Efficiency of metakaolin in steam cured high strength concrete

This paper presents a study on investigating the effect of metakaolin (MK) on the hydration, microstructure and volume stability of steam cured high strength concrete (HSC) with a low water to binder ratio (w/b) of 0.25 cured at 80 °C. The structure of C-A-S-H series with a Ca/Si ratio of 1.0 and Al/Si ratio varying from 0 to 0.3 are investigated to discuss the effect of aluminium on the microstructure of C-S-H gel. The experimental results show that the hydration of cement is accelerated due to the presence of MK. The Ca(OH)₂ is consumed and aluminium substituting silicon in the C-(A)S-H chains can be obtained, which leads to high strength, low average pore diameter and low total porosity of HSC. The cracks caused by steam curing tend to be moderated after adding MK, and no visible interfacial transition zones (ITZ) between aggregate and matrix are observed. The MK decreases the volume expansion of steam cured HSC caused by heat treatment as well as the drying shrinkage, leading to a better volume stability. Moreover, incorporation of aluminium results in modification of the layer stacking of C-(A)S-H and reduction of its porosity, which will support the improvement of strength, microstructure and volume stability of steam cured HSC.