Conservation, repair and strengthening of historic masonry buildings should preserve their significance and ensure their structural stability. The condition of a given structure and the extent of damage determine the type of action needed. Grouting is a well-known remedial technique, which can be durable and mechanically efficient whilst preserving the historic value. Still, the selection of a grout for repair must be based on the physical and chemical properties of the existing materials. Parameters such as rheology, injectability and stability of the mix should be considered to ensure the effectiveness of grout injection. In addition, the bond strength of the grout to the existing material is the most relevant mechanical property. Several commercial lime based grouts are available but it is unclear what are the applicable standards and requirements. This paper evaluates the behavior of commercial grouts under laboratory conditions. First, the properties of the grouts as an independent product are assessed with the objective to perform a comparative analysis of their behavior subjected to different conditions (temperature and working time of grout after mixing). Then, the behavior of the grouts when used in combination with stones used in the construction of masonry buildings is addressed (granite, schist and limestone), again considering different conditions (dry, wet and saturated). It is shown that the performance of the commercial products is rather different and careful selection of injection materials in practical applications is recommended

Conservation and strengthening of historic masonry buildings should preserve their significance and ensure their structural stability. The condition of a structure and the extent of the damage determine the type of actions needed. It is important that the selected strategy maintains the existing aesthetic value of the masonry, as well as its structural integrity and the function of components, both during and after any intervention. Grouting is a well-known technique, which can be durable and mechanically efficient, whilst preserving the historic value. The selection of a grout for repair is based on the physical and chemical properties of the existing masonry. Compatibility between the existing and the injection material is a major factor in the success of the intervention. Parameters such as rheology, fluidity, and stability of the mix should be considered to ensure the effectiveness of grout injection. Many commercial ready-mix grouts are available but the use of lime-based grouts formulated in laboratory, with the addition of materials like fly ashes, silica fume, bentonite, hydraulic lime, or metakaolin, have been proposed by different researchers. This article addresses the development of ternary grouts, which show satisfactory mechanical and physical properties, and are viable low-cost alternatives to the commercial grouts.

Grout injection is an efficient method to improve the mechanical characteristics of masonry walls, in the presence of voids or cracks. Masonry made up of two exterior leaves with the space between them filled with poor infill with a large amount of voids is common in many existing structures. In other cases, dry stack masonry is used. Grouting of these types of vulnerable masonry with lime-based grouts was proven mechanically efficient. The success of this technique depends on several parameters, such as injection pressure, the general condition of the masonry (materials and mechanical properties), and the rheological properties of the grout. The effect of ternary grouts and hydraulic lime-based grouts on the compressive and shear strength of three-leaf stone masonry has been widely investigated. However, fewer studies have been done on walls with one or two leaves, as done in this paper. The present research aims to investigate the mechanical performance of schist masonry walls before and after injection. Six masonry walls of typical schist stone constructions from the north of Portugal were constructed in accordance with the original construction materials and tested under compressive load. Two different grouts were chosen to inject the wall specimens (one commercially available and another prescribed). The results obtained showed that these strengthening techniques were successful in increasing the compressive strength of the walls and improving their behavior under compressive loads.