This paper presents the results of an experimental investigation on strength development and chloride resistance of low-reactivity ground granulated blast furnace slag (GGBFS) incorporated concretes. Fine and coarse GGBFS samples were used to replace the Portland cement at 20 %, and a series of tests were conducted to investigate the effects on conventional and high strength concretes. Compressive strength, RCMT, RCPT and electrical resistivity tests were carried out at different ages up to 180 days. Furthermore, half-cell potential test was conducted weekly on steel reinforced specimens exposed to wet–dry cycles for 30 weeks. The results imply that despite the low activity index of the local GGBFS, it could be used at low levels of Portland cement replacement. Of note, however, is that despite its positive effect on the chloride resistance of concrete, a reduction in compressive strength (mainly at early ages and for conventional concrete mixes) should be expected. Furthermore, the test results indicate that increasing fineness of low-reactivity local GGBFS (within the studied range) does not lead to higher resistance of concrete against chloride attack.