Effect of various additives in activating early age properties of phosphorus furnace slag blended cement

Abstract

Phosphorus furnace slag (PFS) is an industrial by-product of yellow phosphorus production which causes environmental and landfill problems. Researchers have recently shown increased interest in the environmental benefits of incorporating PFS into mortar and concrete mixes. However, the low early strength and extended setting time of PFS restrict its use as a supplementary cementitious material (SCM). Considering this, the effect of different additives, namely sodium sulfate (Na2SO4), calcium sulfate anhydrite, calcined alumina (CA), and calcium aluminate cement (CAC) at up to 5 %, on the setting time, soundness, and early strength (2d) of 25 % and 50 % phosphorus furnace slag blended cement (PFSC) paste and mortar have been investigated. It is observed that chemical activation of PFSC paste is more critical in reducing the setting time. The addition of 3 % Na2SO4 to PFSC mortar resulted in the most significant improvement in early strength, with a 20 % increase in 25 % PFSC and a 27 % increase in 50 % PFSC compared to the reference PFSC mortar. Additionally, the microstructure analysis revealed the additional formation of calcium silicate hydrate (CSH) and ettringite in the 3 % Na2SO4 added PFSC paste compared to the reference PFSC paste. It is established that the relationship between microstructural changes and the enhancement of early strength in PFSC mortar is due to the chemical activation of PFSC paste.