Influence of superplasticizers and retarders on the workability and strength of one-part alkali-activated fly ash/slag binders cured at room temperature

One-part alkali-activated fly ash/slag binders have been promoted as an environmentally friendlier alternative to Ordinary Portland Cement (OPC). However, its tendency towards flash setting and high sensitivity of the mixture’s strength to water content have been sources of major concern in the industry. This study investigates the effects of different w/b ratios, admixtures (such as superplasticizers and/or retarders) and the dosage of the selected admixture on the fresh and hardened properties of one-part alkali activated fly ash/slag based binders. To evaluate the fresh properties of binders, setting time and flowability tests were carried out. The compressive and flexural strength tests were conducted to determine the mechanical strength of the samples. The mineralogy and microstructure of the samples were studied using X-ray diffraction (XRD) and scanning electron microscope (SEM) in conjunction with energy dispersive spectroscopy (EDS) which was used to observe the elemental composition. Additionally, an optical microscope was also used to study the surficial physical properties of the hardened binders. The experimental results showed that an increase in w/b ratio led to an improvement in fresh properties of binder but significantly decreased the mechanical strength. A w/b ratio of 0.3 was deemed to be optimal in this study because at this ratio the hardened samples showed no surficial cracking and had enough compressive strength to be viable as a construction material. Test results also showed borax to be superior to all the other admixtures tested when both mechanical strength and workability are considered.