Abstract
Metakaolin (MK) is one of the most effective mineral admixtures for cement-based composites. The deposits of kaolin clays are wide-spread in the world. Metakaolin is comparable to silica fume as an active mineral admixture for cement-based composites. In this paper, the rheological and mechanical properties of cement paste containing metakaolin are investigated. The effect of MK is more evident at “tight” hydration conditions within mixtures with low water-cement ratio, provided by application of superplasticizers. The cement is replaced with 0 to 15% metakaolin, and superplasticizer content ranged from 0 to 1.5% by weight of cementitious materials (i.e. cement and metakaolin). An equation is derived to describe the relationship between the metakaolin and superplasticizer content and consistency of pastes. There is a linear dependence between metakalolin content and water demand. Second-degree polynomial describe the influence of superplasticizer content. The application of SP and MK may produce cement-water suspensions with water-retaining capacity at 50-70% higher than control suspensions. The investigation of initial structure forming of cement pastes with SP-MK composite admixture indicates the extension of coagulation structure forming phase comparing to the pastes without additives. Crystallization stage was characterized by more intensive strengthening of the paste with SP-MK admixture comparing to the paste without admixtures and paste with SP. Results on the porosity parameters for hardened cement paste indicate a decrease in the average diameter of pores and refinement of pore structure in the presence of metakaolin. A finer pore structure associated with an increase in strength. X-ray analysis data reveal a growing number of small-crystalline low-alkaline calcium hydrosilicates and reducing portlandite content, when MK dosage increases. Scanning electron microscopy (SEM) data confirm, that hardened cement paste containing MK has crystalline structure with dominance of partially crystalized hydrosilicates and gel-like formations.
Highlights
Metakaolin (MK), a pozzolanic admixture of N type [1] consists mostly of amorphous aluminosilicate, obtained by firing kaolin clays with subsequent grinding
The following conclusions are based on the results of this investigation
The normal consistency depends on the content SP and MK, which indicates a positive linear relationship between the water demand increase and MK content growth
Summary
Metakaolin (MK), a pozzolanic admixture of N type [1] consists mostly of amorphous aluminosilicate, obtained by firing kaolin clays with subsequent grinding. One of the common area of its application in construction is high-performance concrete, self-compacting concrete [2,3,4,5]. Self-compacting high-strength concrete is characterized by basal structure with ‘floating’ aggregate particles. In such a structure, grains of aggregate are considerably separated and almost do not interact with each other. Properties of fresh and hardened concrete depend on the rheological, structural and mechanical properties of the cement-matrix. Incorporation of MK as partial replacement of cement leads to significant changes in chemical composition and pore size distribution [6]. Studying of poor Greek Kaolin clays proved positive effect on the cement strength in different age of hardening. MK content of 10% is considered the optimal replacement [7]
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