Effect of reactive MgO on hydration and properties of alkali-activated slag pastes with different activators

Abstract

Activator type significantly affects the properties of alkali-activated slag (AAS) cement. Reactive MgO may have different effects on AAS cement activated with different activators. Four activators, sodium hydroxide (NaOH), water glass (WG), sodium carbonate (Na2CO3) and sodium sulfate (Na2SO4) were used in the present paper. The effect of reactive MgO on the fluidity, setting time of AAS paste and the compressive strength of AAS mortar was studied. The hydration process was assessed with pH and hydration heat. Microstructure and hydrates were also characterized by XRD, SEM and EDS. The results show that the addition of reactive MgO decreases the fluidity and setting time of AAS paste regardless of activator type. In NaOH activated slag, the compressive strength decreases with the increase of reactive MgO. While in the other three activators, there exists an optimal reactive MgO content of about 4%–6%. Now the compressive strength is higher. Reactive MgO increases the pH, heat evolution rate and cumulative hydration heat of AAS cement. Hydrotalcite and C-(A)-S-H gel are the main hydration products. More hydration products are promoted by reactive MgO, which results in the densification of AAS matrix. The above changes due to reactive MgO are especially remarkable for Na2CO3 and Na2SO4 activated AAS cement. AFt is more likely to be formed than hydrotalcite in Na2SO4 activated slag and reactive MgO enhances this trend. Reactive MgO is especially suitable for Na2CO3 and Na2SO4 activated AAS cement. It can accelerate the hydration, adjust the fluidity and setting time, and improve the mechanical properties to a reasonable degree.