Journal of Engineering Geology, Vol. 9, No. 2, Summer 2015

Effect of Mineralogy on Durability and Strength of Hornfelsic Rocks under Acidic Rainfall in Urban Areas
Ghobadi M.H.; Geology Department, Bu-Ali Sina
University, Hamedan, Iran
D. Fereidooni; Earth Science School, Damghan University, Damghan, Iran
Received: 27 Juan 2013 Revised: 8 Sep 2013
Abstract
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In order to assess mineralogical composition influence on durability and strength of rocks, four samples of hornfelsic rocks were selected from southern and western parts of the city of Hamedan, west of Iran. These rock samples were subjected to mineralogical, physical and mechanical tests in the laboratory. Also, they were evaluated in 15 cycles of slake-durability testing in different pH of sulfuric acid solutions and XRD analysis. Based on the results, the type and amount of minerals, their density and hardness had an influence on the uniaxial compressive strength and the slake durability index of tested rocks. That means, presence of non-dissolved minerals such as graphite in studied rocks, decreases Unconfined Compressing Strength (UCS) but increases the slake durability index. The results of slake-durability test indicated that weight loss of the samples at initial cycles was found to be higher than the end cycles. Also, in these samples, initial minerals in the fresh samples were not exchanged by secondary minerals such as clay minerals. Therefore Hamedan hornfelsic rocks are approximately resistant when were put under accelerated chemical weathering and degradation in the laboratory and natural chemical weathering.
Keywords: Hornfels, Durability, pH, XRD, Weathering
-11531510243

*Corresponding author [email protected]
Introduction
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Perhaps one of the major problems for application of different rocks as construction materials and foundation purposes is their susceptibility to degradation or chemical weathering. Weathering is an important geological and engineering geological process resulting in a change in the physical and chemical properties of unweathered rocks once they are exposed to air (Oyama and Chigira, 1999). In other words, weathering can induce a rapid change of rock material from initial properties to soil-like properties. This means that natural weathering processes can lead to the transformation of primary minerals into secondary crystalline and amorphous products (Schiavon, 2007). Therefore, the weathering process is divided into physical and chemical types according to the changes of the rock’s mineralogical compositions. According to Topal and Sozmen (2003), chemical weathering of rocks may cause changes in initial elemental concentrations by leaching and enrichment. Rainfall and availability of water, apart from temperature, are the important factors that influence the intensity of chemical weathering (Sajinkumar et al. 2011). In addition, other environmental factors such as Hydrosphere, topographical and climatic conditions have an important effect on the weathering process of rock materials (Erguler, 2009).
The sensitivity of a rock type against weatherability and the rate of occurrence of such a change can be descripted by a durability parameter, such as the slake-durability index (Ulusay and Sonmez, 2000). The degradation and weathering of rocks are extensive especially in rocks placed in areas where they are subjected to constant wetting and drying (Yavuz, 2006) and where air pollution is heavy. Also, the rate and nature of chemical weathering is governed by many variables such as parent-rock type, topography, leaching conditions and biological activities (Lee et al. 2008).
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Recent urban development and industrialization of cities have changed most environmental conditions. Atmospheric pollutions and the increase in SO2 and NOx gases can be the result of the rise in fossil fuel combustion. These gases combined with water present in the atmosphere will cause a reaction which is rainfall pH occasionally dropping to below 5 values. Furthermore, the produced sulfuric and nitric acid solutions are important agents in degradation of ancient building stones especially those with carbonate minerals (Ghobadi and Momeni, 2011).
An increasing use of hornfelsic rocks as construction stones has highlighted the importance of these materials as an engineering resource. These rocks were used as foundation, kerb, pavement and facade stones on a various engineering projects. Hornfelsic rocks generally have high strength, but when using these rocks as construction materials, the most important aspect to be considered is their degradation mechanism and chemical weathering potential.
Hamedan is one of the most important cities of Iran where hornfelsic rocks have had a presence due to the injection of Alvand batholiths into Hamedan Jurassic schists. Recent developments and industrialization of this city has caused air pollution and acidic rainfalls which have resulted in weathering of different rocks. In this research, we simulated natural conditions in the laboratory and are attempted to report the deterioration and weathering mechanism of hornfelsic rocks in this region.

Site description and geology
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In order to determine the effect of acidic rainfalls on weathering and degradation mechanism of the hornfelsic rocks, the southern and western parts of the city of Hamedan were selected as the study area (Figure 1). This region has an irregular morphology related to its geological history, tectonics and lithology. The geological conditions of the study area, with a longitude of 48° 10′ E to 48° 35′ E and a latitude of 34° 30′ N to 34° 52′ N, is one of the most important and interesting plutonic rock masses with its metamorphic aureole rocks in the earth crust of Iran. This granitic rock mass, called Alvand, is bordered from north and east to Hamedan, from south to Touyserkan and from north-west to Assad-Abad. It covers an area of about 400 km2 which makes the largest plutonic rock mass in Iran.
The metamorphic rocks around this granitic rock mass are pelitic hornfelic rocks such as garnet hornfels, sillimanite hornfels, kyanite hornfels and starlite hornfels. The geology deposits, closing the north and east side of the study area, are the quaternary alluvial. The study area has an elevation of 1950 m above sea level and it has typical continental climate. Summers are warm and dry, whereas winters are cold with high snowfalls, and springs are rainy.
Sampling locations were Heydareh village (HYD), Abbas-Abad valley (ABS), Cheshmeh-Malek village (CMM) and Faghireh village (FAG) in the vicinity of Hamedan which are shown on the geological map of the region.
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Figure1. Geological map of the study area and location of sampling

Methods and materials
In this research, the used samples were hornfelsic rocks collected from the southern and western parts of the city of Hamedan. These locations were Heydareh (HYD), Abbas-Abad (ABS), CheshmehMalek (CMM) and Faghireh (FAG). During the field investigation lithological and structural characteristics of the hornfelses were determined and sampling was done in the site of road cuttings or foundation excavations.
Degradation mechanisms and strength parameters of the hornfelsic rocks affected by acidic rainfalls were investigated through physical and mechanical properties (density, water absorption, porosity, P wave velocity and UCS), optical microscopy, X-ray diffractometry (XRD) and some index parameters. Accelerated weathering tests such as wetting-drying was performed, and durability assessment methods were used to predict the durability of the hornfelsic rocks at 15 cycles in natural water and sulfuric acid solutions with different pH.
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Polished thin sections were prepared for optical microscopic observations to identify the mineral composition and texture of the rock samples. XRD analysis was performed for fresh rocks and cycle powders as well as clay fractions (< 2 mm in size) separated from rock powders. The powdered rock samples were radiated to determine their mineral composition.
Results
The hornfelsic rock samples were tested under laboratory conditions and their different properties were estimated as follows:
1. Petrographical and Mineralogical properties
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Standard petrographic thin sections were prepared from the rock block samples. The mineralogical study of the thin sections showed that the pelitic hornfelic rocks were commonly composed of quartz, biotite, muscovite, garnet, sillimanite, kyanite, Staurolite, graphite and other fine cryptocrystalline matrix materials. Garnet, sillimanite, kyanite, Staurolite and graphite crystallize in metamorphic conditions. The composition and texture of the rocks were porphyroblastic pelitic hornfels. Therefore, the rock textures could be divided into two components: porphyroblasts and matrix. Garnet, Kyanite, Staurolite and Sillimanite porphyroblasts were the dominant types, ranging in size from 0.5 to 2 mm (Figure 2). The matrix was characterized by 100-μm-wide minerals of quartz, biotite and muscovite (50-100 μm dimensions). This pattern defined a fabric characteristic of contact metamorphic rocks. The quartz in the matrix of the rocks was generally less than 70 μm in any dimension and did not exhibit well defined grain boundaries with either white mica or other quartz grains. Graphite occurred at a 400-μm-length, distributing throughout the matrix in CMM sample. Table 1 is a summary of the modal abundance of the minerals and Figure 2 shows macroscopic and microscopic fabric images of the hornfels in the study area that also indicate Mineral composition of the rock samples.
Table1. Mineral composition of the rock samples
Rock mark Rock type Minerals contents (%) Qtz. Bt. Mt. Gt. Slt. Kt. St. Gpht.
HYD Hornfels 35 30 5 10 20 – – –
ABS Hornfels 32 20 8 15 5 15 5 –
CMM Hornfels 25 25 5 15 – – – 30
FAG Hornfels 40 25 11 11 – 7 5 –
Qtz. Quartz, Bt. Biotite, Mt. Muscovite, Gt. Garnet, Slt. Sillimanite, Kt. Kyanite, St. Staurolite, Gpht. Graphite

HYD
HYD
ABS
ABS
FAG
FAG
CCM
CCM
Gr
Slt
Bt
Qtz
Mt
Kt
Bt
Mt
St
Slt
Qtz
Bt
Qtz
Gr
Gr
St
Bt
Qtz

HYD

HYD

ABS

ABS

FAG

FAG

CCM



قیمت: تومان

دسته بندی : زمین شناسی

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