DURABILITY CHARACTERISTICS OF METAKAOLIN IN COMPARISON WITH SILICA FUME IN CONCRETE Farhad PARGAR Senior Researcher, Construction Materials Institute (CMI), University of Tehran, Iran Alireza MIRDAMADI Senior Researcher, Construction Materials Institute (CMI), University of Tehran, Iran ALI DOUSTI Senior Researcher, Construction Materials Institute (CMI), University of Tehran, Iran MOHAMMAD SHEKARCHIZADEH Director, Construction Materials Institute (CMI), University of Tehran, Iran ABSTRACT: This paper reports an investigation in which the performance of plain and metakaolin (MK) in comparison with sili- ca fume (SF) in concrete. The purpose of this study is to evaluate the effectiveness of MK in enhancing the mechan- ical and durability properties of plain concrete. MK and SF were used to replace 0-15% and 10% of Portland cement by weight respectively. The fresh and hardened concrete properties, mechanical properties and durability characte- ristics for all cement replacement are compared with those of the control concrete. The results indicated that the re- placing of MK greatly reduced absorption, chloride permeability and increase extremely electrical resistance of con- crete. Keywords: Concrete, Metakaolin, Silica Fume, Durability characteristics 1. INTRODUCTION Persian Gulf, due to its special climate, is the most ag- gressive exposure conditions for concrete structures. Construction of concrete structures in Persian Gulf has increased during past decades due to its special and strategic location [1]. In Iran, however, the increase of construction costs due to the increasing price of im- ported silica fume compared to other admixtures led re- searchers to turn their interest toward metakaolin, an economical and promising cementitious material pre- senting similar strengthening effects as silica fume. In this region, pozzolanic materials such as silica fume (SF) is necessary for producing high performance con- crete. These materials, when used as mineral admix- tures in high performance concrete, can improve either or both the strength and durability properties of the concrete. In recent years, there has been a growing in- terest in the use of MK as a mineral admixture for a similar purpose. The use of metakaolin as a pozzolanic mineral admix- ture has been known for many years, but has grown ra- pidly since approximately 1985[2]. Metakaolin is a thermally activated alumino-silicate material obtained by calcining kaolin clay within the temperature range 650–800 ºC. It contains typically 50– 55% SiO2 and 40–45% Al2O3 and is highly reactive. The average particle size of metakaolin varies and can be controlled during the processing to change the prop- erties of the fresh concrete. In general, the average par- ticle size of metakaolin ranges from 0.5 to 20 µm [3]. An important difference between MK and natural poz- zolans or other types of artificial pozzolans is that MK is a primary product, while SF and FA are secondary products or by-products. Thus, MK can be produced with a controlled process to achieve the desired proper- ties. When compared with SF concrete at the same level of replacement, MK concrete showed a faster strength development at early ages, but had similar strength after 28 days. With respect to the durability aspects, it has been reported that the resistance of MK concrete to chloride ion penetration was significantly higher than the control concrete but similar to the SF concrete [2]. However, if very large percentages (greater than 20 per- cent) of the SF and MK be used, the levels of calcium hydroxide reduce significantly [4]. The objective of this study was to investigate the me- chanical and durability properties of high performance MK concrete. Concrete mixture is prepared at the wa- ter-to-binder ratio of 0.4. MK and SF were used to re- place 0-15% and 10% of Portland cement by weight in respectively. Fresh and hardened concrete properties such as time of setting and density, compressive strength, water absorption, electrical resistivity and chloride penetrability (according to NT BULT 443) of MK blended concretes are determined and compared with SF concrete. 2. EXPERIMENTAL PROGRAM 2.1 Materials In this study, the aggregates used were crushed granite and natural river sand. The physical characteristics of both aggregates are outlined in Table 1. Portland ce- 915 S4-2-4 4th International Conference on Construction Materials: Performance, Innovations and Structural Implications Performance under Severe Environment and Loading