Assessing the effect of a white Portland cement admixture on the early frost-thaw resistance of NHL-based masonry mortar

Well preserved mortars based on natural hydraulic lime are widely spread in historical buildings throughout the alpine area of Tyrol, Austria. These are frequently restored with mortars oriented on historical models. Portland cement is also commonly used to modify these materials on building sites to speed up the setting time to improve the early frost-thaw resistance, primarily when they are applied around the winter season. Therefore, it was our objective to specify the application potentials of those materials, respecting the intended use and the atmospheric conditions. Moreover, we tried to figure out the possible benefits and risks when using NHL-based materials, optionally modified by cement, yielding stable recommendations for the work with corresponding mortars. Hence, we examined the development of strength and water transport, also the frost-thaw durability in a temporal context. The obtained results were compared with findings of assessing both materials under variable weather conditions around the winter season. Accordingly, the use of pure NHL-based lime mortars resulted as being critical mainly until about two months before intense frost conditions. Cement effectively contributes to the rapid hydraulic setting reaction in NHL-based materials. Hence, we recommend measured cement additions in areas where strong frost is expected after a few weeks and especially targeted in areas where carbonation is hindered because of excess moisture. Nevertheless, the potential use of Portland cement should be properly controlled in historic building preservation. A systematical approach that considers the framework conditions and the corresponding application to an area is essential to assure an appropriate conservation outcome.