Building Materials and Building Technology to preserve the Built Heritage vol:2 issue:33-2 pages:353-368
1st WTA-International PhD Symposium "Building Materials and Building Technology to preserve the Built Heritage" edition:1 location:Leuven, Belgium date:8-9 October 2009
Calcium hydroxide (lime) binder hardens as a result of carbonation reaction. Calcium silicate binders such as lime-pozzolana, hydraulic lime and cement-lime blends harden mainly by hydration reactions while carbonation reaction takes place as the secondary-order reaction. The degree and order of these two reactions in calcium silicate binders depend on the binder composition, the hydraulic reactivity of the binder and the curing conditions. Understanding this combined reaction mechanism and the consequences on the hardened mortar properties are essential while assessing how they affect the overall durability of the historic masonry. In this doctoral research, this combined reaction has been studied in mortars with lime-pozzolana, hydraulic lime and cement-lime binders, and the consequences on the physical properties, mechanical properties and microstructure of the mortars are discussed. In-depth research has been carried out on the hydration and carbonation reactions to provide fundamental insight on the reaction mechanisms.
Hydration reactions are the first reaction order in hydraulic lime and lime-pozzolana mortars, which proceed more gradually when compared with fast cement hydration. Carbonation is the complementary reaction in the strength gain of these mortars and starts once the pore structure is formed to allow the CO2 diffusion. Competitive reaction order between hydration and carbonation can take place in lime-pozzolana mortars depending on the pozzolanic reactivity of the pozzolanic material. As a consequence of the consumption of the calcium hydroxide by the carbonation reaction, the degree of the pozzolanic reactions is lowered. The consequence of this on the mechanical properties is remarkable while the same impact is not observed on the porosity of the mortars. The degree and the order of the hydration and carbonation reactions are strongly influenced by the moisture content. Hydration reactions are promoted under moist conditions while carbonation reaction is retarded. Hydraulic lime and lime-pozzolana mortars require at least 28-day moist curing to improve the hydration reactions and to ensure sufficient strength development.