The effect of fibres on lime mortars : a potential resource for heritage masonry restoration

Abstract

Lime has long been the main binder used in historic mortars across many countries, including Malta. Air lime, in particular, is compatible with limestone and allows breathability, making it also ideal for restoring limestone structures. In restoration practices, it often serves as a sacrificial mortar, where its inherent properties make it beneficial as it deteriorates in preference to stone elements, which may also be structural. However, rapid deterioration is undesirable, leading to issues like frequent maintenance and increased risk of water ingress. More recently, hydraulic lime has gained popularity for its faster setting time, ease of use, and greater durability. Traditionally, natural fibres were added to lime mortars to enhance performance, though this practice has largely been lost. Renewed interest, especially in the Mediterranean, is now exploring the benefits of reintroducing such additives. The primary objective of this study was to assess and compare fibre-reinforced and non-reinforced air and hydraulic lime mortar mixes, using treated Arundo donax L. fibres and also polypropylene fibres as reinforcement options. Through a combination of qualitative and quantitative methods, the research examined the mortars' physical, mechanical, and durability properties. It was found that fibres significantly enhance mechanical performance, particularly by controlling shrinkage and reducing cracking, as has also been reported in literature. These effects are most pronounced in NHL mortars, which, due to their lower water content and fast, rigid set, are less plastic and more prone to early cracking. In air lime mortars, which require more water and are also more plastic, fibres help achieve earlier dimensional stability and also reduce overall shrinkage. It can be concluded that the addition of polypropylene fibres, particularly in NHL mortars, accelerates both drying and water absorption. In contrast, Arundo donax fibres, particularly in air limes, promote more gradual moisture transport and lower permeability. This can be primarily linked to the water absorption properties of the Arundo donax fibres and the water repelling properties of the polypropylene fibres. In addition, initial absorption behaviours can indicate that the fibres can alter internal pore characteristics. These results are very encouraging and open up a whole new area of local research which may eventually result in the production of mixes which may help mitigate the effects of rising damp. By delaying initial water uptake and enhancing resistance to saturation from intense rainfall, such mixes may also reduce salt transport and associated damage, offering a more durable solution for moisture-related deterioration in masonry. Despite these potential benefits, limitations remain. While fibres may mitigate the severity of salt-related damage in some mixes, they do not eliminate it. Their impact, as always, will be highly dependent on the binder. It was found that in air limes, Arundo donax fibres may retain localised moisture providing a persistent medium for the dissolution and re-crystallization of soluble salts. In contrast, in NHL mortars, it was found that these same fibres offer more resistance against sodium sulfate damage, with polypropylene fibres proving most effective. Their overall influence, therefore, appears to be a beneficial mitigation of deterioration processes. Overall, fibre-reinforced mortars present initial promising benefits for heritage conservation, but their performance depends on a clear understanding of the interaction between binder, the water content used, and fibre characteristics, alongside a careful assessment of the historic fabric they are intended to protect and existing deterioration causes, processes and mechanisms.