Abstract: To enhance the performance of compacted clay as the cover barrier material, alkali-activated slags together with or without bentonite are used as amendment materials to amend the compacted clays. The alkali materials include quicklime, calcium carbide residue, and magnesium oxide. Unconfined compression tests and flexible-wall hydraulic conductivity and rigid-wall gas permeability tests are conducted to evaluate the mechanical properties, hydraulic and gas barrier performance of the amended compacted clays. Test results show that unconfined compressive strength (q_u) increased by 5~12 times after 28 days of curing and it increased by 6.0%~56.7% after 60 days of curing as compared to 28 days of curing. All amended compacted clays exhibit acceptably low hydraulic conductivity (k_w) below 10^-9 m/s, and calcium carbide residue-activated slag amendment reduces k_w by over one order of magnitude. All alkali-activated slag amendment lowers gas permeability (k_g) by over one order of magnitude. Calcium carbide residue-activated slag amendment yields the lowest k_w and k_g.