Limestone calcined clay cements (LC3): raw material processing, sulfate balance and hydration kinetics

Limestone calcined clay cements (LC3) are blended cements that combine clinker, limestone, calcined clay and gypsum. The availability of the materials required to produce LC3 and the good performance that it achieves, makes LC3 suitable as a sustainable replacement of Portland cement.

Significant advances have been made to assess the properties of LC3, compare it to other common blended cements and establish benchmark characterization procedures. However, there are still open questions that are relevant for a successful adoption of this technology and consequently, to make a better use of the resources available. This research project addresses some of these questions related to processing, blend design and microstructural development of LC3 cements. The effect of calcite impurities in calcined clay reactivity was explored. It was found that at calcination temperatures below the recrystallization threshold, an intermediate produce was formed between kaolinite and calcite. A slight reduction in reactivity was observed, which can be mostly offset by reducing the calcination temperature of the clay and extending the residence time.

The effect of using grinding aids was studied at the grinding/classification stage and also during hydra-tion. The use of grinding aids significantly improves the efficiency of dry classification of clay particles, which could prevent overgrinding and increase yield in closed circuit milling units. Furthermore, the use of alkanolamines was shown to be effective to enhance the formation of hemicarboaluminate and monocarboaluminate and thus increase strength. LC3 cements require optimization of the calcium sulfate (gypsum). There is an increase in the sulfate needed relative to the clinker content. The mechanism that explains this increased sulfate demand was found to be linked to the enhancement of alite reaction due to filler effect and the adsorption of sulfate in C-A-S-H, rather than the aluminate content of the calcined clay. In addition, the reaction rate of C3A and the dissolution rate of the sulfate source used are also important to describe the sulfate balance of a cementitious system in general.

The effect of hemicarboaluminate and monocarboaluminate on mechanical properties of LC3 was also studied. Metakaolin and sulfate content were found to influence significantly the kinetics of AFm for-mation. Furthermore, the precipitation of AFm between 2 and 3 days of hydration were directly linked to the strength increase observed. The amount of initial space in the system determines the extent to which hydration takes place at a high rate. Afterwards, the porosity refinement leads to a decrease in reaction rate. However, evidence for a continued reaction of metakaolin in the long term was found.

The insights presented in this thesis provide new knowledge that enables a better use of LC3 in the field. Together, they also show the robustness and versatility of this technology, and deliver guidelines for future developments and field implementation of LC3.