Durability of NHL Based Green Labelled Ready Mixed Plasters. A Probabilistic Approach

Commercial plasters for the preservation of the architectural heritage represent a category of materials increasingly used in maintenance and restoration work. Their main objective is to protect and preserve masonry from damage caused by the external environment, including those related to capillary rise and salt crystallization. Salt crystallization is one of the main causes of deterioration of masonry surfaces in particularly aggressive environments. Many of the ready- mixed plasters currently available on the market are labelled as “green” by their manufacturers. These products are distinguished by a more sustainable life cycle than conventional mixes, as the entire production process—from raw material extraction to end-of-life—should have a lower environmental impact. However, for these materials to be considered truly environmentally beneficial, it is essential to demonstrate good durability, even under critical conditions. Resistance to salt crystallization becomes a key parameter to be evaluated to confirm the validity of these green plasters over time. In this study, salt crystallization tests were performed on aged brick masonry samples coated with five commercial NHL-based green plasters to evaluate the relationship between their microstructural characteristics and damage evolution. The variation of the morphology of the surface was measured using a laser profilometer and the interpretation of the data obtained was carried out using a probabilistic approach to study the possible evolution of damage over time. Results showed that pore size distribution strongly influenced salt transport and crystallisation: very fine pores led to swelling and cracking due to subflorescence, while large voids caused detachment at the plaster-mortar interface. Laser profilometry effectively quantified surface degradation, while probabilistic analysis proved suitable to describe damage evolution, although less reliable in cases of extensive material loss.