Sulphate content and compaction water content effects on lime stabilization of clay soils

Expansive soils could be found various places around the world. These soils are known as problematic soils, which could cause heaves and failures in pavements. Swelling potential and its amounts of clay soils could be determined by the correlation. It is known that there are correlations between the swelling potential and liquid limit, plasticity index, clay activity and SPT values (Standard Penetration Test). Chemical stabilization has been applied on soils with calcium based products such as lime and cement since many years. Lime stabilization is occured by four chemical stages, which are cation exchange reactions, flocculation-agglomeration, pozzolanic reactions and carbonation. Cation exchange reactions and flocculation-agglomeration stages are taken place rapidly and decreased plasticity index. Afterwards, pozzolanic reactions are occured which need longer time to originate. During the pozzolanic reactions, hydrated cementitious products such as CAH, CSH and CASH are formed in the soil. These products cause increasing the strength of the soil. Especially, stabilization of highly swelling potential soils provides decreasing in swelling potential and swelling pressure on short term and increasing of strength on long term. The amount of increase in the strength of the soil depends on the parameters such as amount of lime-cement, curing time and temperature. One of the issue that has been faced during the stabilization of the soil with lime or cement is the containing of sulphate in the soil. Thaumasite and Ettringite occurs during the stabilizaiton of sulphate containing soil with lime and cement. The chemical formula of the ettringite is Ca6[Al(OH)6]2.(SO4)326H2O. The structure of the ettringite is similar to tube, which consist of column and canal. In each column consist of calcium, aluminum and hydroxyl and there are also water molecules surrounding them. Columns combined by canals also consist of sulphate ions and water moleculs. Thaumasite (Ca6[Si(OH)6]2.(SO4)2.(CO3)2.24H2O) is very similar to ettringite but it consist of silicium instead of aluminum in its columns and carbonate instead of water molecules in its canals. Thaumasite could be occured directly or it could be occured as a consequence of the isostructure transformation of ettringite. The two main conditons for thaumasite formation are temperature lower than 15 oC and sufficient amount of silicium in the soil. Factors that effect the ettringite formation pH, water content, pore water pressure, aluminum content, void ratio -structure, temperature, clay content, sulphate content, overburden pressure, rigidity of cementitious matrix and time. These compaunds could cause positive impact on soil such as increase in strength or they could also create negative impact such as swelling, heaving or failure. Ettringite has high swelling potential. On the other hand, thaumasite has a swelling potantial which is 45-50% lower compared to the ettringite but it causes loss of strength. It is not sufficient to form of ettringite for sulphate based heave. Ettringite should be grown. There are two different theories for ettringite growing which are crystal growing theory and crystal swelling theory. To this respect, it is cruical to determine the source and amount of the containing sulphate before stabilzation process. Within the scope of this thesis study, lime stabilization and the impact of sulphate on lime stablization of high plasticity clay has been examined. The examined soil has been taken from the near area of 3rd Airport of the İstanbul city. Consistency limits, compaction, unconfined compressive strength and swelling tests have been performed within the scope of the study. Additionally, XRD and ICP-MS test have been performed on some of the soil samples in the Central Labarotary of İstanbul University. The addition of lime changed the consistency limits of the soil. Increase of plastic limit and decrease of plasticity index of the soil has been observed. Consequently, it is obtained that the soil class has changed. It has been determined that the sulphate has not effected too much the consistency limits of the soil. Natural soil is included in high plasticity soil class according to Unified Soil Classification System however lime additive soils are included low plasticity silt class. It is understood that lime addition could change the soil class. Compaction was made by adding lime, it has been obtained that the maximum dry density of the soil has decreased and the optimum water content of the soil has been increased compared to the natural soil. As appropriate to the ASTM D 5102-09 standards, 5 cm diameter and 10 cm height samples are prepared after the compaction curves. Samples have been prepared by using specially manufactured compaction device with having at least 95% compaction ratio and ± 1,25% target water content. Samples have been prepared with three different compaction water content which are 20%, 24% and 28%. Also 5% lime content (5% of dry weight of soil) has been added to the soil and 3,5% of gypsum (3,5% of dry weight of soil) has been added to the soil. Samples have been divided into two subgroups after the preparation. 7 and 28 days curing under 20-25 oC temperature has been applied to the first and second subgroups, respectively. Samples used for swelling tests also stored fully saturated environment for 20 days after curing 7 or 28 days. It also been observed that the adding the lime to the soil has increased the strength and decreased the failure deformation of the soil. The strengths of the sulphated and unsulphated samples have been found very closely after the just curing and unconfined compressive strength. Suplhate with just curing has not much effect to the strength of the soil. After the swelling tests with the sulphated and unsulphated samples, samples has not swelled at the expected level. Subsequent to 7 days of curing and 20 days of fully saturated environment, samples have been observed as fracted from the middle, distributed and swelled at low level. Besides, these samples have lost excessive strength when compared to just curing samples. Samples, subsequent to 28 days of curing and 20 days of fully saturated environment, have not been observed as swelled, fracted or distributed, however, the strength of these samples has increased in comparison to just curing samples. It is obtained that sulphated samples have more strength compared to unsulphated samples. While the maximum strength of a sulpahated sample has been obtained as 1,30 MPa, the maximum strength of a unsulpated sample at the same conditions has been obtained as circa 770 kPa. It also been observed that the strength of the sulphated samples is five times more than the strength of natural soil sample at the same water content. The mineralogical structure of the clay, which has been used in the experiments across the thesis, study and formation of ettringite or thaumasite has been examined through the XRD analysis. The mineralogical structure of this clay has been determined as montmorillonite-muscovite-chlorite-quartz, and not determined the formation of ettringite or thaumasite. Alumina, silica and calcium has been used during the pozzolanic reactions which provides the strength increase. With the ICP-MS analysis, these reactive alumina, silica and calcium amounts has been measured. It has been determined that reactive alumina, silica and calcium amounts would be found higher in sulphate bearing samples than the only lime bearing samples in the same conditions. The results verify the strength difference between the sulphated and unsulphated samples. Turbidimetric analysis is made in order to determine soluble sulphate ion concentration in soil. As a result of this analysis, by using %3,5 calcium sulphate (gypsum) has been concluded %5 soluble sulphate ion. This value correspond approximately 50000 ppm. One of results that are obtained during this thesis study is that the water content above the optimum has better effects on long time curing (28 days) during the lime stabilization. It is also determined that ettringite could not be formed in every clay soil and clay minerology has a crucial role on ettringite formation.