A comparative study on the influence of metakaolin and kaolin additives on properties and structure of the alkali-activated slag cement and concrete

Abstract

The influence of the metakaolin and kaolin additives on the formation and properties of the alkali-activated slag cements and concretes was studied. The influence of the metakaolin and kaolin additives on macro- and microstructure formation of the cements and concretes was studied. A conclusion was drawn that the processes of microstructure formation of the cement stone with the additive flow in a similar sequence but with different intensity. A conclusion was drawn that the addition of the kaolin instead of metakaolin affected as follows: 2.5-10 % by mass reduced the value of NCP by 9.5-8.7 %, respectively; 2.5-5 % by mass did not affect setting times, but with increase up to 10 % by mass the initial setting time was shorter (from 48 min to 40 min); 2.5-5 % by mass did not affect compressive strength at all stages of hardening, but with increase up to 10 % by mass reduced strength characteristics of the cement-sand specimens (from 57.0 MPa to 49.0 MPa). In case of the addition of 5 % kaolin by mass, an optimal macrostructure of the concrete is formed in which the quantities of the "conditionally" closed pores are by 17.7 % higher compared to those of the concretes with the same quantities of the metakaolin. This resulted in the higher freeze/thaw resistance of the concrete (from F400 up to F500). Based on the comparison of properties and structure of the cement and concrete containing the kaolin and metakaolin additives, a possibility to substitute metakaolin by kaolin as a correcting additive was established.