Quantifying 450 years of limestone weathering induced by salt crystallization on fortifications in Malta and Gozo

Structure from Motion (SfM) photogrammetry, on-site hardness and water absorption tests, and petrographical and porosimetric analyses on targeted samples were carried out on representative areas within six different locations of Maltese fortifications affected by salt weathering. The objective was to quantify the limestone loss over the last half-millennium and to attempt to identify the controlling factors of durability. The results highlight the existence of a two-rate weathering regime within the Miocene Globigerina Limestone which is the main building stone of the Maltese Islands. With a median stone recession of 1.4 min/century, the subtype locally called Franka has resisted atmospheric aggression on average 36 times better than the Soil subtype (median stone recession of 50 mm/century).The Oligo-Miocene Coralline Limestone has also resisted well, with a stone recession of on average 1.3 mm/centuty. Two main controls of the varying durability of these limestones have been identified. First, the water uptake capacity of the least durable, already weathered limestone (Soll) is two to three times higher than that of the weathered but more resistant Franka and Coralline Limestone (6.3 g.m(-2).s(-1) against 2.5 g.m(-2).s(-1)). Second, there is a marked difference in cohesiveness between these limestones. Whereas the weathered, susceptible Soil fades is poorly-cemented or miaocradced, the exposed durable Franka displays an abundant micritic and microsparitic matrix. The statistical relationship between durability and surface hardness is not straightforward, and other controls such as the clay content of limestones should be explored. Investigating a larger number of sites would allow the refining of the limestone durability scale inferred from the present study. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

The study conducted photogrammetry, on-site tests, and analyses on Maltese fortifications affected by salt weathering to quantify limestone loss over the past half-millennium. It identified two-rate weathering in Globigerina Limestone, with Franka subtype resisting atmospheric aggression significantly better than Soil subtype. Coralline Limestone also showed good resistance, with identified factors including water uptake capacity and cohesiveness between different limestones.