Recycled cement use to produce fly ash – based geopolymer binders suitable for ambient curing: Comparison with slag effects
D. de Klerk, Abdolhossein Naghizadeh, Stephen O. Ekolu, Megan Welman-Purchase
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.