Recycled cement use to produce fly ash – based geopolymer binders suitable for ambient curing: Comparison with slag effects

Abstract

This paper presents an investigation into potential use of recycled ordinary Portland cement waste powder as a supplementary binder material for accelerating the setting times of the fly ash – based geopolymer binder system, thereby enabling strength development under ambient curing. Typically, fly ash – based geopolymer binders exhibit long setting times and accordingly require curing at elevated temperatures for the system to achieve early – age strength development. In this study, recycled cement material was prepared by crushing and milling hardened Portland cement paste. The recycled cement material was then incorporated into fly ash – based geopolymer mortars at proportions of 0–20 % by weight. The performance of recycled cement was compared with that of mixtures containing 20 % or 40 % ground granulated blast – furnace slag. The activator used to prepare geopolymer mixtures, was a binary alkali solution comprising sodium silicate and sodium hydroxide. Various mechanical properties were measured including flow workability, setting time at ambient temperature, compressive strength and drying shrinkage. Also measured were pore – related physical properties. Analytical studies were performed using X – ray diffraction and scanning electron microscopy. It was found that the optimal proportions of 7.5–10 % recycled ordinary Portland cement, produced ambient – cured mixtures of suitable initial setting time values ranging from 100 to 165 min, along with low drying shrinkage not exceeding 0.50 % and higher 28 – day compressive strength of about 60 MPa, these values being better relative to those of the control fly ash – based geopolymer mortar. Recycled cement was generally more effective than ground granulated blast – furnace slag. • FA – fly ash, ROPC - recycled ordinary Portland cement, GGBS – ground granulated blast – furnace slag. • Below 10 % ROPC, workability of FA geopolymer mortars was not adversely affected. • Optimal 7.5–10 % ROPC or 40 % GGBS gave suitable setting times and strengths. • ROPC adversely increased the pore volume and water absorption values of mortars. • ROPC and GGBS significantly reduced drying shrinkage of FA geopolymer mortars.