Expansive behavior of coal mine spoils treated with calcium-based stabilizers

Abstract

Sulfate-induced swelling, resulting from the chemical treatment of sulfate rich soils, has been known to cause significant damage to pavements and other structures particularly in the south-western United States. This paper addresses the problem of sulfate-induced swelling in coal mine spoils, formed as a result of shallow strip coal mining, after treatment with calcium-based stabilizers. Samples of coal mine spoils were obtained from the Hawthorn coal mine area in Sullivan County, Indiana, and treated with the following calcium-based stabilizers: hydrated lime, quick lime and Portland cement, at dosages ranging from 1.5% to 9% by dry mass of the spoils. The swelling tests, conducted over a period of one month, were performed using the sand bath method in a custom setup housed in an environmental chamber. All specimens of the spoils treated with the stabilizers were found to exhibit swelling greater than that measured on the untreated spoils, with the greatest swelling observed following treatment with 3% hydrated lime. Swelling of the treated spoils was initially delayed and approached a constant value after approximately one month.X-ray diffraction and thermo-gravimetric analyses provide evidence of the presence of ettringite in all spoil–stabilizer specimens. While the formation of this mineral appears responsible for the observed expansion, the measured swelling strain could not be correlated to the amount of ettringite formed, and appears, instead, governed by the restraint provided by the stabilized matrix.Tests were also conducted to examine the effectiveness of the following two mitigation techniques: pre-compaction mellowing and double lime treatment. A pre-compaction mellowing period of 2–3days led to swelling strains comparable to those of the untreated spoils. No advantages could be identified in using double lime treatment.