Abstract: A method for well cementing may include supplying a settable composition including a cement composition and water into a well bore, and allowing the settable composition to set to form hardened concrete. The cement composition may include a hydraulic cement and natural glass, and the hardened concrete may have a strength activity index of at least 125%. A method for making well cement may include combining hydraulic cement and natural glass to form a cement composition, and adding water to form a settable composition. The method may further include allowing the settable composition to set to form hardened concrete, wherein the hardened concrete may have a strength activity index of at least 125%. A cement composition for use in well cementing may include hydraulic cement and natural glass having a median particle size (d50) of less than 12 microns and a top particle size (dm) of less than 30 microns.