Analysis of chemical and physical degradation phenomena in Cultural Heritage materials and development of technologies for their protection and conservation

This PhD research examines the decay mechanisms affecting historic mortars and develops and assesses innovative technologies for their conservation. It addresses the need for repair mortars that are not only high-performing but also appropriate for historic substrates. Within this framework, compatibility is understood in a multidimensional sense—not as strict equivalence, but as a calibrated balance of chemical, physical, and mechanical properties that supports harmonious interaction with historic materials. This balance must preserve breathability, support long-term durability and align with the principle of reversibility. Here, reversibility is not treated as a fixed condition, but as a methodological approach aimed at minimising the impact of the intervention. Accordingly, the research seeks to reconcile conservation requirements with a sustainable design perspective, promoting the reuse of local resources and construction and demolition waste in line with circular-economy strategies. The study focuses on geopolymers as potential alternatives to traditional lime-based mortars. Particular attention is also devoted to cement-based repair mortars, which—despite their widespread use in post-war interventions—have often proved chemically and physically unsuitable for historic substrates. Their excessive stiffness and low permeability can hinder vapour transmission and promote detachment, cracking, and salt accumulation. Ultimately, this research aims to establish a methodological basis for bridging the gap between laboratory experimentation and in situ application by proposing protocols and validation criteria that can be adapted to site-specific conditions, supporting the informed and scientifically grounded use of these materials in built-heritage conservation. Chapter 1 provides a systematic overview of Alkali-Activated Materials (AAM), with a specific focus on geopolymers and their relevance to cultural heritage. The state-of-the-art section first clarifies core definitions and the governing principles that control binder formation and performance. It then translates this knowledge into operational criteria for the design of conservation-oriented mortars, renders and related systems. The discussion moves from general concepts to application-focused issues. It considers the role of precursors and alkaline activators, the influence of curing conditions, and the way microstructure and thermophysical behaviour affect key functions required in conservation practice, including moisture transport, salt dynamics, and hygrothermal response. The chapter then links the material framework to conservation objectives by defining compatibility criteria—physicochemical, microstructural, hygric, mechanical and aesthetic—and by outlining how these criteria can be translated into acceptance thresholds and verification procedures. Mechanical compatibility is framed not as the pursuit of maximum strength, but as the alignment of stiffness and deformational response with the substrate. Hygric compatibility concerns the preservation or restoration of vapour exchange and appropriate capillary behaviour. Chemical compatibility involves avoiding harmful secondary reactions and the formation of soluble species prone to crystallisation. Aesthetic compatibility relates to colour, texture and finish, supporting interventions that remain legible yet non-intrusive. These aspects are discussed alongside sustainability, minimum intervention and reversibility, which position geopolymer-based approaches in relation to original materials and future maintainability. Finally, the chapter reviews the growing body of work on Construction and Demolition Waste (CDW) in AAM systems, summarising selection strategies for recycled precursors and formulation approaches aimed at achieving conservation-relevant density, porosity and water uptake while limiting efflorescence risk and maintaining on-site workability. Chapter 2 moves from this theoretical framework to laboratory experimentation. It examines metakaolin-based geopolymer mortars incorporating tuff aggregates and crushed brick (cocciopesto) to assess their potential as repair mortars for historic masonry. The experimental programme evaluates physical, mechanical, hygric, thermal, colourimetric and durability behaviour under standard conditions and after accelerated ageing. Its overarching aim is to determine, with methodological rigour, whether the selected formulations meet performance requirements typical of conservation practice while offering a technically sound and environmentally responsible alternative to conventional repair materials. Specimen preparation was carried out at the Applied Chemistry Laboratories (ACLabs) of the Department of Chemical, Materials and Industrial Production Engineering (DICMaPI), University of Naples Federico II (UNINA), under the supervision of Professors Domenico Caputo and Barbara Liguori. The work included precursor conditioning, activator preparation, mixing, casting and curing, with attention to batch repeatability, traceability and raw-material characterisation. Subsequent testing was performed at the ETSEM laboratories (Escuela Técnica Superior de Edificación), Universidad Politécnica de Madrid (UPM), under the supervision of Professor Mercedes del Río Merino, using harmonised procedures and consistent instrument settings to ensure comparability among mixes. The starting materials include tuffs with distinct zeolitic mineralogies and a crushed-brick fraction representative of traditional cocciopesto. Their preliminary characterisation supports the interpretation of mortar performance in terms of reactivity, reaction progress and binder–aggregate interfacial behaviour. Mortar performance is assessed through complementary property sets. Physical and hygric measurements—open porosity, apparent density, water absorption and capillary uptake—are used to evaluate moisture transport and storage. Mechanical behaviour is characterised in flexure and compression and is complemented by Shore D surface hardness and dynamic elastic modulus derived from Ultrasonic Pulse Velocity (UPV), providing insight into surface integrity, stiffness evolution, susceptibility to microcracking and interfacial cohesion. Durability is evaluated through accelerated cycles that simulate key degradation mechanisms, including salt crystallisation (with monitoring of mass change, surface condition, damage development, hardness and modulus evolution) and wetting–drying cycles, which isolate moisture-induced stresses and clarify the role of pore structure and aggregate type in damage accumulation. Thermal conductivity is measured on plate specimens to support structure–property relationships and to inform hygrothermal considerations for historic substrates. Colourimetric analysis assesses chromatic coordinates before and after durability tests, total colour difference and appearance relative to reference materials, with regard to perceptibility and acceptance thresholds commonly adopted in conservation practice. Together, these results indicate whether tuff- and cocciopesto-based geopolymer mortars can satisfy conservation-oriented requirements while offering sustainability advantages over traditional repair materials. Chapter 3 addresses the transition from laboratory work to a site-oriented phase. It sets out the design of the pilot intervention and reconstructs the full operational pathway, from characterising the material context to on-site mortar application and the initiation of early in situ checks. The activities were conducted in close collaboration with the Soprintendenza Archeologia, Belle Arti e Paesaggio of Naples (the local Heritage Authority), under the technical supervision of Conservation Officer Barbara Balbi, within the research programme “Archival Survey and Cataloging of Restoration Interventions in the Historical Built Heritage of the City of Naples”. This institutional cooperation enabled the integration of experimental evidence with the operational requirements of heritage protection and conservation. Within the historic centre of Naples, the archaeological complex of “Carminiello ai Mannesi” was selected as the reference site for mortar application, field testing and the long-term monitoring framework. The selection was supported by on-site surveys and an initial phase of historical and technical documentation undertaken jointly with the Soprintendenza, aimed at defining the material context and identifying major degradation patterns. Prior to application, targeted sampling of tuff and historic mortar was carried out and followed by laboratory analyses, enabling comparison between the site’s Neapolitan yellow tuff and the materials used in the experimental programme. X-ray diffraction (XRD) comparison between the collected tuff and the tuff powder adopted in the mixes, together with thermal characterisation of the historic mortar, indicated chemical–mineralogical affinity and supported the selection of the formulation for joint repointing. In this context, compatibility is considered as the convergence of measured performance, composition and microstructure, rather than a purely strength-driven outcome. The in situ pilot intervention was deliberately limited in scale to ensure traceability and controlled observation. The test area was located in an external sector directly exposed to weathering, so that early behaviour could be assessed under realistic service conditions. Three selected joints were repointed locally to reinstate mortar continuity within existing lacunae without further removal of historic material. Site operations included careful surface cleaning, removal of loose deposits and potentially interfering residues, and controlled pre-wetting of the tuff to limit immediate absorption of mixing water and promote interfacial continuity. On-site mortar preparation retained the laboratory design rationale while incorporating adjustments required for repointing. The base components were kept consistent with the experimental mix design, while standardised sand and a coarser tuff fraction were introduced to provide a granular skeleton for compaction, volumetric stability and finishing control. Water was adjusted progressively to achieve workability, and three separate batches (one per joint) were prepared to control setting and consistency. Application was performed by professional conservators using manual repointing techniques, ensuring legibility and morphological continuity with the existing joints. A first post-application observation phase was then carried out as the initial step of a non-destructive monitoring pathway. Documentation of the baseline condition (initial state), supported by visual inspection and photographic recording, established a reference for subsequent readings and helped distinguish physiological maturation from early warning signs. In this perspective, the transition from laboratory to site is not merely a change of scale, but a methodological shift towards assessing system response under real environmental and operational constraints within a replicable and heritage-compatible observational framework. In summary, this thesis positions its contribution within Heritage Science by taking as its reference not only the material but the heritage asset as a complex system shaped by matter, environment, use constraints and conservation responsibilities. Within this perspective, diagnostic tools and non-destructive monitoring are not ancillary, but a methodological prerequisite for making design choices traceable and verifiable over time. At the same time, the design of “waste-informed” geopolymer mortars translates circular-economy principles into an operational strategy for conservation. Progressive validation—from laboratory testing to the application scale—aims to reduce the gap between measured performance and in-service behaviour. The result is a replicable pathway intended to generate evidence that is useful not only for research, but also for the future definition of shared technical standards for the controlled use of geopolymers in the conservation of built heritage.