Effect of Chemically Activated Fly Ash on Concrete

Abstract

Fly ash is most commonly used as a replacement material for cement in concrete. The fly ash based concrete mixes exhibits low initial strength development due to low pozzolanic reactivity of fly ash particles. It limits the usage of fly ash up to a certain percentage in case of cement manufacturing process and also reduces the application of high-volume fly ash structures in construction field. A number of studies had been conducted on the chemical activation of fly ash using various sulphate sources. Phosphogypsum (PG), a particular type of industrial waste material which is an another form of calcium sulphate, has not been used for this purpose. This paper presents an exploratory study on the effectiveness of phosphogypsum on activating fly ash cement system. The initial strength development of chemically activated fly ash mixes is mainly due to the formation of ettringite (AFt) and tobermorite crystals during the early age of hydration. The fly ash cement concrete mixes were prepared by replacing cement with fly ash by 15, 25, 35, 45% and these mixes were treated with 0, 6, 8, 10, 12% of phosphogypsum. The activation effect happens mainly during the first 3–7 days. The strength parameters and durability nature of chemically activated fly ash mixes were compared with that of the normal fly ash mixes. Strength tests such as compressive, split tensile and flexural strengths were conducted at age of 3, 7 and 28 days on various fly ash mixes. Durability nature of activated and non-activated fly ash mixes were studied by acid and base durability tests at age of 90 days. The obtained results indicate that the initial strength development is optimum at replacement of 35% fly ash and 8% PG. Based on the test results the chemically activated fly ash mix shows improved strength and durability nature compared to normal fly ash mixes.