The worldwide demand for high-performance, cement-based materials has increased, and predictions are that it will reach a major industrial dimension during the early 21st century. Economics and environmental considerations have had a role in the mineral admixture usage as well as better engineering and performance properties.
The cementitious materials that are widely used, as concrete constituents, are fly ash, ggbs, and silica fume [1]. Metakaolin, produced by controlled thermal treatment of kaolin, is the most recent mineral admixture to be commercially introduced to the concrete construction industry. It has been claimed that concrete containing metakaolin exhibits premium-level engineering properties comparable to silica fume concrete [2], [3] and [4].
According to the literature, the research work on metakaolin is focused on two main areas. The first one refers to the kaolin structure, the kaolinite to metakaolinite conversion, and the use of analytical techniques for the thorough examination of kaolin thermal treatment [5], [6], [7], [8], [9], [10], [11], [12] and [13]. The second one concerns the pozzolanic behavior of metakaolin and its effect on cement and concrete properties [3], [4], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31] and [32]. Although there is a disagreement on specific issues, the knowledge level is satisfactory and continuously extended.
The concrete performance depends mainly on the environmental conditions, the microstructure, and the chemistry of the concrete. The two latter factors are strongly affected by the concrete components. It is obvious that the metakaolin presence affects the concrete performance. In particular, this effect on concrete properties can be practical, approached by the supplementary cementing materials (SCM) efficiency factor (k-value).
The k-value is defined as the part of the SCM in a pozzolanic concrete which can be considered as equivalent to Portland cement, having the same properties as the concrete without SCM (obviously, k=1 for Portland cement) [33]. The quantity of the SCM in the mix can be multiplied by the k-value to estimate the equivalent cement content, which can be added to the cement content for the determination of the water–cement ratio, minimum required cement content, etc. The property used for the estimation of k-values is the compressive strength [33] and [34]. However, durability properties can also be used, and relative k-values can be calculated. Knowing these k-values, the mix design for the preparation of the building product can be easier and more accurate.
In previous publications, a simplified scheme describing the activity of silica fume and fly ash (of low and high calcium) in terms of chemical reactions was proposed, yielding quantitative expressions for the estimation of the final chemical and volumetric composition of such SCM concretes [35], [36], [37] and [38]. Furthermore, a practical approach to the effect of SCM on the strength of Portland cement systems and on their resistance against carbonation and chloride penetration was presented using the concept of the SCM efficiency factor [39] and [40].
This work forms part of a research project, which aims towards the exploitation of poor Greek kaolins in concrete technology. Up to now, the optimization of the kaolin to metakaolin conversion [11], [30] and [41], the study of the CH–metakaolin system [30], the effect of the crystallinity of the original kaolinite on the pozzolanic activity of metakaolinite [12] and [30], and the properties and behavior of metakaolin cements [42] have been carried out.
The present work deals with the behavior of two metakaolins: a produced metakaolin that originated from poor kaolin and a commercial one of high purity. More specifically, the strength development of metakaolin concrete, the evaluation of metakaolin activity according to accepted quantitative criterion (k-value), and the comparison of the produced metakaolin with the commercial one are studied.
The following conclusions can be drawn from the present study:
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The produced metakaolin, which is derived from a poor Greek kaolin, imparts similar behavior to that of the commercial metakaolin, with respect to the concrete strength development.
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When metakaolin replaces sand, higher strengths than the OPC concrete are succeeded at all ages up to 90 days. When metakaolin replaces cement, its positive effect on concrete strength generally starts after 2 days.
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Both metakaolins exhibit very high k-values (close to 3.0 at 28 days) and are characterised as highly reactive pozzolanic materials that can lead to concrete production with an excellent performance
