Slag was utilized as an additive for fly ash-based geopolymers in this study. It was found that the incorporation of slag could significantly increase the compressive strength of the geopolymer. The compressive strength of geopolymer with 4.0% slag reached 50 and 70MPa when cured for 14days at 30 and 70°C, respectively. The mechanism of slag as additive on the enhancement of compressive strength of geopolymer was investigated using X-ray diffractometer (XRD), Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and mercury intrusion porosimetry (MIP). The XRD and FTIR results showed that the addition of slag could generate more amorphous products and accelerate the reaction rate of raw materials. From XPS results, the decrease of binding energy and a broadening of peaks were observed for Si2p, Al2p, and O1s elements due to the Ca provided by slag. The decrease in binding energy was more favorable to zeolite formation. The results of MIP suggested that the 4% slag addition significantly influenced pore structure of geopolymer. A refinement of pore size was exhibited after 4% slag addition, especially for specimens being cured at 70°C. For these reasons, the compressive strength of geopolymer made with 4% slag addition was greatly improved. © 2007 ASCE.