QUALITY TREND IN INDIAN CEMENTS - A DECADE APPRAISAL

Transcription

1 QUALITY TREND IN INDIAN CEMENTS - A DECADE APPRAISAL U S Sinha, K Bhatnagar, R Gupta and M M Ali National Council for Cement and Building Materials, India Abstract Cement industry plays a vital role in the growth and development of a country. Indian cement industry, second major cement producing in the world underwent rapid technological up-gradation and vibrant growth during the last two decades. The advances in technology and availability of expertise have benefited the cement industry significantly where emphasis has been on utilizing low-grade raw materials, by-product wastes and alternative fuels while maintaining the quality of cement. Due to stringent control on quality parameters, the chemical characteristics and performance evaluation of various cements in the decade revealed that the qualities of Indian cements are comparable with international specifications and competing in market demand particularly in neighbouring countries. The R&D thrust in cement production has been on account of reduction of CO 2 emission resulting in the gainful utilizations of low grade / alternative raw materials, fuels, industrial wastes and pet coke etc. The continuous assimilation of state-of-art manufacturing technology have helped the industry to conserve precious energy, fuel and depleting natural resources without compromising with quality and protecting environment. The parameters modified in standard specifications for compliance of quality of cement were the changes in limits of SO 3 content and insoluble residue.the reason being apparent use of pet coke and 5% performance improver in OPC.Further the rising production of blended cement has led to improvement in energy efficiency by reduction in carbon footprint and making the environment clean. A trend towards the increased use of fly ash and slag is discernible with the share of blended cements increasing to 75% of total cement production in India. The paper presents salient highlights of trend in quality parameters of cements produced in Indian cement plants with emphasis to conserve energy, maximize waste utilization and increase in cement production. The trend reported in this paper are based on testing of cement samples in the Independent Testing Laboratories (ITL) in National Council for Cement and Building Materials (NCB). Introduction Indian cement industry, 2nd largest producer after China accounts for about 7-8 % of the total global production. The first cement industry in India was set up at Porbundar, Gujarat in 1914, with a production capacity of 1000 tonnes per annum, thus making it a century old industry in India. India has a total installed capacity of around 336 mtpa as of financial year ended 12 with about 365 small and 183 large cement plants. Construction activity is one of the largest commercial activities in India. With ever increasing construction activities for industrial and infrastructure development, flourishing real estate business and development of Special Economic Zones (SEZs) across the country, there is a huge demand for different types of cement. To cope-up with the construction demand, the production of cement has been increased many folds by increasing installed capacities and assimilation of state-of-art manufacturing technology by producers for better capacity utilization and stringent quality control. The production of Ordinary Portland Cement in India has currently remains to about 25% and that of Portland Pozzolana Cement has steadily increased to about 67%. The production of Portland Slag cement

2 is limited to 8%. The production trend of different types of cement such as OPC, PPC, PSC and others in last decades 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 specialized 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, realization 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 percent of total. Indian Cement Industry An Outlook India is producing different varieties of cements like Ordinary Portland Cement (OPC), Portland Pozzolana Cement (PPC), Portland Blast Furnace Slag Cement (PBFSC), Oil -well Cement, Rapid Hardening Portland Cement, Sulphate Resistant Portland Cement (SRPC), White Cement and Masonry Cement etc. BIS specifies three grades of OPC (i) IS 269:2013 i.e. 33 grade suitable for all general constructions (ii) IS 8112:2013 i.e. 43/43 S grade and (iii) IS 12269:2013 i.e. 53/53 S grade preferably used for high strength concrete work, such as pre-cast concrete, pre-stressed concrete, long span structures/ bridges, tall structures, etc. Two types of PPC, viz. IS 1489 (Part1): 1991 ( Reaffirmed ) pulverized fuel-based and IS 1489 (Part 2):1991 (Reaffirmed ) Calcined clay -based are suitable for all general constructions, particularly for marine & hydraulic construction and other mass concrete structures. The Portland Slag Cement (PSC) IS 455:1989 (Reaffirmed ) is particularly useful for marine constructions. Indian cement industry is not only meeting the requirements of domestic market but also fulfilling the demands of the neighbouring countries by exporting clinker and cement, qualities of which are comparable with the best in the world. 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 / byproducts or inferior grade raw materials in the cement manufacture, BIS 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 percent in OPC Enhancing the limits of insoluble residue Increasing the limits of SO 3 to 3.5 percent Specifying requirements of cement for pre-cast concrete

3 Quality Trend in Ordinary Portland Cement In an environment of growing competition, higher grades of cement such as OPC-43 and OPC-53 have now conquered the market. BIS specifies the limits for both chemical and physical parameters and among them the compressive strength is more significant and also important parameter for concrete mix design. The 72±1 hrs, 168±2 hrs and 672±4 hrs, compressive strengths are far higher than the BIS specified standards for different grades. In fact OPC-33 Grade has been driven out of market by the easy availability of higher grades. The average values of compressive strengths at 72±1 hrs, 168±2 hrs and 672±4 hrs of OPC 43 & 53 Grades obtained from different offices of BIS, cement plants, marketing department, consumers and tested in Independent Testing Laboratories (ITL) of NCB over a period of 10 years are shown in Fig: 1 and 2. Strength (MPa) OPC 43 G : Compressive Strength hrs 168 hrs 672 hrs Fig:1 Strength (MPa) OPC 53 Gr : Compressive Strength hrs 168 hrs 672 hrs Fig: 2 It has been observed that in case of OPC-43 grade, there is a gradual decrease in average Blaine fineness and compressive strength at 72, 168 & 672 hrs from to and thereafter increasing up to. In recent years the average compressive strength of OPC-43 grade at all ages, is much higher than the specified limits of BIS, which may be attributed to the higher percentage of C 3 S associated with higher fineness and controlled particle size distribution (Fig : 1).

4 Similarly in case of OPC-53 a trend of improvement of compressive strength could be seen at all ages with fluctuations in narrow range. The percentage of C 3 S content could be identified as contributing factors for trends in compressive strength (Fig: 2). The standard deviation data of average compressive strength at all ages of OPC-43 and OPC-53 grades indicate that Indian cement industry is manufacturing consistent quality of cement. The superior performances of OPC-43 & 53 grades have resulted in meeting the requirements of cement for special applications in railway sleepers and pre-cast concrete where the controls are more stringent in regard to fineness and compressive strength as reflected in the standard specifications for 43-S and 53-S grades of cement. Compressive strengths of Indian cements are higher than that specified in ASTM and BSEN standards. Failure rate of compressive strength of OPC 43 & 53 grades have considerably decreased from 03 and now it is more or less constant at around 3 percent. The higher fineness and controlled particle size distribution are contributing to high early strength and fast setting. The C 3 A content in 43 and 53 grades OPC have been found to be in the range of 6.5 to 10.0 percent. During the decade about 80 percent samples of OPC of both the grades show initial setting time minutes and final setting time in the range of minutes. Since 03 less than 3 percent samples are found unsound in autoclave test, out of which more than 1 percent passed after aeration. BIS Specifications for chemical parameters are comparable with other international specifications, though there is difference in the specified limits of LOI, SO 3, IR, MgO, LSF, A/F and alkalies (as Na 2 O), from country to country. The higher limit of IR specified in India as well as in some other countries indicates the use of performance improver in cement. The decade data of chemical parameters indicate that the sulphate (SO 3 ), insoluble residue (IR) and alkalies equivalent as Na 2 O are increasing in both OPC-43 and OPC-53 grade cements due to use of inferior grade raw materials and alternative fuels like pet coke in cement manufacture. It has been found that that the trend of SO 3 varies due to the use of pet coke. Percentage of OPC-43 grade with SO 3 <1.0 percent is decreasing while the SO 3 percentage between 2-3 percent is increasing. In case of OPC 53 grade, percentage of cement with SO 3 <1.0 percent is more or less constant however, the SO 3 percentage between percent is showing sharp upward trend. Quality Trend in Ordinary Portland Pozzolana Cement (PPC) PPC is manufactured by intergrinding / intermixing of Portland clinker with pulversised fuel ash or calcined clay along with gypsum. The air-entraining agents or surfactants may be used to one percent. The advancement in manufacturing technologies has resulted in incorporation of more quantity of pozzolana for PPC production. The BIS, current limits of addition of pozzolana for PPC manufacture is in the range of 15 to 35 percent. It is observed that compressive strength of PPC has also improved while conforming to chemical and mechanical requirements as specified by BIS. Average Blaine fineness of PPC has been in the range of m 2 /kg over the decade. Identical trends were also observed for compressive strengths at 72 hrs, 168 hrs and 672 hrs. Although fluctuations in the average values were observed during the period (Fig: 3). In recent years the compressive strength were found lower than the first half of the decade but showing a stabilizing trend in last three years. Due to higher doses of fly ash in PPC, compressive strength is influenced. The latest technology of grinding the clinker using VRM and better dispersion of blending materials has resulted in higher cement fineness and better particle size distribution attributing to improved mechanical properties. However, the compressive strengths at different ages were much higher than the specified limits of BIS. In case of setting time downward trend has been seen from to - an thereafter it has increased (Fig: 4). The increase in percentage of SO 3 over the years in PPC may be attributed to the presence of higher SO 3 content in Portland clinker due to the use of pet coke. The failure rate of PPC samples

5 compliance has decreased to about 3 percent only. However, none of samples has failed in drying shrinkage and soundness test which has been controlled by using active pozzolana. The characteristics which reflects to the performance of all types of cements such as compressive strength, soundness, setting time & drying shrinkage etc are improving and the Indian cements are fully satisfying the needs of all types of constructions activities in the county. Strength (MPa) PPC (FA Based) Compressive Strength hrs 168 hrs 672 hrs Fig: PPC (FA Based) : Setting Time 220 Time (min) IST FST Fig: 4 Most of the cement plants in India have the state of- the-art technology and are able to achieve their capacity utilization as per the market demand. Cement industry is going ahead with a modification and up gradation 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 utilization 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, palletizing and shrink wrapping for packing & despatch. The R & D activities & 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% followed by Ordinary Portland Cement (OPC) ( 25%). Fly ash based Portland Pozzolana Cement (PPC)

6 contributes the major share (67%) of the total production with 8% of Portland Slag Cement (PSC). In addition to conservation of natural resources, fuel and reduction of green house gases, other advantages of using blended cements include: low heat of hydration superior performance under elevated temperature good long term strength control of alkali - silica reaction resistance to acids and sulphates reduced permeability reduced chlorine diffusivity. Investigations have been carried out on co-processing & energy conservation. The cement industry in India has also shown potential to utilize hazardous waste. The awareness on utilization of wastes and environmental pollutants in the manufacture of cement have increased many folds in last decade. The production of PPC & PSC of uniform quality by utilizing more proportion of fly ash and / or slag could be possible by their assured consumer acceptance & research activities. The other areas which are important for raw materials & energy conservation are low lime and non-alite cements The R&D efforts in the area of utilization of wastes are now directed towards composite cements with more than two blending components. The need for standard specifications for composite cements, using variety of wastes & keeping in view the pragmatic approach of utilization of cement in different types of construction activities has also been realized. BIS specifications for composite cements are the need of time. Behaviour of slag and flyash in composite cements is complementary. Although the use of slag in Portland pozzolana cement and fly ash in Portland slag cement as performance improvers is being explored to take advantage of their complementary nature. At NCB investigations have been carried out on industrial by-product wastes in cement manufacture. Various wastes like lead-zinc slag, copper slag, steel slag, marble dust, which were not allowed for utilization 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. The future of cement industry lies also in products like geopolymers and nano-cement Acknowledgement The authors have freely drawn upon published / unpublished work / data of NCB. The paper is being published with the permission of Director General of NCB. References 1. Visvesvaraya, H C and Mullick, A K; Quality Results of Three Decade Surveys, ASTM Special Technical Publication, STP 961, pp Mullick A K and Ahluwalia S C ; Quality Standards in Indian Cement Industry Progress in Eight decade ( ) 3. Laxmi S and Huria S C; A Decennial Appraisal of Quality Trends in Indian Cements. Sixth NCB International Seminar, 1998 Vol XI, pp 29-40

7 4. Mathur, V K, Gupta, R S and Mohan K; Quality trends in Indian Cements An Appraisal from to 03, Eight NCB International Seminar, Vol XII, pp Ali, M M, Gupta R S, Mandal U K and Vasudeva M; Quality trends in Indian Cements An update, Tenth NCB International Seminar, pp x