Compressive strength, chloride diffusivity and pore structure of high performance metakaolin and silica fume concrete

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Abstract

This study is to relate the mechanical and durability properties of high performance metakaolin (MK) and silica fume concretes to their microstructure characteristics. The compressive strength and chloride penetrability of the control and the concretes incorporated with MK or silica fume (SF) at water-to-binder (w/b) ratios of 0.3 and 0.5 are determined. The pore size distribution and porosity of the concretes are also measured. The effect of MK and SF on the interfacial porosity is discussed based on test results. It is found that MK concrete has superior strength development and similar chloride resistance to SF concrete, and the MK concrete at a w/b of 0.3 has a lower porosity and smaller pore sizes than the control (plain) concrete. The resistance of the concretes to chloride ion penetration correlates better with the measured concrete porosity than with the paste porosity. The differences between the measured and calculated concrete porosity is smaller for MK and SF incorporated concrete than for the control concrete, indicating an improvement in the interfacial microstructure with the incorporation of the pozzolanas. This difference is found to be related to the strength and chloride penetrability of concrete to some degree.

Introduction

It has been well recognized that the use of pozzolanic materials such as silica fume (SF) and fly ash (FA) is necessary for producing high performance concrete. These materials, when used as mineral admixtures 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 interest in the use of MK as a mineral admixture for a similar purpose [1], [2], [3], [4], [5]. A comprehensive review of the studies on the use of metakaolin as a partial pozzolanic replacement for cement in mortar and concrete has recently been presented by Sabir et al. [6].

Metakaolin is a thermally activated alumino-silicate material obtained by calcining kaolin clay within the temperature range 650–800 °C [6]. It contains typically 50–55% SiO2 and 40–45% Al2O3 and is highly reactive. An important difference between MK and natural pozzolans 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 properties. It has been reported that the concrete incorporating 10% MK had a higher compressive strength than the control Portland cement concrete at all ages up to 180 days [3], [4]. 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 [4]. 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 [4]. Overall, as concluded by Sabir et al. [6] after a review of existing studies, MK is a very effective pozzolan. When used as a partial replacement in concrete, it results in enhanced early strength with no detrimental effect to the long-term strength, and greatly improves the resistance to the transportation of water and diffusion of harmful ions.

A number of studies have been conducted on the hydration process and microstructure changes of cement pastes containing MK [7], [8], [9], [10], [11], [12]. These studies show that at early ages the rate of pozzolanic reaction is higher in MK pastes than in SF pastes, but after prolonged curing it becomes slower in MK pastes. When used in cement pastes with a higher water-to-binder (w/b) ratio (e.g. w/b = 0.55), MK results in smaller pore sizes but higher total porosity [8]. However, when used in high performance cement pastes (low w/b ratio), it reduces both the pore sizes and total porosity [12].

The present study is to relate the mechanical and durability properties of high performance MK concrete to their microstructure properties. Two series of concrete mixtures are prepared at the water-to-binder ratios of 0.3 and 0.5. The compressive strength and chloride penetrability of MK blended concretes are determined and compared with those of SF concretes. The porosity and pore size distribution of high performance MK and SF concrete mixes prepared with a w/b of 0.3 are determined using mercury intrusion porosimetry (MIP). The effect of MK and SF on the porosity of the interfaces between the cement matrix and aggregates is evaluated by comparing the data of the present study with the data on the porosity of cement pastes reported in a separated paper [12]. The correlation between the pore structure characteristics and the strength and durability properties are then discussed.

Section snippets

Materials

The cementitious materials used in this study were Portland cement (PC) equivalent to ASTM Type I, metakaolin (MK) named MetaStar 450 obtained from Imerys Minerals; and condensed silica fume (SF) named Force 10,000D microsilica obtained from W. Grace. The chemical and physical properties of these materials are given in Table 1. The aggregates used were crushed granite and natural river sand. A naphthalene-based superplasticizer with a solid content of 38.6% by weight was used for the mixes

Compressive strength

The results of the compressive strength test are shown in Table 3, where each value is averaged from the results of three cubes. The results show that the metakaolin used in this study is superior to silica fume in terms of the strength enhancement of concrete. Among different replacement levels, the use of metakaolin at the replacement level of 10% performed the best, which resulted in the highest strength increase over the control concretes at all the test ages, particularly at the age of 3

Conclusion

The aim of this study is to relate the mechanical and durability properties of high performance metakaolin (MK) concretes to their microstructure characteristics. The compressive strength and chloride penetrability of the control concrete and the concretes incorporated with MK or silica fume (SF) have been determined with the water-to-binder (w/b) ratios of 0.3 and 0.5. The pore size distribution and porosity of the concretes have been determined with a w/b of 0.3. Based on the results and

Acknowledgements

The authors acknowledge the financial support of the Hong Kong Polytechnic University.

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