بررسي رابطه بين محيط تجمع سنگدانه هاي طبيعي كواترنر با مقاومت در برابر تخريب ناشي از تبلور نمك سولفات سديم

Evaluation of the relationship between aggradation environment of natural aggregates and resistance against disintegration by sulfate sodium crystallization

مقاومت سنگدانه ها در برابر تخريب ناشي از تبلور سولفات سديم علاوه بر ويژگي هاي سنگ شناسي مي تواند متاثر از محيط تجمع سنگدانه ها باشد. در اين تحقيق دو محيط واريزه اي و رودخانه اي در دو منطقه دماوند با سنگ هاي آتشفشاني و منطقه ديره با سنگ هاي آهكي مورد بررسي قرار گرفته است. انتخاب محيط هاي مورد بررسي در هر منطقه به گونه اي بوده است كه هر دو محيط از نظر سنگ شناسي شرايط مشابهي داشته باشند و تفاوت سنگ شناسي در اين محيط ها ناچيز باشد. ويژگي هاي تخلخل، درصد جذب آب، درصد سنگدانه هاي داراي ريزترك و مقاومت در برابر تبلور سولفات سديم در نمونه هاي برداشت شده مورد بررسي قرار گرفت. نتايج نشان مي دهد كه ميزان تخلخل، درصد جذب آب و ريزترك در محيط هاي رودخانه اي نسبت به محيط واريزه اي بسيار كمتر است كه مي تواند ناشي از فرايند حمل و سايش بيشر ذرات در محيط هاي رودخانه اي باشد. ميزان افت وزني آزمايش ساندنس در سنگدانه هاي برداشت شده از محيط رودخانه اي كمتر از محيط واريزه اي است. اين تفاوت متاثر از اندازه ذرات نيز مي باشد. در منطقه ديره براي سنگدانه هاي درشت ميزان افت وزني نمونه هاي رودخانه اي حدود 5 درصد نمونه هاي واريزه اي است در حالي كه براي سنگدانه اي ريز (600 تا 300 ميكرون) اين تفاوت حدود 38 درصد است. اين وضعيت در منطقه دماوند نيز وجود دارد اما به علت پايين تر بودن درصد ريزترك شدت اختلاف كمتر است به گونه اي كه سنگدانه هاي رودخانه اي درشت حدود 21 درصد و سنگدانه هاي ريز حدود 65 درصد ميزان افت وزني سنگدانه هاي واريزه اي است.

Aggregates' resistance to degradation caused by the crystallization of sodium sulfate mainly depends on their lithology. Nonetheless، aggregates' accumulation environments can also affect their weight loss induced by crystallization of sodium sulfate. The present study sought to examine weight loss among aggregates in rivers and colluvial areas. To this end، volcanic and sedimentary rocks were collected from Damavand and Direh، respectively. Lithological features have a profound impact on aggregates' resistance to degradation caused by sodium sulfate crystallization. Therefore، care was exercised to select river and colluvial areas with similar lithology or formation. 2-Materials and methods: In line with ASTM D2216-1990، ASTM D2216-10، 1990، 10، and ASTM C 88-99a، the collected samples underwent tests of porosity، water absorption percentage، and weight loss due to soundness. In the sulfate soundness test، the samples were saturated and dried in 5 cycles، followed by measuring their weight loss with a particular sieve. In this standard، the chosen sieves do not follow a specific pattern، and a single sieve may be used to gauge weight loss of aggregates with various sizes. In this way، it is difficult to compare the degree of degradation among aggregates with different sizes. Thus، in order to measure weight loss، the lower limit sieve was used for each range of aggregate sizes. In addition، the microcrack percentage of the collected samples was investigated. To study cracks in aggregates، samples with three different sizes (25-37.5، 19-25، and 12.5-16) were collected from rivers and colluvial areas. In total، 150 samples (50 for each aggregate size) were collected from Direh، while 300 samples were gathered from Damavand. For sampling، the aggregates were sieved and divided into four equal parts. Subsequently، one part was randomly selected for analysis (ASTM C 702-98، 2003). Cracks in aggregates، which were studied through naked eyes، were calculated based on the ratio of aggregates with cracks to the total number of aggregates. 3-Results and discussion: The results indicated that the degree of porosity، water absorption percentage، and microcrack percentage are higher in colluvial fans in comparison with rivers. This can be attributed to different lithological processes that dominate the two areas. In rivers، particles travel through long distances; hence، they are more likely to collide with each other and break down along the microcracks. This process results in a smaller number of cracks in aggregates from rivers. Moreover، the weathered areas at the surface of aggregates are removed through abrasion، hence their lower porosity and water absorption percentage. Conversely، in colluvial fans، particles mainly slide in large masses due to gravity. Since aggregates in colluvial fans do not travel through long distances and are exposed to less abrasion، they have a smaller degree of porosity، water absorption percentage، and microcrack percentage. The results of soundness test also revealed that aggregates in rivers experience smaller weight loss compared to the ones in colluvial areas. Aggregates’ weight loss is a function of their size. In river beds، like Direh، the degree of weight loss for aggregates that are 25-37.5 mm is only 5% as much as that of aggregates in colluvial areas. For the smallest size of aggregates (300-600 micron)، the degree of weight loss for river aggregates is 38% as much as that for colluvial aggregates. The same situation holds true for aggregates from Damavand، though with smaller difference in the weight loss of river and colluvial aggregates. That is، the degrees of weight loss among big and small aggregates from rivers respectively are 21% and 65% as much as those of colluvial aggregates. In rivers، a smaller weight loss is observed among bigger aggregates. In contrast، a direct relationship is detected between size and weight loss among aggregates from colluvial areas. Due to the small number of cracks in river aggregates، microcracks play an insignificant role in degradation caused by sodium sulfate crystallization. The surface to volume ratio is greater in small aggregates (in comparison with big aggregates)، hence degradation caused by sodium sulfate crystallization takes place at a larger surface of aggregates. Thus، given the smaller surface to volume ratio، bigger aggregates experience less degradation. Colluvial aggregates have a larger number of microcracks، which constitute the main factor in degradation caused by sodium sulfate crystallization. The number of microcracks and the degree of degradation go up in bigger aggregates. In such aggregates، the effect of surface to volume ratio is much smaller than that of microcracks. There is a significant difference in the weight loss of big aggregates from Damavand and Direh. In colluvial areas of Direh، weight loss increases by around 100% among bigger aggregates، while the same index for colluvial aggregates from Damavand is around 20%. This can be attributed to the smaller microcrack percentage of aggregates in Damavand. 4- Conclusion: The degree of porosity، water absorption percentage، and microcrack percentage are lower in river environments in comparison with colluvial ones، that can be attributed to the longer transportation and hence abrasion of sediments. The rate of weight loss due to soundness is lower in aggregates obtained from rivers compared to those obtained from colluvial environments. In river beds of Direh، the degree of weight loss for large aggregates (25-37.5 mm) is only 5% as much as that of aggregates in colluvial areas، whereas for small aggregates (300-600 micron)، the degree of weight loss for river aggregates is 38% as much as that for colluvial aggregates. The same situation exists for aggregates of Damavand، but the difference is lower due to the lower microcrack percentage، so that degrees of weight loss among large and small aggregates from rivers are respectively 21% and 65% as much as those of colluvial aggregates.