and limestone are limited and soon these will be finished and on the other hand the requirement for cement is ever increasing. Hence adopting modern technologies and alternate materials is very essential for cement industry to meet this gap.
Cenospheres are one of the most desired by products of fly ash. They are small hollow spheres with roughly 10-1,000 ¦m in diameter and constitute about 1-2 per cent of the fly ash obtained from the coal combustion processes. Because of their specific properties, namely their low density and high mechanical strength, cenospheres are an important subject of coal fired power plants. Research results indicate that cenospheres from coal combustion are constituted by aluminosilicate glasses with some crystalline phases like quartz, mullite and calcite. The high alumina content - roughly 25-27 wt per cent is responsible for the high mechanical strength, while density of most cenospheres is lower than 1 g/cm3. Regarding the formation process, there are correlations between the amount of cenospheres and the sodium and calcium content, in the different fly ash samples.
The economics of blended cements will largely depend on the landed cost of fly ash or slag. The common belief that it is a waste material and can be given away at throw away price has already vanished. Cement companies are prepared to pay better rates for consistent quality of fly ash.
Reference: Quality Trend In Indian Cements - A Decade Appraisal
AUTHORS: US Sinha, K Bhatnagar, R Gupta and MM Ali of National Council for Cement and Building Materials, India (NCCBM)