Technology and behavior of rubble masonry mortars

Rubble masonry mortars are distinct to the masonry mortar joints due to the different construction techniques used and the prerequisites concerning their behavior in the structure. In the present work, physico-chemical and mineralogical examination of rubble masonry mortars is performed on the development of cohesion and adhesion bonds. A working hypothesis on production technology is deduced consequently. Recent collapses of the rubble masonry at several bastions of the knights' period in the Medieval fortifications of Rhodes allow for sampling. Investigations by optical microscopy, scanning electron microscopy, fiber optics microscopy, X-ray diffraction analysis, differential thermogravimetric analysis, porosimetry as well as total soluble salts measurements were performed. Various types of rubble masonry mortars are evidenced and all of them present a very coherent matrix, either 'cementitious' or very finely crystallised and compact. From the top to the bottom of the masonry, different microstructural characteristics are observed in the mortar nucleus, indicating different lime-to-aggregates ratios. The microstructural stratification of the cementing material points out different carbonation and drying levels and implies successive supply of water to stake the lime in situ. The evidence of hydraulic components all around the bulk, but not on the outer top of the walls, is of particular interest and proves the in situ slaking of lime as the energy source to activate either hydraulic reactions among lime and clay minerals or the crystallization of portlandite. The high levels of soluble salts measured indicate the mortars' susceptibility to salt decay, specifically when pore size distribution is in favor, as in the case of the modern cement or the pure calcitic mortar, in comparison with the more resistant hydraulic mortars. (C) 1997 Elsevier Science Ltd.