Having no major application, zinc slag generated during imperial smelting process (ISP) all over the world is simply dumped. Some occasional research attempts, seen in the literature, are limited to its use as a replacement for aggregates in construction related applications. This study is a part of our activities towards complete utilization of slag for building material applications using geopolymerization process, which involves formation of a new rock like species from various aluminosilicate minerals under strong alkaline environment. This slag has been subjected to short grinding in a high-energy AGO-2 mill to enhance its reactivity. The paper particularly focuses on the influence of the grinding atmosphere, viz, air or CO2 on the geopolymers prepared from milled slag. Studies have shown that the particulate characteristics mostly remain unaffected by the milling atmosphere. However, conduction calorimetric experiments have shown that the milling atmosphere has a bearing on the reactivity of slag despite their similarity in particulate characteristics; CO2-milled slag has been found to be more reactive than the air-milled slag. Both air and CO2-milled slag has been found good geopolymerization behavior leading to high compressive strength of the geopolymer products; higher compressive strength values for the geopolymer prepared from the latter further shows its enhanced reactivity over the former. TCLP tests of the prepared samples confirm that the release of toxic metals is within USEPA limits and hence the process is an environment friendly way of utilizing the zinc slag. (C) 2013 Elsevier B.V. All rights reserved.