Natural Lime–Cork Mortar for the Seismic and Energetic Retrofit of Infill Walls: Design, Materials, and Method

Abstract

Recent seismic events have prompted research into innovative and sustainable materials for strengthening and repairing obsolete and vulnerable buildings. These earthquakes have exposed the high seismic vulnerability of existing reinforced concrete (RC) buildings, particularly in secondary structural elements like infill walls. In addition to structural issues, these buildings often face significant energy deficiencies, such as thermal bridges, due to inadequate insulation. Traditionally, structural and energy improvements for residential buildings are addressed separately with different methods and protocols. This preliminary study is part of a broader research initiative at the University of Reggio Calabria (Italy), aiming to design an innovative fiber-reinforced plaster using natural, sustainable, and locally produced materials to enhance the energy and structural performance of existing wall infills. The study investigates two plaster matrices made of natural hydraulic lime and silica sand, with 15% and 30% cork granules added. Mechanical and thermophysical tests on multiple specimens were conducted to evaluate their suitability for seismic and energy retrofitting of infill walls. Results indicate that adding cork reduces mechanical strength by approximately 42% at a 30% cork content without compromising its use in seismic retrofitting. Thermophysical tests show improved thermal performance with a higher cork content. These findings suggest that the lime–cork mixture at 30% is effective, offering excellent ductility and serving as a promising alternative to traditional cementitious plaster systems. The next experimental phase will test matrices with varying percentages of gorse fiber.

1. Introduction

Every year in Europe, there are about 45,000 earthquakes with varying intensities. However, over time, some of these have left their mark with tragic economic and social consequences, as well as numerous losses of human lives in entire territories. The tragic events that have occurred around the world over the last 25 years, starting from the one in L’Aquila in 2009, passing through the central Italy earthquake of 2016, and ending with the one in Turkey in 2023, have highlighted the high seismic vulnerability of existing reinforced concrete buildings, especially concerning non-structural elements [1,2,3,4].

After an earthquake, it is generally possible to find two types of damages to secondary elements: damages to contents in-plane (IP) and damages out-of-plane (OOP) with the expulsion of the masonry infills, as seen in Figure 1 [5,6,7].

The damages to contents in-plane (IP) are subdivided into (a) diagonal compression failure; (b) diagonal cracking failure; (c) sliding shear failure; (d) corner crushing failure; and (e) frame failure crushing.

The damages out-of-plane (OOP) are subdivided into (a) rigid overturning of the masonry without arch effect; (b) rigid overturning of the masonry with arch effect with one yield line; and (c) rigid overturning of the masonry with arch effect with two yield lines.

In addition to the structural deficiencies already mentioned, existing reinforced concrete buildings are also affected by significant thermal bridges due to the lack of an isolated system. It is important to note that most of these buildings were constructed according to seismic and energy codes that are now obsolete compared to current standards. Currently, in the EU, the construction sector accounts for approximately 40% of energy consumption, 50% of raw materials extracted, and 35% of total waste [8,9]. All these factors have a significant impact on existing ecosystems and the surrounding environment, exacerbating the global problem of climate change. To align with the EU target by 2050, the EU’s housing stock should be transformed into a zero-emission building stock, and integrated retrofit measures should be promoted to improve the current condition of approximately 70% of buildings through external interventions.

Usually, structural and energy improvements for the infill walls of RC buildings are addressed separately using distinct methods and protocols, but in the last decades, there has been a strong interest in the field of material engineering in the development of innovative and sustainable composite materials to be used for the structural and energy retrofit of these elements.

Over the years, alongside the refinement of traditional techniques, there has been the development of innovative techniques aimed at improving the structural and energy performance of non-structural elements, such as the infill walls of reinforced concrete buildings. These techniques can be grouped into two main categories: the first includes traditional techniques that involve conventional building materials such as reinforced concrete and steel; the second encompasses new intervention techniques using composite materials.

In the technical–scientific literature, the use of fiber-reinforced composite materials for the seismic and energy improvements of infill walls has been present for over two decades. However, it is only in recent years that the use of this type of reinforcement has represented a significant technological innovation aligned with principles of economic and environmental sustainability, finding increasing applications in the construction sector. Various authors have proposed these reinforcement techniques through the use of composite materials, employing different acronyms that are distinguished by their matrix composition. Some researchers, such as Pohoryles, D. A., et al. [10], have provided a detailed overview of these types of materials, including TRC (textile-reinforced concrete), TRM (textile-reinforced mortar), MBCs (mineral-based composites), FRC (fiber-reinforced cementitious), and FRCM (fiber-reinforced cementitious matrix), as well as fiber-reinforced mortars (SFRMs), which use short fibers dispersed in a mortar on steel meshes to reinforce thin layers of plaster.

Among the materials mentioned, there are the FRCM (fabric-reinforced cementitious matrix) and FRLM (fabric-reinforced lime matrix) composite systems that represent one of the latest generations of composite materials capable of improving the seismic and energy performances of infill walls, as explored in recent research in the literature [11,12,13,14].

These systems are composed of one or more fabrics and two or more layers of a lime- or cement-based matrix, in which reinforcement fabrics are incorporated. These reinforcement aggregates can be made of either natural fibers or traditional fibers. Within this extensive research effort, the main objective is to design FRCM plants using natural aggregates obtained from recycled raw materials or waste produced in order to reduce the environmental impact of the construction sector, reduce the seismic vulnerability of existing buildings, and simultaneously improve thermal properties.

The research aims to investigate the effectiveness of a new composite material, FRLM (fabric-reinforced lime matrix), in mitigating the seismic vulnerability of masonry structures while simultaneously adhering to energy efficiency principles or upgrading masonry buildings. The main objective of this research activity is to design a fiber-reinforced mortar with natural materials.

In recent years, more researchers around the world have turned their attention to the study of natural fiber-reinforced composite materials [15,16,17], but only a few researchers have recognized the use of natural fiber as a reinforced material in the construction industry to attain a sustainable material [18,19].

Many authors have studied the use of natural fiber-reinforced concrete, which is developed by using various types of fibers, such as coir, sisal, bamboo, date, husk, pineapple, and flax [20,21]. Moreover, in recent years, the usage of green materials in structures has developed remarkably due to their advantages in building construction [22,23]. Izquierdo et al. [24] presented a project to study the mechanical properties of masonry elements made of sisal fibers and concrete. Recently, Shah et al. [25] studied the addition of various natural fibers to high-strength concrete, which has aroused great interest in the field of building materials.

In the literature, as highlighted above, there is little evidence of composite mortars with lime and natural fiber. Among these, Brás et al. [26] presented a project to simulate the behavior of cork-based mortars in the minimization of energy consumption and condensation effects in an existing dwelling from the 1980s built in Lisbon, Portugal.

This paper serves as a preliminary step to studying the behavior of the matrix with the addition of cork and analyzing the mechanical and thermal features.

This study is articulated in three steps: The first part focuses on selecting different materials, including a binder to serve as the base of the matrix, an insulator to be incorporated into the mixture, and sand with characteristics compliant with code requirements. In the second part, two mixtures are prepared: one with a 15% cork content and the other with a 30% cork content. The third step of the study involves evaluating the thermophysical and mechanical properties of the mortars with varying percentages of cork. Ultimately, the thermophysical and mechanical results demonstrate the effectiveness of the matrix design with the addition of 30% cork.

2. Materials and Methods

The experimental procedure under consideration began with the design of a thermal and ecological mortar based on lime and cork granules to be applied to the infills of existing reinforced concrete buildings to improve both their structural and energy performances. For reasons of economic and environmental sustainability, it was chosen not to use a conventional industrially produced cement mortar available on the market. Along the same lines, it was preferred to develop an innovative matrix using sustainable natural materials of Calabrian origin.

2.1. Selection of Materials

The materials for the design of lime–cork mortar were selected based on various characteristics, including composition, type class, grain size, aggregate size, natural context, and recycled content; in particular, mechanical properties (compressive strength) and thermophysical properties (thermal conductivity) were also considered. The selection of materials was conducted according to UNI EN 998-1 [27].

The choice of binder to be used as a matrix base was made by selecting among the products of natural origin available on the market; among these, the only one that met the requirements mentioned in the preamble was the B-Fluid X/A lime. This is a natural hydraulic lime NHL-5, classified according to EN 459-1:2010 [28], produced completely naturally through the firing of calcareous clay [marl] and without any chemical additives; it is composed mainly of calcium silicates, aluminates, and calcium hydroxide. The stone was fired at temperatures between 900 °C and 1100 °C. Subsequently, a slow and careful hydration process of the stone was carried out to obtain a final product that leads to better preservation of the environment.

Table 1. Features of the selected binder.

Binder	Granulometry (mm)	Density (kg/m3)	Compressive Strength σ (N/mm2)	Thermal Conductivity λ (W/m2K)
Natural hydraulic Lime NHL 5	from 0 to 4	1350.00	5.00	0.066

shows the features of the selected commercial product.

The insulating material used in the designed matrix is granulated cork, a sustainable local product of natural origin. Cork granulate was chosen for its excellent thermal properties compared to commercial products, with a thermal conductivity of 0.043 W/m²K and an apparent density of 105 kg/m³, as detailed in the

Table 2. Features of the selected insulating material.

Insulating	Typology	Granulometry (mm)	Density ρ (kg/m3)	Thermal Conductivity λ [W/(mxK)]
Cork	Granules	1.5	150.00	0.043

.

The production process of insulating material is shown in Figure 2.

As seen in detail in Figure 2, the cork oak is born spontaneously in the Mediterranean basin; the cork is obtained from the decortication of the oak trees of Calabria, which can be performed by forestry regulation every 9 years on the same plant after the first harvest, so it must wait up to 25 years from planting.

The industrial process of cork extraction is simple and has a low environmental impact; in fact, it is divided into several stages, as described in Figure 2. Once the bark has reached the right degree of humidity, it is crushed, separated from woody parts or impurities, and then placed in metal blocks, closed, and heated with steam at about 350–380 °C. After about 20 min, the result is a block of cork that has become dark due to temperature in a completely natural agglomeration process that uses the suberine and other waxy substances contained in the bark. These substances dissolve under the effect of heat and allow the granules, which have expanded in the meantime, to settle without any other chemical substance being added [30,31].

The choice of cork was made because this material has low density, low thermal conductivity, and water resistance, which, when mixed with mortar, can effectively improve the thermal insulation and moisture properties of mortar materials.

Moreover, the component related to fine aggregates was instead natural river sand from Lake Massaciuccoli, having a rounded particle size of 1.16 mm in diameter, according to UNI EN 1015-1 [32]. Natural sand (bulk density: 1590 kg/m³) was selected using a sieve with a size of 0.1–2 mm.

2.2. Specimen Preparation

Two lime–cork mortar mixtures were designed using natural hydraulic lime NHL5, water, silica sand from Lake Massaciuccoli, and cork granules with a diameter of 1.5 mm.

Specifically, two different matrices were designed, which involved reducing the percentage of silica sand by 15% and 30%, respectively, and adding cork granules in the same percentages. The dosing and mixing operations were conducted according to UNI EN 1015-1 [32]. The dosage was adjusted by adding cork granules, which reduced the volume of sand. The dosage percentages of the two lime–cork mortar matrices are shown in detail in

Table 3. Composition and dosage of mortar specimens.

Matrix Name	Lime [g]	Water [g]	Cork Ratio [%]	Cork Amount [g]	Sand [g]
Matrix15	450.0	225.00	15	9.7	1340.30
Matrix30	450.0	225.00	30	19.4	1330.60

.

In this case, as shown in Table 3, the dosage involves adding cork granules to the mixture, which reduces the volume of sand used.

The specimen production process is detailed in Figure 3.

The mixing operations of the lime–cork mortar matrices, followed by the preparation of the samples, were carried out at the Tecno-Sud material testing laboratory in Reggio Calabria, using a 20-L mixer equipped with three different speeds [125 rpm (1st cycle), 220 rpm (2nd cycle), 450 rpm (3rd cycle)], which conforms to EN 196:2005 [33].

The mixing and molding phases were carried out according to UNI EN 1015-1 [32]. In detail, two mixing processes were carried out, each with the same duration but with different dosages, which we differentiated into type A matrix for 15% of cork and type B matrix for 30% of cork. Both mixing operations lasted approximately 3 min and 30 s inside a SYK-10 cement mixer. The first step of mixing involved low-speed mixing (1st cycle) with the addition of lime and water for approximately 10 s. Successively, after about 25 s, the cork granules were uniformly added while increasing the mixer speed to the 2nd cycle, and in the last steps, after 40 s from the start of mixing, silica sand was added while maintaining a medium speed. At 1 min 40 s, the mixer was paused for about 0.90 s to allow the scraping of the mortar that had stuck to the walls of the container. After 2 min and 40 s at high speed (3rd cycle), it was then switched to medium speed (1st cycle) for 30 s. Finally, the mixing continued for another 30 s at medium speed to allow for better homogenization of the mixture.

After completing the mixing operations, the matrices produced were poured into the respective metal molds of size 40 × 40 × 160 mm³ for mechanical tests and 300 × 300 × 30 mm³ for thermophysical tests, filling half of their volume for both. Subsequently, the samples were placed on a shaken table for about 2 min to ensure the homogeneity of the matrix inside the mold. After the first sample vibration, the remaining mold volume was filled, followed by another vibration for about 1 min.

Once the sample preparation operations were completed, the samples were left to rest for 24 h, after which they were removed from their metal molds. The samples for testing the compressive strength were put into a constant temperature and humidity chamber with a temperature of 20 °C and a relative humidity of 95% for 28 days. For thermophysical tests, the samples were covered with a curing blanket and naturally cured in an indoor environment at a temperature of 20–25 °C and a relative humidity of 60–80% for 28 days.

2.3. Mechanical Characterization of the Composite Matrix

After 28 days, the samples were subjected to experimental tests as follows:

• Three-point bending tests
• Uniaxial compression tests

In detail, average flexural strength values were obtained on three specimens of mortar, and average compressive strength values were obtained on six specimens of mortar.

2.3.1. Three-Point Bending Tests

The three-point bending tests were conducted on six parallelepiped-shaped specimens with section dimensions of 40 × 40 × 160 mm³ (Figure 4). The first three specimens subjected to the bending test were composed of lime mortar matrix with 15% cork, while the last three were composed of lime mortar matrix with 30% cork.

For the bending test, a Shimadzu AGX 100 kN universal testing machine was used. The load cell used in the bending test had a capacity of 50 kN, and the testing speed was set to 0.3 mm/min. During the bending tests, each specimen was placed with its lower surface resting on two steel cylinders with a diameter of 10 mm, positioned 50 mm from the centerline and 100 mm apart from each other. Additionally, on the upper part of the test specimen, at the center, another cylinder, also with a diameter of 10 mm, was aligned with the vertical load to effectively concentrate and transfer the load onto the specimen.

The bending tests were carried out according to UNI EN 1015-11:2019 [32]. The calculation of bending strength, indicated in the present case as Rt, was computed by Equation (1):

$$R_t={\displaystyle \frac{1.5·F_t·l}{b·h^2}}$$

(1)

where:

-

l is the length of the specimen;

-

b and h are the respective two dimensions of the cross-section in millimeters;

-

F_t is the maximum force (in kN).

The results of the bending tests carried out on the specimens made of the matrix with 15% cork are reported in

Table 4. Results of the bending tests for three points of the matrix with 15% cork matured for 28 days.

Specimens Matrix	Mass [g]	Lo [mm]	l [mm]	b [mm]	h [mm]	Mv [kg/m3]	Ft [N]	Rt [MPa]
M15-C1	512	160	100	40	40	2000	1650	3.87
M15-C2	519	160	100	40	40	2027	1195	2.80
M15-C3	510	160	100	40	40	1992	1450	3.40

.

The results of the bending tests with 15% cork granules showed a flexural strength variable between 3.87 MPa and 2.80 MPa.

The results of the bending tests of the matrix with 30% cork granules are shown in

Table 5. Results of the bending tests for three points of the matrix with 30% cork matured for 28 days.

Specimens Matrix	Mass [g]	Lo [mm]	l [mm]	b [mm]	h [mm]	Mv [kg/m3]	Ft [N]	Rt [MPa]
M30-C1	503	160	100	40	40	1965	1600	3.75
M30-C2	509	160	100	40	40	1988	1000	2.88
M30-C3	515	160	100	40	40	2012	800	2.34

.

The results of the bending tests with 30% cork granules showed a flexural strength between 2.34 MPa and 3.75 MPa.

2.3.2. Compressive Strength Tests

To determine the strength of the mortar under examination, it was also necessary to conduct compression tests after 28 days of curing.

The compressive tests were carried out according to UNI EN 1015-11:2019 [32].

The test concluded with the failure of the specimens.

The set-up for the compression tests is shown in Figure 5.

Force-controlled tests were carried out using a universal hydraulic machine with a 600 kN hydraulic actuator. The results obtained from the mechanical tests are summarized in terms of compressive strength (Fc) in

Table 6. Results of the compressive tests of the matrix with 15% cork after 28 days.

Specimen Code	Mass [g]	Fc [N]	Rc [MPa]
M15-C11	254	3900	2.76
M15-C12	258	4050	2.83
M15-C21	258	3800	2.16
M15-C22	261	3920	2.25
M15-C31	253	3750	2.44
M15-C32	257	3900	3.35

and

Table 7. Results of the compressive tests of the matrix with 30% cork after 28 days.

Specimen Code	Mass [g]	Fc [N]	Rc [MPa]
M30-C11	250	4100	2.56
M30-C12	253	4300	2.69
M30-C21	252	3800	2.38
M30-C22	254	4000	2.50
M30-C31	250	3900	2.44
M30-C32	260	4000	3.19

.

In detail, the mortar with 15% cork granules showed a flexural strength between 2.80 MPa and 3.87 MPa and a compressive strength between 2.16 MPa and 3.35 (N/mm²), with an average value of 2.25 MPa. The mortar with 30% cork granules showed a flexural strength between 2.34 MPa and 3.75 MPa and a compressive strength between 2.38 MPa and 3.19 (N/mm²), with an average value of 2.78 MPa. Consequently, according to EN 998, the plastering mortar can be classified as CSII.

2.4. Thermophysical Analysis

Based on the experimental results obtained from the samples, in terms of flexural and compressive strengths, the 30% cork mortar had higher compressive strength than the 15% cork mortar, as well as higher than expected values. It was decided that thermophysical tests would be carried out only on the 30% cork samples. This decision was also based on studies conducted by other authors who obtained lower strength values than those obtained in the present study.

Therefore, in this section, the results of the thermophysical properties of the designed matrix were summarized on natural lime mortar with 30% cork granules. Tests for measuring thermal conductivity were conducted using thermal flow meters (HFMs) to measure steady heat transfer through flat materials according to the following codes: ASTM Standard C518-17 and EN 12667 [34,35].

For the test measuring thermal conductivity, the FOX 314TM and FOX 50TM apparatuses were utilized, as shown in Figure 6 [36]. A preliminary test run (Test Run 1) was conducted using the Fox 314 instrument for the evaluation of the thermophysical properties of the lime–cork mortar 300 × 300 × 30 mm³ specimen (S1), while the homogeneity of the specimen was estimated on two circular specimens (S2 and S3) extracted from the specimen of Test Run 1 using a drilling procedure (diameter d = 50 mm and depth de = 30 mm).

The first phase of the thermophysical test was conducted on a 300 × 300 × 30 mm³ (S1) specimen using the Fox 314 instrument (see Figure 7a).

The results of this analysis, along with the set temperatures, are presented in

Table 8. Results of Test Run A—specimen S1—matrix with 30% cork.

Measure ID	tlow (°C)	tup (°C)	tavg (°C)	k W/mk
1	0	20	10	0.4088
2	15	35	25	0.4060
3	25	45	35	0.4009
4	35	55	45	0.3958
Average				0.4029
Standard Deviation				0.0057

.

It is worth noting that, although the thermal gradient across the specimens was set to 20 °C in accordance with the requirements of the relevant technical codes [37], each measurement run involved several average temperatures to examine the parameter’s influence on thermal conductivity.

The second phase of the experimental procedures, as mentioned above, aimed to measure the homogeneity of the specimens. Specifically, specimen 2 (S2) was taken from the edge, while specimen 3 (S3) was obtained from the core (Figure 7b). The Fox 50 TM apparatus was used for the measurements.

The results of the second phase of experimental procedures are reported in

Table 9. Results of Test Run B—specimen S2.

Measure ID	tlow (°C)	tup (°C)	tavg (°C)	k W/mk
1	5	25	15	0.3900
2	15	35	25	0.3929
3	25	45	35	0.3919
Average				0.3916
Standard Deviation				0.0015

and

Table 10. Results of Test Run C—specimen S3.

Measure ID	tlow (°C)	tup (°C)	tavg (°C)	k W/mk
1	5	25	15	0.4459
2	15	35	25	0.4455
3	25	45	35	0.4455
Average				0.4456
Standard Deviation				0.0002

.

3. Conclusions

The behavior of masonry infill walls under seismic action is often characterized by many out-of-plane failures, which result in high repair costs and the possible loss of human lives. The use of natural materials such as cork and hydraulic lime NHL 5 for retrofitting for the seismic and energetic retrofit of infill walls offers sustainable solutions, improving seismic performance and thermal and acoustic insulation, ensuring the breathability of structures, and reducing the risk of condensation and mold. These materials are not only environmentally friendly but also help to maintain a healthy microclimate inside buildings.

This preliminary study aimed to investigate the mechanical and thermophysical properties of a matrix of natural hydraulic lime mixed with natural aggregates.

In this preliminary study, two matrices of lime–cork composite mortars (appointed as M15 and M30) were designed by reducing the volume of sand with the addition of cork granules in fractions of 15% and 30%.

The results of the mechanical characterization showed that the compressive strengths of M15 and M30 specimens were very close to each other and that specimen M30 showed a reduction in compressive strength of about 42.2% compared with the average strength of the product data sheet (of 5 MPa); consequently, according to EN 998, the plastering mortar can be classified as CSII.

The results of the thermophysical characterization showed that the insulation features of the specimen can be improved compared to the main products on the market. In detail, the study of thermophysical aspects has shown that the thermal conductivity of natural lime–cork mortar decreases when increasing the percentage of cork volume in the mix, with a value of 0.39 W/mK for mortar containing 30% cork granules.

Based on these findings, the new composite material can be considered a promising candidate for further research and future applications in engineering practice. More comprehensive experimental studies and numerical simulations are necessary, employing an integrated thermo–mechanical approach, to validate these data on different masonry substrates.

The results indicate that the new composite material ensures excellent ductility and can be viewed as a promising alternative to traditional cement plaster systems.

A more in-depth experimental investigation is underway, which also includes the insertion of gorse fiber and tests adhesion, water vapor permeability, and capillary absorption.