Use of fly ash to enhance performance of expansive concrete

The use of expansive additive, a kind of mineral admixture, is an effective method for reducing the shrinkage cracking due to the induced expansion at the early age by the production of Ca(OH)2 and ettringite which compensate the effect of subsequent shrinkage. Although the use of expansive additive is effective to reduce the shrinkage cracking, the high price of expansive additive is a factor resulting in the high cost of concrete. At present, some substandard fly ashes consisting high amounts of free lime and SO3 have been occasionally produced and have eventually become a main reason of unused fly ash in Thailand due to the problem of undesirable expansion when used in concrete. In order to find the way for relieving the problem of construction due to expansive additive usage and the problem of unused high free lime and SO3 fly ash in Thailand, the possibility for a combined use of the substandard fly ash and expansive additive in concrete was challenged. This research aims to study performances and clarify performances of expansive concretes containing fly ashes. Both effects of expansive additive types and fly ash types were focused. Setting time, slump, free expansion, autogenous shrinkage, total shrinkage, and unconfined and confined compressive strength were preliminary investigated and microstructural or mineralogical investigation were investigated for clarifying the results. The standard fly ashes which are a low CaO fly ash (FAR) and a high CaO fly ash (FAA), and substandard fly ashes which are high free lime and SO3 fly ashes (FAB and FAC) were used in this study. All types of fly ash were used as a cement replacing material at 0 and 30% by weight of total binders. Four expansive additives referred to as EAD, EAT, EAA and EAB were also used in this study. All types of expansive additive were used for replacing binder at 0 and 5% by weight of total binders for paste and mortar mixtures, and 0 and 20 kg/m3 for concrete mixtures, respectively. The test results on the concrete containing different types of expansive additive revealed that when compared with Non-EA mixtures, the EAT, EAA and EAB mixtures showed better performances in all properties. The EAT, EAA and EAB mixtures showed the comparable autogenous and total shrinkages, and in order, showed slightly lower slump, faster setting, higher free expansion, and higher unconfined and confined compressive strength. When the EAD mixture was cooperatively considered (only compared in case of Non-FA mixture), the EAD mixture showed best performances in results of free expansion, autogenous shrinkage and total shrinkage when compared with the Non-EA and expansive additive mixtures, but the compressive strength of EAD mixture was the lowest in case of unconfined condition, but could turn into the same level with EAT mixture when was made under confined condition. The test results for the effect of concrete containing different types of fly ash revealed that the low CaO fly ash mixtures showed slower setting than that of the Non-FA mixture, but the Non-FA and low CaO fly ash mixtures showed comparable performances in the results of slump, free expansion, and autogenous and total shrinkages. All above mentioned results continuously showed better performances in the high CaO fly ash, and high free lime and SO3 fly ashes mixtures, respectively. The results of unconfined and confined compressive strength showed the same tendency. The early age compressive strength showed the largest in the Non-FA mixtures and respectively followed by the high free lime and SO3 fly ashes, high CaO fly ash and low CaO fly ash mixtures. At long term, the compressive strength of low CaO fly ash mixtures showed the largest or the same with the Non-FA and high CaO fly ash mixtures, and the high free lime and SO3 fly ashes showed the lowest. The results of free expansion and shrinkage also additionally showed that the use of 10% high free lime and SO3 fly ash could enhance the performance of expansion in a similar level to the use of 30% high CaO fly ash. In addition, the use of 30% high free lime and SO3 fly ash, 30% high CaO fly ash, and 10% high free lime and SO3 fly ash in EAT and EAA concretes could reduce the expansive additive usage approximately to 22, 12 and 8 kg /m3, respectively. Moreover, when the expansion at the early age was not considered, the use of expansive additives in concrete was not beneficial to the shrinkage reduction at long term, but it became effective when the expansive additive was applied with fly ash. The different expansion in each mixture could be proved by the results of produced expansive products which were analyzed by X-ray diffraction. The cause for expansion in each type of expansive concrete mainly occurred from different types of produced expansive products which were results of the amounts of different constituents for producing expansive products in each type of expansive additive. The expansion enhancement of expansive concrete containing different types of fly ash was not due to the increased amount of Ca(OH)2, but well correlated with the increased amount of ettringite. The highest ettringite formation was observed in the high free lime and SO3 fly ash mixtures, followed by the high CaO fly ash and low CaO fly ash mixtures, respectively. For the clarifications in the results of unconfined and confined compressive strength, in case of unconfined compressive strength, the different unconfined compressive strength results in each type of expansive concrete was mainly caused by the different porosity values. The mixtures with lower porosity, indicating to a denser paste structure, resulted in higher compressive strength. Comparing between the specimens made under unconfined and confined conditions, the results of compressive strength were higher when the specimens were prepared in confined condition. The confinement of the specimens led to the reduction of porosity of concrete, indicating a denser paste structure, therefore resulted in an enhancement of compressive strength.