In this work, the properties and the hydration procedure of cements containing metakaolin were monitored for periods up to 180 days. Four metakaolins, derived from poor Greek kaolins, as well as a commercial metakaolin of high purity were used. Cement mortars and pastes, with 0%, 10% and 20% replacement of cement with the above metakaolins, were examined. Strength development, water demand and setting time were determined in all samples. In addition, XRD and TGA were applied in order to study the hydration products and the hydration rate in the cement–metakaolin pastes. It is concluded that metakaolin has a very positive effect on the cement strength after 2 days and specifically at 28 and 180 days. The blended cements demand significantly more water than the relatively pure cement and the water demand increase is higher, the higher the metakaolin content. The produced metakaolins as well as the commercial one give similar hydration products after 28 days and the pozzolanic reaction is accelerated between 7 and 28 days, accompanied by a steep decrease of Ca(OH)2 content. Finally, it is concluded that a 10% metakaolin content seems to be, generally, more favorable than 20%. The produced metakaolins, derived from poor Greek kaolins, as well as the commercial one impart similar properties with respect to the cement strength development, the setting and the hydration.
The most common cementitious materials that are used as concrete constituents, in addition to Portland cement, are fly ash, ggbs and silica fume. They save energy, conserve resources and have many technical benefits [1]. Metakaolin, produced by controlled thermal treatment of kaolin, can also be used as a concrete constituent, since it has pozzolanic properties [2] and [3].
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], [3], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29] and [30]. Although there is a disagreement on specific issues, the knowledge level is satisfactory and is being continuously extended.
In this study, the properties and the hydration procedure of cements containing metakaolin were monitored for periods up to 180 days. Four metakaolins, derived from poor Greek kaolins, as well as a commercial metakaolin of high purity were used. This work forms part of a research project, which aims to exploit Greek kaolins in concrete technology.
The following conclusions can be drawn from the present study:
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The produced metakaolins derived from poor Greek kaolins, when combined with PC to produce blended cements, impart similar behavior to that of commercial metakaolin, with respect to cement strength development, setting times and hydration.
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The studied metakaolins have a very positive effect on the cement strength after 2 days and specifically at 28 and 180 days. The blended cements demand significantly more water than the relatively pure cement and the water demand increase is higher, the higher the metakaolin content.
•
The pozzolanic reaction of metakaolins is accelerated between 7 and 28 days, accompanied by a steep decrease of Ca(OH)2 content. The hydration products, after 28 days, are similar in all samples.
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Based on the mechanical and physical properties of metakaolin cements, a 10% metakaolin content seems to be, generally, more favorable than 20%.
