Heritage Building Preservation in the Process of Sustainable Urban Development: The Case of Brasov Medieval City, Romania

Abstract

This paper aims to present a comprehensive review of the literature on the definition and development of the concepts of heritage and sustainability. The harmful effects of various pollutants on the materials widely used in the construction of monuments/buildings, which are part of the national and international cultural heritage, are also highlighted. In addition, the paper draws attention to modern techniques for investigating the composition and diagnosis of the alteration of buildings materials with the focus on stone, limestone, and mortars/concrete. The present research also emphasizes that in the case of heritage buildings, different skills are needed not only related to heritage conservation and rehabilitation, but also skills related to heritage planning processes, and to sustainable constructions. For exemplification, the manuscript proposes specific conservation principles based on the case of Brasov city, located in the heart of Romania and being par excellence a medieval town with representative buildings for that period.

1. Introduction

The 1972 United Nations Conference in Stockholm stressed that the economic development model based on continuous growth in consumption and overdevelopment in several developing countries cannot be a sustainable process and can have irreversible effects on the ecosystem and heritage [1]. These ideal policies should follow not only the perspective of preserving the past but also the one of securing resources for the future.

The literature addresses the concept of heritage and sustainability closely, exploring the different consequences or benefits of the various principles and approaches followed in heritage conservation processes and policies and sustainable development projects. Both concepts are elements that are part of a value system that defines them and gives them a specific status in a social context. Therefore, heritage conservation and the interest in sustainable development are closely influenced by political action, which may value one heritage monument more or less than another.

A widespread example is the case of Place Royale Square in downtown Quebec, Canada. This part of the city has enjoyed extensive urban renewal and emergence from anonymity due to nationalist motivations since the end of the 20th century [2]. Such examples can easily be found in each country’s history, which show that the concepts of heritage and sustainability are influenced mainly by ideological contexts’ social and historical phenomena.

Heritage restoration and conservation processes and the development of intervention programs that focus on sustainability involve daily decisions about the interpretation, access, and valorization of these elements [3]. This process consists of increased financial and human resources, often challenging to obtain and implement correctly. At the same time, even if a heritage site, for example, represents a unique element in the context of World Heritage, conservation and restoration decisions will be anchored in an overall strategy that reflects the concerns of the respective communities when considering interventions in their heritage [3].

On the one hand, air pollution continues to be a big challenge for cities, and this is true not only when it comes to formulating laws that must address traffic efficiency in urban expansion, but also when it comes to issues of wellness and cultural heritage preservation. On the other hand, the developments in the heating systems of homes or other buildings and industry, which are no more primarily dependent on the well-known fossil fuels, have made significant progress in reducing emissions of pollutants such as sulphur dioxide (SO₂) during the past few decades [4,5]. As a result, new challenges have hastened the negative impacts of pollution in the atmosphere.

In recent years, deficient construction management planning in the relevant urban locations induced important changes in the standards and quality of people’s life [6]. Moreover, the natural conditions imposed by the climate/climate changes in some regions had multiple and disastrous visible/consequences (destructions of buildings, landslides, the acute need to improve the management of different types of soil, etc.) [7,8]. Moreover, traffic emissions are considered as being an harmful source of pollutants, resulting in particulate matter (PM), SO₂, nitrogen dioxide (NO₂), etc., all of them having a considerable contribution to the construction materials deterioration, especially in the cases of structures that are considered to be part of a country’s cultural heritage [9]. Finally, there are national and international energy saving requirements that must not to be forgotten [10,11,12].

This research describes an extensive literature review concerning the definition and development of heritage and sustainability concepts and revealing at the same time the damaging actions of numerous/various air pollutants on most used constructions materials related to buildings, monuments etc. that are a representative part of the cultural heritage. The research was based on the idea of making a comparison between the European approach to the restoration of heritage buildings and the way in which medieval buildings were restored in a medieval city, emblematic for Romania, namely Brasov. Our research describes and suggest also specific conservation principles based on the case of that Romanian town. To our knowledge, no such parallel approach implying Romanian heritage buildings has been published.

2. Heritage and Sustainable Development

The concept of heritage and sustainable development are closely linked historically. Different political ideologies and social contexts have led to approaches based on value systems that are often radically different from today. Although the two concepts may appear to be a recent emergence in the literature, it is easy to see that heritage and its sustainable development have been an essential concern since pre-classical times. In the 5th century BC, in Athens, an enthusiastic approach to preserving ancient buildings was developed due to a public need to glorify the city’s power [13]. In essence, sustainable development is about managing the relationship between people’s needs and their environment so that critical environmental limits are not exceeded and modern ideals of social equity and fundamental human rights, including the right to development, are not constrained. Sustainable development processes aim to avoid the collapse of the environment and society and ensure a sustainable future [1]. Different ideas about urban heritage and preservation have come and gone since the turn of the 20th century [13]. Conservation and development of public spaces and monumental public heritage constitute urban heritage conservation in its broadest sense. There are several criteria involved in preserving historic sites, according to research. An example of this is the economy and the number of visitors. Furthermore, ideology plays a role when it comes to embedding cultural legacy into a set of core principles that form the foundation of political discourse. Today, the concept of sustainable development benefits from over 300 definitions and is a part of almost any social, economic, or environmental policy or program [14]. At the same time, the concept of heritage and sustainability, in particular, can also be defined in the light of countless specific aspects and indicators depending on the proposed objectives. A set of 20 more important indicators for sustainable heritage conservation have been proposed by Tanguay et al. [2]. They chose the indicators from five categories:

• characterization (attachment to location/place; traditional or perceived aesthetic, artistic, and harmonious values; building fabrics, insulation and ability to adapt them to the construction requirements, etc.);
• protection (viability of existing recyclable materials, authenticity of the concept, integrity, spatial compatibility);
• enhancement (environmental/ecological awareness, promoting actions for further knowledge about historical/cultural heritage, improvement of living conditions and quality of life, benefits of reuse vs. redevelopment);
• use and impacts (locals or visitors’ interests and implication in conservation, functional and business use, investments, tourists drawing, increasing the urban density);
• policy and regulation (public perceived opinion, appropriate protection and/or management system, compliance with regulations in force and building codes established by national/international agreements, stakeholders’ inclusiveness, and partnership). The more complex the intervention required the more indicators are recommended.

The choice of one or more indicators to start an intervention process, or the preliminary research of this process, will be made considering their usefulness in the overall project. Indicators quantify and synthesize complex phenomena that fall within the constitutive dimension of sustainable development and organize information for policy action, intervention, or postponement of intervention on the heritage asset. Indicators should allow for environmental and socio-economic diagnosis to support designed sustainable development strategies.

The main materials on which specialists focus their attention to preserve heritage buildings are stone, wood, and metal. From the characteristic materials of historical brickwork (namely stone and brick), several types of brickwork can be made, which differ from each other in the way they are built and the dimensions of the elements, but also in the way, they should be preserved.

The conservation of the original materials that the heritage building is made of turns out to be significant for appreciating the age and value of the works of art. From the point of view of the general theory and methodology of restoration, the knowledge of the causes of deterioration plays an essential role in assessing the state of a monument both in terms of resistance, changes in structure, and expression suffered over time [15]. To establish the proper series of preventive measures and direct interventions and stop a destructive process, a correct assessment of the causes of deterioration is essential. Once the causes of damage are identified, the key actors involved in preserving and conserving heritage buildings can propose interventions. For example, easing traffic around a specific heritage building can reduce pollution and contribute to the long-term preservation of that building, while adapting modern construction techniques to the restoration process can lead to discovering the most cost-effective ways of intervention that are sustainable. These new techniques involve different digital approaches, such as Building Information Modelling, that allow designing and coordinating different professional figures faster and considerably reducing modelling errors [16].

Long-term funding, specific development of heritage-related products, marketing, visitor, community, employee management, interpretation, and evaluation—these are just some of the elements that require proper planning and structuring when discussing heritage asset management. Ensuring sustainability is about finding the right balance that reconciles all aspects of a heritage asset’s and a building’s life cycle. Ultimately, the potential challenges in ensuring the sustainability and durability of cultural heritage can be seen in trying to prescribe the same recipe to all possible contexts. However, no matter how transferable a concept is, its nuances can significantly affect the outcomes [17].

3. Air Pollutants Damaging Actions on Heritage Buildings

In this sense, the primary challenge in urban areas is air pollution. In the case of designing optimal policies to ensure the fulfilment of both the proposed objectives of urban development, traffic efficiency, and to ensure the correct approach to issues of good preservation/conservation of cultural heritage, air pollution is inevitable. This is true not only when it comes to the designing the best policies that must support these goals [18]. Significant climate change due to its component factors (precipitation, air currents/winds, variations in seasons and temperatures, etc.) caused and obviously correlated with the uncontrollable increase in pollution and the adverse effects caused by these climate changes, leads to the visibly accelerated deterioration of the cultural heritage constructions with historical value.

Related to the above mentioned, PM, SO₂, and NO₂ are considered the main pollutants in air causing the most obvious, visible, and detectable degradation of building heritage. These pollutants determine the acidic rain and acidic deposition on the surface of the building, which, together with water infiltration, mold, and fungi, are the primary sources of the irreversible structural and aesthetic damages [19,20,21].

SO₂ concentrations in the air, due to the burning of fossil fuels, are a significant concern for the health, sustainability, and preservation of cultural heritage, especially heritage buildings that are made of extensive surfaces sensitive to corrosion, soiling, and other destructive processes because of their exposure to external factors.

Nitrogen oxides (NOx) represent an essential factor in air pollution, especially in urban areas, produced during fuel burning processes. In the atmosphere, nitrogen oxides contribute to the formation of tropospheric ozone and urban smog through photochemical reactions with organic hydrocarbons. Moreover, nitrogen oxides and sulphur oxides play an important role in forming the so-called “acid rain” and wet acid deposition.

In the environment, PM is considered a complex solid/liquid mixture of particles, suspended in the air, and varying in their size, chemical composition/structure, and other physical and biological features (depending on their location and time of origin). The modification in pollutants’ levels is due to the diversity of pollution sources. Natural causes or human implying factors (e.g., instant forest fires or human-caused ones, driving vehicles, operating manufacturing, plants/animals residues/waste) causes an increase in the amount of waste of any kind. Additionally, reactive species in the atmosphere combine to form secondary particles such as sulphates, accounting for a substantial portion of total PM. In sum, the pollutants in the form of gas, meteorological conditions, geographical location and seasonal types in a given area unduly influence PM levels [22].

3.1. The Influence of Air Pollutants on Stone, Limestone, and Marble

Stone is one of the oldest building materials, usually with a porous structure and a chemical composition based on various minerals such as carbonates, silicates, and aluminosilicates. Although it has special properties, stone can be significantly affected by the natural environment and air pollution [19]. Stone can undergo structural modifications on the surface due to the synergistic action of some physical, chemical, and biological factors. According to Vergès-Belmin et al., stone weathering is defined as being a result of the atmospheric, natural phenomena on that specific surface. In contrast, degradation is defined as physical and/or chemical change in the intrinsic properties, a fact that leads to losing value or decreased usability [23,24]. Aucouturier et al. considered that the degraded surface of the stone is a thin surface layer with a different composition from that of the bulk and has several specific properties induced by several disturbances that have occurred (i.e., roughness, coloring, etc.) [25]. The degradation of the stone’s surface is achieved by three well-defined physicochemical mechanisms: the attack of air pollutants, the dissolution in clean rain (karst effect), and the dissolution caused by the neutralization of rain acidity [26,27,28]. Natural agents such as rainwater, strong winds, airborne particles, and temperature variations can induce a series of physical mechanisms in the surface structure of stone that can cause surface erosion or granular breakdown. According to Vidal et al., those effects are more likely to occur in soft stones such as limestone or sandstone than in stones with better mechanical properties, such as crystalline granites [19]. Natural atmospheric factors and air pollutants act in combination, causing damage to historic buildings, monuments, and sculptures. This phenomenon is shared, especially in urban areas [29,30,31]. The surface degradation of the stone monuments and buildings can occur everywhere or only in certain areas that are more exposed and more reactive.

Limestone is a soft carbonate rock of sedimentary origin that is frequently found in nature. It is composed mainly of calcite (calcium carbonate, CaCO₃), but in some cases, dolomite (double calcium and magnesium carbonate, [CaMg(CO₃)₂]) may be present [32,33]. Under the action of major geological events, some natural limestone deposits have developed a series of metamorphic transformations resulting in recrystallization in the form of travertine and marble. Since ancient times, limestone and marble have been widely used in architectural applications for walls and ornamental-decorative or sculptural materials [34,35]. Rainwater, combined with gaseous atmospheric pollutants, leads to acid rain; the sulphurous, sulphuric, and nitric-formed acids can react with the calcite, causing the carbonate stones’ destruction [36]. The action of acid rain on exposed areas of buildings and statues from limestone and marble leads to the appearance of rough surfaces, the removal of calcareous material, and the loss of some carved fine details. In addition, acid rain can dissolve limestone by direct contact [5].

3.1.1. Black Crusts

It is well known that atmospheric SO₂ interaction with materials of construction occurs through the dry and wet deposition (acid rain) processes. Acid deposition due to SO₂ presents a significant role in carbonate stone decay [19]. The sulfuric acid formed in the atmosphere reacts with CaCO₃ from the limestone leading to CaSO₄·2H₂O formation, which is the main component of gypsum. Due to its higher water solubility (2.4 g/L at 25 °C) than calcite, the major part of gypsum can precipitate within the pores of carbonate stone surfaces that are not washed directly by rain, and, if these surfaces are open to the dry deposit, they may darken over time [30,37,38]. Another part of gypsum is removed from the limestone surface by dissolving in rainwater, leaving new layers of calcite exposed to acid deposition, and consequent more gypsum will be formed [19]. The gypsum formation on the carbonate stone surface is a rapid process that can be accelerated by the metals and metal oxides presence, which can act as a catalyst in the sulphation reaction [30,39]. During the gypsum formation and crystallization on the surfaces, some particles from the atmosphere (aerosols particles, airborne carbonaceous particles, particles resulting from anthropogenic activities, etc.) are entrapped in gypsum and calcite crystals matrix, forming the black crust (scabs), which represents one of the most dangerous degradation forms caused by air pollution [30,40,41]. Schiavon et al. showed that the scabs grow both inwards and outwards compared to the original surface of the stone, but the growth inwards is the predominant one [42,43].

Vergès-Belmin et al. proposed a three-step mechanism and distinguished between the layer of gypsum that grows inward through pseudomorph replacement and the black layer, which is a deposit that develops on the stone’s surface and extends outwards to explain the formation of black crusts [23]. The black crust formation results in the darkening of the buildings and statues’ external surfaces (black patina), a pervading phenomenon on many historic structures in urban environments worldwide [37,44,45]. The black patina is an indicator of the degradation of historic monuments due to environmental pollution. It defines the accumulation areas of both the compounds resulting from the chemical modification of stones and atmospheric deposits [45,46]. In many cases, the black crust formation is a complex process strongly related to the growth of bacteria, algae, lichen, and fungi [4].

Along with SO₂, nitrogen oxides (NO_x) are also important pollutants with acidifying effects on carbonate stone building materials. Nitric acid is formed in the atmosphere by NO_x oxidation and has an essential role in wet acid deposition. Although nitric acid can react with calcium carbonate in limestone, leading to calcium nitrate [Ca(NO₃)₂] formation, still, its role in the deterioration process of carbonate stone is not very clear [5,19,28,47]. According to Fassima, the amount of sulphate in the black crust is 40–69 wt%, that of chlorides is 0.09–0.52 wt%, while the nitrates content is between 0.01–0.25 wt% [48]. The low amounts of nitrates generally found in the black crust of limestone monuments indicate that nitric acid’s attack is not very important [38,44,48,49,50]. Many authors reported that it is difficult to determine how nitric acid attacks limestone because the high solubility of calcium nitrate leads to removing it from the stone’s surface, thus explaining the low amounts that have been found [26,47,48]. Some authors consider that calcium nitrate can form on limestone under the biological action of some nitrifying bacteria [19,27,42,47,51,52]. On the other hand, other authors have pointed out, based on the results of laboratory tests, that NO_x can cause an increase in the weight of limestone, contributing to its degradation [53,54]. In addition, nitrogen oxides enhance SO₂ adsorption in stone and can function as an oxidant leading to a significant increase in the sulfation reaction rates [27,55].

Considering its minimal porosity, marble is less prone to deterioration than limestone [9,31,48,56]. Bugini et al., evaluating the crust formation rate at the surface of two sculptural groups made from Carrara marble, reported that the amount of gypsum formed per unit surface was 5–13 mg/cm², and the rate of gypsum formation in the black crust was about 0.2 mg/cm² per year [56].

Due to the differences between the characteristics (microstructure and porosity) of the black crust and the limestone substrate, the crusts can come off, leading to the gradual weakening of the material’s surface of heritage objects and buildings [21,27,34], which can produce several irreversible structural and aesthetic damages [11]. The degree or depth of the degradation actions on the heritage constructions depends to a large extent on the quantity/percentage/content of these stone materials, related to the entire specific surface of the respective building [57]. The dissolution of carbonate minerals (carbonates), the secondary formation of gypsum, as well as the production of cracks (in the most severe cases) are caused even more accentuated by a higher porosity of the stone material in the construction, because the high porosity involves/results in -higher water absorption (high absorption power of porous material) [58,59].

The analysis of the black crusts formed on the built patrimony provides essential information on the microstructural characteristics and their chemical and mineralogical composition. It also includes information on the degree of degradation of the carbonate stone material, and the data obtained can be used to establish appropriate procedures for cleaning and restoring artifacts [5,30,31,45,60,61]. The analyses have shown that black crusts may contain traces of iron oxides, heavy metals, and elements of the stone substrate, like quartz. In some cases, even traces of calcium oxalate have resulted from partial oxidation of organic carbon [5,19,39,62,63]. In addition, the analysis of black crusts can provide interesting information about the degree of pollution of the environment around buildings and heritage objects because the black crusts themselves can act as passive samples of pollutants [5,17,30,49,60,61]. However, based on the above, it should be emphasized that proper monitoring and control of black crusts can lead to a better understanding of changes in the different levels of pollution in a given area. Moreover, their follow-up can contribute to the progressive but faster implementation of urban management programs, designed to support both the quality of life of people and the protection of the environment, with all that it contains in a city [5,59].

3.1.2. Investigative Techniques

The reported studies have shown that several complementary investigative techniques have been employed to characterize the stone materials from historical buildings or monuments and the acid deposition products resulting from the action of SO₂. Such techniques are presented in

Table 1. Complementary investigative techniques used to characterize the stone materials from historical buildings/monuments and the acid deposition products resulting from the action of SO₂.

Technique	Application	References
Optical microscopy (OM) Polarized light optical microscopy (PLOM) Fluorescence optical microscopy (FLOM)	Petrographic and topographic analysis to determine the textural characteristics of carbonate stone and to evaluate the morphology, and the growth rate of black crusts	[25,30,49,62,64]
Scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectrometry (EDX)	Obtaining information about the micromorphology and chemical composition (in terms of significant elements) of black crusts, and specifically used to acquire images of carbonate stone at high magnification	[25,30,49,65]
X-ray powder diffraction (XRPD) X-ray fluorescence (XRF)	To investigate the carbonate stone and black crusts’ mineralogical composition	[26,30,32,62,65,66]
Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS)	Allows determination of geochemical composition (considering here the trace elements) of the black crusts	[25,30,49,67]
Atomic force microscopy (AFM)	It shows sample surface properties by providing real 3-dimensional topographies	[65]
Fourier transform infrared spectroscopy—attenuated total reflectance (FT-IR ATR) spectroscopy	Carried out to identify the mineralogical phases and possible organic compounds, and also to examine black crusts constitution	[26,29,39,49,62]
Ion chromatography (IC)	Employed for the quantification of the main ions	[29,62]

.

3.1.3. Pollutants’ Impact on Heritage Stones Structures

Although the existence of these black crusts has been found on the surface of many heritage structures around the world [28,58,59,60,68], the most extensive studies concerning the impact of pollutants on stones from historical artifacts have been carried out especially in Europe, which is highlighted by a vast cultural heritage located in the urban environment [5,9,27,29,30,35,38,40,49,51,61,62]. Some significant examples are provided in

Table 2. Heritage structures/buildings exemplifying the impact of pollutants on stones.

Heritage Structures [69]		Observation Regarding the Impact of Pollutants on Stones	Ref.
St. Rumbold’s Cathedral in Mechelen, Belgium		Gypsum formation was correlated to limestone surfaces erosion	[58]
The Church of Santa Maria Mater Domini in Venice, Italy		Results indicated that the crusts contain mainly gypsum and less calcite and oxalates, and the degradation of the stone is due to pollution in the Marghera industrial area and maritime traffic	[38]
The interior of the colonnades in Piazza San Marco, Venice, Italy
San Domenico Maggiore complex, Naples, Italy		The analysis of the collected black crusts was employed in order to detect the variability in the degradation forms, mainly due to atmospheric pollutants	[30,70]
Sculptures of the cloister of San Marcellino e Festo, Naples, Italy
Fontana di Trevi, Rome, Italy		The black crust analysis provided information on urban air pollution and on the impact of air pollutants on stone degradation	[62]
Seville Cathedral, Spain		The analysis showed that the black crusts are composed mainly of gypsum, but some traces of calcite from the substrate were also identified, as well as several oxides from both the substrate and external sources	[49]

, pictures being Reprinted from Ref. [69].

Of course, many processes caused by pollution can be highlighted, which can act and change the appearance of the surface of stone buildings and monuments. For example, by the side of the black crusts, the deposits of dust and soiling in general on surfaces significantly affect the aesthetics of historic buildings and structures, and ultimately even their durability over time [71]. Although it is not a chemical deterioration, soiling plays an important role in blackening the facades of buildings, resulting from deposits and accumulations of atmospheric PM. The presence of PM from the combustion of solid fuels (i.e., coal) has been observed in the structure of black crusts formed on some limestone buildings and historical monuments, having an essential role in their blackening [37,63,72]. Regarding the PM emitted by the combustion of liquid fuels such as diesel, Searle et al. reported a more intense blackening of Portland limestone than that produced by coal particles [37].

Since the dirt does not affect the base substrate of the stone, the accumulated particles can be removed by a simple wash, while the removal of black crusts requires more complicated procedures. After all, dirt can only be considered an aesthetic issue, largely dependent on public acceptability [43,63,73,74]. In addition to air pollutants, several other factors such as pore salt growth, biodegradation, fires, floods, terrorism, vandalism, neglect, tourism, temperature fluctuations, inadequate chemical treatments, freeze-unfreeze cycles, climate change, etc. may cause degradation or damage to historic stone buildings and monuments [71,75,76,77,78,79].

3.2. The Influence of Air Pollutants on Concrete and Mortars

Mortars are an anthropogenic material used since ancient times to construct buildings [80,81,82]. Throughout history, masonry mortars have been obtained by mixing a binder material in the presence of water with various aggregates and additives, the latter having the role of improving the hardening properties of the mortar. The most common binders used over time were based on pozzolanic materials (used in ancient Greek and Roman constructions), lime, hydraulic lime (since the 18th century), Roman cement (also called “natural cement”), and Portland cement (19th century) [83,84,85,86]. The properties and characteristics of the mortars are correlated primarily on the used binder nature. The air lime consists primarily of calcium oxide/hydroxide (un-slaked/slaked lime); meanwhile, calcium silicates/aluminates/hydroxide can be found predominantly in the hydraulic lime [87]. Considering air lime vs. hydraulic lime, the difference is given by the fact that in the first case, adding water just facilitates mixing the components and the processing, not being involved in any chemical reaction [81,83].

Along with the binders and silicate aggregates (finely crushed rock and natural sands), many other additives were used to prepare mortars [83]. These additions were biological systems in ancient and medieval times (e.g., blood, egg whites, fig juice, lard, manure, etc.). Still, now, several industrial products or by-products are used, such as ash, blast furnace slag, epoxy resins, etc. [84,88]. In the concrete case, large aggregate particles are used. Mortars based on lime or lime with pozzolana were frequently used in Europe until the beginning of the 20th century, when Portland cement, hydraulic lime, clay binders, and gypsum were introduced [81,83].

3.2.1. Deterioration and Degradation of Mortars and Concretes

The study of mortars in archaeological sites provides essential information on their composition and degradation state and the sustainable conservation techniques that must be chosen [80,89,90,91]. The main chemical compounds identified in lime-based mortars used in heritage constructions are calcium oxide and calcium hydroxide (CaO, Ca(OH)₂), magnesium oxide and hydroxide (MgO, Mg(OH)), silica (SiO₂), alumina (Al₂O₃) and iron oxide (Fe₂O₃) [80,83].

Along with the material factors (i.e., chemical nature of materials, design workmanship and construction procedures, maintenance), the environmental factors (i.e., freeze-thaw cycles, salt crystallization cycles, wind and aquatic erosion, hygroscopic absorption of moisture, bio-deterioration, air pollutants, etc.) have a significant influence on the degradation and aging of mortars and concretes [85,89].

Carbonation is one of the most influential factors in the compounds in the gaseous atmosphere, in the immediate vicinity above the lime mortar, this reaction implying a chemical process between Ca(OH)₂ from cement products and the atmospheric CO₂, resulting in CaCO₃ formation [83,84,89]. CaCO₃ is formed during the hardening of the mortar and, in contact with rainwater, will dissolve until equilibrium is reached. If rainwater contains higher amounts of dissolved CO₂, the solubility of CaCO₃ will increase, consequently reducing the pH on the surface of the cement and leaving it without real protection in the case of corrosion [19]. Martínez-Ramírez reported that in wet conditions, the degree of CaCO₃ formation was higher than in dry conditions because in the presence of water, the dissolution of CO₂ gas is improved, and the carbonation of Ca(OH)₂ is promoted to higher levels [92]. Palomo et al. considered that the dissolution of the binder leads to an increase in the porosity of the mortar, affecting its permeability which causes a decrease in mechanical strength and an increase in the susceptibility to being attacked by other aggressive agents [83]. Therefore, the structural strength of the construction will decrease and will result in cracking and detachment of pieces, along with the weakening of the strength of the mortar/concrete layers [93].

On the other hand, CaCO₃ salts formed on mortars can lead to several aesthetic problems due to precipitation with the formation of white efflorescence on the surfaces [83,94]. Variations of the temperature and increased water content (caused by humidity) will affect the mortar and concrete surfaces because of the volume contractions. These processes can lead to the physical deterioration of the artifacts [59,95].

The action of the atmospheric SO₂ on the mortar binder is a complex interaction influenced by several factors (i.e., atmospheric concentration/quantity of the polluting gas, winds direction, rains intensity, binder nature, microstructure of the mortar, porosity and permeability of the cement material, etc.) [83]. The studies reported that the interactions of SO₂ with mortars and concretes, in general, are similar to those that take place on carbonate materials giving rise to the formation of both calcium sulphite hemihydrate (CaSO₃·1/2H₂O) and calcium sulphate dihydrate (CaSO₄·2H₂O) [89,92,96,97,98,99]. Zappia et al. demonstrated that the interaction of atmospheric SO₂ with lime, lime + pozzolana and cement mortars occurs through two mechanisms involving the formation of CaSO₃·1/2H₂O followed by its oxidation or the direct formation of CaSO₄·2H₂O [100]. Some authors indicated that the amount of sulphate and sulphite formed depends mainly on the properties and physicochemical characteristics of the mortar and less on the CaCO₃ content [100,101,102]. In any case, CaSO₃·1/2H₂O can be considered an intermediate compound because its oxidation leads to CaSO₄, the main component of gypsum, which is regarded as the only stable compound formed in the mortar–SO₂ interaction [97]. According to already published results, the gypsum produced is a consequence of the oxidation/hydration of dry deposition of SO₂ and the subsequent interaction of sulfuric acid with CaCO₃ in the substrate [83]. The formation of gypsum plays a vital role in the deterioration of cement and mortars since it generates the appearance of black crusts [97,103,104]. On the other hand, gypsum can dissolve in rainwater, thus being washed from the surfaces, which leads to the weakening of the mortar structure and degradation of the artefact surfaces [89,98,105].

Using climate simulation chambers to study the effect of dry and wet deposits on cement and mortar components, Martinez-Ramirez et al. found that in the case of the dry deposition, the rate of reaction between the hydraulic mortar components and the SO₂ increased by 35 times in the presence of water and ozone (O₃) than in the absence of those elements [106]. The same authors also pointed out that for mortars exposed to a polluted environment, the rate of conversion of SO₂ to sulphate increases when the binder/water ratio of mortars decreases, and in the case of acid rain, the rise in the water/cement ratio affected the surface of the mortar [106]. Bochen et al. have shown that wet SO₂ deposition generates a higher amount of sulphates than dry deposition [105].

Published data have shown that the gypsum can interact with the calcium carbonate and C-S-H gel from mortars exposed to an atmosphere containing SO₂ leading to the formation of some expansive compounds such as ettringite or thaumasite [85]. Thaumasite sulphate (CaCO₃·CaSiO₃·CaSO₄·15H₂O) is a compound without binding properties that are formed in the presence of calcium carbonate and calcium (mono) silicate from hydraulic lime mortars in conditions of high humidity, relatively low temperature (generally <5 °C), and high sulphate contents [83,85,107]. According to other authors, the formation of thaumasite may be the result of the ettringite modification by incorporation into its structure of Si⁴⁺ or by replacement of Al³⁺ ions in Ca₆[Al(OH)₆]₂ [83]. The studies have shown that both mechanisms can coincide in the same material. The destructive effect of thaumasite sulphate formation is due to the expansion phenomenon or the destruction of the C-S-H gel, which causes decohesion of the mortar or cement [83,85,99,104,107]. Palomo et al. mentioned that in the lime + pozzolan mortars, atmospheric SO₂ led to the formation of syngenite CaK₂(SO₄)₂·H₂O, as the first and main phase formed by sulphation [83].

3.2.2. Investigative Techniques for Mortars and Concretes Degradation

Several analytical techniques have been employed to characterize mortars and concrete and investigate the deterioration and aging of old mortars due to air pollutants (

Table 3. Investigative techniques frequently used to characterize the mortars and concretes degradation due to atmospheric pollutants.

Technique	Application	Ref.
Optical microscopy (OM), Polarizing and fluorescence microscopy	Understanding of the named compounds, determination of the composition of the mortar	[85,108]
Fourier infrared spectroscopy (FT-IR and Diffuse reflectance FTIR), Raman spectroscopy	Carbonate and sulphate salts identification, black crusts characterization, molecular information, and the identity of organic constituents	[92,95,109]
X-ray diffractometry (XRD)	Identification the mineral crystalline phases of the mortars	[85,92,109]
Scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectrometry (EDX)	Examination of SO2 interactions with the mortars and concretes surface, and formation of expansive compounds like thaumasite	[99,107]
Inductively coupled plasma–mass spectrometry (ICP-MS)	Element analysis at high resolution	[87]
Thermogravimetric analysis (TG), Differential thermal analysis (DTA), and Differential scanning calorimetry (DSC)	Characterization of mortar and cement compositions, and quantitative evaluation of carbonate salts formed on their surface	[84,85,87,89,95,110]

).

3.2.3. Pollutants’ Impact on Mortars and Concretes from Heritage Structures

Many scientific studies have been carried out on historical mortars and concretes from heritage structures and buildings throughout the world. These studies were focused on the archaeological aspects, the origin of the raw materials, the chemical, mineralogical and physical characteristics, the degradation phenomena due to air pollutants, as well as on the appropriate conservation/restoration techniques [111,112]. Some significant examples are provided in

Table 4. The composition of ancient mortars and concretes from some heritage structures/buildings and the impact of pollutants on their structures.

Heritage Structures [69]		Observation Regarding the Impact of Pollutants on Stones	Ref.
Church of the Cross in Gerasa, Jordan		The obtained results by FTIR and TGA/DTA analysis showed the existence in the mortar samples that were taken from the walls and under the column structure of a higher amount of amorphous calcium carbonate than that of crystalline calcite. In contrast, the mortar samples taken from the mosaic floor showed higher proportions of silicate. Protein materials (e.g., egg whites) have also been identified because they have been used as additives in the preparation of mortars. The presence of calcium sulphate was observed in all samples taken from the surface of the artifacts as an effect of pollution.	[113]
Palace of Knossos in Crete, Greece		The results obtained by (OM), (XRD), (FTIR) and (TG/DTA) investigative techniques showed that most of the analyzed samples were both aerial and hydraulic lime mortars. The presence of gypsum was also observed in some samples as an effect of atmospheric SO3 action.	[114]
Ancient farmhouses and mills, Penyagolosa massif, province of Castellón, Spain		The characterization of vernacular mortars by FTIR, XRF and XRD analysis allowed the identification and quantitative characterization of the predominant minerals in the samples (quartz, calcite, halloysite, apatite, sodium carbonate, hematite feldspar, hanksite and hornblende). The results obtained provide valuable information for establishing an optimized procedure for the restoration and conservation of these heritage monuments that consider their chemical composition and the degree of degradation due to atmospheric components	[115]
Casa di Diana Mithraeum from Ostia Antica, Italy		SEM-EDS and X-ray analysis showed the existence of a pozzolanic mortar that contained both fragments of crushed pozzolana and mineral phases (i.e., analcime and clinopyroxene). The binder used was obtained by mixing carbonates (lime) with a series of aluminosilicate-based components. In addition, some reaction products between the silicate minerals (analcim and quartz) were observed.	[116]
Ancient city walls of Xindeng in Fuyang, China		The results showed that the mortar from the historic wall was prepared mainly from calcite (hydrated lime) and quartz with small amounts of clay. The great strength and high apparent density of the samples are due to the used aggregates and to the presence of blue brick fragments.	[117]
Ming Great Wall, China		The chemical and microscopic analysis showed that the dolomitic lime binder was dominantly used on mortar composition around Hebei and Beijing; at the same time, in some western provinces of China, the use of calcium-enriched lime has been observed, as well as aerial lime with low natural hydraulic reactive phases. The mineralogical investigation demonstrates that the lime mortars can be considered as being almost aggregate-free. The recent preservation principles emphasize the role of mortar in conservation strategy of the ruin of Ming Great Wall, recent studies showing that (chemical) reactions between air pollutants and dolomitic lime mortars have led to important damages of its structure. Consequently, in order to restore the parts originally built of dolomitic lime, the optimal restoration strategy will have to take into account the use of binders (natural hydraulic lime-based and calcium-rich lime with pozzolan-based).	[118]

, pictures being Reprinted from Ref. [69].

4. The Brasov Cultural Medieval Buildings

The city of Brasov is in the center of Romania, 161 km from the capital Bucharest. The medieval town is situated at an average altitude of 625 m, in the internal curvature of the Carpathians, delimited on the South and South-East by the Postăvaru Massif, which penetrates through a spur (Tâmpa) into the town, and Piatra Mare. The municipality has an area of 267.32 km² (Figure 1).

Gradually, in the process of development, Brasov has incorporated into its structure the villages of Noua, Dârste, Honterus (today the Astra district), and Stupini. By incorporating the Postăvaru mountain peak into its structure, Brasov is the city at the highest altitude in Romania. The inhabited medieval fortress of Brașov is one of the most crowded fortresses in Europe, preserving historical and cultural relics of unique architecture. Medieval Brasov is known for its uniqueness, thanks to the preservation and restoration of the buildings preserved on old sites. The Fortress of Brasov has sights of great tourist attractions: the Black Church, the Council House, the Ecaterina Gate, the Cojocarilor Bastion, Bastion of the Postăvarilor, Bastion of the Weavers, Bastion of the Blacksmiths, Graft Bastion, White Tower, Black Tower, Butchers Tower, First Romanian School in the courtyard of the St. Nicholas church. All these medieval historical and cultural sites have a sustainable development, which meets the needs of the present generations without harming the interests of the future generations.

The degradation phenomenon of construction stones and the black patina of heritage buildings due to air pollutants were also observed in the medieval cities of Romania, such as Brasov, where some historic structures present black crusts on the surface, as can be seen in Figure 2.

To choose an adequate program for the restoration and conservation of Brasov heritage constructions and implement more well-thought urban management programs, careful and rigorous research on buildings’ stone degradation is required. In addition, the characterization of the black crust’s structure and composition in correlation with air pollutants concentration from Brasov city must be done. Even if the average yearly concentrations of the main atmospheric pollutants related to building destructions (NO₂, SO₂, and PM) over the last ten years do not exceed the maximum permissible concentrations, these pollutants’ presence still influences Brasov’s medieval heritage (Figure 3, personal photos of the first author).

Turnu Negru (Figure 2), made of stone, is part of the assembly of the four defense and observation towers of Brasov Fortress. Designed as a point of defense and observation to prevent the enemy from approaching the defensive walls, which are 5 m away, it has six firing gaps arranged in three rows on its faces. It is 11 m high, its walls having a thickness of 2 m at the base, dominating the Brasov Scheii with its dimensions. On 23 July 1559, the tower burned down in a lightning fire, the smoky color giving it its name. In July 1991, heavy rains caused the side of the wall facing the fortress to collapse. In 1996, the most extensive restoration work was carried out, when the south wall was restored, and the existing strange glass roof was installed.

Cultural heritage rehabilitation and conservation must deal with particular difficulties in dating and identifying construction techniques and technologies to propose an appropriate intervention [18]. These difficulties arise because of the lack of methodological paths that systematically define the diagnostic study’s typical phases and the intervention proposals. Moreover, the above-mentioned issues can lead to an inadequate or even incorrect time allocation or funding assessment solution. Specific load-bearing structures belonging to historic buildings have not received the same attention or consideration as the buildings in which they are located and have therefore not been treated with the same professionalism. Numerous structural repairs in recent years have highlighted the principle of optimal conservation, as appropriate as possible, often necessitating the demolition (sometimes unjustified) of some secular roofs (where the situation in situ needed it).

In the case of these arbitrary (sometimes excessive) interventions, it is often a matter of difficulties in correctly assessing the actual condition of the respective material, as well as its loading capacity. Consequently, the result is an obviously incorrect assessment of the structural condition of the elements in question or a consideration of too simplistic procedures (based more on financial, profit considerations than on the requirements imposed by the state of the works required). Recently, given the reopening of interest in green building, wood has managed to refocus interest and studies; its load-bearing capacity is still questioned by those who are not at all or less informed who consider innovative materials, techniques and materials/supports are the optimal answers to all structural difficulties [119].

The White Tower, located like the Black Tower, on Straja Hill or Romurilor Hill, was built of stone and brick in the 15th century for defense and observation purposes (Figure 4, pictures being Reprinted from Ref. [69]). The White Tower has the shape of a horseshoe and was intended for the guilds of the pewterers and the ploughmen, the top being finished in crenelations. Its name comes from the color with which it is painted, nothing else but white lime. The restoration of the White Tower was carried out between 2005 and 2006. It was returned to the public a year later and was introduced into the sustainable museum and tourist circuit. The interior of the tower has been arranged with the possibility of exhibitions and cultural evenings.

The cover materials used for restoration proved to be incorrectly chosen, as the color of the tower changed and the surface of it was visibly damaged. After only 16 years, advanced decay, namely the appearance of black crust on the tower’s surface, can be observed. The conditioning strategy did not consider the air pollution indicators of the area due mainly to car traffic.

Unfortunately, many examples of bad practices worldwide threaten cultural heritage values, that is, their aesthetic, cultural, scientific, and educational values, putting some of the world’s unique heritage sites or cultural objects at risk. Poor maintenance of cultural heritage assets can significantly affect their sustainability and completely lose their values. In the case of restoration, the answer to this challenge is the involvement of relevant experts in restoration works and the use of appropriate materials and techniques in the renovation process [17,120]. How long a cultural heritage asset will last depends mainly on how it is managed. Thus, sustainability is seen as a component of sustainability and has proved to be a significant issue for most European heritage assets, especially in developing countries. Proper heritage management requires different skills related to heritage conservation and rehabilitation, and skills associated with planning heritage processes.

5. Conclusions

Heritage is a legacy that we all must participate in through a continuous process, being passed on from one generation to the next, providing an absolute value to the individual and the community. Cultural heritage represents us as individuals, groups, societies, and communities and makes us feel different and unique, depending on each person’s culture, customs, and religion. As a major factor of destruction, the actions of pollutants on construction materials used in the field (especially in the case of heritage buildings that are part of and represent the culture of a country) can be considered as affecting both the structural strength of the buildings in question and the aesthetic appearance of the areas in which they are located; moreover, the effects are lasting, targeting living conditions/quality of life, tourism, as well as other aspects of local daily and economic life. In Brasov, a medieval town, many buildings have been affected by air pollution and degradation. Fortunately, some medieval buildings have been under conservation and preservation even if the restoration strategy does not follow all indicators. On the other hand, the public administration strategy on heritage conservation could include a large part of ecological indicators (such as traffic restrictions around historic buildings), but also coherent policies that encourage people’s attitudes and force the population to be aware of the inestimable value of these buildings.