DEVELOPMENT OF STRENGTH AND DURABILITY OF CONCRETE INCORPORATING LOCAL METAKAOLIN

Abstract

Ordinary Portland cement concrete is a very popular construction material used in developed/underdeveloped countries. The basic ingredients of concrete are cement, fine and coarse aggregates bounded by water. Fine and coarse aggregates are generally inert materials which do not react with cement during hydration process rather act as filler material. Ordinary Portland cement (OPC) concrete is good for normal construction; however, for industrial construction OPC concrete may come in contact with different types of aggressive environment such as acid producing or acid based industries, fruit and vegetable processing industries, underground structures subjected to water logging and salinity which results in degradation of concrete. Corrosion of reinforcement for exposed portion of concrete structure is also very important especially in case of concrete highway bridges or other important heavy structures. Supplementary cementing materials (SCM’s) are commonly used to improve strength and durability of concrete. The incorporation of these materials also reduces the cost of the concrete. The most commonly used SCM’s are silica fume, fly ash, blast furnace slag, metakaolin, rice husk ash etc. These SCM’s are finer than cement and hence improve the packing of the concrete mixture, resulting in increased compressive strength. The durability of concrete containing SCM’s is improved due to the chemical reaction of various compounds present in supplementary cementing material with cement during the hydration process. The increase in replacement level of SCM’s by weight with the cement also influences the strength and durability properties of concrete. Kaolin clay, a source of metakaolin; has been frequently used in pottery industry and it is abundantly available in Pakistan; however, it has never been used as supplementary cementing material by local construction industry. This study was undertaken to assess the potential of locally produced metakaolin for using as pozzolan especially for durability of concrete against acid and carbonation attack. The main objective of the research includes a) to develop reactive metakaolin from kaolin clay, iib) to study the performance of metakaolin concrete against acid and carbonation attacks, c) to evolve Strength Degradation Model for metakaolin concrete against acid attack. During the first phase of study, metakaolin was successfully developed from the Nagar o Parkar kaolin. The kaolin was calcined at several temperatures ranging from 600 C to o 1000 C for variable durations of 6 to 10 hours. The calcined clay in each case was finely divided to specific surface of approximately 645 m 2 /kg. The powdered samples were subjected to X-ray Diffraction Tests (XRD) as well to Strength Activity Index Tests o (SAI) for 7 and 28 days. It was found from XRD tests that kaolin calcined at 800 C for 8 hour duration was transformed into the most reactive metakaolin. The strength activity Index tests supported this finding as well. The performance of metakaolin concrete against acid and carbonation attacks was studied during the second phase of the study. A huge and exhaustive test program was designed in which the most reactive metakaolin developed during the first phase was used as pozzolan. Two broad classes of concretes were prepared with binder contents of 300 kg/m 3 and 400 kg/m 3 of concrete respectively; the former represented the normal class concrete and the latter as a rich class concrete. Four metakaolin-binder ratios (0%, 15%, 20% and 25%) were combined with three water-binder ratios (0.45, 0.55 and 0.55) to produce 12 concrete mixtures from each of the normal and rich class concretes. One hundred twenty cubes of 100mm size were case from each mixture. Four cubes from each mixture were crushed at 7 & 28 days to determine the compressive strength. Fifteen cubes were reserved for carbonation experimentation and three each were used for determination of carbonation depth in open atmosphere at 4, 7, 13, 25 and 52 weeks. Sixteen cubes each were immersed in 2, 5 & 8% concentrated solution of sulfuric acid and an equal number of cubes were placed in 2, 5 & 8% solution of acetic acid. The compressive strength was obtained after 7, 28, 91 and 182 days of immersion in each case. It was revealed from the test results that metakaolin concrete better resisted the acid attack in comparison with the control plain cement concrete The strength of concrete was degraded due to immersion in acid solution of variable concentrates. A strength degradation model was proposed using statistical approach. The iiimodel was based on physical parameters like binder content, metakaolin-binder ratio, water-binder ratio, solution strength of acid and immersion period. One half of the experimental data was used in the calibration of the model and the other half was used for validation of the model. The model prediction agrees quite closely with the experimental data. Carbonation depth was measured for concrete cubes exposed to open atmosphere at 4, 7, 13, 25 & 52 weeks interval. The carbonation depth increases with increase in water to binder ratios but drastically decreases for metakaolin concrete due to the improved packing of the concrete matrix. The graphical presentation of carbonation depth clearly demonstrates the role of variable dosage of metakaolin. These charts may also be used as a ready reference for a fair estimate of carbonation depth for different mixtures of metakaolin concrete other than specified.