تاثير تركيب زئوليت و الياف بر بهبود خصوصيات ژئومكانيكي خاك تثبيت شده با سيمان تحت چرخه هاي يخ و ذوب

Effect of zeolite and fibers on the geo-mechanical properties of cemented soil under the freeze-thaw cycles

در پژوهش حاضر تاثير استفاده زئوليت همراه الياف پلي­پروپيلن در پايايي مشخصات مهندسي خاك­هاي نرم تثبيت شده با سيمان تحت چرخه­ هاي يخ و ذوب (F-T) بررسي شد. بدين منظور مقادير صفر تا 30% وزني، سيمان و مخلوط سيمان-زئوليت (با نسبت­هاي متفاوت)، بطور جداگانه به خاك مورد مطالعه اضافه، سپس با افزودن الياف و عمل­آوري تا 90 روز، مجموعه­اي از آزمايش­هاي ژئوتكنيكي در كنار آناليزهاي ريزساختاري صورت گرفت. نتايج بدست آمده مويد آن است استفاده از مقادير اندك سيمان مي­تواند سبب بهبود اوليه پارامترهاي ژئومكانيكي خاك شود؛ اما با افزايش چرخه­هاي F-T مقاومت نمونه­ها بعضاً تا 60% كاهش مي­يابد. تأمين شرايط مطلوب، نيازمند مصرف زياد سيمان و نگهداري طولاني بوده كه علاوه بر افزايش هزينه تثبيت، رفتار مصالح را به شدت تردشكن مي­كند. از طرفي، با افزودن زئوليت (حداكثر تا يك چهارم سهم سيمان) علي­رغم امكان رشد ظرفيت باربري (تا 3/1 برابر)؛ اما همچنان مقاومت مصالح تحت چرخه ­هاي F-T افت زيادي نشان مي­دهد. مشاهده شد حضور الياف در اين سري از نمونه­ ها، تاثير قابل ملاحظه­اي بر افزايش پايايي رفتار خاك به ويژه در زمان­ه اي محدود نگهداري (كمتر از 7 روز) و مقادير كم افزودني داشته و شاخص خرابي ناشي از فرآيند يخ و ذوب حدود 50% كاهش مي­يابد. علاوه بر اين، مقاومت كششي سيستم اخير نسبت به سيمان تنها، بيش از 5/1 برابر بهبود و مقاومت پس­ماند آن تا 200% بيشتر است. علت اين تغييرات با استناد به طيف­هاي اشعه ايكس و تصاوير ميكروسكوپ الكتروني، تشكيل ماتريس پيوسته­تر خاك ناشي از گسترش نانو ساختارهاي سيماني و پيوند مستحكم الياف با ذرات ارزيابي شد. در مجموع بر اساس نتايج حاصل، استفاده از تركيب CZF، به دليل امكان مصرف كمتر سيمان (حدود 30%) و همچنين كاهش زمان مورد نياز عمل­آوري (تا 3 برابر)، به عنوان يك گزينه موثر براي بهسازي خاك­هاي نرم تحت چرخه­هاي يخ و ذوب، پيشنهاد مي­شود.

Although cement stabilization is used extensively to modify the soft clays, it may show limited success in some applications. Hence, this paper presents a multiscale investigation on the viability of employing zeolite and fiber to enhance the durability of cement treated soil against the freezing-thawing (F-T) cycles. In so doing, a wide range (0 to 30%) of additives including sole cement and cement-zeolite mixture (CZ) with different cement replacement were separately added to a soft soil sample and then mixed with the optimal fiber content of 0.75% (by weight), which was determined by the indirect tensile strength test. A set of experiments at various curing days (up to 90 days) were performed to study the mechanical and microstructural changes of the stabilized soils. The results indicated that while a low level of cement can modify the geo-mechanical parameters of soil sample, the compressive strength of cemented soil could decay up to 60% when the specimens exposure to the successive F-T cycles. Such changes may be ascribe to the F-T-induced particles rearrangement and degradation of the cementation structure-bonding, forming many new voids and cracks subsequently decreasing the interlocking of matrix. As a result, to get the strength guidelines threshold and make the composite water proofing a high dosage of sole cement and a long time of curing (at least 28-day) are needed, which may be uneconomical and lade to the brittle behavior. Adding zeolite (≤ 25% proportion) to supplant part of cement could effectively enhance the engineering properties of cement-mixed soil, due to an increase in the cementitious products [e.g. Calcium-aluminate-hydrate (CAH) and Calcium-silicate-hydrate (CSH)] induced by the pozzolanic activity, subsequent reduction of the inter pore-spaces and eventually a more compacted microstructure, as confirmed by the X-ray diffraction (XRD) patterns and scanning electron microscope (SEM) images. It should be emphasized that the zeolite/cement ratio is a very influential factor on the behavior of cement-zeolite mixture. Therefore, the cemented soil mixes with Zopt showed a further (up to 1.3 folds) resistance relative to the mere cemented soil as well as a greater tensile strength; however, the binary system was still vulnerable under the F-T action. In this case, the insertion of fiber could significantly enhance the soil durability (decrease the degree of damage by an average value of 50%), which was more evident at the small binder dosage and early stage of curing time. Incorporating fiber into the system also led to a higher tensile strength (nearly 1.5 times) than those deduced from the stabilization alone. Moreover, this strategy was effective to overcome the brittle nature of stabilized mixes, resulting an increase the post-strength up to 270%. These observations can be justified by the extended cementing gels formation and the enhanced interlocking of matrix through the CZ-fiber application. Overall, the combination of CZ blend and fiber can be considered as an effective technique for the soft soil modification with the fact that triggered a prominent reduction (~ 30%) in the needed amount of cement an time of curing (up to 3 folds) for the successful treatment against the F-T cycles.