Stones and archaeological buildings’ preservation is one of the most important considerations when planning or implementing any action that results in the recovery of the material. Without specifying the environmental and deterioration factors affecting these buildings and their conservation procedures, most of them will evanesce and perish, and important historical data will be lost. According to [44] and [3], marine aggression is mostly caused by physical effects, dissolved oxygen, various types of ionic corrosion, and biological attacks. In coastal zones, marine environments significantly impact the deterioration of stone and archaeological buildings, as marine waters and aerosols contribute to deterioration cycles through three main mechanisms dependent on different climatic variables, such as AT, RH, RW, WE…etc. [45, 46]. In addition to the effects of underground water, sea levels rise, air pollution, and tides play a crucial role [47]. The main affecting mechanisms are mechanical and chemical. In addition, the participating organic matter has biological effects [48, 49].
4.1. XRD analysis and lithotype and petrographic characteristics
Based on the XRD data, Fig. (3-a, b), and petrographic investigation, it could be noted that the essential building materials are divided into two types. The 1st type used in the fortress bedrock (Oolitic limestone) is composed of biosparite limestone. It is a type of calcite characterized by light beige-grey sandy friable textural characterized and loosen features (very low degree of coherence). It contains biosparite and foraminiferal grain-stone that consists mainly of sub-angular to sub-rounded calcareous grains, with about 0.3 mm with no cement materials except some points with meniscus cement [50], created through the precipitation of aragonite or calcite cement as meniscus-shaped discs at grain. This type of stone is light-colored deposits called calcarenite limestone belonging to the Alexandria formation of the Pleistocene Age [51]. In addition, it is characterized by the presence of fine silty/sandy grains (Quartzose sandstone) and a high porous index with non-skeletal carbonate grains containing up to a small amount of dolomite [50]. The 2nd type of stone (Sandy dolostone) used in the fortress foundation is fine grains of quartz cemented by dolomitic cement. According to [52], it is characterized by the yellowish to brownish colors and the presence of (30–70% Ca content) and (60–70 to 20–30% Qz content measured 60–120 µm). By comparing the different analytical data, the lithological features presented by [53] could be affirmed that this type's provenance does not belong to various limestone quarries (Mokattam, Minia, or Drunka, etc.). In the same context and through site visits, it may be quarried from El-Gabal el-Ahmar (The Red Hill) quarry belonging to the Oligocene formation [54]. It is located within the boundaries of modern-day Cairo, in the area of Nasr city due to the notable similarities in both petrographic features and mineralogical constituents; quartz, calcite (micrite & sparite), and iron oxides, as argued by [55]. Furthermore, the resulting XRD data of the investigated samples, Fig. (3-a, b) showed some minor minerals, such as halite, sylvite, and gypsum. They are essential minerals resulting from salt crystallization mechanisms affecting the main building materials. According to [56, 57], halite (NaCl) is quite common in coastal environments and abundant compounds in the Egyptian soil. It belongs to chlorides species and appears as a main component in the investigated samples. In this regard, halite is considered an important destructive agent of fortress walls due to its high solubility index and its good ability to penetrate the porous network of the wall components, Fig. (5-a). Then, it produces disruptive pressure forces that lead to micro-cracks after a crystallizing process, as argued by [58, 59] in similar cases. On the other hand, it could be claimed that halite, as a prevalent component of the white stain with minor amounts of other salts in semi-deteriorated samples collected from some affected areas, is due to the chemical reaction between seawater and marine aerosol with the fortress building material [60]. These efflorescent stains sometimes disappear and appear again after heavy and prolonged rain due to the high index of water solubility of the salts present in the efflorescence [61], in addition to a typical sub-florescence defect with other serious damaging effects, which cause the cracking and detachment of mortar layers due to the alternative crystallization and hydration cycles and their generated pressures [62]. In the same context, gypsum [CaSO4.2H2O] salty crusts were created through the effects of the high level of sulfur agents dominated in the study area as a pollutant, especially with changes in dry/wet cycles [63]. This formation may be attributed to the combined action of particulate matter deposition and sulfation process or the significant influence of fungal growth in converting metal sulfide particulate matter to sulfate [64]. This effect can be attributed to sulfur cycle bacteria that can convert limestone into gypsum, especially in sulfur-polluted environments [65, 66]. Moreover, detecting gypsum in highly-deteriorated samples is mostly related to complex deterioration mechanisms resulting from the combination between the dominating sources of salts, such as sulfate-contaminated materials and some microorganism’s enzymes in the study area [67]. This mechanism is exhausted after some wetting-drying cycles of rainy and sunny days. Finally, these salts and their related mechanisms, especially those resulting from repeated wetting and drying phases, can be greatly amplified or reduced and lead to severe dramatic effects [68], such as volume expansion [69] and stone crumbling resulting from sub-efflorescence [70]. Stone bursting results from the hydration action after water absorption [71] depending on several variables, especially the dominating rates of air temperature and relative humidity [56] and other ranges of environmental fluctuations. These fluctuations can activate damage related to several factors (amount of salt undergoing transitions and frequency of environmental fluctuations). Sylvite [KCl] is one of the most common components of marine K-Mg salts [72, 73]. It belongs to the halides group, with halite as an essential mineral ingredient of evaporates sediments (salt deposits) [74]. It is composed of arid saline areas in the isometric system [75]. In our case, it is mostly observed when humidity is over 50% RH and leads to conversion reaction accompanied by volume loss to the stone mineral due to dissolution and recrystallization processes [76]. The observed trace minerals contain anhydrite, margarite, periclase, and k-feldspar attributed to the mortar sand, as noted by [77]. Anhydrite (CaSO4) forms mostly in the presence of acid sulfide, water, and calcium ions [78]. It is an anhydrous calcium sulfate-often component of saline evaporite deposits, which mostly results from dehydrated gypsum and vice versa depending on the temperature and humidity of the air [79]. This reaction is accelerated by other salts, such as halite and sylvite [80]. In our case, it is considered a common feature affected carbonated rocks due to water evaporation, particularly with continuous alternative wetting and drying cycles [81]. It mostly occurs because of mixing acid-sulphate water with the neutral chlorides, especially in areas rich in calcium similar to the area under the study, as attested by [82]. Margarite is a calcium rich member of the mica group consisting of CaAl2(Al2Si2)O10(OH)2, and occurs commonly as an alteration product of some aluminous minerals existed with feldspars in archaeological mortars containing brick fragments [83], or owing to the combination effects between air pollution and dominating humid agents [84]. It is characterized by a white to pinkish or yellowish gray color and crystallized in the monoclinic system [85]. In our case, it is largely formed by means of local ion-exchange reactions between the Al-rich precursor and Ca-rich fluids (seawater), as Al-silicates and chloritoid have high Al/Si ratios similar to those of margarite that may facilitate the margaritization of these minerals, as attested by [86]. Periclase is a cubic form of magnesium oxide (MgO). In our case, it is attributed, on the one hand, to its usage as constitutive of building materials, particularly those composed of dolomitic limes [87]. Moreover, using the Portland cement in recent conservation works [88], especially with the effect of speed hydration process of the cement paste [89] characterizes the study area. On the other hand, periclase may be due to ion exchange by the source of magnesium in seawater with calcium hydroxide, as attested by [90]. K-feldspar, or orthoclase feldspar (KAlSi3O8), is a monoclinic type of potassium feldspar that may be interpreted due to equilibrium reaction between silicate fragments in mortar and seawater at low temperatures with the presence of marine microorganisms [91]. Regarding the XRD data of mortar samples, Fig. (3-c), it could be asserted that it is essentially composed of calcite, halite, gypsum, and aragonite (the main components of Islamic mortars). It is known by Kosromil and composed of lime with pozzolanic additives, as argued by [92, 93]. It is well known that the main reason for mortar desegregation is the effect of efflorescence processes [94] attributed to alternative crystallization/hydration processes of salt species, especially chlorides (halite) and sulfates (gypsum) [57]. Our results show that this mortar is desegregated due to lime leaching between the stone courses. This mechanism is attributed to the effect of seawater, marine aerosol, or dominated seepage and leakage water that contains some corrosive agents [57, 95–97]. Significant mortar deterioration is dependent essentially on the action of salt, in addition to the direct effects of dominated AT and RH, as argued by [98]. It was proven that it is difficult to separate specific mechanisms of salt deterioration on the effects of temperature and relative humidity [99], in addition to the evaporation cycle, which is considered as the main factors that cause the decay of mortar components [100]. Additionally, some aggressive deterioration features result from using new materials during various preservation interventions (2000–2002). It may be desegregated because of bio-colonization and their resulting enzymes [101], especially in rough surfaces in the presence of cavities. These factors are critical for moisture retention in outdoors or open environments [102]. This symptom can occur between stone courses due to the pulverization of mortars to create hollow areas and loss of adhesion among the building elements that finally lead the mortar to be detached [103].
4.2. SEM investigation results
SEM photomicrographs, Fig. (5-a, b) asserted that the studied samples were exposed to aggressive deterioration factors due to many severe mechanisms that created eroded ages of calcite platy crystals and some other previously mentioned types of salt crystals. Moreover, the presence of some aggressive complex layers, consisting of a mixture of proton and organic materials resulting essentially from the combination between evaporated salts and organic materials, characterized the Mediterranean basin exposed to salt water and salt spray, especially in its more arid parts [104]. In addition, it could be claimed that these layers reacted with stone surfaces through interferences between the minerals either in seawater [105], saline aerosols [106], or crusts of the stone surface itself. This process caused new deteriorated layers, consisting mainly of some new salt species [57, 107]. In the same context, other severe deterioration forms, such as abrasion and attrition affected stone surfaces, Fig. (5-c), attributed to the direct effects of hydraulic or water wave action due to the rush of sea waves (powerful waves) into the cracks of the rock causing mechanical weathering. The air layer traps at the bottom of the crack, compressing it and weakening the rock when the wave retreats. This trapped air is suddenly released with explosive force, cracking away the fragments at the rock face, growing the micro-cracks, and deepening the crack, as noted by Zhang and Shao [108]. Also, it could be asserted that this mechanical action led to loose internal stone cohesion and corroded stone grains [109, 110], leaching processes [111, 112], and decreasing the mechanical strength of the stone, particularly with increasing exposure [113]. Figures (5-d) shows the presence of honeycomb as one of the most severe, common, and cavernous weathering forms [114]. This form is a serious symptom affecting calcareous materials in foggy and humid environments, as attested by [80, 115, 116]. In our case, the presence of this symptom is attributed mainly to the combination of effects between relative humidity and salt crystallization as main reasons [117], the effects of wind erosion [118], the temperature variations [119], and the freeze/thaw actions [120]. They refer to wind as one of the critical factors in the generation of honeycomb weathering, considering the effects of intrinsic factors, especially petrographic features and petrophysical characteristics, as attested by [118, 121]. Finally, seawater mechanical actions cause an indirect effect as physical damage in the deterioration processes that generally undergo a cycle of the soil aggradation (beaches) under the building. This mechanism occurs through erosion actions (sedimentation cycles) by sea waves under stable structure (northern wall of the fortress), as proved by [122]. The differences of these cycles up and down lead to net sediment loss, eroding the wall therein/thereon as argued by [123], and finally creating rough surfaces. Also, abrasion might be attributed to the effects of the wind on monumental stones and could be changed into other severe forms, such as undercutting and shaping.
4.3. Atomic absorption spectroscopy (AAS) results
AAS analytical data presented in Table (1-a) clarified that there is a good balance in the qualitative and quantitative ratios of chemical components of seawater samples both in the study area [124] and other samples studied by many authors, such as [125–128] in similar cases. These data claimed that different salty crusts were composed according to the occurring of the main ions dominated in the analyzed water. These salty crusts contained 73.86% (NaCl), 10.78% (MgCl2), and 7.19% (Na2SO4). In addition, some minor salts were rated 7.37% and composed of (Ca2HCO3), (Ca Cl2), and (K2SO4). It could be asserted that the grains of calcite samples, Fig. (5-c,d), were affected through a dissolution process enhanced under saline conditions, as could be noticed in Table (1-b,c) & Fig. (7). These features suggested that the salty etching components observed in some calcite grains could be due to the effect of salt loading from seawater and marine aerosol, as observed by [59]. It could be concluded that the chemical, mechanical and biological effects of seawater and marine aerosol cause complicated chemical reactions between their different sources of salts and the constituents of monumental buildings. These reactions may lead to the disintegration of the building material constructions, which can further lead to the weakness of the mechanical properties of these materials.
4.4. Microbiological investigation results
The extreme environment of rocks has long been thought to have limited microbial diversity, as the isolation and characterization of bacteria and fungi give an insight into Qait Bey’s rocks. Microorganisms play an important role in mineral transformation in the natural environment, especially in the formation of soils from rocks and element cycling [129]. Many studies looked at the variety and distribution of microbial communities found on ancient stone monuments with no conclusive results [130, 131]. Based on figs. (2-f) and SEM, Fig. (5-a), microbial symptoms affected most of the northern wall of the fortress in numerous tightly adjoining pits of several centimeters, as argued by [132–134] in similar cases. Table (2) shows that the microbes isolated from deteriorated and moisten walls of the fortress contain some dominated fungal and bacterial species. On the one hand, bacterial species created some colors on the walls of the fortress that attributed essentially to differentiating degrees in Pseudomonas species, mainly P. aeruginosa (yellow to bluish green) and P. clacis (granular appearance having brownish ting in the center). It is also possible that this coloration resulted from the secretion of the metabolic product called pyocyanin of the Pseudomonas species. Pyocyanin is a blue-green phenazine pigment produced in large quantities by active cultures of Pseudomonas species [135]. Moreover, Pseudomonas aeruginosa is considered one of the most beneficial organisms responsible for producing soluble pigments, such as pyocyanin (blue), pyoverdine (yellow-green), pyorubin (red) and pyomelanin (brown) [136]. On the other hand, those colors on the stone surface might result from the secretion of some isolated genus related to Bacillus spices. The natural pigmentation of the Bacillus genus sporulating colonies is therefore brown. Still, other colors have been documented in spores, e.g., a red-pigmented Bacillus megaterium, a pink pigment in some isolates of Bacillus firmus, and red‐ and grey‐pigmented Bacillus atrophaeus [137]. Some fungal species belonging to the most common fungi genus growing on limestone surfaces [138] were obtained: Aspergillus sp. and Penicillium sp. They led to different fissures, holes, and cavities through both pore structure and grain bodies. The pores offer a suitable advantage for fungal hyphae penetration, while providing a more hospitable microhabitat depending on the substrate's configuration, chemical composition, and state of conservation (Gadd [139]. In the same context, the development of metabolites, which react with stone to form secondary minerals, is a part of biogeochemical processes through producing organic acids, such as e.i. oxalic, citric, acetic, formic, gluconic, and fumaric) by fungi [140]. In addition, their products from volatile organic compounds and pigments affected the stone surfaces of the fortress considered as a good indicator of biological growth in monumental buildings, as mentioned by Bhatnagar et al. [141]. Based on our previous studies (e.g., El-Gohary and Yousef, 2004), it could be asserted that colored crusts affected the surface walls attributed fungi species are (black) resulting from the effect of dematiaceous (black) fungi, combined for their capability of forming dark pigments and resulting from the growth of fungi, such as A. niger, A. phoenicis, Cladosporium cladosporioides, and Alternaria alternata. This finding agrees with the results of Chang et al. [142], Houbraken et al. [143], Llorente et al. [144] and [145]. Green and yellow pigments may be referred to as the growth of green colored fungi, including A. chevalieri, A. flavor-furcatis, and Trichoderma viride, rather than algal and lichens growth, which agree with [146]. Other pigments may be due to the growth of pigmented fungi or acid-producing fungi, including Penicillum species and Trichothecium roseum; they can cause biodeterioration process [147], especially through their high ability for acid production [148].
Finally, it can be affirmed that Qait Bey Fortress has been exposed to three types of weathering mechanisms: 1) Physio-weathering is one of the main deterioration mechanisms affecting the porosity, surface roughness, and permeability of building materials [117]. It represents a common problem that may increase when rising sea level. It creates severe corrosion events due to a set of conditions: high tide, waves, particular wind direction [45], and strong large waves and storm surge [149–151], especially at low atmospheric pressure that causes a rise in the sea level. Moreover, water changes (rising/downing) primarily affect the rock's original physical composition and spatial structure, closely related to the change in stone microstructure [152], especially after wet-dry cycles. 2) Chemo-weathering results due to seawater and marine aerosol. As mentioned earlier, it is considered the main problem that aggressively affects the fortress through several reaction mechanisms. These mechanisms could be enhanced by the oxidation process through the effects of dissolved oxygen and various types of ionic corrosion [153]. In addition, there are further problems through penetrating some salt types and marine deposits, such as calcareous shells and marine borers within the stone pores, as discussed by Pearson [3] and Cady [154]. These aggressive materials attack both the interior and exterior walls of the fortress, especially with the notable roles of synergetic effects between seawater and/or spray with wind actions that create some ionic species in the surrounding environment that led finally to crystallizing some harmful salt species, particularly chlorides and sulfates, as demonstrated by many authors [155, 156]. 3) Bio-weathering (bio-deterioration) of marine structures is mainly attributed to the biofouling effect on the surface and borers. Organisms are able to bore, then penetrate deeply into the substrate and destroy the building material [157]. In addition, marine aerosols cause the most critical factors of mineral damaging and provide the suitable culture medium for the biodegradation and accumulation of organic matters [158]. These matters are present with significant concentrations in marine aerosols [159, 160] and influence water flow between stones and ground [161]. These aggressive mechanisms create surfaces roughness, high values of pores and provide convenient homes for various types of organisms, such as bacteria, algae, fungi, and lichen [162]. They are naturally present in sedimentary rocks, usually between (0.2% and 2%) [46]. They are essentially due to their content of dissolved organic carbon, as attested previously by Kleefeld et al. [159, 163].