Salt crystallization is one of the most common causes of decay in porous building materials. Plasters and renders, mainly due to their location at the surface of walls, where most often salt accumulation occurs, are particularly subjected to salt decay. Despite the relevance of the problem, there are no standard salt weathering tests for the assessment of the durability of plasters and renders against salt decay. This research proposes a salt weathering
procedure, based on an adaptation of the RILEM TC 271-ASC recommendation, and it validates it by applying it to three plaster types (based on hydrated lime, natural hydraulic lime and hydrated lime-cement). Combined stone/plaster specimens are prepared and contaminated with 10 % (weight salt/weight solution) sodium sulphate and sodium chloride solutions. The damage development is assessed by visual and photographic monitoring, gravimetric measurements of the material loss, assessment of salt distribution in the specimens, and SEM
observations supported by EDX mapping on thin sections. The results show that the test procedure is effective in causing damage in the tested plasters, within the time period of the test (about 4 months), and that damage increases with subsequent cycles. The decay severity and type differ depending on type of salt and/or of plaster.
Suggestions for further improvement of the procedure are provided.

Salt crystallization is a major cause of damage in porous building materials. Accelerated salt weathering tests carried out in the laboratory are among the most common methods to assess the durability of material to salt decay. However, existing standards and recommendations for salt weathering tests have limitations in terms of effectiveness and/or reliability. In the framework of the RILEM Technical Committee 271-ASC, a procedure has been developed which proposes a new approach to salt crystallization tests. It starts from the consideration that salt damage can be seen as a process developing in two phases: accumulation of the salt in the material and propagation of the decay. In the first phase, salts are introduced in the material and accumulate close to the evaporation surface, while in the second phase damage propagates because of repeated dissolution and crystallization cycles, induced by re-wetting with liquid water and by relative humidity changes. In this paper, the procedure is described and the results of a first round robin validation of the test, carried out on 7 materials and involving 10 laboratories, are presented. The results show that the procedure is effective to cause decay within the time period of the test (about 3 months) and that the decay increases with subsequent cycles. The decay observed differs in type and severity depending on the salt type and concentration and on the type of substrate. The decay types detected in the laboratories are generally representative of those observed in the field for the selected substrates. The differences in durability between the various substrates, as assessed at the end of the test, are in line with the durability expected based on field observation. The reproducibility of the results in terms of decay type is good; some differences have been observed in terms of material loss. These are more significant in the case of NaCl contaminated specimens. Based on the results, proposals for fine-tuning of the procedure are given.

The RILEM TC ASC-271 is developing a new laboratory test to assess the durability of porous building materials to salt crystallisation. The test encompasses two phases: salt accumulation and damage propagation. This paper focuses on designing a procedure for the accumulation phase; this is considered successful when salts crystallise at the material's evaporative surface (common situation observed on site) without the occurrence of damage. Two procedures were developed and tested on two limestones with different porosity: (1) capillary absorption of a salt solution followed by drying, and (2) continuous capillary absorption. Sodium chloride or sodium sulphate solutions were used. Several methods for assessing the salt distribution were employed: ultrasonic pulse velocity, drilling/scratching resistance, hygroscopic moisture content, ion chromatography, scanning electron microscopy, and micro X-ray fluorescence. The results enabled the selection of the most effective protocol for the salt accumulation phase.

Surface repair mortars are used for the compensation, or repair, of lost portions of surface materials in historic masonry buildings. It is recommended that their design and application should be performed in a wider context of conservation values related decision making, to prioritise preservation of original fabric, authenticity of approach and maintenance of integrity, and not just on technical principles alone. However, a technical context for their design does exist, that requires understanding of the properties of the substrate that they will be applied onto, and adherence to minimum aesthetic (colour and texture) requirements. The principles of physical, mechanical and chemical compatibility of repair apply and the attributes of the repair mortar should be carefully matched to the substrate alongside a sacrificial behaviour (not more durable than the material being replaced). Guidance is given on the design, application and the functional requirements that must be met when using surface repair mortars.