Effects of Steam-Slaking on the Characteristics of Lime from Three Different UK Manufacturers

The practice of producing lime mortars by slaking the quicklime directly in the sand rather than water is known as hot-mixing and, according to numerous historic accounts, was much more common in the past centuries than is generally appreciated by the majority of practitioners. During the last decade, a renewed interest has been dedicated toward the study of the hot-mixed mortar technology, as these mortars are regarded by most craftsmen of superior quality with respect to putty-based mortars in terms of workability, durability and other physical and mechanical properties. In such systems, the slaking of the quicklime is carried out by the moisture surrounding the sand grains and a small amount of added water. The steam developed by such initial slaking promotes further slaking which is supposed to be crucial in determining the characteristics of the lime, and, consequently, of the mortars. However, there are very few in-depth investigations regarding the role of steam on the characteristics of slaked lime. In this study, we have investigated the effects of steam slaking on the characteristics of lime produced by slaking the quicklime from three different UK manufacturers: Lhoist UK, Singleton Birch and Tarmac. Microstructural and mineralogical characteristics of the steam slaked limes are compared with those of the same limes slaked in excess of water and of the related oxides. Analyses were performed using X-ray fluorescence, scanning electron microscopy, and crystallite size and platelet abundance measured by X-ray diffraction data. The results provide useful information on the relations between the characteristics of the limestone used to produce the oxides and the effects of different slaking methods on the characteristics of the slaked lime. This can aid professional conservators to choose the right material and slaking method for the production of compatible mortars in conservation works.