Metakaolin for High-Performance Concrete

Transcription

1 Metakaolin for High-Performance Concrete Special Correspondent Cement concrete is the most extensively used construction material. Maintenance and repair of concrete structures is a growing problem involving significant expenditure. As a result carried out worldwide, it has been made possible to process the material to satisfy more stringent performance requirements, especially long term durability. High performance is generally assumed to be synonymous with high strength, although this is not true in every case. Unacceptable rates of deterioration due to environmental effects indicate that only compliance with strength requirements, although need, is not adequate to ensure long-term, durability, which is the primary requirement for high performance. It is generally accepted, that the high performance of the very concrete contributes to low permeability, stronger and denser transition zone between aggregate and cement paste in the concrete. This also adds to the abrasion resistance of concrete. According to ACI High Performance Concrete is defined as concrete which meets special performance and uniformity requirements that cannot always be achieved routinely by using conventional materials and normal mixing, placing and curing practices. High Performance Concrete (HPC) Concrete is probably the most extensively used construction material in the world. However, when the high range water 102 The Masterbuilder - July

2 reducer or super plasticizer was invented and began to be used to decrease the water/cement (w/c) or water/binder (w/b) ratios rather than being exclusively used as fluid modifiers for normal-strength concretes, it was found that in addition to improvement in strength, concretes with very low w/c or w/b ratios also demonstrated other improved characteristics, such as higher fluidity, higher elastic modulus, higher flexural strength, lower permeability, improved abrasion resistance, and better durability. This fact led to the development of HPC. HPC is the latest development in concrete. It has become more popular these days and is being used in many prestigious projects such as Nuclear power projects, flyovers, multistoried buildings etc. Since 1990s, HPC has become very popular in construction works. At present, the use of HPC has spread throughout the world. In 1993, the American Concrete Institute (ACI) published a broad definition for HPC and is defined as the concrete which meets special performance and uniformity requirements that cannot always be achieved by using only the conventional materials and mixing, placing and curing practices. The addition of mineral admixture in cement has dramatically increased along with the development of concrete industry, due to the consideration of cost saving, energy saving, environmental protection and conservation of resources. However, environmental concerns both in terms of damage caused by the extraction of raw material and carbon dioxide emission during cement manufacture have brought pressures to reduce cement consumption by the use of supplementary materials. Mineral admixtures such as fly ash, rice husk ash, metakaolin, silica fume etc are more commonly used in the development of HPC mixes. They help in obtaining both higher performance and economy. These materials increase the long term performance of the HPC through reduced permeability resulting in improved durability. Addition of such materials has indicated the improvements in the strength and durability properties of HPC. Jayant Basu Ray MD, Constromat Consultancy & Services India PVT LTD As far as India is concerned metakaolin actually came into the market as a cheaper material than micro silica. It was only after reputed companies started using metakaolin that the product began to generate a buzz in the market. About Metakaoline & Its Role in Concrete and High Performance Concrete Metakaolin differs from other supplementary cementitious materials (SCMs), like fly ash, silica fume, and slag, in that it is not a by-product of an industrial process; it is manufactured for a specific purpose under carefully controlled conditions. Metakaolin is produced by heating kaolin, one of the most abundant natural clay minerals, to temperatures of C. This heat treatment, or calcination, serves to break down the structure of kaolin. Bound hydroxyl ions are removed and resulting disorder among alumina and silica layers yields a highly reactive, amorphous material with pozzolanic and latent hydraulic reactivity, suitable for use in cementing applications. Metakaolin reacts with portlandite (CH) to form calcium-silicate-hydrate (C-SH) supplementary to that produced by portland cement hydration. This reaction becomes important within the interfacial transition zone (ITZ) located between aggregate and paste fractions. This region typically contains a high concentration of large, aligned CH crystals, which can lead to localized areas of increased porosity and lower strength. Metakaolin can react with some of the CH produced by cement hydration, there by densifying the structure of the hydrated cement paste. The rates of pozzolanic reaction and CH consumption in metakaolin systems have been shown to be higher than in silica fume systems, indicating a higher initial reactivity.6 Because this reaction with CH occurs early and rapidly, metakaolin incorporation may contribute to reduced initial and final set times. In addition, this refinement in the ITZ can result in increased strength in metakaolin concrete. As portlandite in the ITZ and elsewhere in the paste is water soluble and is susceptible to deterioration in aggressive chemical environments, metakaolin has great potential for improving concrete durability. Also, because (a) (b) Figure 1: (a) Metakaolin, (b) Self-consolidating concrete using metakaolin The Masterbuilder - July

3 the supplementary C-S-H formed during the pozzolanic reaction with metakaolin has a lower Ca/Si ratio than ordinary C-S-H, these products are believed to be better able to bind alkali ions from the pore solution, thus reducing concrete s susceptibility to alkali-silica reaction (ASR). This potential beneficial use of metakaolin is particularly relevant, as silica fume agglomerates have been shown to contribute to ASR expansion in some cases. Metakaolin has also been shown to decrease concrete permeability, which in turn increases its resistance to sulfate attack and chloride ion ingress. Additionally, metakaolin may reduce autogenous and drying shrinkage, which could otherwise lead to cracking. Thus, when used as a partial replacement for Portland cement, metakaolin may improve both the mechanical properties and the durability of concrete. In general, metakaolin offers a set of benefits similar to those imparted by silica fume, including comparable strengths, permeability, chemical resistance, and drying shrinkage resistance. Physically, metakaolin particles measure approximately one-half to five micro meters across, making them an order of magnitude smaller than cement grains and an order of magnitude larger than silica fume particles. Both metakaolin and silica fume are typically used to replace 5 to 20 weight % of the cement. Metakaolin is white in color, whereas standard silica fume ranges from dark grey to black (although white silica fume is available at higher cost). This makes metakaolin particularly attractive in color matching and other architectural applications. For these reasons, metakaolin is increasingly used in the production of high-performance concrete. ASTM C618 and AASHTO M 295 classify metakaolin as a Class N (or natural) pozzolan. Figure 1 (a) and (b) shows metakaoline and self-consolidating concrete using metakaolin. Metakaolin s Contribution in HPC - Improved Strength Metakaolin s reaction rate is rapid, significantly increasing compressive strength, even at early ages, which can allow for earlier release of formwork. As stated by Mr. Atil Parikh, Joint Managing Director of 20Microns Globally metakaolin Atil Parikh Joint Managing Director of 20Microns Globally metakaolin is a very well accepted product. In India too the demand is fast picking up. Initially we had to educate people. We first targeted Mumbai the high rise capital of India. This is because metakaolin finds use in the M60 or M80 grade concrete that is typically used in high-rise construction projects is a very well accepted product. In India too the demand is fast picking up. Initially we had to educate people. We first targeted Mumbai the high rise capital of India. This is because metakaolin finds use in the M60 or M 80 grade concrete that is typically used in high-rise construction projects (a) Control (b) With Meta Star 501 Figure 2: (a) Control Mix floor, (b) Metakaolin mix floor - Improved Durability In addition to increasing strength, the densification of the microstructure that results from the pozzolanic and hydraulic reactions of metakaolin also leads to greater impermeability. In concretes containing metakaolin at 8 to 12% of the total cementitious materials, 50-60% decreases in chloride diffusion coefficient suggest that significant improvements in service life can be achieved through metakaolin utilization in chloride environments. In addition, metakaolin has been shown to be highly effective in mitigating expansion due to alkali-silica reaction (ASR) and sulfate attack. - Improved Early Age Behavior The relative fineness of metakaolin can result in decreased slump, but the use of water reducing admixtures or use in combination with fly ash in ternary mixes can compensate for this. Slumps of 125 to 180 mm have been achieved with metakaolin at water cementitious materials ratio (w/ cm) of 0.36 to 0.38, using 25-35% less high-range water reducing admixture than comparable mixes. Metakaolin concrete tends to exhibit a creamy texture, resulting in better finishability compared to other finely divided SCMs. This quality also improves pumpability and can be used to impart detailed surface textures to cast surfaces. In addition, the cohesiveness provided by the metakaolin allows for relatively simple formulation of self-consolidating concrete, when using an appropriate dosage of polycarboxylate water reducer as shown in the photograph at the beginning of this article. Data on the potential contributions of metakaolin to chemical, autogenous, and drying shrinkage are inconsistent, with authors reporting both decreases and increases in each form at various ages and at various addition rates. For applications with restrictions on shrinkage, additional testing, including the assessment of shrinkage-reducing admixtures and fiber reinforcement, may be advised. 104 The Masterbuilder - July

4 - Contributions to Sustainability Because of the lower processing temperature compared to cement clinker, use of metakaolin can contribute to sustainability through energy savings, as well as reductions in greenhouse gas emissions. After examining various SCMs alone and in combination and considering performance, economic, and environmental criteria, metakaolin concrete was identified as a very promising solution for the precast industry for reducing clinker content in concrete. Available Brands and Applications 1. MetaStar 501: METASTAR 501 is a metakaolin pozzolanic additive that: - Improves strength, durability, and workability of Portland cement concrete - Makes Portland cement easier to apply - Provides smoother finish - Has bright white color for white and color plasters - Reduces permeability, efflorescence, and cracking - Reduces the porosity of hardened concrete - Contains no undesirable impurities, such as carbon or sulphur, which could affect the curing rate or strength of the final PC product - Readily disperses in cement-based systems - Is safe and easy to handle Refer Figure 2 (a) and (b) for judging the difference of control and Metakaolin mix. 2. In India 20Microns is considered one of the leading producers of white minerals. The company offers a wide range of products including functional fillers, extenders, and specialty chemicals. The company s mines are spread across the country. The company offers a diverse range of customized products based on specific industry requirements. 20microns has its calcined clay mines located in Gujarat, Bhuj. For the production of calcined kaolin the material has to be put through heating up to a certain temperature around 1200 o C. Metakaolin is an intermediary product which is produced at about o C during the process. Figure 2: (a) Control Mix floor, (b) Metakaolin mix floor 3. PALAIS ROYALE, Mumbai: First project in India to use M80 grade of self consolidating concrete. The Project is around 300 metres high comprising of Duplexes and Villas in it. The developers have used metakaolin as one of the supplementary cementitious material. Conclusion The beneficial effects of using metakaolin is it reacts with calcium hydroxide almost as fast as it is formed in the cement during hydration. The overall effect of removing calcium hydroxide, refining the pore structure and densifying the interfacial zone is to reduce: - Re-bar corrosion - Sulphate attack - Acid attack (e.g. silage clamps, food factories) - Freeze-thaw damage - Alkali-silica reaction (even when using active aggregates, 106 The Masterbuilder - July

5 high alkali cementsand where the concrete is exposed to salt solutions) - Efflorescence Till three-four years ago, hardly anybody in India was aware of the use of metakaolin in concrete. During these four years, the developments that have taken place include increased awareness of the huge potential of production of metakaolin in the country (with huge mineral resource, that is, kaolin availability across the country), start of indigenous commercial production and many investigations on the development of concrete mixes containing metakaolin. The metakaolin is a low cost, locally produced, highly effective pozzolonic material, particularly for the early strength development, without appreciable loss in workability. It makes finish easier, reduces efflorescence, increase resistance to sulphate and chlorine attack. It maintains colour of concrete, especially in white concrete. So it can be very well used in Architectural work. As silica fume is to be imported, it is dare necessity to find locally available material at an affordable price, substitute of it, Metakaolin could be an answer to it seeing its vast exploring opportunity in India. It can bring a huge export possibilities to India; as quoted by Mr. Atil Parikh, The US is a high consumption market. It was during the last decade or so that people started to switch from micro silica to metakaolin there in a big way. Canada is another major market. Reference - Justice, J. M. and Kurtis, K. E., Influence of Metakaolin Surface Area on Properties of Cement-based Materials, ASCE Journal of Materials in Civil Engineering, September 2007, Vol.19, No. 9, pp Tafraoui, A. et al., Metakaolin in the Formulation of UHPC, Construction and Building Materials, Vol. 23, 2009, pp Justice, J. M. et al., Comparison of Two Metakaolins and Silica Fume Used as Supplementary Cementitious Materials, Seventh International Symposium on Utilization of High- Strength / High Performance Concrete, Ed. Russell, H. G., Publication SP-228, Vol. 1, American Concrete Institute, Farmington Hills, MI, 2005, pp Also on Compact Disc. - Gruber, K. A. et al., Increasing Concrete Durability with High- Reactivity Metakaolin, Cement and Concrete Composites, Vol. 23, 2001, pp Khatib, J. M. and Wild, S., Sulphate Resistance of Metakaolin Mortar, Cement and Concrete Research, Vol. 28, No. 1, 1998, pp Ramlochan, T., Thomas, M., and Gruber, K. A., The Effect of Metakaolin on Alkali-Silica Reaction in Concrete, Cement and Concrete Research, Vol. 30, 2000, pp Garas, V. Y. and Kurtis, K. E., Assessment of Methods for Optimising Ternary Blended Concrete Containing Metakaolin, Magazine of Concrete Research, September 2008, Vol. 60,No. 7, pp Caldarone, M. A., Gruber, K. A., and Burg, R. G., High- Reactivity Metakaolin: A New Generation Mineral Admixture, Concrete International, Vol. 16, No. 11, November 1994, pp Cassagnabere, F. et al., Metakaolin, A Solution for the Precast Industry to Limit the ClinkerContent in Concrete: Mechanical Aspects, Construction and Building Materials, Vol. 24, 2010, pp The Masterbuilder - July