2.1.1. Archeological site

Witzinah, dated 800 A.C., is located in Peto county in south Yucatan State in Mexico also known as “South Cone” (Cono Sur) approximately 164 Km from Merida Capital City. Witzinah presents a total of 50 Prehispanic structures distributed in a surface of 77,243 m² and it is formed by edifications generally oriented at 10º NE, meanwhile the rest exhibit small variations (349º NW). Main structure (UO-1) is a pyramidal basement in a 14 m high slope, its architectural plant is 35 m by side and above there is a “C” shape structure that was constituted by five rooms with overhang covers, three in the main part of the building and one in each lateral side. Figure 1 exhibits Witzinah archeological site and location of evaluated stuccos.

Fig. 1 a) Witzinah location, Yucatán peninsula, b) Archeological site scheme, c) UO-1 building structure. d) Stuccos and mortars sample position. 

Stony archeological samples to be studied were classified in two materials, first corresponding to mortars and second referring to stuccos. Mortars: Sample 1 (noted as M-1) corresponds to the mortar from the constructive filler found in the overhang cover, sample 2 (M-2) was obtained from the mortar on the posterior loading wall, sample 3 (M-3) was taken from the mortar on the frontal loading wall and finally sample 4 (M-4) refers to the mortar that joined the stones in the overhang cover. Stuccos: Sample 1 (S-1), corresponds to screed stucco from de posterior loading wall, sample 2 (S-2) was obtained from the coating of the north wall. Natural resin extracted from Hevea brasiliensis was also obtained for chemical characterization in order to be compared with extracted residues obtained from stony archeological samples. This latex, although it is believed that was not available in Mesoamerica in Prehispanic time, was chosen for comparison purposes since a plus intention of this research is to assist in producing a durable yet accurate modern mortar that could be used in present archeological building reconstruction and preservation. Despite the various possible sources of naturally occurring rubbers, latex from Hevea brasiliensis remains the most widely used (^{Bhowmick & Stephens, 2001}), therefore its availability and affordableness make it an excellent option to be used as preserving and conserving material in building restoration.

2.1.2. Soxhlet Extraction

Four samples of each archeological mortar and stucco were chosen to undertake Soxhlet extraction as selective removal. 30 g of the material (granules and powder) were placed in a filter cone to be introduced into the main chamber in the Soxhlet system. Toluene was used as organic solvent and the system was heat at 55°C in order to initiate the filtration cycles during 6 hr. When cycling time was finished, the solvent remained inside the flask was carried to gradual evaporation until the obtaining of wax-like solid residues. Such deposits were taken to be analyzed by FTIR and ¹H NMR.

2.1.3. Scanning Electron Microscope (SEM) with EDX spectroscopy

Stuccos and mortars image analysis were carried out in a SEM model JEOL JSM-6360LV in order to study granular surface and distribution at different magnifications at high resolution. Mortar and stucco samples were taken in powder and granules to obtain additional granulometric and particle dimensions information. Each sample was analyzed and several images were captured with a power and resolution of 15 kV to X15, 15 kV to X50, 10kV to X20,000 and X30,000; 10kV to 5,000; 10kV to 1,000 and 1,500; 10 kV to 10,000; 10 kV to 500. Such images were used to identify representative patterns in the materials samples. Energy-dispersive X-ray spectroscopy (EDX) analysis was also performed in SEM equipment to obtain quantitative and qualitative identification of the main constituent elements in the studied materials.

2.1.4. Thermogravimetric Analysis (TGA)

This technique senses weight changes in a material in function of the temperature (or time) under a controlled atmosphere and allows the evaluation of mass loss in order to evaluate thermal stability and materials composition. A thermogravimetric analyzer model TGA 8000 from Perkin Elmer was used. An amount of 5 to 10 mg approximately of the petrous material were placed on the TGA to be analyzed under a Nitrogen atmosphere with an interval of temperature between 50 to 650°C on a heat speed of 10°C/min.

2.1.5. Fourier Transform Infrared Analysis

FTIR spectrogram was carried out in Nicolet 460 equipment placing the wax-like residue, obtained by Soxhlet extraction, in KBr to prepare a solid sample that was examined at 100 scans in a specter interval of 4000 a 400 cm^-1 with a resolution of 4 cm^-1.

2.1.6. Nuclear magnetic resonance ( ¹ H NMR)

¹H NMR spectra was obtained in a Varian 600 MHz equipment using deuterated chloroform as diluent. Wax-like extracts were also analyzed by ¹H NMR.

2.2.1. Elaboration of experimental mortars

Experimental mortars were manufactured using the fundamental elements found in the archeological petrous samples analyzed such as sahcab (calcareous stone powder), lime, water and natural polymer (latex), in order to represent the viability of reproduce such materials and compare them in applicability. Sahcab was obtained from a quarry nearby to Chan Chen archaeological site belonging to Cholul area in Yucatan. The lime used was commercially available as "Lime American finish" while piped drinking water and latex obtained from local tree (Hevea brasiliensis) were also used as components in the experimental mortar. Samples preparation included sieving, hydration of lime, mixing and finally mechanical compression tests. Sahcab was dried at room temperature (35°C - 37°C) exposed under natural sunlight for 10 h during three consecutive days previous to separation. Subsequently, standardized mesh sizes of 40, 50, 60 and 70 having an aperture 0.420, 0.297, 0.250, and 0.210 mm respectively were used in order to obtain a granulometric separation. Powder passing through the sieve was collected and used in the mortar manufacture, retained material was discarded. Lime (CaO) was hydrated by mixing in a water/lime ratio of 2:1 and the samples were left on the lab bench for 6 months to obtain slaked/hydrated lime (Ca(OH)₂). Latex was obtained by bleeding tree conventional process and treated with ammonia (NH₃) as a stabilizing agent, with a ratio of 3 to 4 g/l of latex (polyisoprene). All materials were mixed in a mechanical blender adding 275 g of hydrated lime, 525 g of calcareous powder (Sahcab), 100 g of latex and 100 g of water in cycles of 3 to 5 min per aggregate to obtain the ideal mixture, that is, with good plasticity and malleability. The experimental mortars system was poured in 2" cube molds elaborated according to ASTM (C 349-14) previously surface cleaned and covered with a polymer film based on alcohol in order to avoid mortar adhering to the surface of the molds. Cube molds were placed in a vibrating table for a period of 10 min. to allow the mortar system to cover all the mold area. Finally, the mold is left for curing during 14 days until it hardens and solidifies for further testing. Cubic mortar samples obtained were mounted into the mechanic test machine by ensuring that the upper and lower faces of the cubes, that are being loaded, are parallel in order to homogenize the compression stress through the sample.

2.2.2. Mechanical testing of experimental mortars

Compression test was carried out in five experimental mortars samples per formulation using a universal testing machine Shimadzu AG-I, with a load cell of 5 KN at room temperature with humidity of 80 to 90% and a crosshead speed 0.4 mm/min until fracture. Acoustic emission equipment, model MICRO 11PCl-2 was used in order to monitor the mechanical tests in real time to identify the failure mechanisms. Piezoelectric peak sensors were fixed on one side of the cube leaving 20 mm between them using silicone grease as coupling agent. Threshold was set at 20 dB, with a gain of 40 dB to determine the onset of failure (Figure 2).

Fig. 2 a) Compression test for experimental mortars, b) Sample dimensions with AE sensors attached. 

3.1.1. Scanning Electron Microscope with EDX spectroscopy

EDX is a method of spectroscopy that is useful in identifying the composing elements of materials; however, it does not reveal chemical bonding information. Hence EDX is more suited for the analysis of inorganic materials, rather than organic materials, which contain for the most part all the same elements bonded differently. For this reason, EDX analysis results carried out to mortar and stuccos revealed high values of C,O and Ca mainly forming part of the stony material namely calcium carbonate, 3(CaCO₃) (Sahkab). Although organic materials cannot be reliably identified using EDX, it can bestow a broad idea about the possibility of encountering organic molecules that can be useful to localize characteristic structures and allow a comparison and potential identification of them supported by additional characterization techniques such as FTIR. Previous studies carried out by ^{Ruiz Martín (2010)} on endemic trees in Yucatan peninsula such as Pixoy, Chucúm, Chacá and Jolol indicated that resin extracts from those trees were used by Mayan builders as adhesive in their building constructive materials. Therefore, is highly possible that organic substances are also being sensed by EDX as fragments of the analyzed samples. Figures 3a and 3b show SEM images examination in mortars and stuccos, which determined the presence of nanometric clayey material such as Palygorskite displaying a long and tubular morphology. These nanoclays are reported to be found in the south west of Yucatan State and are associated to tertiary calcareous material that are exposed as fine Palygorskite lentils. They use to be found along Sepiolite in larger amounts. Those mineral formations are due directly to precipitation of marine waters (^{Martín, 2002}).

Fig. 3 SEM micrographics of archeological stony particles, a) and b) Palygorskite nanoclay, c) and d) nanoscale clay structures agglomerates. 

SEM/EDX analysis allowed identifying Palygorskite 4H₂O((Mg₂Al₂)Si₈O₂₀(OH)₂·4H₂O)) that exhibited, as chemical elementary constitution, C, O, Mg, Al and Si, and has been reported locally as Sak lu’um clay (^{Soberanis Monforte, Gordillo Rubio, & González Chi, 2015}). Morphologically, too many particles are mostly ovoid although it is also possible to find shapeless solids (Figures 3c and 3d). All samples showed clay particles in fiber-like shape in a nanometric scale of about 0.1 μm a 35.09 μm in length and 0.02 μm a 0.99 μm in diameter.

3.1.2. Thermogravimetric Analysis

Figure 4 presents TGA analysis results performed to mortar and stuccos. This study allows estimating the possible presence of organic material in the analyzed constructive materials. Results exhibited in Figure 4 identified a mass loss between 3 and 5% in samples indicating elimination of humidity around 70°C, then and apparent stabilization is seen until 250°C where initiates a stage in which a notorious declination in the curve appears indicating organic mass loss until 600°C. Mineral solid residue remains after the test (96%).

Fig. 4 Thermogravimetric analysis (TGA) thermogram for petrous materials samples. 

3.1.3. Fourier Transform Infrared Analysis

Archeological mortar samples underwent Soxhlet extraction in order to separate and obtain possible organic component present on them. A yellowish waxy residue was acquired to be analyzed qualitatively by FTIR and ¹H NMR techniques. As shown in Figure 5a the FTIR spectrum exhibits bands at 2950, 2920 y 2850 cm^-1, which are typically associated to C-H stretching of methyl and methylene groups. Bands at 1460 y 1380 cm^-1 related to bending vibration confirm the existence of these groups. A signal in carbonyl zone at 1730 cm^-1 suggests the presence of ester groups which is also supported by C-O stretching bands at 1280 cm^-1. Natural latex used to elaborate the experimental mortars was also analyzed by FTIR in order to identify functional organic groups and correlate them to original stuccos characterization (Figure 5b). C-H bands, typically associated to methyl and methylene stretch vibrations were observed at 2932 cm-1. At 1460 cm-1 additional bands, related to bending vibrations, confirm the presence of such groups. Esters groups were also observed at 1730-1750 cm-1and 1280-1300 cm-1 linked to C-O stretching vibrations. Additionally, the presence de of bands associated to C=C a 832 cm-1and 3036 cm-1, suggest the occurrence of isoprenoids, characteristics of natural rubber cis 1-4 isoprene.

Fig. 5 FTIR spectra a) waxy residue obtained after Soxhlet extraction, b) natural rubber extracted from Hevea brasiliensis tree. 

3.1.4. Nuclear magnetic resonance

¹H NMR spectrum displayed in Figure 6 exhibits intense signals at 1.5, 1.18 y 0.8 ppm along with a weak signal at about 3.57 ppm. Signal at 0.8 ppm is generally associated to methyl group terminal protons in hydrocarbonated chains. On the other hand, signals at 1.18 and 1.5 ppm can be related also to methyl group protons affected by different environments. Band at 3.57 ppm could also be due to methylene protons but bonded to an oxygen atom (as in an ester group as shown in FTIR) which promotes the signal location to be lower due to the electronegativity of O.

Fig. 6 Proton nuclear magnetic resonance (1H NMR) spectra for waxy residue obtained after Soxhlet extraction. 

Considering the information obtained by FTIR and ¹H NMR, it is possible to state that mortar and stucco systems are constituted by a mixture of fatty acids esters since the presence of sutured hydrocarbonated chains and carbonyl compounds were identify. However, it was not found any clear evidence to link the wax-like extractions to the incidence of natural rubber (polyisoprene) due to the lacking of signals related to double bonds (C=C) characteristics of terpenoids such as stretching bands absorbed around 1664 cm-1, or through unsaturated methyl protons (HC=C) of cis-1,4-isoprene localized around 5.1 ppm (^{Chaikumpollert et al., 2011}). Those signals were not evinced by the applied techniques. While it is true that there are literature reports (^{Rose & Steinbüchel, 2005}) that indicate it is possible the identification of carbonyl groups such as ketones, carboxylic acids and esters during natural rubber degradation, in all mechanisms reported, invariably double bonds of isoprenoids remain. Therefore, it is highly probable that the analyzed samples are formed by a mixture of fatty acids esters proceeding from natural rubber degradation which is reduced to elemental chains (oligomers) that make difficult the C=C bond identification. It has been demonstrated that biotic degradation originates a significant decrement in double bonds in the polymeric chain and that its final chemical groups are rich in carboxylic fatty acids (^{Linos et al., 2000}; ^{Sakdapipanich & Rojruthai, 2012}). In contrast, ^{Fletcher (2002)} carried out FTIR and ¹H NMR analysis in stuccos taken from Maquixco Bajo archaeological site at Teotihuacan in order to search for the presence of organic gum binders. However, such results did not produce any definitive evidence for the presence of organic binders. So they proposed that calcite was used as binder in those stony materials.

3.2.1. Particle Size distribution

Experimental mortars were elaborated following the sieving of calcareous stone powder (Sahkab). Figure 7 exhibits the particle size distribution where is notorious the diminishing of powder dimensions when the mesh size augments. For this analysis a Coulter counter equipment was used by placing the particles in question into distillated water presenting a hydrophilic character that generates a rapid absorption by the sample and continues changes in the particles affecting the determination of its average size. Despite such variations, results displayed in Figure 7 gives a comprehensive impression of the majority distribution and the differences between the sieved particles. Sieve number 70 presented the lower particle size meaning a higher contact surface.

Fig. 7 Calcareous stone powder (sahcab) particle size distribution to manufacture experimental mortars. 

3.2.2. Mechanical Testing

Mechanical compression tests were carried out on 10 samples of each formulation (mesh size). Results showed that experimental mortars exhibit fracture mode II, which basically are shear strength, since the load was uniformly distributed over the sample generating first cracks in the cube corner sections that act as stress concentrators within the mortar causing it to crack. Porosity and internal imperfections affect the cracking progression along the sample which in some cases led and directed the cracking development both internally and externally.

Based on the mechanical tests results shown in Table 1 it is clear the effect induced by the particle size in the experimental mortars. As the calcareous stone powder (sahcab) particle size is smaller, such grains exhibit higher surface area that interacts favorably providing higher modulus.

Acoustic emission signals displayed in a representative stress-strain-AE amplitude curves in Figure 8 exhibit those events involved in experimental mortars compression tests. Mechanical damage events are intimately related to signal characteristics such as amplitude, energy, etc. The amount and type of events also depends on failure mechanisms from crack initiation and propagation until fracture. Results determine that amount of events are low in all cases since the mortars are porous materials they are fractured rapidly with the first internal cracks. Mortars made of 60 and 70 sieve particle size showed lesser events with lower energy.

Fig. 8 Acoustic emission signals displayed in stress-strain-amplitude curves for experimental mortars according the particle size.