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MECHANICAL PROPERTIES & DURABILITY OF CEMENT-STABILIZED EARTH MORTARS FOR APPLICATION IN PREHISTORIC MONUMENTS

The present work deals with the properties, mainly physical and mechanical, of repair mortars destined for the repair and strengthening of prehistoric masonry. The paper focuses on a specific prehistoric building type, the tholos tomb. Mixtures composed of Portland cement in limited quantities, clay and calcareous sand are examined. Aim of the mix design is to produce repair materials with Portland cement content as low as possible, to avoid mainly durability failures. Those materials should at the same time be able to ensure effective repair and strengthening, which is required in earthquake-prone areas such as Greece. The follow up of the evolution of some key parameters with time, such as compressive and tensile strength and apparent porosity, has shown that the cement-stabilized mortars can develop a great variety of properties, some of which can satisfy historic masonry requirements. Moreover, the study of the microstructure, using Scanning Electron Microscopy and X-Ray Diffraction, revealed the development of pozzolanic reactions, which is a key-factor for durability. However, a minimum percentage of cement is required, in order to improve the setting time and to ensure durability against sulphate attack as well. Cement-stabilized earth compositions permit the reduction of the Portland cement content and are able to produce a variety of mechanical and physical properties, sufficient for the repair and strengthening of prehistoric masonry structures as well as for use in archaeological sites. INTRODUCTION Earth-based binders were often used in the construction of prehistoric buildings in Greece. The function of those binders was either bedding mortars or plasters. The paper focuses on a specific prehistoric building type, the tholos tomb, where such binders were occasionally used. A tholos tomb consists of a circular burial chamber, called thalamos, initially roofed by a corbelled vault (Figure 1) and approached by an entrance passage, the dromos, which narrows abruptly at the doorway, the stomion. The thalamos is built of stone courses. The “prehistoric” way of assembling the stone blocks consists in the direct application of a stone on top of other, without the mediation of a mortar, as in Roman, Byzantine and later masonry assemblages. In the case, however, of irregular masonry, this technique results in the creation of, smaller or bigger, voids between the stone blocks which are often found filled with earth (Figure 2a). The fact that this earth fill was not systematically found during the excavations, led some researchers to the hypothesis that the tholos tombs were constructed solely of dry-stack masonries. The size, however, of many tholos tombs as well as stability requirements during construction call in favor of the use of this earth fill between the building stones as well. This assumption could be applicable to the majority of the tholos tombs, mainly those made of roughly carved stones, and it still remains an open subject. Figure 1: Example of a Tholos Tomb: interior of Tholos IV in Thorikos, Attica Focusing on the structural pathology of the masonries envountered at the interior of many tholos tombs, it is systematically observed that the long-term exposure to environmental strains resulted in the creation of voids at the interstices between the stone blocks and layers (Figure 2b). Deterioration is then rapidly initiated, and sometimes leads to severe structural problems. A very common structural problem, in case of material loss, is the bending failure of the building stones, which were initially meant to lye one over the other (Figure 2b). If deformation proceeds, then local instabilities could take place, and eventually lead later to collapses. In this case, one of the techniques that could be used for the structural restoration of the tholos tomb masonries is the application of a mortar at the interstices between the stones, in replacement to the original earth fill (if it was) or simply in order to fill the voids created by material deterioration. This measure permits the increase of the frictional internal forces of the system and the overall Modulus of Elasticity. (a) (b) Figure 2: (a) Example of irregular masonry (tholos tomb of Acharnae, Attica), (b) severe stone deterioration and loss of material between the stones (Tholos IV in Thorikos, Attica) Consequently, a more uniform stress distribution is expected to be restored, reducing the risk of stress concentrations, which lead to unwanted bending or even out-of-plane deformations, such as the ones shown in Figure 2b. On the basis of the aforementioned considerations, it is decided to study and develop suitable earth mortars for application in prehistoric masonries. EXPERIMENTAL Selection and characterization of the materials It is well-known that one of the requirements for the design of mortars for application in historic masonries is the study of the original materials in order to produce mortars, physicochemically and mechanically compatible to the original fabric [1]. Earth mortars are, however, vulnerable to environmental conditions and require protection to achieve durability. An alternative could be the design of stabilized earth mortars [2-4]. These are earth-based mortars, in which a part of the binder is typically replaced by cement or lime. To produce the mortar, an earth binder industrially produced in Attica was selected. Its grain size distribution is shown in Figure 3 [5]. In Figure 4 the morphology and the composition of the clay binder are also presented. The material mainly consists of albite and silica as it is also confirmed by the XRD pattern (Figure 5). Earth grain-size distribution 0 10 20 30 40 50 60 70 80 90 100 0,001 0,01 0,1 1 10 100 Diameter [mm] P a ss in g [ % ] Figure 3: Grain-size distribution of earth binder Figure 4: SEM image (x3000) and EDX Spectrum of earth binder 0 100 200 300 400 500 600 700 800 90