Analysis of the Mortar of Early Islamic Era on Tomb Towers and Castles of Eastern Mazandaran by XRD Method

After the political collapse of the Sassanid Empire, a kind of national resistance against the Arabs came into being. Tabaristan was one of those areas that the invading tribes did not have a good chance of reaching due to its special geographical location. Inscriptions in the Pahlavi language indicates that until the fourth century AH in some areas of Tabaristan, this language was trusted and can be a sign of a cultural resistance of the people of this land against the Arab invasion. According to this, The hypothesis of this research is that the study of mortar of the remaining buildings related to the early Islamic period and its comparison with the Sassanid period mortar can show the evolution of the mortar structure up to several centuries after the collapse of the previous system of government. The selection of Mazandaran as the spatial territory of this research is for the existence of three tomb towers (Figure1,2) that were built in the fourth century AH and have two common elements, which are the structure of brick and mortar with Kufic and Pahlavi inscriptions. Apart from these towers which are located in the east of present-day Mazandaran, there are many castles (Table2) in these areas that a comparative study of their mortar structure can be helpful in achieving the goal of this research. The purpose of this study is to find the answer to the question whether after the fall of the Sassanid Empire and the disruption of the long-standing political order of Iran, there has been a change in the technology of mortar production and how long and in which parts of Iran this can be done. We assume that the land of Tabaristan, as mentioned before, can be a good place to seek this technical transformation. The laboratory method selected for the study is called XRD analysis method. In the case of mortars and mineral powders, the XRD method is a reliable method. The XRD method is also effective for different granulations, as all the minerals in the mortar show an X-ray scattering pattern that is easily detectable in phase detection software. If you need quantitative analysis of mortar, you should use complementary methods of elemental analysis such as "SEM" or even XRF method, which is not the purpose of this study; This is because the same quantitative standard was not used to make mortar in different geographical areas, and only the method of construction was the same over time, which changed over time, depending on the needs of the architects. The hardening of the mortar and its adhesiveness depend on the reabsorption of the initial crystallization water of the raw mortar. Therefore, the use of any organic matter, whether plant material, or food additives such as egg white or egg yolk, etc., has no effect on the chemical structure of the mortar and does not help it to harden and stick. As a result, the use of organic matter detection methods such as "Chromatography", UV and IR spectroscopy, etc. will not be effective in this study. According to the analysis (Table3), it can be said that in general, the mortar of all three tomb towers, like most Sassanid structures, is a mortar of gypsum. Among the mortar samples of the studied castles in East Mazandaran (Table4), the predominant phase with the composition of "gypsum" has been identified only in the samples of "Hammam Kojoor Castle", "Varazan Noor Castle" and "Kanglo Castle". Also in all the samples in which the "gypsum" composition forms the leading phase, we encounter the "dolomite" or "calcite" phase together with the "quartz" phases with different crystal shapes, which are reminiscent of the same structure of Sassanid mortars. Copper-based sulfur compounds from natural copper ores and Aluminum silicate compounds in some samples are also due to natural impurities in the raw ore. Due to the phase diagrams and the number of phase adaptations, it is not possible to confirm the manual addition of sand or clay to the gypsum mortar in these samples. The results of XRD studies by the author on samples of raw gypsum (Table5), clay and sand stones (Table6) indicate that at least throughout the Sassanid era, the predominant mortar used in the construction of castles, palaces and Fire temples was gypsum mortar, which according to special heating conditions, the composition of calcite remains in the mortar and this has caused the stability of this mortar. But due to the fall of the empire, still in the early Islamic period, we see the use of the same gypsum mortar in the eastern regions of Mazandaran. Considering the date of construction of Sassanid tombs and castles such as "Kanglo Castle", and the similarity of mortar composition of all these samples, it can be concluded that the method of processing, heating and baking gypsum has not changed from the Sassanid period to at least 400 years later in Mazandaran. It seems that the structure of the mortar, in order to reach the next more resistant mortars, goes through a transitional period, the evidence of which can be found in the mortar used in the three tombs of the fourth century AH in East Mazandaran.