Investigation of cement substitution by blends of calcined clays and limestone

This thesis investigates the replacement of Portland cement by addition of high amounts of calcined clays and limestone. Such ternary blends can become more ecological and economical binders to answer the worldwide housing shortage. It could be demonstrated that up to 60% of cement could be replaced by such combined addition of pure metakaolin and limestone with acceptable compressive strengths after 7 and 28 days. With natural clays, with about 50% kaolinite content, cement can still be replaced by 45% with mixtures of calcined clays and limestone without significantly impairing strength development. The good compressive strengths obtained at 7 and 28 days have been explained by the synergetic formation of carboaluminates phases and the porosity refinement. The aluminates from the metakaolin react together with the carbonates from the limestone and calcium hydroxide, from cement hydration and could be confirmed by thermodynamic calculations. The synergetic reaction could be optimized by carefully adjusting the sulfate content and controlling the alkali content of the cement. The durability aspects of these novel blends were studied. The STADIUM ® multiionic transport model has been used and successfully adapted to the specificities of our materials. The ternary blends have very low ionic diffusion coefficients, more than one order of magnitude lower than Portland cement. These explain the good resistance to chloride ingress observed in ponding tests. Carbonation resistance was also investigated. The first results tend to demonstrate that the ternary blends have somewhat lower carbonation resistance than Portland cement, due to the lower Portlandite content. However, in the short term investigated here, performance in natural carbonation condition is still good. Accelerated carbonation leads to the formation of aragonite rather than calcite in the blended system. The different densities of these polymorphs means that the porosity after carbonation will be higher in the accelerated conditions than in natural carbonation. Considering that the rate of carbonation in the long term is determined by the diffusion through the carbonated structure, this indicates that accelerated tests are likely to overestimate long term rates of carbonation.