Development and testing of repair mortars for floor mosaic substrates

Mosaics were diachronically used as a mean to decorate floors of historic constructions. Their durability and resistance to loading and environmental parameters, was mostly attributed to their substrate, which composed of three to four very well compacted mortar layers. The construction of these substrates usually followed specific requirements and criteria, concerning both the selection of raw materials and the application techniques followed. This paper concerns a systematic study of double-layered mortars, designed and manufactured according to the characteristics found in ancient floor mosaic substrates. Specific parameters were taken into account, such as the binding system, the aggregates' type and gradation, the Binder/Aggregate (B/A) and Water/Binder (W/B) ratio, as well as the layers' thickness and the application technique. A series of test were performed at the age of 28, 90 and 180 days. The properties measured concerned porosity, apparent specific gravity, water permeability, capillary absorption, dynamic modulus of elasticity, flexural, compressive strength, while bond strength by pull off and tensile bond strength were conducted. From the evaluation of the results, it was concluded that good compaction and low W/B ratio ensured relatively high compressive strength (5-10 MPa), as well as increased adhesion between the mortar layers, while the addition of brick dust and crushed brick enhanced the layered mortars' performance.