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

Pavel Krivenko,Oleksandr Kovalchuk,Oleg Petropavlovskyi

doi:10.15587/1729-4061.2018.119624

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.

Highlights

Environmental concerns arising from cement production due to CO2 emissions and high energy consumption required for high-temperature firing can be eliminated by a wider use of blended and composite cements [1]
Since metallurgical slags vary in chemical composition [7], correcting additives are to be added in order to give the required properties to a cement stone [8]
The addition of the kaolin instead of the metakaolin was found to act 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 setting times were shorter; 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

Summary

Introduction

Environmental concerns arising from cement production due to CO2 emissions and high energy consumption required for high-temperature firing can be eliminated by a wider use of blended and composite cements [1]. The AACs vary in proportions of basic oxides in the system N(K)2O–CaO–Al2O3–SiO2–H2O and, according to this, can be divided into three types: high, low and medium-calcium alkali-activated cements [5] This predetermines a phase composition of the hydration products, properties of the cements and concretes on their basis [6]. High quantities of reactive metakaolin promote the formation of large quantities of gel phases of mixed composition like CASH, NASH, CNASH along with the CSH phase at the initial stages of hardening [18] This may result in danger associated with the lower deformative properties and “deteriorated” pore structure of the formed cement stone. Various clays can interact with alkali metal compounds to form alkaline alu-

The aim and objectives of the study

Constituent materials and testing techniques

Findings

The influence of the additives on setting time of the cements