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

## TL;DR

This study examines how adding sulfate carriers affects the microstructure and strength of cements blended with different types of clay.

## Contribution

The paper explores the microstructural effects of sulfate carrier addition in clay-blended cements, comparing 1:1 and 2:1 clay types.

## Key findings

- Sulfate carrier addition improves microstructural properties more in 1:1-dominated clay blends than in 2:1-dominated ones.
- Higher sulfate carrier addition increases bound water and reduces porosity in all blends during early hydration.
- Compressive strength is not strongly correlated with water binding or porosity, making strength prediction unreliable using these parameters.

## Abstract

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.