Surface coating on solidified water-based drilling waste materials and its mechanism for resource reuse

BACKGROUND: Stabilization/solidification treatment has been used as the final step for water-based drilling waste materials (WBDWMs) worldwide; however, solidified WBDWMs possess potential secondary leaching pollution and environmental adaptability problems. The objective of this study is to develop a surface coating technology for deeply treating solidified WBDWMs. RESULTS: The mechanical, wettability, durability and environmental protection properties of uncoated and coated solidified WBDWMs were conducted in this work. Test results showed that after surface coating the solidified WBDWMs exhibited remarkable water-proof performance, water absorption rate decreased from 0.06489 to 0.0314 mm min^−1/2 and surface contact angle increased from 36.25° to 65°, resulting in the reduction of strength loss rate from 59% to 16% after 1-day soaked in water. Moreover, after immersed in 10% hydrochloric acid (HCl), 10% sulfuric acid (H₂SO₄) and 10% sodium hydroxide (NaOH) for 7 days, both external and internal structures remained intact and strength loss rates were below 50%. Furthermore, environmental performances related to the leachate of coated WBDWMs achieved PRC national Grade I discharging standard and contents of heavy metal ions were too low to be detected. Microcosmic tests showed that the high strength of solidified WBDWMs was mainly from mechanical interlocking of hydration products. CONCLUSION: Based on the surface coating treatment, a sealed hydrophobic layer tightly bonded to substrate can be formed, which prevented degradation of strength when exposed to harsh environment and leaching of inside contaminants. Therefore, the application of surface coating can enhance durability and environmental performances of WBDWMs for better reuse.