Investigation of the grade of calcined clays used as clinker substitute in Limestone Calcined Clay Cement (LC3)

The combination of calcined kaolinitic clays and limestone in Limestone Calcined Clay Cement (LC3) is a promising approach to reduce the cost and the CO2 emissions of cement production by reducing the clinker content of cement. This thesis investigates the feasibility of using various grades of calcined clays in LC3. LC3-50 blends with a clinker content reduced to 50% are studied. The factors controlling the reactivity of LC3-50 blends containing various grades of calcined kaolinitic clays were first studied. A benchmark test of mortar strength was developed. Similar strength to plain Portland cement (PC) can be obtained even for clays with 40% of calcined kaolinite only. Moreover, strengths are strongly dependent on the calcined kaolinite content of the calcined clay. The development of the new Rapid, Relevant and Reliable (R3) pozzolanic test allows the evaluation of the reactivity of calcined clays after only 24 h by isothermal calorimetry and 3 days simply using an oven, and it allows the prediction of the strength development of LC3-50 mortars. To explain strength results, a phase assemblage study was carried out. In order to determine the amount of reacted metakaolin, three methods were tested and mass balance was found to be the most reliable one. The phase assemblage study showed that a critical refinement of pore connectivity is reached already at 3 days for LC3-50 blends with high calcined kaolinite content. From this point on, clinker hydration is slowed down, and the formation of crystalline hydration products is limited. The on-going reaction of metakaolin leads to the higher incorporation of aluminium in the calcium alumino silicate hydrate (C-A-S-H). The C-A-S-H was fully characterized in terms of composition, morphology and density. No change in morphology was observed by Transmission Electron Microscopy. The C-A-S-H density determined by 1H-Nuclear Magnetic Resonance was also found to be similar between PC and LC3-50 with different calcined kaolinite content. Combining all this information, a good relationship is obtained between strength and gel space ratio for PC and for the LC3-50 blends. Finally, the chloride resistance was tested through ponding and chloride binding isotherm tests. The results also support previous findings for the use of calcined clays with a calcined kaolinite content of at least 40% to get a better chloride resistance than PC. These results are mainly explained by the pore connectivity refinement of LC3-50 blends compared with PC. The chloride binding is the highest for clays with 40-50% of calcined kaolinite.