ارزيابي ريزساختاري ميزان مصرف آهك و پيشرفت واكنش‌هاي پوزولاني خاك‌ واگراي تثبيت‌شده با آهك

Microstructural Assessment of Lime Consumption Rate and Pozzolanic Reaction Progress of a Lime-Stabilized Dispersive Soil

وجود خاك‌هاي واگرا در محل اجراي سازه‌هاي مهندسي به‌عنوان يكي از مشكلات ژئوتكنيكي شناخته شده‌است. هدف اصلي اين تحقيق، تعيين نرخ رشد و پيشرفت واكنش‌هاي پوزولاني خاك واگرا و آهك در كوتاه‌مدت و بلندمدت از منظر ريزساختاري و درشت ساختاري و همچنين تعيين ميزان آهك مصرفي در طي زمان و تأثير آن بر پارامترهاي مكانيكي بوده‌است. در اين راستا آزمايش‌ها با درصدهاي مختلف آهك هيدراته (0 تا 10 درصد) انجام شد. از مهم‌ترين نتايج اين پژوهش مي‌توان به ارائه يك معيار ساده براي شروع واكنش‌هاي پوزولاني و تعيين ميزان آهك مصرف‌شده در طي واكنش‌هاي پوزولاني بر اساس pH و ضريب هدايت الكتريكي (EC) اشاره نمود. بر اساس نتايج آزمايش‌هاي pH و ضريب هدايت الكتريكي (EC)، پراش پرتو ايكس (XRD) و مقاومت فشاري محدود نشده (UCS) در مقادير mS/cm 4 ≤ EC واكنش‌هاي پوزولاني صورت مي‌پذيرد و تشكيل تركيبات جديدي مانند ساختار هيدرات سيليكات كلسيم (CSH) و هيدرات آلومينات كلسيم (CAH) موجب افزايش خصوصيات مقاومتي خاك مي‌شود. در طي زمان با كاهش mS/cm 4 ≥ EC و 12.4 ≥ pH نرخ واكنش‌هاي پوزولاني و افزايش مقاومت خاك محدود مي‌شود. در خاك واگراي مورد مطالعه حدود 3 تا 4% آهك براي واكنش كوتاه‌مدت كه شامل تبادل كاتيوني است، كافي است. بر اساس نتايج، افزايش 6% آهك در طي 14 روز اول (mS/cm 4 ≤ EC و pH ≥ 12) مقاومت فشاري نمونه تثبيت‌شده را حدود 10 برابر افزايش مي‌دهد اين در حالي است كه پس از 14 روز (mS/cm 4 ≥ EC و pH ≥ 12) افزايش مقاومت تنها حدود 5% است.

The existence of soft clay and dispersive soft clay at the site of engineering structures is regarded as one of the geotechnical problems. This study is performed on silty soft clay that according to the experimental results showed 100% dispersivity potential. Due to the low bearing capacity of this type of soil in the site, the slaked lime was used to stabilize the soil geotechnical properties, to increase its strength, to decrease plasticity behaviour of soil, and to overcome its dispersive properties. The main goal of the present study is to determine the growth rate and progress of limesoil pozzolanic reactions in short and long terms from micro and macro structural point of view, as well as the measurement of the consumed lime rate over the time and its effect on mechanical parameters of the stabilized soil. The results of this study allow determining the minimum percentage of the lime that is necessary to react with clay minerals for making an acceptable change in longterm properties of stabilized soil. In this regard, a number of tests carried out with different percentages (0 to 10 percent) of hydrated lime. The pH, electric conductivity (EC), unconfined compressive strength, and lime consumption rate determination by Xray diffraction analysis were the tests used in this study to observe the progress of lime reaction with clay. In order to determine the microstructural and mineralogical changes, and reaction products formed in the modified soil, Xray diffraction (XRD) evaluation and scanning electron microscopy images have been used. Among the most important results of the present study, this paper propose a simple criterion for the onset of pozzolanic reactions and determination of the consumed lime rate during the pozzolanic reaction process based on pH and electric conductivity measurements. Based on the results from pH, EC, XRD, and unconfined compressive strength (UCS) tests, the pozzolanic reaction occurs at EC ≥ 4 mS/cm. Following that, the formation of new components such as calcium silicate hydrate (CSH) and calcium aluminate hydrate (CAH) causes an increase in soil strength. Over the time, with the reduction of EC ≤ 4 mS/cm and pH ≤ 12.4, the progress rate of pozzolanic reaction and the progress rate in soil strength suspend. The results show that for the dispersive soil around 34% lime is sufficient for its shortterm reaction, which includes cation exchange. Based on the achieved results, the use of 6% lime for stabilization of sle gives EC ≥ 4.0 mS/cm and pH ≥ 12 after the first 14 days period. The unconfined compressive strength of the stabilized sle increased around 10 times in the same period, while only 5% increase in strength observed after 14 days (EC ≤ 4.0 mS/cm and pH ≤ 12).