Advanced Lime Mortars for Historical Architectural Structures ^†

Abstract

The utilization of lime mortar to connect the masonry wall elements of historical architectural structures, to overlap and protect these structures, is an ancient technique that has prevailed until today as a compatible solution based on the principles of restoration. In recent years, scientists have studied the modification of lime mortars with new, sustainable and environmentally friendly materials that respect the value of monuments and are aligned with the principles of restoration. In the present study, the existing international literature on advanced lime mortars with improved mechanical and physicochemical properties is presented, and the knowledge gap is identified. Finally, new materials for improving lime mortars are proposed as a basis for successful restoration and further protection of architectural heritage.

1. Introduction

From ancient to modern times structures are build using natural materials such as lime mortars [1,2,3]. The importance of lime is demonstrated by the existence of stable lime mining sites in Europe, driven by additional knowledge of limestone quality (CaCO₃) and repeated effective empirical use [4]. Lime mortar was used as a binding material and as a coating material for facades. Even today, this material is used in the restoration of existing ancient and modern monuments following the principles of restoration. One of the most important restoration principles is compatibility, as materials should have specific characteristics. In the case of integration, new materials promote mechanical characteristics and physical properties such as compression and flexural strength improvement, freeze–thaw resistance, compatibility, thermal insulation, self-healing, self-cleaning, durability and elasticity [5,6,7,8]. State-of-the-art advanced lime mortars for historical architectural structures represent a convergence of traditional craftsmanship with modern materials science and engineering principles. These advanced lime mortars aim to address the unique challenges associated with the conservation, restoration, and sustainable enhancement of historical buildings while preserving their architectural authenticity and cultural significance.

Non-destructive methods for evaluating the physicomechanical, electrochemical and mineralogical characterization of lime mortars have already been employed, such as X-ray diffraction, Fourier-Transform Infrared spectroscopy, scanning electron microscopy, or thermogravimetric analysis [9,10] (Figure 1).

Researchers have explored the incorporation of nanoparticles such as silica nanoparticles, calcium carbonate nanoparticles, and nanosized fibers to enhance the mechanical properties and durability, bond strength, and resistance of mortars to environmental degradation. On the other hand, the aim was to reduce porosity and thus enhance the sustainability of historical architectural heritage. Moreover, the integration of natural and pozzolanic materials, such as metakaolin, rice husk ash, and volcanic ash, has gained attention for its ability to improve the workability, strength, and durability of lime mortars. These materials react with lime to form additional binding phases, enhancing the mortar’s performance while maintaining compatibility with historic substrates [12,13,14,15,16,17,18].

The various proportions of hydraulic lime mortars offer a beneficial enhanced setting and hardening properties concerning traditional non-hydraulic lime mortars. Through careful selection and possible combination of two types of hydraulic limes, the lime mortar developed has properties that could meet the specific requirements of historical architectural heritage restoration. The embodiment of natural fibers, bio-fibers, or synthetic fibers and textiles from cellulose, jute, hemp, or polypropylene can enhance the tensile strength, crack resistance, and ductility of restoration mortars [19,20,21,22,23,24,25]. This strengthening method contributes to the wall stabilization of vulnerable masonry, converting it into an earthquake-resistant structure.

With advanced digital simulation tools, including finite element analysis (FEA) or computational fluid dynamics (CFD), the performance of lime mortar can be optimized under various loading and environmental conditions. This fact leads to accurate mechanical and physico-chemical performance predictions, aligned with the conservation strategies for historical structures. Finally, interdisciplinary research on sustainable and environmentally friendly products with innovative properties and a low environmental footprint is provided, in combination with the promotion of cultural heritage conservation [11,13,26].

In the subsequent sections, fundamental knowledge concerning the existing international literature is presented to establish the research on advanced lime mortars with improved mechanical and physicochemical properties, and the knowledge gap is identified. Additionally, the key properties essential for assessing the admixture performance are outlined, with following the principles of successful restoration and the further protection of architectural heritage.

2. Classification of Additives in Lime Mortar

The classification of the additives used in lime mortar for historical architectural heritage, is divided into several categories based on their specific functions and properties. Some usual types of additives including pozzolans, nanoparticles, fibers, plasticizers, air-entraining agents, accelerators or retarders, waterproofing agents, antifungal and antimicrobial agents, and coloring agents are illustrated in Figure 2.

Pozzolanic constituents such as silica fume, metakaolin, fly ash, and volcanic ash act with lime to arrange supplementary binding composites, refining the mortar’s strength, durability, and environmental resistance. Natural fibers (jute, straw, or hemp) or synthetic fibers (polypropylene, nylon) can be integrated into the lime mortar to improve its flexural strength, cohesion and ductility, serving to provide crack prevention. Plasticizers can improve the workability or consistency of lime mortar without significantly altering its chemical properties, reducing the water content and improving the mortar’s flow, making it easier to apply and ensuring better adhesion to substrates. Air-entraining agents are additives that introduce microscopic air bubbles into the mortar mixture, enhancing the mortar’s freeze-thaw resistance by providing space for water to expand and contract without causing damage.

Accelerators are additives that speed up the curing and setting time of lime mortar, allowing for faster construction or repair work. Retarders, on the other hand, slow down the setting time, providing more time for application and adjustment. Waterproofing additives such as silicone-based compounds or admixtures containing water repellents can be added to lime mortar to improve its resistance to water penetration and moisture-related deterioration. Additives with antifungal and antimicrobial properties can be incorporated into lime mortar to inhibit the growth of mold, mildew, algae, and other microorganisms that can cause staining and deterioration over time. Coloring additives such as pigments or mineral oxides are used as coloring additives to tint lime mortar to match existing historic mortars or achieve specific aesthetic effects. These additives can help preserve the visual integrity of historical structures during restoration or repair work.

By carefully selecting and incorporating these additives into lime mortar formulations, preservationists and conservators can ensure the longevity, structural integrity, and visual authenticity of historical architectural heritage.

3. Mechanical, Physical and Chemical Properties of Advanced Lime Mortar

The physical and mechanical properties (Figure 3) of lime mortar play a crucial role in its performance and suitability for use in historical architectural structures. A crucial indicator of the capacity of a mortar to be used in anti-seismic structures is its compressive, flexural and tensile strength. These significant properties are necessary for earthquake-resistant mortar applications to withstand lateral or bending forces. Another property is the ability of lime mortar to adhere strongly to brick, tile, and stone masonry substrates, ensuring detachment and delamination prevention.

Durability measures the resistance of lime mortar to deterioration and weathering over time. It encompasses various factors, including resistance to freeze-thaw cycles, moisture ingress, chemical exposure, and biological growth. Durable lime mortar formulations ensure the long-term performance and protection of historical structures. Moreover, successful workability allows easy application, establishing the correct consolidation of the lime mortar within masonry wall joints. Permeability and porosity are important properties of lime mortar that influence its ability to absorb and release moisture. Controlled porosity allows lime mortar to breathe and accommodate moisture movement, reducing the risk of trapped moisture and the associated deterioration, such as efflorescence or spalling. Thus, the risk of entrapped moisture and efflorescence or spalling occurring is decreased. Volume and shape stability reduce the risk of shrinkage and expansion, which can lead to cracking, distortion and displacement, ensuring the integrity and functionality of masonry walls. Finally, nano-enhancement [5,7,9,10,12,14,16,17,25], fiber tailoring [9,14,18,19,20,24] or using conductive materials [5,7,10,17,19,20] as additives can improve the electrical resistance or piezo resistivity of lime mortars. Through the systematic assessment and optimization of electromechanical and physicochemical properties, preservation professionals can decide on the appropriate lime mortar formulations according to the specific requirements of historical architectural anti-seismic structures.

In

Table 1. Indicative experimental research, showcasing various types of advanced lime mortars with additives.

Authors + Year	Type of Mortar	Type of Addition	Properties	Results
Tsampali et al., 2024 [24]	Lime, clay-based	Untreated and hydrothermally treated hemp fibers and crystaline admixtures	Physicochemical strength, self-healing	Enhanced the mechanical strength and reduced the mortars’ shrinkage. For clay mortars, an increase of 24% for untreated fibers, while for the hydrothermal process, fibers exhibited an increase (43%)
Drougkas et al., 2023 [12]	Conductive micro and nanofillers	Graphite (G), carbon nanotubes (CNT) and carbon microfibres (CMF)	Electro-mechanical properties (piezoresistivity)	Significant strength recovery and self-healing potential in lime-pozzolan mortars with hemp fibers
Nalon et al., 2023 [16]	Lime/cement	Carbon black nanoparticles (CBN) contents (0%, 3%, 6%, 9% by weight of binders)	Electro-mechanical properties (self- sensing, electrical conductivity)	Reduction of electrical resistivity of dry masonry mortars by more than 4 orders of magnitude with the addition of CBN contents > 6%. Self-sensing response in masonry mortars containing 6% or 9% of CBN
Diaz et al., 2022 [17]	Lime	Silica (SiO2) nanoparticles	Physical-mechanical and mineralogical properties, mineralogical compositions	Improvement of mechanical properties up to 4 wt.%
Dimou et al., 2022 [6]	Lime/cement	Graphene oxide (GO), reduced graphene oxide (rGO) and carboxylated graphene (GCOOH)	Electro-mechanical properties (self-sensing, electrical conductivity)	The highest (+33%) increase of the compressive strength is noted for the rGO-reinforced paste
Stefanidou et al., 2022 [23]	Lime	Substitution of natural pozzolan by perlite by-products	Physico-mechanical properties	Reduction of the W/B ratio around (15–30%). Reduction of porosity, absorption and capillary absorption (up to 75%). Enhanced 90d mechanical characteristics and Dynamic Modulus of elasticity
Dimou et al., 2020 [7]	Natural hydraulic lime	Silica (SiO2) nanoparticles	Electro-mechanical properties (self-sensing, electrical conductivity)	The addition of MWCNTsCOOH at 0.15 wt.% led to a 56% increase in flexural strength. Improvement of compressive strength (20%) while the flexural strength remained almost constant with rGO. The electrical resistance drops in both cases (~15%)
Fernandez et al., 2020 [26]	Lime-Pozzolan Plasters	Lime–metakaolin and hydraulic lime–metakaolin with the addition of nano-TiO2 and perlite	Mechanical performance and durability	Decreased mechaniclal properties but enhanced the durability and energy
Kesikidou and Stefanidou, 2019 [22]	Lime/cement	Natural fibers jute, coconut and kelp were used as additives in 1.5% by mortar volume	Mechanical, physical and microstructure properties	Jute and kelp, reduce the compressive strength of cement mortars (15%), but increase it in lime mortars (250%). Coconut fibers, work better with cement but they do not favor lime
Slížková et al., 2019 [18]	Lime	Calcium hydroxide (lime water) or barium hydroxide (barium water)	Mechanical properties (compression and surface cohesion)	Barium water treatment significantly increased mainly the tensile strength of the tested lime mortar. Improved surface cohesion
Rosato et al., 2017 [14]	Lime	Bio-fibrils are nano-structured cellulose fibers (nano-fibrils)	Physico-mechanical properties	Mechanical tests showed a decrease in flexural and compressive strength as the percentage of nano-fibrils increased

, indicative examples from recent bibliographic research, showcasing various types of advanced lime mortars with additives at the nano- and micro-scale, as well as hybrids, are presented. Also, the different properties being investigated and the experimental results are listed for each author.

4. Discussion and Recommendations for the Future Research

This review presents the body of existing research on enhancing lime mortar performance through the use of various additives. It is evidence that several areas remain unexplored. For instance, while it has been demonstrated that various nanoparticles improve the compressive and flexural strength of lime mortars, the results are still uncertain, given. In particular, for the mechanical properties, a slight drop in flexural and compressive strength was evidence. Considering their physical and chemical efficiency, it is suggested that future studies investigate the efficacy of different types and concentrations of nanoparticles on the mechanical properties of lime mortars. Additionally, a comprehensive assessment of the durability and weathering resistance of advanced lime mortars with nanoparticles is essential to gain valuable insights into practical applications in historical architectural structures.

Standing at a pivotal point of tradition and innovation, it is imperative to recognize the deep impact that advanced lime mortars can exert on the future trajectory of architectural preservation. It would be beneficial for future research to explore novel, cost-effective and environmentally friendly methods for fortifying historical heritage and enhancing the resilience of masonry constructions.

By understanding the implications of heterogeneity and variability in traditional materials is crucial, to investigate how these variations may impact the effectiveness of different strengthening techniques and identify strategies to mitigate their impact. Different organic additives and other materials could be identified for effective lime mortar strengthening, exploring an area of interest in future studies.

Future studies could explore the environmental impact of using enriched mortar with natural additives. The low environmental footprint of mortars essential globally, should incorporate eco-friendly materials and methods for building restoration.

5. Conclusions

This review focuses on basic and preliminary knowledge about lime mortar enhancement using various additives to protect the historical architectural heritage. Specifically, there are three categories of geometry scales, the nano-, the micro- and the hybrid combination of the two scales, used to study the lime mortars’ mechanical or physicochemical properties. Classifying additives in lime mortar is vital for understanding their miscellaneous roles and effects on performance. By systematically classifying additives and detailing their properties and functions, the purpose of choosing individual additives for lime mortar strengthening becomes more obvious. Additionally, additive optimization enables the efficient application of advanced materials in several architectural preservation and restoration projects, according to historical heritage preservation principles.

Lastly, fostering robust interdisciplinary collaboration and embracing innovative approaches are essential to fully grasp the potential of advanced lime mortars and ensure their resonance in the present and -for future generations.