Effect of Sulfate Carrier Addition on the Microstructure of Calcined Clay Blended Cements

This publication builds on a previous paper proving the importance of sulfate carrier addition (sca) on the early compressive strength of calcined clay blended cements, especially when using a 1:1-dominated clay. This paper now aims to identify the background of these preceding findings at the microstructural level. For this purpose, a Portland cement is replaced by a calcined kaolinitic, smectitic and illitic clay to different levels at various sca. The microstructural investigations focus on hydrate phases, porosity and scanning electron microscopy of hardened pastes at 2 and 28 days. The microstructural properties of 2:1-dominated clay blends can be improved by the sca, but the benefit is small compared to 1:1-dominated clay blends. On the other hand, their portlandite content does not decrease as much, but can even increase slightly. At early hydration, the amount of bound water increases, and the porosity decreases with increasing sca for all blends. Neither the correlation of the compressive strength with the water binding nor with the porosity is high enough for a reasonable strength prediction using these two parameters.