GYPSUM AND LIMESTONE DISSOLUTION WITHIN FATHA FORMATION (MIDDLE MIOCENE) AT VARIOUS pH SOLUTIONS : A LABORATORY STUDY

The studied rocks (gypsum, limestone, & marl) of Middle Miocene Fatha Formation situated beneath the foundations of the study site in Mosul city. These rocks were obtained from four boreholes, at various depths up to 30 meters. The aim of work is to study the dissolution of rocks at various conditions, and then obtain the effect of the dissolution on their physio-mechanical properties, in addition, to amelioration our understanding of the consequences of the dissolution by acid-sulphate solution which is indisputably linked to sulphur oxidation. Gypsum and limestone rocks (10 samples of each type of rocks) and immersed in three solutions at various pH environmental conditions; acidic (0.0001N H 2 SO 4 ), neutral (tap water), and alkaline (0.0001N NaOH) solutions. Chemical analysis results of specific elements (Ca 2+ , Mg 2+ , Na + , K + , S 2- , Cl - ) at the acidic, neutral, & alkaline solutions show that: The dissolution (concentration of Ca 2+ & S 2- ) was higher in alkaline solution of gypsum rocks, while the dissolution (concentration of Ca 2+ ) of the limestone rocks was higher in neutral (tap water) solution than other solutions. The physio-mechanical properties of gypsum and limestone rocks samples such as, bulk specific gravity, indirect tensile strength, and unconfined compressive strength were determined. The compressive strength of water-saturated samples of gypsum rocks decreased by 25.56% and of limestone by 23.97% when compared with the dry samples, but the indirect tensile strength of water-saturated samples of gypsum rocks decreased by 36.62% and of limestone by 36.34% compared with the dry samples.


INTRODUCTION
Dissolution of the common rocks (rock salt, gypsum, anhydrite, limestone and dolostone) produces caves, caverns, sinkholes and creating a landscape known as karsts. These rocks constitute parts of the foundations of many structures and buildings throughout the world and Iraq, so different parts of areas are suffering from karstification. In the vicinity of the Mosul city, gypsum and limestone rocks alternate with marl or claystone in cyclic nature in the Fatha Formation (Middle Miocene), Fatha rocks are highly karstified, especially the Lower Member of this Formation (Sissakian and Al-Mousawi, 2007). Parts of Mosul city built over an area where the Fatha Formation is mostly exposed. Because the continuous dissolution of gypsum and limestone beds, many buildings have collapsed after being highly cracked. In addition, roads and sewages piping continuously repaired due to continuous subsidence (Thabet et al., 1986;Adeeb, 1988). The dissolution by acid-sulphate solution, which indisputably linked to sulphur oxidation by acidophilic bacteria in the proximity of the left bank of Tigris River in the south of Mosul city (Al-Taa'i, 2002). Porcelainite and the associated Alunite and Jarosite minerals are found within the Fatha Formation and the Lower-most part of Injana Formation.
These minerals are formed due to sulphuric acid seepages. The sulphuric acid (H2SO4) may be produced at depth due to oxidation of sulphur (H2S) by Thiobacillus bacteria, which raises to the surface through fractures (Jassim and Al-Naqib, 1989;AlJuboury et al., 2006). Awadh  (Krauskopf, 1979). The solubility of sulphate (gypsum) or carbonates (limestone) rocks is affected by many factors like temperature, the pressure of CO2, pH, the grain size of rocks, oxidation-reduction potential and the concentration of dissolved ions in solution (acid, base, salt) (Krauskopf, 1979;Lasaga et al., 1983). In general, for any salt in equilibrium with its saturated solution, the solubility is less if an excess of one of its ions is present, this decrease in the solubility of salt due to the common-ion effect. The presence of ions different from those furnished by the salt itself generally makes the salt more soluble (Krauskopf, 1979). The increase in salt content in soft soil and rocks causes a decrease in undrained shear strength and an increase in compression index, liquid limit of soft soil (Nu et al., 2020). One of the most important variables that affect the solubility of minerals is pH; the pH of pure water (like rainwater) in equilibrium with the atmosphere is < 7, caused by dissolved CO2 forming bicarbonate ion (HCO3 ‾ ), carbonic acid (H2CO3), and H + . This water may dissolve the calcite in limestone as the water moves along fractures and other partings or openings in the rock; this results in the dissolution of much of the limestone, and leaves behind void spaces if the reaction continues to take place over a long period of time (Martinez and White, 1999;Craft et al., 2007) (Langmuir, 1997). Gypsum also is highly soluble in the water, if gypsum rocks contain many cracks, so the higher number of cracks increases the dissolving rate in lesser time (Aljubouri and Khattab, 1997). The chemical composition of carbonates provides clues for the physicochemical conditions during deposition. The major oxides and some of the trace elements provide information about the overall composition of the carbonate rocks and depositional conditions to conclude the nature of seawater from which they originated (Tobia and Mustafa, 2019). The different physio-mechanical properties of limestone and gypsum rocks such as; uniaxial compressive strength, tensile strength and specific gravity, which plays an important role in the planning and designing of civil constructional works. An acidic and alkaline environment adversely affects these properties, where the rocks expose for a longer time. The rocks dissolution at different pH values of moisture conditions may experience a range of problems including cracking and deforming. These lead to crush and weakened rocks and forms many subsurface caverns, which have increased the dangers of the site and are unfavorable for foundation (Sharma et al., 2006). Natural cavities in limestone develop by the same processes that form cavities in evaporite (gypsum) rocks, albeit much more slowly. The high solubility of gypsum permit cavities to form in days to years, whereas cavity formation in limestone bedrock is a very slow process that generally occurs over centuries to millennia (Galloway et al., 1999).
The purpose of this research is to study the dissolution of gypsum and limestone rocks in a closed system (at atmospheric pressure and 28 ±2℃), at three solutions of various pH environmental conditions; acidic (0.0001N H2SO4), neutral (tap water), and alkaline (0.0001N NaOH) solutions. Then evaluating the influence of dissolution on the physio-mechanical properties of water-saturated samples in comparison with dry samples.

LOCATION AND GEOLOGY
The samples of the present study, which belong to the Middle Miocene Fatha Formation, lie beneath the construction site of a water tank project, near Al Yarmuk intersection on the western (right) side of Mosul city (Fig.1). The sediments of the Fatha Formation are exposed 53 (2B), 2020: 71-88 74 along the western (right) side of Tigris River, which divides and passes through Mosul city.
The rocks of the formation are the oldest in the area of the study at the (or near) surface in small places (Adeeb, 1988;Ismail and Al-Naib, 1992). The Mosul city is situated on the transitional belt between Hemrin-Makhul, Butmah-Chemchemal Subzones, and belongs to the foothill zone of the unstable shelf units according to the Tectonic Divisions of Northern Iraq.
the Fatha Formation is deposited during the middle Miocene sequence of the evaporite lagoons systems (Jassim and Buday, 2006). The Fatha Formation represents a cyclic repetition of gypsum (anhydrite), limestone and marl. However, the studied litho-log of boreholes ( Fig.2) represents part of the formation; it consists of three types of rocks: -White colour to dark grey gypsum with fractures, pockets of green marl, fragments of rocks and has cavities.
-Brown colour to grey solid limestone with fossils, high porosity and cavities.
The rock descriptions in the studied section show an indication for existence of subsurface karst features like the absence or dissolution of gypsum beds. Many gypsum horizons were not detected (cavities) during drilling (Fig.2), because they were karstified and in their places there were cavity fillings with materials, such as marl, claystone and limestone fragments (Sissakian and Abdul-Jabbar, 2005).   hours. The results of physical and engineering properties are shown in Table 4.  which are calculated according to Aljubouri (2011). The analytical results of some elements in solutions of the three types environments (acidic, neutral (tap water), and alkaline) in closed system (at atmospheric pressure and at 28 ±2℃) of gypsum and limestone rocks samples are shown in Table 5 and Table 6 respectively.  The Ca 2+ concentration of the three types of solutions, which was determined, considered as a measure of dissolved gypsum or limestone in closed system at flow velocity equal zero and constant temperature. Therefore, the effect of solution-pH on the dissolution rate is greater as the time is increased. The results of the gypsum rocks (concentrations of Ca 2+ and S 2-) of the three types of experiments (Table 5) show that, the dissolution at the time 28 days was higher in alkaline environment than neutral (tap water) and acidic (Figs. 3, 4 and 5).

. Relation between pH and concentration of calcium in three solutions at time (28 days) of gypsum and limestone rocks
The gypsum rocks have other ions of salt combinations as (Mg 2+ , Na + , K + and Cl -) were dissolved also. The dissolution of gypsum and salts combination is not constant but increases with time (Al-Dabbagh, 1989;Al-Dabbagh et al., 1990;Aljubouri and Al-Kawaz, 2007).
However, in the present study, the dissolution decreases in acidic solution (Table 5) and (Fig.3). Due to the common-ion effect, because the excess of SO4 2would lower the solubility. So, the concentration becomes constant with longer time (Krauskopf, 1979;Al-Dabbagh, 1989 andLangmuir, 1997).
The results of the limestone (Ca 2+ concentrations) of the three types of solutions (Table   6) explain that, the dissolution was higher in neutral (tap water) environment than the other solutions ( Figs. 5 and 6). The concentrations of Ca 2+ , Mg 2+ , and Clin neutral (tap water) solution at the time of 28 days were more than the concentrations in the alkaline and acidic solutions. The ions of salt combinations (Na + , K + ) and (S 2-) have high concentrations in acidic and alkaline solutions respectively (Table 6).
Generally, dissolution of limestone is faster at low pH (acidic solution) especially at low acid concentration, so the acid dissolves calcium carbonate to form the very weak acid by reaction equation (Huminicki, 2006): CaCO3 + H + → Ca 2+ + HCO3 -. The forward reaction shows, what happens when limestone subjected to weathering by acidic solution, the low pH 53 (2B), 2020: 71-88 83 accelerates weathering processes and dissolves to form caves (Krauskopf, 1979 andSharma et al., 2006). The dissolution rate of limestone will decrease as the pH increasing (alkaline solution), because the hydroxide of alkaline solution reacts with bicarbonate which resulting from dissolving limestone (Lasaga et al., 1983). Because carbonate ion (HCO3 -) readily forms, can be reverse the process, represented by the precipitation of calcium carbonate (like common-ion effect). Therefore, the dissolving of limestone slows or stops (Krauskopf, 1979;Cravotta, 2005 , which may affect to decrease its dissolution rate over time (Oard, 1998;Alkattan et al., 1998 andHuminicki, 2004).  The study expects that the physio-mechanical properties of rocks are adversely affected by the acidic and alkaline (different pH environments) moisture conditions (Sharma et al., 2006). A statistical technique (Regression analysis) was used in an attempt to explore and modeling the relationship between two or more variables. The regression-equation can check how good the model (in terms of predictive ability) by examining the coefficient of determination (R 2 ), the closer R 2 is to one, the better is the model and its prediction (Navidi, 2008). Table 7  formation. The high coefficient of determination such models may using to predict the degree of dissolution of the rocks with time, which will lead to the cavity formation. However, the alkaline solution makes the reaction of limestone to reverse the process 53 (2B), 2020: 71-88 86 of calcium carbonate precipitation, or carbonate are not sensitive to alkaline solution, so the dissolution of limestone slows or stops.
3-Dissolution of gypsum rocks in a closed system (at atmospheric pressure and at temperature 28 ±2℃) is not a constant; it increases as the pH of the dissolving solution increases.
4-Analytical results (concentrations of Ca 2+ , K + , and S 2-) of three various pH environmental conditions, of closed system (at atmospheric pressure and at temperature 28 ±2℃) appears that the dissolution of the gypsum rocks was higher in alkaline environment, while the dissolution of the limestone (concentrations of Ca 2+ , Mg 2+ , and Cl -) was higher in neutral (tap water) environment than others solutions.
5-From the statistical models (simple linear regression), the high coefficient (regression sum of squares) of gypsum rocks is in alkaline solution, and of limestone is in neutral (tap water). It indicates good correlation between the dissolution (Ca 2+ concentration) and others elements with time, and could be used to predict the dissolution of rocks, which could be considered the same as the cavity formation. 7-The study expects that, the dissolution of gypsum and limestone rocks may occur, when surface water from rains and others domestic uses seep to underground, or from sewage and tap water leakage of main pipes. As well as, when sulphuric acid may form at depth due to the oxidation of sulphur by acidophilic bacteria. The prevention of surface water seepage into the ground is important. Surface water should be piped away by establish surface water drainage to avoid the rapid subsidence of unstable deposits over cavities in the karst.