The Strength, Leaching And Microscopic Mechanism Of Ground Granulated Blast Furnace Slag And Magnesium Oxide Stabilized Lead-Contaminated Soils

Ground granulated blast-furnace slag (GGBS) has been widely used in the additive of cement base materials as a low carbon and sustainable development material. However, cement materials are unsustainable and polluting the environment. Besides, there are many heavy metal contaminated sites which are caused by urban industries removal. Based on this background, with the financial support of National 863 plans projects under Grant No. 2013AA06A206, National Natural Science Foundation of China under Grant No.41330641, No.51278100 and No.41472258, and Natural Science Foundation Outstanding Youth of Jiangsu Province No. BK2012022, a number of tests are done on GGBS-MgO solidified/stabilized lead contaminated clay as a binder. The characteristics of physical-chemical, strength, environmental security and microscopic mechanism are discussed by acid buffering capacity test, unconfined compressive strength test, toxic characteristic leaching procedure and semi-dynamic leaching test. In addition, the comparison between GGBS-MgO and cement solidified/stabilized lead, zinc and cadmium composited contaminated soils are also researched. The main research contents are as follows.(1) The physical-chemical characteristics: Under standard curing and semi-dynamic leaching test, pH of GGBS-MgO stabilized lead-contaminated soils are lower than that of stabilized non-contaminated soils. The soil pH at the specimen subsurface, when the leachant pH being 2.0, is approximately 50% of that when the leachant pH values are 3.0 to 7.0; however, the internal pH is almost the same. The acid buffering capacity of GGBS-MgO stabilized lead-contaminated soils is higher than that of Portland cement stabilized lead-contaminated soils.(2) The strength characteristics:The needle penetration depth of GGBS-MgO stabilized lead-contaminated soils is 1.4 to 3.2 times of stabilized clean soils; however, the unconfined compressive strength (qu) is lower. The needle penetration depth decreases and qu increases with the binder content increases. After semi-dynamic leaching test, the needle penetration depth decreases with the initial leachant pH increase; specifically, qu is 2% to 53% lower than that of 39 days standard curing. Under the same condition, the qu of GGBS-MgO stabilized clean soils is 12% to 43% higher than that of Portlant cement stabilized clean soils; besides, it is 1.3 to 1.8 times of that of cement stabilized lead-contaminated soils with the 18% binder content.(3) The environmental security characteristics:The toxicity characteristic leaching procedure shows that the fraction of leaching lead decreases with the binder content increases. Besides, the semi-dynamic leaching test shows that De of GGBS-MgO stabilized lead-contaminated kaolin is 1-2 orders of magnitude lower relative to cement stabilized kaolin, when the leachant pH is 7.0. Three methods are used to calculate diffusion coefficient. The lead is released by dissolution when the leachant pH is 2.0. Besides, with the pH increases, the mechanism is from dissolution turn to diffusion.(4) The microscopic mechanism: X-ray diffraction (XRD) results shows that GGBS-MgO stabilized lead-contaminated soils produce hydrated magnesium silicate and Ht obviously. The surface adsorption and precipitation are the mainly fixed form of lead. Scanning electron microscopy (SEM) presents that the main hydration products of GGBS-MgO stabilized soils are C-S-H and Ht; however, the presence of lead causes morphological change of C-S-H and Ht. Moreover, mercury intrusion porosimetry (MIP) shows that the cumulative pore volume reduces with curing day increase. More binder contents may cause the structure denser. However, the presence of lead causes the pore size increases. After semi-dynamic leaching test, the pore size is larger than that of the same standard curing specimen. The structure of GGBS-MgO stabilized soils is denser than that of cement stabilized soils.The investigation results reveal that GGBS-MgO, being a binder to solidified/stabilized soil, is better than cement, both in strength and environmental security aspects. It can be used as a binder to solidified/stabilized lead contaminated soil instead of cement.