Effect of free lime content on properties of cement-fly ash mixtures

This research aims to clarify the limitation of free lime content in fly ash according to EN-450. The form and existence of free lime in fly ash by SEM, EDX, and free lime content by a titration method, were observed. Some basic properties of fly ashes with various free lime contents and durability properties of mortar incorporating the fly ashes were also preliminary studied. It was found that free lime particles were mostly distributed outside the fly ash particles and that a minor amount of free lime was encapsulated inside the fly ash particles in the fly ash formation process, meaning that the originally high free lime and added free lime similarly affects properties of fly ash mixtures. A free lime addition method was selected by adding free lime to the originally collected fly ash in order to vary free lime content of the fly ash. The studied basic properties consisted of normal consistency, autoclave expansion, initial and final setting time, water requirement, and compressive strength. In terms of durability, alkali-aggregate reaction, carbonation, shrinkage, and chloride and sulfate resistances, were investigated. Test results revealed that physical properties such as normal consistency and water requirement were unaffected by free lime, and that a free lime content up to 4.51% had only slight chemical effects on fly ash-cement mixtures, i.e., faster setting, higher compressive strength, especially at early age, and higher autoclave expansion. However, the values of autoclave expansion were still within the standard limit. Regarding durability, mixtures with the high free lime fly ash led to higher expansion due to alkali-aggregate reaction. The expansion was smaller than that of a cement-only mixture. Carbonation depth, shrinkage, and chloride and sulfate resistances were only slightly affected by the added free lime. (C) 2012 Elsevier Ltd. All rights reserved.