Rheological, Fresh State, and Strength Characteristics of Alkali-Activated Mortars Incorporating MgO and Carbon Nanoparticles

This study presents a comprehensive assessment of the fresh state, rheological, and mechanical properties of alkali-activated mortars (AAMs) developed by incorporating magnesium oxide (MgO) and nanomaterials. A total of 24 AAM mixes with varying content of MgO, multi-walled carbon nanotube (MWCNT), and reduced graphene oxide (rGO) were developed following the one-part dry mix technique using powder-based activators/reagents. The effects of the types/combinations of source materials (binary or ternary)/reagents, MgO (0 to 5%), MWCNT (0 to 0.6%), and rGO (0 to 0.6%) were evaluated in terms of the mini-slump flow, setting times, viscosity, yield stress, compressive strength, ultrasonic pulse velocity (UPV), and microstructural properties. The results showed that the addition of finer MgO/nano-fillers produced a higher viscosity and yield stress accompanied by a lower slump flow and setting times. The addition of 5% MgO resulted in the lowest slump flow of 80 mm, 2-2.5 times higher viscosity, and the reduction in the initial and final setting times of about 21% and 16%, respectively. Mixes with MWCNT showed about 5-10% higher viscosity whereas for mixes with rGO, the values were noted to be 8% higher, on average, than the mixes with no MWCNT or rGO. All the developed AAMs exhibited shear-thinning behavior. The 28-day compressive strength of the AAMs ranged from 37 MPa to 49 MPa with 5% MgO and up to a 0.3% MWCNT/rGO addition increased the compressive strength. Correlations among the fresh state, rheological, and mechanical properties such as the viscosity, slump flow, setting time, compressive strength, and UPV are also described.