Abstract
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.
Highlights
Recent years have seen a growing interest in a traditional method of mortar production, generally referred to as ‘hot-mixing’, that was widely used in the past but is onlyPesce et al Herit Sci (2021) 9:72 just enough water to bring the mixture to a workable mortar [4, 5].The increasing interest in hot-mixing within the conservation industry is demonstrated by the growing number of events such as conferences, workshops, training courses and public demonstrations organised over the past years on this topic, that have attracted hundreds of attendees and much attention from specialists
The pore diameter observed by scanning electron microscopy (SEM) is included in the range of values measured in literature for other calcined limestones [33], it should be noted that there are several factors that can contribute to the microstructure of the calcined material, including the morphology of the parent material and the calcination conditions [31, 33,34,35]
Results of the X-ray diffraction (XRD) analysis used to investigate the carbonation in mortars are reported in Fig. 13, where diffractograms of the mortar samples after 42 days of curing are shown, and in Fig. 14, where the calcite:portlandite (C/P) ratios of the mortar samples are plotted against the curing time, from 21 to 42 days
Summary
The increasing interest in hot-mixing within the conservation industry is demonstrated by the growing number of events such as conferences, workshops, training courses and public demonstrations organised over the past years on this topic (e.g. events organised by the Building Limes Forum and by the Scottish Lime Centre in the UK), that have attracted hundreds of attendees and much attention from specialists. Several contractors currently offer the use of hot-mixed mortars in conservation and restoration projects. According to the work of Schmidt [6], who analysed the database of over 3500 historic mortar samples of the Scottish Lime Centre Trust, hot-mixed mortars comprised over 80% of all mortar samples dating from before the 17th century, more than 60% of samples dating from the 17th to the 19th century. More and more peer-reviewed papers focused on the analysis of historic mortars suggest the use of this technology in the past centuries, such as in the case of the 16th century mortars used in the Amaiur Castel (Navarre, Spain) [7] and medieval wall painting plasters in Denmark [8]
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