بهره برداری از کائولن ضعیف یونانی: دوام بتن متاکائولن

In this paper the effect of metakaolin on concrete durability is investigated. A Greek kaolin of low kaolinite content was thermally treated at defined conditions and the produced metakaolin was finely ground. In addition, a commercial metakaolin of high purity was used. Eight mixture proportions were used to produce high performance concrete, where metakaolin replaced either cement or sand in percentages 10% or 20% by weight of the control cement content. Durability of metakaolin concrete was evaluated by means of resistance to chloride penetration, air permeability, sorptivity, porosity and pore size distribution. Metakaolin concrete exhibits significantly lower chloride permeability, gas permeability and sorptivity. The addition of metakaolin refines the pore system of concrete, leading to a decreased mean pore size and improved uniformity of the pore size distribution. The produced metakaolin, derived from the poor Greek kaolin, imparts similar behavior to that of the commercial metakaolin, with respect to the concrete durability.

Metakaolin is the most recent mineral admixture to be commercially introduced to the concrete construction industry [1], [2] and [3]. Unlike other pozzolans, it is a primary product, not a secondary product or by-product, produced by controlled thermal treatment of kaolin. This allows manufacturing process of metakaolin to be optimized, ensuring the production of a consistent pozzolanic material. 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 [4], [5], [6], [7], [8], [9], [10], [11] and [12]. The second one concerns the pozzolanic behavior of metakaolin and its effect on cement and concrete properties [2], [4], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32] and [33]. Concrete durability depends mainly on the chemistry (cement hydration process), and the microstructure of the concrete. Metakaolin addition affects positively both factors; Metakaolin consumes rapidly and effectively the Ca(OH)2 that is produced from the cement hydration process and additional to CSH, phases like C2ASH8 (stratlingite), C4AH13 and C3ASH6 (hydrogarnet) are produced. These pozzolanic products contribute to a total pore refinement [13], [18] and [32]. The refined pore system results in a more compact concrete, through which transportation of the water and other aggressive chemicals is significantly impeded and therefore a decrease in the diffusion rate of harmful ions is reported [17], [27], [33] and [34]. This work forms part of a research project, which aims towards to the exploitation of poor Greek kaolins in concrete technology. Up to now, the optimization of the kaolin to metakaolin conversion [10], [29] and [35], the study of the CH–metakaolin system [36], the effect of the crystallinity of the original kaolinite on the pozzolanic activity of metakaolinite [11] and [29], the properties and behavior of metakaolin cements [37], the effect of metakaolin on the corrosion behavior of cement mortars [30] and the evaluation of strength development of metakaolin concrete by means of k-value [38] have been carried out. In the present work, two metakaolins, a produced metakaolin originated from poor Greek kaolin and a commercial one of high purity, are examined and their effect on concrete durability is investigated.

The following conclusions can be drawn from the present study: • The produced metakaolin, derived from a poor Greek kaolin, imparts similar behavior to that of the commercial metakaolin, with respect to the concrete durability. • Metakaolin concrete, compared to PC concrete, exhibits significantly lower chloride permeability, gas permeability and sorptivity. • The addition of metakaolin refines the pore system of concrete, leading to a decreased mean pore size, improved uniformity of the pore size distribution and decreased volume of pores with size more than 160 nm. • The above improved properties would improve the durability of the concrete with respect to most forms of attack.