Effect and limitation of free lime content in cement-fly ash mixtures

This study emphasizes the limitation of free lime content in fly ash. A detailed experimental program was carried out in order to evaluate the extent to which free lime content in fly ash can be tolerated, particularly for fly ashes with different sulfur trioxide contents. Some basic and durability properties of fly ash mixtures with varied free lime contents were considered. Four distinct types of fly ashes were obtained from two different sources, and free lime was added to obtain overall free lime contents of 5%, 7% and 10% for each type of fly ash. Water requirement, initial and final setting times, compressive strength, autoclave expansion, alkali aggregate reaction (AAR), and sulfate resistance tests of fly ash mixtures, containing various free lime contents, with two fly ash replacement percentages (20% and 40%), were conducted. Experimental results revealed that an increase in the free lime content caused an increase in water requirement. Higher free lime content also lead to faster setting times, improved compressive strength, and higher autoclave expansion. Mixtures with 20% fly ash replacement and free lime content up to 10% as well as mixtures with 40% fly ash replacement and free lime content up to 7.72% experienced autoclave expansion within the specified limit of ASTM C618. Similar trends of expansion were observed in cases of alkali-aggregate reaction and sulfate resistance tests where fly ash mixtures with high free lime led to higher expansion. In alkali-aggregate reaction testing, the mixtures with 20% fly ash replacement and free lime content up to 7.95%, as well as mixtures with 40% fly ash replacement and free lime content up to 10%, expanded less than the cement-only mixtures. The effect of added free lime was more severe in the sulfate resistance test, as fly ash mixtures tend to expand more than cement-only mixtures, especially in the case of fly ashes with very high SO3 content (>5%). Test results also revealed that it is possible to utilize 20% of tested fly ash in a mixture as binder, with SO3 content <5% and free lime content up to 10% while not compromising the basic and durability properties. In the case of 40% fly ash replacement, tested fly ashes with SO3 < 5% and free lime content up to 4.23% can be utilized. In the case of high SO3 content in fly ash, i.e., 8.53% and 9.44% in this study, the limit of free lime content of fly ash is reduced to 5.31% and 3.73%, for 20% and 40% fly ash replacements, respectively to satisfy the durability requirements.