This recommendation is devoted to testing the resistance of natural stone and fired-clay brick units against salt crystallization. The procedure was developed by the RILEM TC 271-ASC to evaluate the durability of porous building materials against salt crystallization through a laboratory method that allows for accelerated testing without compromising the reliability of the results. The new procedure is designed to replicate salt damage caused by crystallization near the surface of materials as a result of capillary transport and evaporation. A new approach is proposed that considers the presence of two stages in the salt crystallization test. In the first, the accumulation stage, salts gradually accumulate on or near the surface of the material due to evaporation. In the second, the propagation stage, damage initiates and develops due to changes in moisture content and relative humidity that trigger salt dissolution and crystallization cycles. To achieve this, two types of salt were tested, namely sodium chloride and sodium sulphate, with each salt tested separately. A methodology for assessing the salt-induced damage is proposed, which includes visual and photographical observations and measurement of material loss. The procedure has been preliminarily validated in round robin tests.

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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.

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The RILEM TC 271-ASC is currently developing a new ageing test to assess the
resistance of porous building materials to salt crystallisation. The new test consists of two phases: salt accumulation and damage propagation. This paper focuses on the salt accumulation phase; this phase should promote salt crystallisation close to the evaporative surface of the substrates (common situation onsite) without leading to salt efflorescence or damage. Damage should take place in the propagation phase, which is not addressed in this paper. This work compares the effectiveness of three contamination procedures for salt accumulation: P1) salt contamination by capillary absorption of salt solution, followed by drying; P2) continuous capillary absorption of salt solution; P3) placement of salt crystals on the surface of the materials followed by the conditioning of the specimens at high relative humidity until complete dissolution of the salts. The results of P1 and P2 procedures presented in this paper are detailed in a separate publication, while this paper focuses on P3 procedure and compares the results. The effectiveness of each procedure has been evaluated by assessing the salt distribution in the specimen using ion chromatography and scanning electron microscopy. The results show that P3 is technically the least complicated to set up and does not entail the risk of development of salt efflorescence or damage. However, it can lead to salt migration within the specimens to a greater depth, hence future research is proposed to counteract this effect.@en

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.

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