Effect of excess dosages of superplasticizer on the properties of highly sustainable high-volume fly ash concrete

It is quite common for researchers to use an excess dosage of superplasticizer to achieve the desired very low water to binder (w/b) ratio required for sustainable high-volume fly ash (HVFA) concrete mixes in order to obtain early strength without reporting on the effects of the excess dosage. This study investigates the effects of such excess dosages on the properties of highly sustainable HVFA concrete. Four series of concrete mixes were designed, with Series 0 being the control concrete mix containing no fly ash and no superplasticizer. Series 50, 60 and 65 contained HVFA concrete mixes that had 50, 60 and 65% fly ash content, respectively. Series 50, 60 and 65 contained three similar mixes; in each series, the three mixes were prepared with the maximum dosage of superplasticizer at 2% of the binder by mass, and excess dosages at 3% and 4%, respectively. The effect of the excess doses on slump, flowability, compressive strength, flexural strength, tensile splitting strength, and abrasion resistance were investigated. The results show that an excess dosage of superplasticizer helps to achieve increased workability, causes a decrease in abrasion resistance and has no decisive effect (good or bad) on the compressive, flexural and tensile splitting strengths of HVFA concrete mixes. An increase in fly ash content in the HVFA concrete mixes resulted in reduced overall flexural strength, tensile splitting strength and abrasion resistance. Not using a very low w/b ratio, which can be achieved by using an excess dose of superplasticizer, results in HVFA concrete mixes struggling to meet the minimum required compressive, flexural and tensile splitting strengths of various standards. It is concluded that although HVFA concrete, which is normally prepared with a high dosage of superplasticizer, is highly sustainable, it is not the best for applications such as industrial floors, where wear and abrasion resistance is of vital importance.