Development of new strategies for the conservation of natural pozzolanic mortars using metakaolin, nano-silica and silica fume

1. Introduction The integration of pozzolanic materials into ancient Roman mortar mixtures marks a pinnacle of engineering innovation, pivotal in revolutionizing construction practices across the empire. Volcanic sand, readily sourced from the geologically rich environs of Italy, particularly near Mt. Vesuvius, were integral to the Roman architectural success story. The volcano’s abundant eruptions provided a wealth of materials, such as volcanic ash and sand, which were harnessed to produce mortars with hydraulic properties. These mortars not only set and hardened in wet conditions but also supported the construction of enduring monuments and infrastructure. The resilience and longevity of structures utilizing natural pozzolanic mortars can be attributed to the chemical interactions between the hydrated lime and volcanic byproducts. These interactions foster the formation of calcium silicate hydrates; compounds renowned for their stability and resistance to environmental aggressors. This pozzolanic reaction was a cornerstone of Roman material technology, giving end-products with enhanced mechanical strength and durability, allowing their architecture to withstand the rigors of time, including factors like water infiltration, thermal cycles, and seismic forces. The strategic application of these materials underscored a profound understanding of geo-resources, enabling the Romans to optimize the structural durability and functional longevity of their constructions. By studying these ancient techniques, we gain invaluable insights into sustainable material use and architectural resilience, guiding contemporary efforts in historical conservation and modern building practices. This legacy not only celebrates Roman ingenuity but also serves as a foundation for advancing current approaches to durable and sustainable construction. 2. Research Aims Despite their proven durability, natural pozzolanic mortars are susceptible to environmental degradation, which is exacerbated by their intrinsic material properties such as porosity, and their mineralogical and chemical composition. This interaction with the environment highlights the need for meticulously planned conservation strategies, emphasizing the need for a detailed historical and scientific understanding of the original materials before any intervention is undertaken on historical structures. Inadequate restoration practices can lead to irreversible damage, making it imperative to ensure that any new mortars used in the restoration process are fully compatible with the original materials. This compatibility must encompass several key aspects: chemical alignment, physical compatibility and mechanical suitability. This study aims to address these challenges by developing innovative, compatible mortar mixtures that preserve the structural and aesthetic integrity of heritage buildings constructed with Somma-Vesuvius based pozzolanic mortars. Additionally, with the original volcanic materials becoming scarce, this research will explore alternative sustainable admixtures that can replicate the historical properties of traditional pozzolana. These new mixtures are intended not only to match the original materials in terms of appearance and texture but also to provide enhanced durability and resilience to environmental pressures, thereby reducing the frequency of maintenance and improving the long-term preservation of our architectural heritage. 3. Materials and Methods The binder used for the mortar renders is a seasoned slaked lime, supplied by CTS srl (Europe), and commonly used in restoration efforts. This material, adhering to standard and classified as CL90-S PL, is predominantly composed of calcium hydroxide (>97% CaO) with a minor inclusion of magnesium oxide (ca. 2%). It features a semi-solid consistency, with a density of 1.3 kg/L and a solid-to-water ratio of 40:60. To authentically replicate Roman pozzolanic mortars, three distinct volcanic aggregates were sourced from an ancient quarry located on the Somma-Vesuvius volcanic complex near Alveo Pollena (Naples, Italy). These aggregates are derived from pumices corresponding to different historic eruptions: Avellino (ca 4000 years ago), Pompei (79 AD), and Pollena (472 AD). Prior to their inclusion in the reference mixtures, these volcanic aggregates were meticulously milled and sieved to a maximum grain size of 2 mm. For the experimental mixtures, a CEN Standard Sand with particle size under 2 mm, supplied by NormenSand (Germany), was used. Additionally, three pozzolanic additives were incorporated into the study to evaluate their efficacy: Metakaolin (Metaver N, supplied by NewChem), Nano-Silica (Aerosil 200, provided by Evonik), and Silica Fume (Sprayset SF, supplied by Fosroc). The mixtures were formulated and prepared in the laboratories at the University of Navarra, adhering to ancient recipes from historical literature as Vitruvius’ De Architectura (vol. 7). The dry raw materials, comprising aggregate and pozzolanic additives, were mixed for 5 minutes using a BL-8-CA solid mixer (Lleal S.A.) Subsequently, the required amount of water was gradually added to achieve a predefined workability of 160±5 mm on the flow table test. The fresh mixtures were then casted into cylindrical moulds, each 40 mm in diameter and 36 mm in height, and stored for 7 days in curing chamber (RH>95%). After this initial curing phase, the moulds were transferred for curing under laboratory conditions (T=20°C, RH=60%). The properties of the hardened samples are evaluated after 28 days of curing. To ensure the reliability and representativeness of the results, three samples per mixture and curing time are analysed. Once the hardened state is achieved, the samples will undergo extensive analytical assessments to evaluate their physical and mechanical properties. Initial characterization involves techniques such as Particle Size Determination by Laser Diffraction, X-Ray Powder Diffraction, Scanning Electron Microscopy with Energy Dispersive X-Ray Spectroscopy, Thermogravimetric Analysis and Mercury Intrusion Porosimetry. Further test will evaluate the compressive strength, open porosity and apparent density, water absorption coefficient, as well as resistance to salt crystallization and freeze-thaw cycles at intervals of 28, 90 and 180 days of curing. 4. Results A complete characterization of the mortars was carried out through the aforementioned techniques. Values of the compressive strength were obtained and correlated with the mineralogical and chemical composition of the mixes, and the effect of the pozzolanic additions was also ascertained. Some of the samples are currently being studied and their future analysis are expected to shed light on the impacts of varying additives and aggregate types on the mortar’s properties, such as strength, durability, and environmental resilience. Detailed findings from this study will be presented at the subsequent conference session, where we will discuss the implications of our results for the conservation of historic structures. Acknowledgements The authors would like to acknowledge the support of Sapienza University of Rome, University of Naples Federico II and University of Navarra for providing the facilities and resources necessary for this research. Special thanks to Dr. Mario Cesarano and Cristina Luzuriaga for their valuable assistance.