Studying the Influence of Metakaolin on Self-healing Processes in the Contact-zone Structure of Concretes Based on the Alkali-activated Portland Cement

This paper reports results from comparative testing the reaction "alkali ‒ active silica" in traditional Portland cement and alkaline Portland cement with the addition of metakaolin. The research is based on studying the process of structure formation in cements in the contact zone "cement stone – basalt".The research results allow us to conclude that the dynamics of the process of interaction of the reaction "alkali‒silica" in cements may be constructive and destructive in character. That depends on the content of components that are able to actively interact with alkalis in the presence of reactive silica. The so-called "constructive" processes are accompanied by the binding of corrosion products during the formation of alkaline hydroalumosilicates. The research results were used as the basis for developing the mechanism of preventing the reaction "alkali – active aggregate" in concretes based on alkaline cement by introducing to the cement composition additional amounts of materials containing active aluminum, in particular, metakaolin.Our study has shown that the introduction of a metakaolin additive could effectively control the processes of structure formation in the contact area "cement stone – active silica", thereby changing the character of new structures. The mechanism of an alkaline corrosion process of an active aggregate in the presence of metakaolin has been established, according to which metakaolin enters the reaction at a rate of microsilica, providing for a very Fast binding of the Na+ and K+ ions. Silicate gel of alkaline metals binds into insoluble zeolite-like new structures and hybrid hydroalumosilicates. As resilient structures, the latter condense and strengthen the contact area by enhancing its microhardness and strength.We have investigated the natural shrinkage deformations (expansion) of the developed compositions of concretes based on the traditional and alkaline Portland cements. It has been shown that the introduction of a metakaolin additive to the system formulation makes it possible to reduce the system expansion indicators from 0.44 to 0.01 mm/m, thereby maintaining the defect-free structure of cement stone and concrete and improving the durability of concret