Effects of metakaolin, silica fume and slag on pore structure, interfacial transition zone and compressive strength of concrete

• We investigate effects of metakaolin, silica fume and slag on properties of concrete.
• We analyze the mechanism of effects of metakaolin, silica fume and slag on concrete.
• Metakaolin, silica fume and slag have positive impact on properties of concrete.
• Traditional properties of concrete are closely related to microstructure.
• Metakaolin, silica fume and slag influence the stability of hydrate phases.

The pore structure and interfacial transition zone (ITZ) of concrete incorporating slag, silica fume and metakaolin were analyzed in this study. Some techniques such as mercury intrusion porosimetry (MIP), microhardness testing and scanning electronic microscopy (SEM) were employed to characterize the effects of slag, silica fume and metakaolin on the pore structure, microhardness and morphology of the ITZ at 28 and 180 days. The traditional properties such as compressive strength were experimentally evaluated in relation to pore characteristics and ITZ. Meanwhile, the influence of silicon, as the major component of slag, silica fume and metakaolin, on thermodynamic stability of hydrate phases was further investigated. The experimental results show that mineral admixtures have positive impact on pore refinement and ITZ enhancement of concrete especially at later curing stages. The effect of the mineral admixtures on microstructure is in the sequence: metakaolin > silica fume > slag. The development of the compressive strength is related to the evolution of the pore structure and ITZ. Thermodynamic stability analysis indicates that if mineral admixtures are added to the CaO–Al2O3–CaSO4–H2O system, the phase assemblage of C6AsH32 and C3AS0.8H4.4, which represents a member of the solid solution series Ca3Al2(SiO4)3−x(OH)4x, has a lower Gibbs free energy of reaction and is therefore thermodynamically more stable than monosulfoaluminate at temperatures from 1 to 99 °C.