Fabrication and Analysis of an Effective Composite Desiccant for the Rapid Dehydration of Shield Waste Slurry

Massive shield waste slurry generated in shield tunnel construction is difficult to deal with because of its high water content (WC). Only when the WC of waste slurry is reduced to 40%–60% it can be further disposed and reutilized. Three inorganic materials including attapulgite (AT), montmorillonite (MT) and water-washed kaolin (WK) were utilized to dehydrate the shield waste slurry and found that the WC of shield waste slurry dropped to 48.3%, 48.2% and 49.6% with 12.5% AT, 10% MT, and 12.5% WK as the desiccant, respectively. To enhance the dehydration efficiency, a composite desiccant was prepared with AT, MT and WK for the rapid dehydration of waste slurry. The optimized ratio of composite desiccant for AT: MT: WK was decided as 3.23%: 3.50%: 3.40% by response surface methodology based on Box-Behnken design, corresponding with the predicted WC as 44.9%. The WC of shield waste slurry can be decreased to 44.1% after the waste slurry has been dehydrated with the optimal composite desiccant after 6 days. The pH value of solidified shield waste slurry cured with the optimal composite desiccant was approximately 10.5, benefiting the further disposal and resource reutilization of shield waste slurry. The shield waste slurry was characterized by X-ray diffraction, Fourier transform infrared spectroscopy and scanning electron microscopy. The results revealed that hydration products came into being during the hydration reaction when the shield waste slurry was cured with desiccant. The hydration products refabricated the microscopic lamellar structure of the waste slurry, squeezed the interlayer gap and drove the interlayer water away, leading to the depression of the WC of waste slurry.