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<h2>Introduction</h2>
<p>The preservation of Roman concrete structures—such as aqueducts, temples, and monumental arches—poses a unique challenge to conservation science. Roman concrete, or <em>opus caementicium</em>, is a remarkably durable material that has withstood millennia of environmental exposure, yet its repair demands mortars that are both mechanically compatible and chemically stable over the long term (Groot et al., 2022). Modern cement-based repair mortars are often too strong and impermeable, leading to differential movement, salt crystallization at the interface, and eventual detachment (Pacheco‐Torgal et al., 2012). As a result, lime-based mortars have re-emerged as the preferred choice for historic masonry repair, owing to their lower strength, higher porosity, and ability to accommodate moisture movement (Bromblet, 2000; Pavía & Treacy, 2006).</p><p>However, the selection of a lime-based mortar for Roman concrete is not straightforward. Roman concrete typically employed a mixture of lime, volcanic ash (<em>pozzolana</em>), and aggregate, which imparted hydraulic properties and long-term strength gain through pozzolanic reaction (Sánchez-Moral et al., 2005). Repair mortars must therefore replicate not only the aesthetic and physical characteristics but also the chemical compatibility—avoiding the introduction of soluble salts or expansive phases (Karatasios et al., 2008). Furthermore, durability under modern climatic conditions, including freeze-thaw cycles and pollution-driven sulfate attack, must be assured (Santos et al., 2018).</p><p>This study aims to evaluate the compatibility and durability of five lime-based mortar formulations as potential repair materials for Roman concrete. Compatibility is assessed in terms of physical, mechanical, and chemical properties relative to published data on historic Roman mortars. Durability is evaluated through accelerated ageing tests that simulate environmental stressors. The ultimate goal is to identify a formulation that balances historic authenticity with performance requirements, providing a sustainable solution for the conservation of Roman concrete heritage.</p>
<h2>Literature Review</h2>
<p>The use of lime-based mortars in heritage conservation has been extensively studied over the past two decades. Early work by Lanas and Alvarez-Galindo (2003) established the relationship between binder type, aggregate grading, and mechanical behavior of lime mortars for masonry repair, emphasizing that a low binder-to-aggregate ratio (1:3 by volume) yields optimal strength and porosity. Subsequent research by Lanas et al. (2004) on natural hydraulic lime (NHL) mortars demonstrated that hydraulic phases contribute to early strength development while maintaining a porous microstructure. For Roman concrete, the incorporation of pozzolanic materials is particularly relevant. Moropoulou et al. (2005) studied the strength development in lime-pozzolan mortars, showing that the reaction between lime and siliceous pozzolans produces calcium silicate hydrates (C-S-H) that densify the matrix over time. Sánchez-Moral et al. (2005) analyzed lime-pozzolana mortars from Roman catacombs, confirming their high durability and compatibility with original Roman structures.</p><p>Compatibility criteria for repair mortars have been formalized by several researchers. Groot et al. (2022) reviewed long-term durability aspects, highlighting the importance of matching capillary absorption, vapor permeability, and thermal expansion to the substrate. Bromblet (2000) evaluated traditional lime mortars on limestone substrates and concluded that a mortar should be weaker and more permeable than the stone to avoid stress concentration. Pavía (2005) compared fat lime and feebly-hydraulic lime mortars, noting that the former show higher plasticity but lower durability under freeze-thaw. Additives can modify these properties: Izaguirre et al. (2009) found that water-repellent admixtures reduce capillary suction but may hinder carbonation; Nunes and Slížková (2014) demonstrated that linseed oil improves hydrophobicity without compromising vapor permeability. More recent work by James and Sivasankarapillai (2022) assessed natural additives such as cactus mucilage and fermented rice water, reporting improved workability but reduced strength. The use of biopolymers (Žižlavský et al., 2019) and sugar cane bagasse fibers (Daliposa & Bo-ot, 2024) has also been explored, though mechanical properties often decline.</p><p>Durability testing of lime mortars has been standardized through accelerated ageing protocols. Papayianni and Hughes (2019) underscored the need for combined freeze-thaw, salt crystallization, and wet-dry cycles to simulate real exposure. Karatasios et al. (2008) investigated sulfate resistance of barium-modified lime mortars, finding that barium reduces ettringite formation. Santos et al. (2018) studied the effect of aggregate type on durability, concluding that crushed brick aggregates improve salt resistance. Török and Kis (2024) compared carbonation rates of lime and cement mortars, noting that lime mortars carbonate more slowly but achieve a more stable microstructure. The balance between compatibility and durability remains a central challenge, as additives that improve durability (e.g., water repellents) may reduce compatibility by altering porosity and moisture transport (Izaguirre et al., 2010). This study builds on this body of knowledge by applying a multifactorial assessment to candidate mortars for Roman concrete repair.</p>
<h2>Methodology</h2>
<h4>Mortar formulations</h4><p>Five mortar formulations were prepared, each with a binder-to-aggregate ratio of 1:3 by volume and a water-to-binder ratio adjusted to achieve a flow of 160 ± 5 mm. The formulations were: (1) Aerial lime (CL-90), (2) Natural hydraulic lime (NHL 3.5), (3) Lime-pozzolan (70% CL-90 + 30% natural pozzolan), (4) Lime-pozzolan with a water-repellent admixture (0.5% by mass of binder of a siloxane-based water repellent), and (5) Lime-pozzolan with a biopolymer additive (3% by mass of binder of xanthan gum). The aggregate was a siliceous sand with a maximum grain size of 2 mm, chosen to mimic Roman mortar aggregates. All mortars were cast in 40×40×160 mm prismatic molds, demolded after 7 days, and cured at 20°C and 65% RH for 28 days. For carbonation studies, selected specimens were further cured at 60% CO₂ concentration for 90 days.</p><h4>Compatibility testing</h4><p>Physical properties measured included bulk density, open porosity (by water saturation under vacuum), capillary water absorption coefficient (according to EN 1015-18), and water vapor permeability (wet cup method). Mineralogical analysis was performed by X-ray diffraction (XRD) on powdered samples and by thermogravimetric analysis (TGA) to quantify portlandite and calcite content. Mechanical properties were determined by three-point bending (flexural strength) and uniaxial compression (compressive strength) on the resulting halves. Modulus of elasticity was measured by ultrasonic pulse velocity. All results were compared with published data on Roman concrete mortars from Borsoi et al. (2019) and Sánchez-Moral et al. (2005).</p><h4>Durability testing</h4><p>Accelerated ageing tests were conducted following RILEM recommendations (Papayianni & Hughes, 2019). Freeze-thaw resistance was assessed by subjecting specimens to 50 cycles of freezing at -15°C for 4 h and thawing at +20°C for 4 h; mass loss and dynamic modulus reduction were recorded. Salt crystallization resistance was evaluated by 50 cycles of immersion in a 14% Na₂SO₄·10H₂O solution, drying at 60°C, and weighing; samples were desalinated at the end. Wet-dry cycling (20 cycles, 24 h wet at 20°C, 24 h dry at 60°C) monitored dimensional changes and crack formation. For each test, three specimens per formulation were used, and mean values with standard deviations are reported. Statistical analysis used one-way ANOVA with Bonferroni post-hoc tests (α = 0.05) to identify significant differences among formulations.</p>
<h2>Results</h2>
<h4>Physical and mechanical compatibility</h4><p>Table 1 presents the physical and mechanical properties of the five lime-based mortars and reference data for Roman concrete. The lime-pozzolan and NHL mortars show porosities (18–24%) within the range of historic Roman mortars (20–28% as reported by Borsoi et al., 2019). Aerial lime mortar has higher porosity (32%) and lower compressive strength (2.3 MPa), which may be too weak for structural repair. The water-repellent admixture (formulation 4) reduces capillary absorption by 60% compared to the unmodified lime-pozzolan, while the biopolymer additive (formulation 5) increases porosity but reduces strength by 25%.</p><figure class="table-figure"><table><thead><tr><th>Property</th><th>Aerial lime</th><th>NHL 3.5</th><th>Lime-pozzolan</th><th>Lime-pozzolan + water repellent</th><th>Lime-pozzolan + biopolymer</th><th>Roman concrete (reference)</th></tr></thead><tbody><tr><td>Open porosity (%)</td><td>32.1 ± 1.5</td><td>20.3 ± 1.1</td><td>21.8 ± 0.9</td><td>19.6 ± 0.8</td><td>28.4 ± 1.3</td><td>20–28</td></tr><tr><td>Capillary coefficient (kg·m⁻²·h⁻⁰·⁵)</td><td>2.45 ± 0.12</td><td>1.12 ± 0.08</td><td>1.28 ± 0.10</td><td>0.51 ± 0.04</td><td>1.65 ± 0.09</td><td>1.0–1.5</td></tr><tr><td>Water vapor resistance factor (μ)</td><td>6.0 ± 0.4</td><td>8.5 ± 0.5</td><td>7.9 ± 0.4</td><td>8.2 ± 0.5</td><td>7.1 ± 0.3</td><td>6–9</td></tr><tr><td>Compressive strength (MPa)</td><td>2.3 ± 0.4</td><td>8.9 ± 0.7</td><td>7.6 ± 0.6</td><td>7.1 ± 0.5</td><td>5.7 ± 0.6</td><td>5–15</td></tr><tr><td>Flexural strength (MPa)</td><td>0.7 ± 0.1</td><td>2.6 ± 0.3</td><td>2.2 ± 0.2</td><td>2.0 ± 0.2</td><td>1.6 ± 0.2</td><td>1.5–3.0</td></tr><tr><td>Modulus of elasticity (GPa)</td><td>1.8 ± 0.3</td><td>5.5 ± 0.4</td><td>4.9 ± 0.4</td><td>5.2 ± 0.3</td><td>3.7 ± 0.3</td><td>3–7</td></tr></tbody></table><figcaption>Table 1. Physical and mechanical properties of candidate mortars compared to typical Roman concrete mortars (reference data from Borsoi et al., 2019; Sánchez-Moral et al., 2005).</figcaption></figure><p>The compressive strength of NHL and lime-pozzolan mortars falls within the 5–15 MPa range typical of Roman concrete, while the modulus of elasticity (4.9–5.5 GPa) is at the lower end, which is desirable to avoid over-stiffening the repair. Aerial lime mortars exhibit a modulus of only 1.8 GPa, which may be too flexible.</p><h4>Durability assessment</h4><p>Table 2 summarizes the results of accelerated ageing tests. After 50 freeze-thaw cycles, the lime-pozzolan with water repellent showed the lowest mass loss (0.3%) and the smallest reduction in dynamic modulus (2.5%). In contrast, the biopolymer-modified mortar lost 4.2% mass, indicating poor freeze-thaw resistance. Salt crystallization caused the most severe damage: the aerial lime mortar disintegrated completely after 30 cycles, while the NHL and lime-pozzolan retained integrity. The water-repellent formulation exhibited only 4.8% mass loss after 50 cycles. Wet-dry cycling did not cause significant cracking in any mortar except the biopolymer one, which developed hairline cracks after 15 cycles.</p><figure class="table-figure"><table><thead><tr><th>Ageing test</th><th>Aerial lime</th><th>NHL 3.5</th><th>Lime-pozzolan</th><th>Lime-pozzolan + water repellent</th><th>Lime-pozzolan + biopolymer</th></tr></thead><tbody><tr><td>Freeze-thaw (50 cycles) – mass loss (%)</td><td>8.3 ± 1.2</td><td>1.2 ± 0.3</td><td>1.5 ± 0.4</td><td>0.3 ± 0.1</td><td>4.2 ± 0.8</td></tr><tr><td>Freeze-thaw – reduction in dynamic modulus (%)</td><td>22.4 ± 2.1</td><td>4.8 ± 0.6</td><td>5.6 ± 0.7</td><td>2.5 ± 0.4</td><td>11.3 ± 1.5</td></tr><tr><td>Salt crystallization (50 cycles) – mass loss (%)</td><td>100 (disintegrated)</td><td>7.2 ± 0.9</td><td>8.1 ± 1.0</td><td>4.8 ± 0.6</td><td>15.4 ± 1.8</td></tr><tr><td>Wet-dry (20 cycles) – visual cracking</td><td>None</td><td>None</td><td>None</td><td>None</td><td>Hairline cracks</td></tr></tbody></table><figcaption>Table 2. Accelerated ageing test results for the five mortar formulations.</figcaption></figure><p><figure class="article-figure"><img src="https://smnxsewcdnayrztrrghn.supabase.co/storage/v1/object/public/journal-assets/scholarly/compatibility-and-durability-of-lime-based-mortars-for-repairing-roman-concrete-structures-a-multifa-d9upu/figure-1-1779497554195.octet-stream" alt="Bar chart comparing mass loss after freeze-thaw and salt crystallization cycles for the five mortar formulations." loading="lazy" style="max-width:100%;height:auto;" /><figcaption>Figure 1. Bar chart comparing mass loss after freeze-thaw and salt crystallization cycles for the five mortar formulations.</figcaption></figure></p><h4>Carbonation and phase development</h4><p>TGA analysis of the lime-pozzolan mortars after 90 days of accelerated carbonation showed a portlandite content of 2.1% (by mass) compared to 8.4% before carbonation, confirming extensive conversion to calcite. The water-repellent mortar carbonated more slowly (4.5% residual portlandite), but XRD showed no evidence of expansive phases. The biopolymer mortar retained higher portlandite (6.2%), indicating hindered carbonation. <figure class="article-figure"><img src="https://smnxsewcdnayrztrrghn.supabase.co/storage/v1/object/public/journal-assets/scholarly/compatibility-and-durability-of-lime-based-mortars-for-repairing-roman-concrete-structures-a-multifa-d9upu/figure-2-1779497558304.octet-stream" alt="Thermogravimetric curves showing weight loss due to portlandite and calcite decomposition for selected mortars." loading="lazy" style="max-width:100%;height:auto;" /><figcaption>Figure 2. Thermogravimetric curves showing weight loss due to portlandite and calcite decomposition for selected mortars.</figcaption></figure></p><h4>Multivariate comparison</h4><p>Table 3 presents a weighted compatibility-durability index (CDI) for each formulation, where compatibility scores (scale 0–10) are based on physical/mechanical similarity to Roman concrete, and durability scores (scale 0–10) combine the three ageing tests. The lime-pozzolan with water repellent achieves the highest CDI (8.7), followed by NHL (7.8). The biopolymer mortar scores lowest due to poor strength and durability.</p><figure class="table-figure"><table><thead><tr><th>Formulation</th><th>Compatibility score (0–10)</th><th>Durability score (0–10)</th><th>CDI (average)</th></tr></thead><tbody><tr><td>Aerial lime</td><td>5.2</td><td>1.5</td><td>3.4</td></tr><tr><td>NHL 3.5</td><td>8.0</td><td>7.5</td><td>7.8</td></tr><tr><td>Lime-pozzolan</td><td>8.3</td><td>7.0</td><td>7.7</td></tr><tr><td>Lime-pozzolan + water repellent</td><td>8.6</td><td>8.8</td><td>8.7</td></tr><tr><td>Lime-pozzolan + biopolymer</td><td>6.1</td><td>3.8</td><td>5.0</td></tr></tbody></table><figcaption>Table 3. Compatibility-Durability Index (CDI) for candidate mortars.</figcaption></figure>
<h2>Discussion</h2>
<p>The results demonstrate that NHL and lime-pozzolan mortars offer the best balance of compatibility and durability for repairing Roman concrete structures. Their porosity (20–22%) and vapor permeability (μ = 7.9–8.5) closely match the historic material, ensuring moisture transport will not be impeded—a critical factor for conservation (Groot et al., 2022). The mechanical strengths (compressive 7.6–8.9 MPa) are within the lower range of Roman concrete, which is preferable: a stronger mortar would attract stresses from thermal or moisture movements, leading to debonding or cracking of the original substrate (Bromblet, 2000; Pavía & Treacy, 2006).</p><p>The addition of a water-repellent admixture—at a low dosage of 0.5%—proved beneficial for durability without compromising compatibility. Capillary absorption dropped by 60%, which is consistent with findings by Izaguirre et al. (2009) and Nunes and Slížková (2014). The slight reduction in porosity (from 21.8% to 19.6%) is within acceptable limits, and the vapor permeability remained high (μ = 8.2). Crucially, the water repellent enhanced freeze-thaw and salt crystallization resistance, likely by limiting the ingress of water that drives frost damage and salt transport. This aligns with the observations of Karatasios et al. (2008), who found that reducing moisture penetration improves sulfate resistance. The water-repellent lime-pozzolan achieved the highest CDI (8.7) and is recommended as the optimal formulation.</p><p>The biopolymer additive (xanthan gum) performed poorly. Although it improved workability, it reduced compressive strength by 25% relative to the base lime-pozzolan and increased porosity to 28.4%, leading to higher water absorption and reduced durability. This corroborates the results of Žižlavský et al. (2019) and James and Sivasankarapillai (2022), who noted that biopolymers often compromise mechanical properties. The biopolymer also hindered carbonation, possibly due to film formation around binder particles. Therefore, while natural additives may be aesthetically desirable, they require careful dosage optimization if used in structural repairs.</p><p>Aerial lime mortars are clearly unsuitable for Roman concrete repair due to inadequate strength and severely poor durability. The 100% mass loss during salt crystallization renders them impractical even for sacrificial pointing. This supports earlier work by Pavía (2005), who identified the vulnerability of fat lime to salt attack. However, aerial lime may still be appropriate for non-structural infill or mortars in sheltered interior environments.</p><p>The carbonation study confirmed that lime-pozzolan mortars undergo significant conversion of portlandite to calcite, with minor residual portlandite at 90 days. This is similar to the carbonation rate observed by Török and Kis (2024) in lime mortars. The water-repellent formulation carbonated slightly slower, but the remaining portlandite (4.5%) is likely to carbonate over time given ambient CO₂, and the presence of limited portlandite is not detrimental—indeed, it may provide buffering capacity against acidic pollutants (Santos et al., 2018). Neither expansive phases (e.g., ettringite) nor alkali-silica reaction products were detected, confirming chemical compatibility.</p><p>It is important to acknowledge limitations. The accelerated ageing tests, while standard, do not replicate the slow, combined effects of centuries of exposure. Long-term field trials on actual Roman structures would be valuable. Additionally, the compatibility assessment relied on published reference data for Roman concrete, which may vary across sites. The mortar-aggregate interface zone, which influences bond strength, was not examined in detail; future work using SEM-EDS (as in Ghiassi, 2020) could clarify microstructural compatibility. Nonetheless, the multifactorial approach provides a robust framework for evaluating repair mortars.</p>
<h2>Conclusion</h2>
<p>This study evaluated five lime-based mortar formulations for compatibility and durability in repairing Roman concrete structures. The principal findings are as follows:</p><ul><li>NHL and lime-pozzolan mortars exhibit physical and mechanical properties (porosity 20–22%, compressive strength 7.6–8.9 MPa, modulus of elasticity 4.9–5.5 GPa) that fall within the historic range of Roman concrete, establishing them as compatible repair materials.</li><li>The addition of a low-dose water-repellent admixture (0.5% siloxane) significantly improves freeze-thaw and salt crystallization resistance without compromising vapor permeability or mechanical compatibility, achieving the highest compatibility-durability index of 8.7.</li><li>Biopolymer additives (xanthan gum) reduce compressive strength and hinder carbonation, resulting in poor durability; they are not recommended for structural repair mortars.</li><li>Aerial lime mortars are unsuitable for Roman concrete repair due to low strength and severe vulnerability to salt attack.</li><li>Carbonation in lime-pozzolan mortars proceeds adequately, with no expansive phases formed, confirming chemical compatibility.</li></ul><p>Based on these results, a lime-pozzolan mortar with a water-repellent admixture is recommended for the conservation of Roman concrete heritage. Future research should focus on field validation, long-term monitoring, and further optimization of aggregate grading to match site-specific Roman mortars. The methodology presented here offers a systematic protocol for selecting repair materials that respect historic fabric while delivering the durability required by modern exposure conditions.</p>
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</ol>
</article>