After activated at 850 degrees C under air condition, calcium hydroxide and calcium carbonate in carbide slag are transformed into calcium oxide. The prepared transesterification catalyst, labeled as CS-850, gains surface area of 8.00 m(2) g(-1), functional groups of vanishing O-C-O and O-H bonds, surface morphology of tenuous branch and porous structure and basic strength of 9.8 < H_ < 15.0. From aspects of the molar ratio of methanol to oil (gamma), the catalyst added amount (zeta) and the reaction temperature (T-r), transesterification catalyzed by CS-850 is optimized through the Box-Behnken design of the response surface methodology (BBD-RSM). A quadratic polynomial model is preferred for transesterification efficiency prediction with coefficient of determination (R-2) of 0.9815. The optimal parameters are predicted to be gamma = 13.8, zeta = 6.7% and T-r = 60 degrees C with the efficiency of 94.70% and validated by experimental value of 93.83%. Meanwhile, gamma is demonstrated to be the most significant variable for the minimum p-value. Besides, CS-850 performs acceptable reusability and for the fifth time reusage, efficiency of 82.61% could still be supplied. Aluminium oxide is proved to have the greatest effect on the catalytic activity of CS-850 among other small quality oxides. Physicochemical properties of the purified biodiesel meet American Society for Testing and Material (ASTM) standard. (C) 2014 Elsevier Ltd. All rights reserved.