MICROSTRUCTURAL FEATURES OF INDIAN PORTLAND CEMENT CLINKER SAMPLES. V P Chatterjee and S K Chaturvedi

Transcription

1 MICROSTRUCTURAL FEATURES OF INDIAN PORTLAND CEMENT CLINKER SAMPLES V P Chatterjee and S K Chaturvedi National Council for Cement and Building Materials India ABSTRACT The broadening of raw material base, use of low grade deposits use of industrial byproducts and alternate fuel has affected the cement manufacturing along with clinker microstructure substantially Majority of the Indian cement plants in present day scenario are producing OPC clinker utilizing their captive limestone mines along with supplementary raw materials and additives as mineralizer/flux for the conservation of limestone and fuel. It is observed over the years that the resultant impact is on the granulometry, morphology, distribution pattern and percentage of clinker mineral phases. Detailed microscopic analyses of clinker samples of various sizes of clinker nodules obtained from different Indian plants have been carried out. It is observed that the phase formations in various sizes of nodules varied to a large extent and the morphological and microstructural features indicated improved characteristics in terms of quality of the clinker samples particularly in small sized nodules. These fractions most of the time were found to show better phase formations. Their morphological and microstructural features indicated tendency to improve the cement hydration and strength development. The microscopic studies showed large variation in grain sizes of alite, belite, aluminate and ferrite phases. Such phase formations were attributed to effects on clinker liquid properties and kinetics of reactions taking place because of varying quality of limestone and other materials being used in raw mix preparations. The present paper deals with microstructural features of clinker samples from different cement plants. 1.0 INTRODUCTION In present day scenario most of the Indian cement plants are using marginal grade limestone to produce OPC clinker along with various types of additives and fluxing materials. Over the years the clinker samples of different Indian cement plants were studied under optical microscope. In several instances it was observed that when there was a problem with clinker production or cement performance, the clinker microstructure provided clue as to the cause. Routine physical and chemical tests often provided insight, but microscopy provided a complete solution. If a change in any aspect of clinker production was anticipated (especially new raw materials), analysis of the microstructure became important before during and after the change. Records of clinker microscopy date back to 1887 but the technique has gained great deal of popularity in the past three decades. Polished section is the most common means of evaluating the clinker. In this method the clinker nodules are either uncrushed or partially crushed are

2 impregnated in epoxy. These samples are properly polished to reveal the various phases developed in the clinker nodules. Many studies are reported in literature regarding clinker microscopy (1, 2, 4, 6 & 7). Detailed characteristic features of OPC clinker and their role in clinkerisation, impact of raw material, raw mix and kiln feed are also compiled in literature (3). The National Council for Cement and Building Materials (NCB) has the state-of-art equipment to conduct microscopic analysis of clinkers. Over a period of last four decades clinker samples of various cement plants of India are examined from time to time in NCB. The present paper deals with the distribution pattern of phases, morphology, microstructure and granulometry of some of the clinker samples of Indian cement plants. Correlation of these features with the raw material, kiln feed and clinker were also made including their role during cement hydration and strength development. 2.0 SAMPLE PREPARATION TECHNIQUE In this technique, 3 to 4 sliced or crushed clinker nodules thus sampled are embedded in 1:5 epoxy resin (1 part hardener and 5 parts resin). This is subjected to vacuum impregnation which is a technique used to fill the pores and cracks by removing the air. The impregnated sample is kept in an oven at 50 C for 10 minutes and allowed to cool at room temperature. Thus, the hardened sample is ready for grinding and polishing. The preparation of polished section for reflected microscopic study is done in different stages. The coarse grinding is done on a silicon carbide disc using mesh carborundum powder and the fine grinding is done on a glass mounted disc using 600 to 1500 mesh carborundum powder. Polishing is done on a satin cloth using 9, 6 and 3 micron diamond lapping compound on a grinding disc. The whole process of sample preparation takes 2 to 3 hours. 3.0 PETROGRAPHIC INVESTIGATIONS The polished sections of the clinker nodules were observed with a polarizing microscope (NIKON, ECLIPSE LV100POL) using reflected light. Grain size analysis, microstructural and morphological investigations were done with the Image Analysis System attached to the microscope. In this paper microscopic results of clinker showing typical characters were mainly considered. The phases observed in OPC clinkers were alite (C 3S), belite (C 2S), tricalcium aluminate (C 3A), tetra calcium alumino ferrite (C 4AF) and free lime (CaO). Porosity and liquid phase analysis of typical clinkers were only shown. Optical character of clinkers clearly indicates that there is a relationship between clinker microstructure, the kiln feed and burning conditions.

3 Alite: Alite grains present in the clinker was from hexagonal to needle in shape. But majority of alite grains of pseudo hexagonal in shape with sharp grain margins. In majority of OPC clinker produce in India contain polymorphic alite phases with compositional zoning and partitioning of mineral tracers. Grain size variation in alite was normally too large (2 µm to 80 µm with an average of 25 µm -35 µm) this indicated that it would give larger variation in strength of the cement. This was also observed that if fuel used was pet coke (100%) and Laterite as raw material component, formation of larger phenocrysts of alite was noticed. 25 µm to 50µm size alite grains crystallize at higher temperature zone. Alite crystal size indicates rate of temperature rise in sintering zone and fineness of kilnfeed. Quick heating rate of fine kiln feed promote formation of smaller alite grains. Finer alite crystals are quick to react and this affects early strength. Belite: Belite grains were developed mostly as rounded to sub rounded and elliptical grains with sharp grain margins. In majority of clinkers belite grains also developed in different polymorphic forms. Belite grains were distributed both in clusters as well as in disseminated forms. In many instances development of semi crystalline belite grains were noticed. Size, shape and distribution pattern of the belite nests varied from clinker to clinker. Minor minerals constituents present in all the raw material components used for clinkerisation played a very vital role in the development of the belite nests, morphology and distribution pattern in the clinker. In few cases inclusions were also present in belite grains. Radiating needles were observed in few clinkers. Partially transform belie into alite grains were a common feature observed in clinker. Average grain size of belite varies from 25 to 40 µm with numerous micro structural features. Grain size indicates residence time in the higher burning zone. Long burning time produces large belite grains with multiple microstructures, whereas shorter burning time produces smaller belite grains. Since belite reacts slower than alite hence this hydration characters of belite show an effect on later age compressive strength. Tricalcium aluminate (C 3A): Size of C 3A grains is a function of cooling rate. In this slow cooling conditions grain size of calcium aluminate increases. The raw mix with high alumina ration produces great amount of aluminate than ferrite. Presence of alkali aluminates in the clinker indicates either excess alkalies or possibly reducing conditions in the kiln. C 3A grains were developed mostly as complex twinned grains. In certain clinker samples structural modification were also observed. These structural modifications in C 3A were developed in presence of complex alumino silicates in the raw mix components. Tetra Calcium Alumino Ferrite (C 4AF): Lath to needle shaped C 4AF grains with sharp grain margins were normally distributed in association with C 3A grains.c 4AF grains were developed mostly either as complex twinned grains or fused grains. Free Lime: Presence of free lime in the Indian clinker was uncommon. However, in few instances pseudo free lime grains were observed. In many instances presence of actual free lime did not match with the standard bouge s method. Free lime grains were observed as small clusters with

4 multiple colours. Free lime grains were normally rounded to sub rounded in shape with sharp grain margins. In most of the cases free lime were present in association with alite grains. Liquid Phase: Liquid phases observed in the clinker were mainly fine grained (<1µm) glassy phases. In most of the cases liquid phases were developed in the cores surrounded by belite grains. In many instances development of very fine grained well developed alite and belite (grains <1µm) were also observed in some of the clinkers. Higher percentage of liquid phases were noticed in the clinkers due to improper maintaining the retention time, complex raw material components, grinding and improper cooling. Distribution: In most of the clinkers the phases were developed moderately and distributed inhomogeneously. But well developed clinker phases and homogeneously distributed alite and belite grains were also observed. Rarely poorly developed and distributed phases of clinker were also noticed in the sample. Microstructures: Pitted structures were commonly developed in alite grains. In many clinkers compositional zoning in the form of micro grains were observed. These complex alite grains were developed due to complexity of the raw material components. In many instances formation of radiating needles, bulb and tubers were noticed on the margins of the alite grains. Twinned and fused grains were very common in majority of clinker sample. In majority of clinker samples it was observed that transformation of belite into alite did not reach the equilibrium condition. In many instances development of one and two set twinning also observed in belite grains. In many clinker samples growth of branched alite and belite grains were developed on the margins of both alite and belite phenocrysts structures like pine, fern and flowery etc. were developed. Majority of these grains were of alite and belite. In majority of clinker samples development of very fine (<1µm) grains of alite and belite were also observed. In few instances percentage of these grains varied from 5 to 10%. Fig 1: Homogeneous distribution of clinker phases. (20X) Fig 2: Well developed Alite grains. (20X)

5 Fig 3: Well developed Belite grains. (20X) Fig 4: Development of C 3A & C 4AF grains. (50X) Fig 5: Cluster of Free Lime grains. (20X) Fig 6: Development of Liquid Phase. (50X) Fig 7: Development of Liquid Phase. (50X) Fig 8: One and two sets twinning in Belite. (50X) Fig 9: Alite grains showing twinned & pitted structures and compositional zoning. (100X) Fig 10: Fern structured belite on the tip of alite phenocrysts. (50X)

6 Fig 11: Pine, flowery and Fern structured alite grains developed in the matrix. (50X) Fig 12: Flowery and Fern structured belite grains developed in the matrix. (50X) 4.0 CONCLUSIONS 1. The grinding of the raw materials should be done into an optimum fineness to produce burnable kilnfeed. However, variances in grindability and burnability of the raw mix components after many challenges. The minerals present in the limestone crystalline in nature with higher hardness may generate clusters of crystals in the clinker, which may lead to decreased clinker grindability. 2. Proper homogenization of the kiln feed is must for proper burning and uniform pyro processing of the feed during clinkerisation. 3. Burning conditions in the kiln develop the desired hydraulic compound from the raw materials. Even the raw materials have great bearing in the preprocessing. Burning conditions are related to development of micro structure. Rate of heating and cooling, kiln conditions, maximum temperatures etc. normally determine the size, morphology and development of complex compounds in the clinker. 4. The characteristic features of phases, their distribution pattern and formation decide how these phases hydrate, which determines the early performance of cement. The characteristic of the phases which react slowly will dictate how the cement performs at later stage. 5.0 ACKNOWLEDGEMENT This paper pertains to R&D works carried out by Centre for Cement Research and Independent Testing at National Council for Cement Building Materials. The Authors acknowledge the cooperation of the staff of the institute in carrying out the works. The paper is published with the permission of the Director General of the Council.

7 REFERENCES 1. Anwander, A. and others: New Method for Clinker Phase Recognition Using Automatic Image Analysis, Proceedings of the 20 th International Conference on Cement Microscopy, International Cement Microscopy Association, Guadalajara; Jalisco 1998, pp Butt, Y.M. and Timashev, VV: The mechanism of Clinker Formation Processes and Ways of Modification of Clinker Structure, Sixth International Congress on the chemistry of Cement, Sec.I, Moscow, 1974, PP Donald H. Campbell: Microscopical Examination and Interpretation of Portland Cement and Clinker Campbell D.H. and Galehouse, J.S.: Quantitative Clinker Microscopy with the Light Microscope, Cement, Concrete and Aggregate, ASTM, winter 1991, pp Chatterjee, V.P, Rao, L.H. and Sinha, S.K.: The Role of Morphology and Microstructure of Clinker Phases on Clinker Grindibility and Cement Strength, 6th NCB International Seminar on Cement and Building Materials, New Delhi, 1998, vol 3, IX Maki, J, Masaki, K and Suzuki, M: Burning and Nodulization Process of Clinker in the Rotary Kiln as Viewed from the Fine Textures of the Constituent Minerals, Proceedings of the Sixth Euro seminar on Microscopy Applied to Building Materials, Reykjavik, Iceland, 1997, pp Maki, I. and others, Kinetics of Formation and Fine Textures of Alite in Portland Cement Clinker, Proceedings of the 17th International Conference on Cement Microscopy, International Cement Microscopy Association, Calgary, Alberta, 1995, pp Norris, D., Kiln Feed Homogeneity and Clinker Microscopy, Proceedings of the 10th International Conference on Cement Microscopy, International Cement Microscopy Association, San Antonio, Texas, 1988, pp Ono, Y., Observation of Clinker Texture by Reflection Microscope, Reviews, 45th General Meeting, Cement Association of Japan, 1991, pp Ono. Y: Ono s Method, Fundamental Microscopy of Portland Cement Clinker, Chichibu Onoda Cement Corporation, No-2-4-2, Ohsaku, Sakura, Japan 1995, pp-229.