Efficiency of mineral admixtures in concrete: Microstructure, compressive strength and stability of hydrate phases

• Effects of metakaolin, silica fume and slag on properties of concrete were studied.
• Mineral admixtures improve microstructure as well as compressive strength.
• Metakaolin presents the most distinct improvement effects on microstructure.
• The mechanism of addition effects was analyzed from thermodynamic stability aspects.

The use of mineral additives in concrete such as fly ash, silica fume, natural pozzolan, metakaolin and calcined clay has become widespread due to their pozzolanic reaction and environmental friendliness. The microstructure characteristics of concrete including pore structure and interfacial transition zone (ITZ) with addition of metakaolin, silica fume and slag were investigated in this study. The experimental results show that the addition of mineral admixtures results in the denser ITZ, optimized pore structure and reasonable pore size distribution especially at later curing stages. Metakaolin presents the most distinct improvement effects on microstructure of concrete. The development of the compressive strength is quantificationally related to the total porosity and average microhardness of the ITZ. Importantly, the influence of metakaolin, silica fume and slag on concrete was analyzed from thermodynamic stability of hydrate phase aspects.