Towards a better understanding of hot-mixed mortars for the conservation of historic buildings: the role of water temperature and steam during lime slaking

According to various historic accounts and material evidence, the practice of producing lime mortars by mixing the quicklime with the sand (i.e. hot-mixing) before first slaking it with water was much more common in the past centuries than appreciated by most contemporary academics, conservation professionals and craftsmen. However, in the last 10 years, there has been resurgence in interest in hot-mixing. In such systems, the steam developed during the mixing is supposed to be crucial in determining the superior characteristics of the mortars, but in-depth investigations on the role of steam in hot-mixing are very few. This study reports the results of some experimental work investigating the effects of water temperature and steam used for lime slaking on the characteristics of lime and related mortars. In these tests, calcic quicklime was slaked in water at 20 and 75 °C, and with steam at 90 °C. Microstructure and mineralogical characteristics of the hydrates were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Mortars produced with these limes were tested for fresh (water retention and flowability) and hardened (compressive and flexural strength) properties. Carbonation was assessed using SEM, XRD and phenolphthalein tests. Results show that steam-slaked lime is characterised by portlandite crystals with smaller crystallite size and significantly different microstructure compared to that of water-slaked lime. Results also show that mortars made with steam-slaked lime have higher water retention and flowability than the mortars produced with water-slaked lime. Under conditions of comparatively low relative humidity (c 40–50%), carbonation is slower in the steam-slaked lime mortar due to the lower water content compared to water-slaked lime mortars. Overall, these results confirm anecdotal reports of better workability and water retention and suggest that this production technology, which is only rarely used nowadays, can produce mortars with improved characteristics, and provide a means by which to match the performance of some historic mortars, and create compatible materials for conservation and restoration work.