Strength and microstructural characteristics of cement solidified lead-contaminated kaolin exposed to leaching circumstances

The strength and microstructural characteristics of cement stabilized/solidified lead-contaminated kaolin clay are investigated. The leaching of calcium (Ca) is studied under the targeted leachant pH of 2, 4, and 7 via a series of semi-dynamic leaching tests. The results reveal that the unconfined compressive strength (q_u) of the stabilized/solidified soils experiencing the semi-dynamic leaching tests is 1% to 42% lower than that of the soils cured for 39 days under standard conditions (SC). The cumulative leaching fraction of Ca (CFR_Ca) at the leachant pH of 2 is 2 to 7 times greater than that at the leachant pH of 4 or 7. The difference in q_u or CFR_Ca of the soils experiencing the semi-dynamic leaching tests at the lechant pH of 4 and 7 is marginal. When the cement content increases from 12% to 18%, q_u increases by 35% to 98%; whereas CFR_Ca decreases by 40% to 58%. Furthermore, q_u of the stabilized/solidified lead-contaminated soils is 50% to 68% lower, and CFR_Ca is 29% to 175% higher than that of the stabilized/solidified clean soils. An empirical equation is proposed for predicting the loss of unconfined compressive strength (i.e., ratio of q_u obtained after leaching tests to that obtained at curing time of 39 days under standard conditions) using CFR_Ca. It is shown that the proposed method has high accuracy. The results of X-ray diffraction, scanning electron microscope, and mercury intrusion porosimetry analysis show that the relatively high concentration of Pb has remarkably retarded hydration/pozzolanic reactions in the stabilized/solidified lead-contaminated soils. Therefore, the strength development, leaching of Ca, and microstructural characteristics of the stabilized/solidified lead-contaminated soils differ from those of the stabilized/solidified clean soils in a notable manner.