Effects of free lime content in fly ash on sulfate expansion of cement-fly ash mixtures

This study emphasizes the effects of free lime content in fly ash on sodium sulfate expansion of mixtures containing fly ash with different CaO and SO3 contents, designed to have different w/b ratios and fly ash replacements. Two types of fly ash, Mae Moh (high CaO) fly ash and BLCP (low CaO) fly ash were used in the study. Free lime contents of the fly ash were varied at 4%, 7%, and 10% by adding an external free lime source to the fly ash. Anhydrite was added to obtain SO3 contents in fly ash of 5 and 10%. The mortar samples were cast with 0-60% of fly ash replacement. The w/b ratios were varied between 0.35 and 0.65. The experimental results indicate that replacing fly ash reduces sulfate expansions, nevertheless, increasing free lime contents, SO3 contents, and w/b ratios increase the expansions. Higher fly ash replacement levels decrease sulfate expansion when compared with OPC even though the fly ash contains high free lime and SO3 contents up to 10%, except for the mixture containing 20% high CaO fly ash replacement with w/b of 0.55. This indicates that the effects of pozzolanic reaction of fly ash and cement dilution dominate over the free lime adverse effects.