High amounts of fly ash generated as a by-product of coal combustion in thermal power generation plants have been a concern for years due to their negative environmental impact; in Colombia, for example, this fly ash represents 6,000,000 tons per year. The use of this material in industrial processes is among the many solutions proposed, provided that it has pozzolanic properties useful in cement and concrete industries. The aim of this study is evaluating concrete resistance to compression at different ages, replacing cement by fly ash of 40%, using alkaline activation with NaOH, Sodium Sulfate, and limestone in different proportions. The results show that activation improves fly ash performance and it is possible to achieve better mechanical properties than with mixtures without activation.