1 Introduction

Industrial masonry chimneys saw widespread adoption in the United Kingdom during the mid-19th century and subsequently spread globally to Europe, America, Asia, and Oceania. Characterized by a typically conical shaft, they served to eliminate combustion smoke from industrial processes, extending in height to obtain sufficient draught-power for efficient combustion and to drive the upward movement of smoke. This design helped reduce the likelihood of smoke being spread or recirculated back into the building or nearby structures, and it minimized the risk of fires caused by sparks or embers. However, in the 20th century, this type of chimney experienced a decline as new construction materials and systems, for instance forced extraction, gained prominence. At the same time, as urban areas expanded, these chimneys became integrated into cityscapes, gaining significance as symbolic historical remnants of local activities and, in many cases, achieving architectural heritage protected status. This new context presents specific challenges, particularly in terms of safety considerations. Ensuring structural integrity becomes mandatory, namely in regions prone to seismic activity, to safeguard nearby urban populations, goods and constructions, as well as to protect the historical object itself [1,2,3,4,5,6].

Here, we present the case study of the industrial brick masonry chimney of former cork processing factory Mundet in Montijo, Portugal. A case study is a detailed examination of a specific real-life scenario, aimed at providing a holistic understanding of the subject matter within its context. It retains the complexities and specificities of the individual case, offering insights that may not be captured through broader studies that generalize findings across multiple cases. Case studies contribute to the advancement of knowledge in the field by delving into the practical applications and challenges encountered in real-life conditions. They also provide valuable data that becomes accessible to other researchers, facilitating collaboration and preventing the loss of valuable information collected through practical experience, often on a one-off basis. Additionally, case studies enable the establishment of hypotheses and serve as educational tools, offering valuable insights into the complexities of real-world scenarios. The present case study, of the Montijo chimney, explores the pathology and deterioration processes of this structure and the corresponding possible mitigation strategies. It fills a gap in the literature by offering a detailed examination that contributes to understanding of a typology that is understudied, in particular as regards the Portuguese territory.

The height of industrial masonry chimneys typically falls within the range of 20 to 50 m [1, 3] although some extend up to 100 m [1]. In Spain most of these chimneys are between 20 and 30 m [6]. In Portugal, the situation is similar, with the ones surpassing 30 m generally associated with important industrial companies [4]. The latter is the case with the Montijo chimney focused on this article, which measures 45 m in height (Fig. 1).

Fig. 1
figure 1

Overall view of the industrial masonry chimney of former cork processing factory Mundet, in Montijo

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Typical types of deterioration observed in industrial masonry chimneys include structural damage, as well as material damage and efflorescence [1, 2, 4, 6]:

  • The most prevalent form of structural damage is displacement, with the shaft going out of plumb. This can arise from various factors, including faulty construction methods, foundation problems, differences in mortar drying on different sides of the chimney due to prevailing winds, the wedge effect of steel elements, and differential expansion in lime mortar due to condensation. Another form of structural damage is longitudinal cracking in a vertical or zig-zag pattern along the shaft, induced by thermal gradients. Seismic damage may also pose a critical threat to these slender structures and is typically concentrated near the base and in zones affected by the second and third vibration modes—the upper third of the chimney. Lightning strikes affect mostly the upper part of the chimney, particularly the crown, causing the loss of bricks in irregular patterns. Severity of structural damage increases where material damage has occurred, particularly where the mortar between bricks has already deteriorated.

  • Material damage and efflorescence are typically caused by chemical reactions due to gases, smoke, moisture or high-temperature during the activity of the industry (affecting the inner surfaces), or to atmospheric contamination and sea-water exposure (affecting the external surfaces). Efflorescence accompanied by spalling or flaking of the bricks appears mainly on the chimney base, into which moisture seeps from the ground by means of capillary action, as noted by López-Patiño et al. after inspecting over 500 chimneys across Spain [6]. Mortar loss may be particularly severe in the upper parts of the chimney, where loose bricks may occur because of the harsher exposure to wind and humidity.

Chimneys that are already cracked or are at risk of developing this kind of damage may be reinforced, for example with fibre reinforced polymers (FRP) [1, 20].

López-Patiño et al. [6] mention specifically the damage caused by living organisms, which can be of the mentioned types (structural damage, material damage and efflorescence). According to these authors, birds cause the most damage to chimneys: (i) droppings contain nitric acid and phosphoric acid, which react with bricks and mortar with deleterious effects; (ii) large birds such as storks can cause structural damage when they build their large nests on the crowns. But damage by vegetables can be relevant too. Wind-blown seeds give rise to plants whose roots can penetrate the mortar and get between the bricks, causing the first to disintegrate and the second to break up. Finally, human action is also not a negligible origin of damage. Some of the damage due to humans may have occurred when the chimney was in use as part of the factory. It was, for example, common practice to support roofs and ceilings against the chimney and attach electric cables and pulleys to it. These often remained after the factory was demolished, or were removed, leaving behind cavities, remnants of mortar, or damaged materials due to the strain of supporting heavy components. Inadequate restoration work or repairs are also a common source of damage.

The case study of the industrial masonry chimney of former cork processing factory Mundet, presented in this article, was aimed at characterizing the pathology and conservation status of this chimney and providing recommendations for its preservation. The initial phase of the study comprised a structural survey conducted in July 2016 using height access equipment [7]. Subsequently, a second phase was undertaken in the wet season in March 2017, during which samples were collected at various depths and heights on three sides of the chimney base [8] These samples allowed obtaining profiles of current and hygroscopic moisture distribution and mineralogical characterization, employing X-ray diffraction, of the ceramic brick and mortar in zones representing the main degradation patterns.

This article begins with an overview of the chimney, encompassing its history, location, geometry, and materials (Sect. 2). It then proceeds to discuss the base (Sect. 3), describing the observed pathology (Sect. 3.1), detailing the experimental diagnostic methods used and their respective outcomes (Sect. 3.2 and 3.3), and concluding with a discussion of the degradation causes (Sect. 3.4). Subsequently, the focus shifts to the shaft and crown (Sect. 4), outlining the surveying methodology (Sect. 4.1), and presenting the corresponding results (Sect. 4.2 and 4.3). Finally, the article summarizes the conservation state of the chimney (Sect. 5.1) and presents key measures for damage mitigation (Sect. 5.2).

2 Description of the chimney

The chimney of the former Mundet cork processing factory is situated on the southern bank of the Tagus River, opposite Lisbon. Adjacent to a modern supermarket, it lies less than 500 m from the waters of the Tagus estuary (Fig. 2). Operating between 1905 and 1988, Mundet stood as one of Portugal’s largest companies in the cork sector, eventually becoming the world’s largest cork exporter. Mundet was also known for some innovative social policies. In the 1930s, it built a nursery and a kindergarten, as well as cafeterias that served the factory workers and needy people in the area. At its peak, the company employed over 2500 individuals, but it eventually struggled to adapt to the proliferation of new materials, such as plastic, which gained prominence in the early 20th century. The chimney under analysis is one of the remnants of the local cork processing activity, and the Municipality of Montijo strives to preserve it for future memory.

Fig. 2
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Location of the chimney (image obtained from Bing Maps, 20160919)

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The chimney, built in 1951, is 45 m high and, as seen in the drawn piece of the original design drawing (Fig. 3), is divided into four main parts: foundation, base, shaft and crown. The foundation, with a thickness of 0.80 m, is constructed of reinforced concrete atop a layer of screed, according to contemporary records detailing the project’s inception [9] The base features a square section measuring 4.3 m on each side and stands 5.8 m tall. The shaft rises to a height of 39.2 m and boasts a hollow circular section with an inner mouth diameter of 1.75 m and a flare of 5% (0.05 m per meter of height). Finally, the crown constitutes the topmost section of the shaft and has a height of 0.95 m.

The thickness of the masonry wall decreases along the height of the chimney. Due to the unavailability of scaffolding, chimneys had to be constructed from the inside in rings. These rings had constant height and gradually narrowing width [1] At the base, the masonry thickness of the Mundet chimney ranges from 1.01 m in the lower zone, mostly below ground level, to 0.90 m in the upper zone. In the shaft, it varies from 0.84 m near the base to 0.24 m near the crown. This gradual variation in masonry wall thickness in the shaft occurs in a regular manner, with the thickness decreasing by 0.12 m between sections of constant thickness, which are generally 6.50 m long.

The brick laying mortar is a light-coloured air lime mortar. The same type of the mortar is observed in masonry joints, where white lime granules are often visible (Fig. 4). In numerous areas, the lime mortar, likely from the original construction, is disappearing or has already disappeared entirely due to erosion, leaving the joints unprotected. Repairs made with cement mortar are also evident, particularly at the base of the chimney.

Fig. 3
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A piece of the chimney’s project [9]

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Fig. 4
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Masonry joints filled with air lime mortar at the shaft (left) and air lime joint mortar with white lime granules at the base (right)

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3 Base of the chimney

3.1 Survey of the conservation state

The base of the chimney is made of solid red bricks of prismatic shape that form a mixed bond with one course of headers for every three courses of stretchers. (Fig. 5). The bricks are about 23 cm x 11 cm x 7 cm. The name and location of the manufacturer, “S. J. Oliveira – Montijo”, are engraved on each brick, acknowledging the Montijo ceramics factory José Salgado de Oliveira (Fig. 8, bottom).

Fig. 5
figure 5

General view of the four faces of the base of the chimney

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As seen in Figs. 5 and 6, the masonry at the base of the chimney is very degraded. Decay is most pronounced on the outer faces facing north and east, where the surface layer of bricks has often completely disappeared. The main types of anomalies observed in the decayed areas are the following:

  • Efflorescence (Fig. 6, top left);

  • Flaking of the ceramic brick (Fig. 6, top right);

  • Arenization of the joint mortar (Fig. 6, bottom);

  • Erosion (Fig. 8);

  • Cracks (Fig. 9);

  • Collapse of the masonry on edges and vertices of the chimney base (Fig. 10);

As shown in Fig. 6, the two masonry materials undergo disaggregation through processes of flaking (the brick) or arenization (the mortar). Flaking refers to the detachment of thin scales with submillimetric to millimetric thickness [10] whereas arenization involves a loss of cohesion, causing the material to revert to a sand-like granular state.

Fig. 6
figure 6

Fundamental decay features of the external surface of the masonry at the base of the chimney: efflorescence (top left); flaking of the brick (top right); arenization of the mortar (bottom)

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The interior surfaces of the chimney also exhibit decay, characterized by erosion of the brick surfaces, areas where the laying mortar has receded to significant depths, occasional missing bricks, and white efflorescence. However, no large cracks or collapse of the masonry are observed (Fig. 7).

Fig. 7
figure 7

Inner face of the base of the chimney

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The process of masonry erosion is depicted in Fig. 8, illustrating its initial and advanced phases. In the initial phase, efflorescence, flaking of the brick, and arenization of the joint mortar are evident. In the advanced stage, numerous bricks have nearly disappeared, and the collapse of the masonry at the edges is visibly underway as a result of joint mortar erosion.

Fig. 8
figure 8

Masonry erosion process: initial phase (top left) and advanced phase (top right); exposed bricks with the manufacturer’s marking (bottom)

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Visible cracks mar the outer faces of the chimney base. Notably, two large vertical slits are prominent, roughly situated in the middle of the south and east faces, as depicted in Fig. 9. The slit on the south face (left image) intersects the arch of the entrance to the chimney’s interior. However, these two large cracks are not visible on the inner surface of the chimney, which means they do not span the entire thickness of the masonry.

Fig. 9
figure 9

Cracks on the south (left) and east (right) faces of the base of the chimney

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Smaller or less developed cracks are also visible on the other outer faces, accompanying the disintegration and erosion of mortars and bricks, as well as the collapse of complete sections of masonry. These cracks and collapsed areas could be due to ruptures along the joints of masonry sections subjected to certain stress conditions. The collapse occurs in structurally less important areas, such as corners (Fig. 10), and areas that are not directly located under the shaft and are therefore less compressed. The reduced compression leads to greater ease of disaggregation of the masonry, explaining why these areas are the first to collapse.

Fig. 10
figure 10

Collapsed masonry at the corner between the South and East faces of the base

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Finally, areas with a greyish-coloured mortar, probably cement mortar, are seen on the top and on the faces of the base of the chimney. On the top, the mortar forms a capping which is quite degraded, with biological growth and cracks on both the surface and the interfaces with the masonry (Fig. 11). On the south face, remnants of an old cement plaster are visible, likely from a pre-existing adjacent construction (Fig. 10). Note that in the same area, the cornice of the chimney base was destroyed, probably to create a flat surface serving as a wall for that pre-existing adjacent construction.

Fig. 11
figure 11

Capping of the base of the chimney with biological growth

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3.2 Actual and hygroscopic moisture content

The actual moisture content (MC) corresponds to the amount of water present in the sample. The hygroscopic moisture content (HMC) is the moisture content that the sample acquires by capturing moisture from the air until it reaches a state of equilibrium. The MC and HMC were measured in the laboratory using samples taken at various heights and depths of the base of the chimney. The values were then used to draw distribution profiles that reveal how the two types of moisture content vary in the building element, in height and in depth.

The MC profiles indicate the level of dampness in the masonry and provide clues about the origin of the moisture. For example, in the case of rising damp, which is the most frequent source of moisture in old constructions, the moisture content typically decreases in height but almost does not change along the thickness of the wall. Rising damp corresponds to a (tendentially) steady-state moisture flow condition, with intake of moisture through the base and evaporation through the surface of the wall.

The HMC profiles allow accessing the presence and distribution of soluble salts in the masonry because the hygroscopicity of building materials is negligible compared to that of soluble salts [11, 12]. The presence and distribution of salts can then be confirmed by chemical analysis focusing on selected points. In addition, the HMC profiles may also provide information about moisture flow patterns [13].

To obtain MC and HMC distribution profiles, 105 samples were collected by drilling at various heights and depths, at the base of the chimney. The collection was scheduled for after the rainy season and took place on March 8, 2017. Two complete profiles were obtained from the East-facing and West-facing walls, which exhibited the highest and lowest material damage intensity, respectively. Additionally, since there was still some time left for the sampling operation, an extra drill could be performed. It was thus decided to investigate the horizontal distribution of salts at the base of the North-facing wall, which ranked second in terms of material damage.

In each drill, samples were collected at depth indents of 2 cm close to the surface and of 5 cm deeper inside the walls, up to a final depth of 35 cm. Vertically, a spacing of circa 50 cm between drills was adopted, except for the uppermost drill, which was located at the maximum possible height below the cornice. This kind of distribution of sampling points has been previously used successfully [13] and represents a good compromise between, on one hand, obtaining as many samples as possible to characterize the situation adequately, and on the other hand, avoiding excessive destruction and completing the work within the available time for the operation. In this case, all the work was carried out in a single day, which included: travel from LNEC in Lisbon to Montijo and back; organization and arrangement of material and equipment at departure and upon return to LNEC, as well as at the arrival and departure from the construction site; assembly and disassembly of scaffolding; observations; sampling, storage, and individual labeling of samples; weighing of samples upon arrival at LNEC.

The sampling procedure was the following (Fig. 12):

  • the element was drilled at low speed, using a drill with a diameter of 16 mm;

  • at the same time, the material was collected directly into a polyethylene bag placed below the hole;

  • immediately after the collection, the sample was carefully wrapped in the polyethylene bag;

  • this first bag was wrapped in a second polyethylene bag, which was sealed with adhesive plastic tape to prevent moisture loss, and then labelled.

Fig. 12
figure 12

Sampling by powder drilling

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All the samples were collected from the outer faces of the walls for reasons of operability and safety of the technicians. Access to the highest levels was made possible using mobile scaffolding (Fig. 12).

The use of powder drilling as a sampling method is a well-established practice, with documented usage dating back many years, following guidelines such as BRE’s Digest 245 [14]. This technique of sampling by perforation for subsequent determination of moisture contents has also been vouched for by authors such as Hall and Hoff [15]. It was also previously and thoroughly tested by one of the authors and it was concluded that the loss of moisture during drilling is negligible [16].

The technique also allows for qualitative characterization of the distribution of salt along and across a construction element by measuring the hygroscopic moisture content (HMC) of a large number of samples. This is known as the HMC method, a front-line approach that is based on the fact that the HMC of soluble salts is significantly higher than that of typical building materials. Although the method doesn’t yield quantitative data due to variations in HMC among different salt species and cannot guarantee the complete absence of contamination during drilling, it provides an overview of the element’s general condition and helps to identify the most promising points for further detailed chemical analyses [11,12,13,14].

The MC and HMC of the samples were measured in the laboratory.

The actual moisture content (MC) corresponds to the moisture present in the masonry at the time of sampling and was measured in the laboratory, by difference in masses, according to the following procedure:

  • the samples were placed in glass petri dishes and their wet mass (Masswet) was determined by weighing;

  • the samples were then dried in a ventilated oven at 60ºC for about 24 h, a period of time that experience shows is sufficient to achieve constant mass; after cooling, the dry mass of the samples (Massdry) was determined by weighing;

  • the MC was then calculated by the following expression:

$${{Mas{s_{{\rm{wet}}}}{\rm{ - }}Mas{s_{{\rm{dry}}}}} \over {Mas{s_{{\rm{dry}}}}}}$$
(1)

The possible loss of moisture of the samples, from the moment they were collected until they were tested, was controlled by weighing. The polyethylene bags containing the samples were weighed immediately after arrival at the laboratory. These bags were weighed again two days later, before drying the samples in the oven, and it was confirmed that the loss of moisture between the two weighings was negligible. This indicated the tightness of the plastic wrappers was sufficient.

The hygroscopic moisture content (HMC) is the moisture content the samples have when they are in hygroscopic equilibrium under certain environmental conditions of temperature and relative humidity (RH). The HMC of the samples taken from the chimney was determined according to the following procedure:

  • after determination of the current moisture content according to the procedure already described, the dried samples were stored in a climatic chamber (FITOCLIMA 500 EDTU© from Aralab) at 20ºC and approximately 95% RH;

  • the samples remained in this environment, absorbing moisture from the air, and were weighed weekly until their mass (Masswet hygroscopic) stabilized;

  • the HMC was then calculated according to the following expression:

$${{Mas{s_{{\rm{wet hygroscopic}}}}{\rm{ - }}Mas{s_{{\rm{dry}}}}} \over {Mas{s_{{\rm{dry}}}}}}$$
(2)

The resulting in-height and in-depth MC and HMC profiles [8] are presented in Figs. 13, 14 and 15.

Fig. 13
figure 13

East face: (a) location of the sampling; (b) vertical MC profiles; (c) horizontal MC profiles; (d) vertical HMC profiles; (e) horizontal HMC profiles

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Fig. 14
figure 14

West face: (a) location of the sampling; (b) vertical MC profiles; (c) horizontal MC profiles; (d) vertical HMC profiles; (e) horizontal HMC profiles

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Fig. 15
figure 15

North face: (a) location of the sampling; (b) MC and HMC profiles

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3.3 Mineralogical analysis

Mineralogical analysis by X-ray diffraction (XRD) of two samples was performed in order to identify degradation agents, in particular, soluble salts. XRD is a technique that provides information about the minerals present in a sample and is based on the property of crystalline materials to diffract X-rays in a specific way. In addition, the height of the diffraction peaks allows for a semiquantitative assessment of the presence of these minerals in a sample [8].

The two samples were collected to represent the primary degradation patterns of the two masonry materials, respectively: (1) sanding of the mortar; (2) flaking associated with the occurrence of efflorescence in the ceramic brick. The sampling sites are shown in Figs. 13a and 14a, which show that the samples were collected at a height close to that of the highest HMC.

Sample 1 consisted of disaggregated material, mainly mortar, and was collected by brushing. Two diffractometric records were obtained from this sample: one from the collected material as it was, and another mainly from the binder after removing the sand by sieving. Sieving, which removes as much sand as possible from the mortar, enhances the detection of soluble salts, as they typically migrate extensively through the capillary pores of the binder paste. This step is crucial because XRD only permits the identification of substances present in a reasonable percentage (minimum amount of about 2–4%, by weight), whereas the percentage of soluble salts capable of damaging masonry is often lower. Sample 2 consisted of brick fragments exhibiting white efflorescence, collected using a hammer. In this instance, XRD analysis was performed on the efflorescence scraped from the brick surface [8].

Table 1 depicts the results of these XRD analyses [8]. Soluble salts with the greatest potential to cause degradation are indicated in bold and highlighted in grey.

Table 1 Results of mineralogical analysis by X-ray diffraction (XRD)
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3.4 Discussion on the causes of degradation

The current moisture content (MC) in the East and West-facing walls is elevated near the ground, diminishes with height, and remains relatively constant across the wall (Figs. 13, 14 and 15). These patterns suggest that the primary source of moisture within the masonry at the time of sampling was rising damp from the soil. Indeed, it is plausible that the groundwater level beneath the chimney is elevated due to the proximity of the Tagus River (Fig. 2). Observations also indicate stormwater runoff from the nearby supermarket toward the chimney, which could potentially exacerbate the situation.

The hygroscopic moisture content (HMC) reaches very high values in these walls (Figs. 13, 14 and 15). That is consistent with the visual evidence suggesting that masonry erosion is caused by the crystallization of soluble salts. The highest HMC values are reached at mid-height and near the surface, especially on the East facing wall, which is typical of rising damp and salt crystallization decay, as explained below [13, 14].

Soluble salts induce the decay of porous building materials, such as mortars and brick, when they crystallize as efflorescence or subflorescence, which typically occurs during evaporative processes. Efflorescence arises when the salt crystallizes on the material surface. This happens when the moisture content of the material is high enough to maintain a saturated condition at the surface while evaporation proceeds. Efflorescence itself does not cause material breakdown but it does impact the element’s aesthetics, attracts hygroscopic moisture, and can further dissolve and recrystallize as subflorescence. Subflorescence, on the other hand, occurs when the salt crystallizes in the microscopic pores of the material, introducing internal stresses that can lead to rupture. This type of salt deposit forms when the moisture content inside the material is insufficient to generate a liquid flow capable of compensating for the evaporative demand, causing the liquid front to reach an equilibrium position below the material surface. Figure 16 illustrates these two situations.

Fig. 16
figure 16

Schematic representation of the mechanism of soluble salts crystallization

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The degradation patterns observed at the base of the chimney are characteristic of salt crystallization:

  • Sanding of the air lime mortars happens because crystallization introduces internal stresses that break down the lime matrix, reducing the mortar to a sand-like granular material.

  • Flaking of the bricks occurs because subflorescence causes successive detachment of the material surface layers.

  • Brick erosion tends to occur in the upper areas of the walls whereas near the ground a whitish veil of efflorescence occurs associated to the highest moisture contents (Fig. 5).

  • The differential erosion of the bricks (Figs. 5, 6, 7 and 8) may be attributed to two classes of factors that are currently difficult to fully evaluate: (i) variations in physical characteristics of the bricks, such as porosity and mechanical strength, stemming from fluctuations in the quality of the raw material and temperature variations in old kilns; (ii) hydric discontinuities within the masonry.

The shape of the vertical HMC profiles, like that of the MC profiles, is also typical of rising damp. In the case of rising damp, the salt content is higher in the surface layers, reaches a maximum at a certain distance from the ground and then decreases with height, as occurs in Figs. 13 and 14. This happens because in a wall with rising damp, water rises up to the level where the loss of water through the wall surface (evaporation flow) balances the inflow of water through the base of the wall (liquid flow). The moisture content of the wall decreases with height, leading to the crystallization of salts as efflorescence near the base of the wall and as subflorescence higher up. The amount of subflorescence decreases with height due to the decrease in liquid flow. The whole process is as follows:

  • Closer to the ground, the high liquid flow from inside the wall causes salt to crystallize on the surface as efflorescence, which is then regularly removed by the action of gravity, wind, and rain.

  • The liquid flow diminishes with height, and beyond a certain distance from the ground, salt begins to crystallize inside the material pores as subflorescence. This salt accumulates in the material until internal stresses lead to rupture, which is followed by erosion due to wind and rain.

  • At the higher levels, the liquid flow is low, so the amount of accumulated salt is also reduced.

Another important clue pointing to rising damp is provided by the horizontal MC profiles (Figs. 13c, 14c and 15b). These profiles demonstrate that the MC:

  1. (i)

    is high near the ground;

  2. (ii)

    remains relatively constant or slightly increases towards the interior of the wall;

  3. (iii)

    is clearly higher than the HMC, which indicates that the actual moisture cannot solely be attributed to moisture absorption from the air.

Of course, some water ingress is also expected through the surface of the masonry due to the porous nature and deteriorated state of both the brick and mortar. In addition, on the south face, the cornice was removed and therefore the water that falls on the shaft and on the top of the base of the chimney can more easily flow down the wall surface. Furthermore, the cementitious capping of the chimney base is decayed (Fig. 11). The slight increase in the MC and HMC at the top of the walls, towards the top of the chimney, as revealed by the vertical distribution profiles (Figs. 13 and 14), suggests that there is indeed some ingress of water from above.

It is also notable that the HMC is significantly high in depth in the East-facing wall, which exhibits more degradation, whereas it is low in depth in the West-facing wall. The predominance of surface salts in the latter wall is consistent with its considerably lower material degradation.

The intensity of degradation at the base of the chimney varies from face to face, possibly due to differential water ingress or inconsistencies in the masonry’s hydric continuity. Variations in evaporation conditions across different faces may also contribute to these discrepancies. The evaporation flow depends on environmental factors such as temperature, relative humidity, and air velocity. Indeed, the height of capillary rise will be greater, for example, in walls less exposed to sunlight or shielded from the wind. However, predicting how these factors interact and affect a real case like that of the chimney is challenging. This is particularly true considering the possibility of differences in water intake through the base and the influence of local conditions, such as the nearby supermarket. But other things being equal, the data suggests a greater influence from the wind than from sunlight exposure. In fact, despite facing more intense sun exposure, the West face shows the least degradation, possibly due to the protective influence of the nearby supermarket pavilions against wind. This protective effect of the supermarket pavilions in terms of wind may also explain why the most degraded areas of the chimney base face Northeast (Fig. 17), despite the prevailing winds in Montijo coming from the Northwest.

Fig. 17
figure 17

Identification (in red) of the most degraded areas of the base of the chimney

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Regarding the nature of the salts responsible for the degradation, XRD results (Table 1) reveal that these are sodium chloride (NaCl) in the mortar and sodium sulfate (Na2SO4) in the brick. It is likely that sodium chloride comes from contamination with seawater from groundwater or from the soil itself because in this zone, of estuary, there is a mixing of river water with sea water. Sea salts may also be deposited through fog or wind. Sodium sulfate, on the other hand, has probably its origin in the ceramic brick itself. This salt is commonly found in efflorescence in ceramic materials and is typically produced during calcination by the chemical action of flue gases. However, the hypothesis that a part of the salt came from the ashes deposited in the masonry over time, when the chimney was in operation cannot be ruled out. The verification of this hypothesis implies performing additional chemical analyses.

The occurrence of distinct salts in brick and mortar can be explained not only by sodium sulfate originating from the brick itself, but also by the differential migration of moisture in the two materials. The mortar, being more absorbent and forming a continuous network in the masonry, serves as the primary pathway for moisture. These characteristics allow mortars to act as sacrificial materials, which is crucial for the durability of this type of construction as they minimize the degradation of the masonry elements (ceramic bricks), which are considerably more challenging to repair.

4 Shaft and crown of the chimney

4.1 Inspection methodology

Inspection of the chimney’s outer surface was carried out through comprehensive visual observation and a photographic survey. The base was examined from ground level. The shaft and crown were observed using a self-crane equipped with a basket from which the authors conducted their observations and photographed the chimney (an appropriate drone was at the time not available). To ensure safety and prevent any damage to the chimney caused by the basket’s wind-induced oscillations, the upper sections were observed from a minimum distance of approximately 1 m. The interior of the chimney was visually inspected from the region adjacent to the opening on the south face of the base.

4.2 Chimney shaft

The shaft consists of bricks with dimensions similar to those of the base of the chimney but of toroidal shape and having the same manufacturer mark J. S. Oliveira (Fig. 18). The bricks are arranged in a stretcher bond and its coloration is heterogeneous (Fig. 19).

Fig. 18
figure 18

Toroidal shaped brick fallen from the shaft

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No vertical slits or significant deviations from verticality were observed along the shaft. However, the recession of the joint mortar becomes more pronounced with increasing height. This heightened recession is likely due to increased exposure to rain and wind, leading to greater erosion of the mortar at higher levels (Fig. 19).

Fig. 19
figure 19

Different level of erosion of the mortar of the joints in the lower zone of the shaft, near the base of the chimney (below) and in the upper zone of the shaft, next to the crown (above)

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Regarding the interior of the shaft, as far as it was possible to observe, here too there is a significant surface degradation of the masonry (Fig. 20), similar to that observed inside the base of the chimney (Fig. 7).

Fig. 20
figure 20

Inside the chimney shaft

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An opening of rectangular shape which does not exist in the original blueprints, and appears to have been added later without much care, is also observed in the lower part of the shaft (Fig. 21). The opening arched the path of vertical loads in the opening zone, giving rise to cracks, with detachment of the bricks below the arch. Some careless repairs in this area, including filling of cracks and joints and the reconstitution of eroded brick zones, were carried out with cement mortar.

Fig. 21
figure 21

Opening in the lower area of the chimney shaft

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4.3 Chimney crown

The chimney crown, located at the top of the shaft, 45 m from ground level, is constructed using toroidal-shaped bricks similar to those used in the shaft, arranged in a stretcher bond. Additionally, in the thickest area, it incorporates prismatic bricks like those used in the base of the chimney, arranged in a header bond (Fig. 22).

The main decay pattern observed in the crown is the structural collapse of the masonry itself, as a result of the laying and joint mortar disappearing, causing the bricks to collapse while remaining largely intact. The entire structure is in an advanced state of degradation, with approximately half of the perimeter having already collapsed and the remaining portion at imminent risk of collapse (Fig. 22). In the area still standing, which faces NE, the laying and joint mortar are nearly non-existent, leading many of the bricks to be simply stacked on top of each other. Additionally, vertical cracks intersecting the total thickness of the masonry are evident, indicative of the collapse mechanisms.

Fig. 22
figure 22

Partially collapsed crown of the chimney

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The high level of degradation of the crown is consistent with it being the zone of the chimney most exposed to atmospheric agents. Furthermore, the zone that has already collapsed corresponds to a higher incidence of these agents, particularly wind, according to its dominant direction. The incidence of atmospheric rays could also be adding to the damage (an old lightning rod is still present but appears to be damaged). Additionally, as the masonry in the crown is less compressed, it disaggregates more easily than other more confined areas.

In the portion that has not yet collapsed, remnants of an outer mortar coating can be observed (Fig. 23). This coating was likely applied near the mouth of the chimney to facilitate the runoff of rainwater, thereby minimizing the risk of infiltration. However, it is unclear at which point during the construction’s lifespan this was applied. The mortar used for this purpose has a greyish colour and appears to possess greater mechanical strength than the lime mortar observed in the joints. Therefore, it is plausible that it is a cement or hydraulic lime mortar applied at a later stage to the original construction.

Fig. 23
figure 23

Surface coating on the upper area of the chimney crown

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5 Damage mitigation measures

5.1 Chimney base

In the base of the chimney, the crystallization of salts is a key degradation process. Measures to mitigate this type of damage usually involve controlling one or both of its predisposing factors: soluble salts and moisture [16]. In the present case, since it is not feasible to eliminate the salts, especially sodium sulfate that probably comes from the brick itself, it is essential to control the presence of water.

First, it will be necessary to: (i) understand the possible contribution of phreatic water to the rising damp in the chimney; (ii) investigate the possible contribution of the adjacent supermarket to the soil moisture content in the chimney area, considering the possibility of poor storm water drainage, as well as the possibility that the construction or presence of these buildings has induced changes in the water table or the local evaporation conditions.

There are several techniques that could be used to limit rising damp, such as the following, all of which however have advantages and disadvantages [17, 18]:

  • It is possible to inject products (water repellents or sealants) into the masonry, close to the ground, to create a barrier that prevents capillary rise. However, it is important that the barrier be effective in all its extension, which sometimes does not happen, particularly in thick and heterogeneous masonries. In addition, if there are leaks, the consequent reduction (without elimination) of the liquid flow that penetrates the masonry can lead to a retreat of the evaporation front, leading to the occurrence of destructive subflorescence where before only efflorescence appeared [16].

  • Another method to prevent capillary rise of water involves introducing watertight barriers such as metal sheets at the base of walls. However, this technique may compromise the structural resistance of chimneys concerning horizontal actions such as wind and seismic forces. The vulnerability to seismic activity is particularly pertinent in this case, given that the Montijo chimney is situated within the zone of higher seismic risk in Portugal. Furthermore, these types of structures were not initially designed to withstand seismic action, and their base is susceptible to failure due to the fundamental mode of vibration [19,20,21]. Therefore, introducing vulnerability at the base of the chimney is not recommended.

  • There are also solutions that involve ventilating the lower levels of walls in order to increase the evaporation flow and thus limit the amount of moisture that progresses to the upper levels [17]. The chances of success of this type of solution needs to be well evaluated, preferably through preliminary site tests and monitoring.

  • Drainage of waters may also be considered, namely through the construction of wells [22]. But these techniques also require careful planning and monitoring, as there is a risk of inducing differential settlements in the constructions located throughout the whole affected area. In the case of surface water, drainage is simpler and can be carried out, for example, through peripheral ditches.

Whichever method is implemented to reduce the water content of the masonry at the base of the chimney, it must be accompanied by the application of a sacrificial layer on the chimney surface. This layer is intended to prevent that the progressive reduction of the liquid flow during drying give rise to destructive subflorescence in the masonry. Cellulose or clay compresses may be used for this purpose. These should be tested beforehand in the chimney, to evaluate their effectiveness and ease of removal.

In addition to those methods meant to control rising damp, it is recommended that measures be taken to minimize the ingress of rainwater through the surface of the masonry, which implies the reconstruction of the cornice of the south facing wall, or through the top, namely to repair the capping of the upper surface of the base.

5.2 Shaft and crown

Consolidation of the masonry in the shaft and crown should be carried out to limit the infiltration of rainwater and the consequent progression of decay. This should be carried out with a mortar that is compatible with the materials of the masonry [23, 24]. It should include appropriate closing of the joints between bricks and filling of the cracks.

In the crown, this may involve dismantling the masonry and rebuilding it using the technique of anastylosis, which is used for reassembling existing but dismembered parts of a historical structure [25]. This technique requires thorough identification and cleaning of the bricks beforehand, and operations must be conducted with care to avoid damaging them, preferably under supervision of a specialized professional. Under no circumstances should the bricks be cut.

The filling of the joints and repair of the capping of the crown should contribute to the reduction of the penetration of rainwater into the masonry. If metal or plastic sheets are used to protect the top of the masonry, these should preferably be separated from the masonry by an air space. To avoid damage to the masonry, the sheets can be fastened to elements specifically embedded in the masonry for this purpose.

The damage detected in the crown of the chimney may also be in part due to the incidence of atmospheric rays, so a new lightning rod should be installed.

5.3 General care

The first general care measure is to restrict people’s access to the area surrounding the chimney, minimizing the risk of falling brick blocks from the crown until adequate consolidation works are completed.

Subsequently, in addition to controlling the presence of water in the masonry, restoration of the damaged areas of the base and crown of the chimney can be considered. This should be carried out by technicians with training and experience in conservation and restoration of historical monuments. Conservation and restoration are delicate operations that, according to the Venice Charter [25], must preserve both the work of art and the historical testimony that it conveys. Therefore, the works should be planned and carried out with regard for the ancient materials and their aesthetic and historical values. That means restoration should not be a reconstitution that totally eliminates the marks of time, which would lead to loss of historical character.

It is crucial to avoid applying water-repellent products to the masonry surface. Despite their intended use of preventing efflorescence by creating a surface layer of water-repellent material, these products unintentionally trap saline solutions within the masonry. As the water in these solutions evaporates, salts crystallize behind the water-repellent layer, resulting in more damaging subflorescence rather than efflorescence, ultimately worsening material degradation [26, 27]. Instead, methods that reduce the water content of the masonry, as discussed in the previous section, should be utilized.

It is also necessary to use compatible materials and methods as a deficient intervention may potentiate an increase in degradation [27]. Ordinary cement mortars, for example, should not be used as they generally have mechanical properties too strong for this type of brick masonry, and may also be an additional source of salts. Cements with a high alkali content, such as Portland cement, give rise to destructive carbonate and bicarbonate salts.

6 Conclusions

The chimney of former cork processing factory Mundet, in Montijo, showcases varying types and degrees of degradation across its different zones.

At the base, salt crystallization causes efflorescence and leads to erosion of the masonry associated with flaking of the brick and sanding of the mortar. The main source of moisture is probably rising damp, as evidenced by the distribution profiles of moisture content (MC) and hygroscopic moisture content (HMC). The primary types of salts are sodium sulfate in the brick and sodium chloride in the mortar. Sodium sulfate may originate from the brick itself, while sodium chloride may come from the sand used in the mortar, the surrounding terrain, or the coastal environment. As the bedding and joint mortar disintegrate, the masonry collapses in less confined areas, such as the corners.

The shaft exhibits no significant signs of erosion, particularly in the brick and at lower heights. The contrast in erosion levels at the transition from the base to the shaft could stem from a hydric discontinuity in the masonry or differing wind dynamics around the cylindrical shaft and the parallelepiped base. Mortar erosion becomes more pronounced as we ascend, causing the joints between the bricks to recede further from the masonry surface. This variation likely results from environmental factors, particularly wind, intensifying with increasing distance from the ground. No other forms of degradation, such as structural risks like deviations from verticality, are observed.

At the crown, situated atop the shaft, mortar erosion is extensive, with many bricks now simply overlapping. Given the limited confinement by compression in this area, masonry collapse is already evident. Approximately half of the crown, facing NE, has already collapsed, with the remainder at risk of imminent collapse.

In the interior surface of the chimney, efflorescence and masonry material erosion are observed in the base and shaft areas, but to a lesser extent than on the outer surfaces of the base.

Regarding the damage mitigation measures, since eliminating salts is not feasible, moisture control becomes paramount. To address this, it is essential to investigate the contribution of phreatic and surface water, as well as the possible influence of the recent nearby supermarket. Further, various techniques can be used to limit rising damp, each with pros and cons. Injecting water repellents or sealants into the masonry can create a barrier against capillary rise, but effectiveness may vary, and it could lead to destructive subflorescence if leaks occur. Introducing watertight barriers like metal sheets may compromise structural resistance to horizontal forces, particularly seismic activity, a concern given the high seismic risk in the area. Drainage options like wells or surface water diversion are also viable, but require careful monitoring to prevent issues such differential settlements. Ventilating lower wall levels to increase evaporation flow requires specific project and preliminary testing. Regardless of the method chosen, applying a sacrificial layer on the chimney surface to prevent destructive subflorescence is crucial, with materials like cellulose or clay compresses being potential options. Additionally, measures to minimize rainwater ingress through masonry surfaces, such as reconstructing the cornice or repairing the capping, are recommended.