Fresh State, Rheological and Microstructural Characteristics of Alkali-Activated Mortars Developed Using Novel Dry Mixing Technique under Ambient Conditions

Abstract

Ambient cured alkali-activated mortars (AAMs) are developed through the activation of supplementary cementitious materials (SCMs) by powder form reagents with silica sand using a novel dry-mixing method. The fresh state, rheological, compressive strength and microstructural characteristics of eight AAM mixes are comprehensively investigated. The effects of binary/ternary combinations/proportions of SCMs, different combinations/dosages of powder form reagents and the fundamental chemical ratios (SiO2/Al2O3, Na2O/SiO2, CaO/SiO2 and Na2O/Al2O3) present in the precursors and the reagents are investigated. The AAM mixes obtained compressive strengths ranging from 34 to 42.6 MPa with initial and final setting times between 122 and 458 min and 215 and 483 min, respectively. The yield stress and viscosity of the mixes decreased with the increase in the slump flow spread. All the mixes demonstrated pseudoplastic behavior. The microstructural analysis revealed the formation of more longer polymeric chains comprising Si-Al linkages in N-C-A-S-H/N-A-S-H gels for reagent one (calcium hydroxide:sodium metasilicate = 1:2.5) mixes, which resulted in a lower slump flow, higher yield stress, higher plastic viscosity and quicker setting times compared to their reagent two (calcium hydroxide:sodium sulfate = 2.5:1) counterparts.

Strength, Shrinkage and Early Age Characteristics of One-Part Alkali-Activated Binders with High-Calcium Industrial Wastes, Solid Reagents and Fibers

“…This is consistent with previous research [14,15], where no significant improvements in compressive The binary binders without fibers obtained compressive strengths ranging from 41 to 57 MPa at 28 days, as shown in Figure 4a,b. The formation of the C-A-S-H/C-S-H binding phase/gel in binary mixes was responsible for high early strength and faster strength gain, as verified from the SEM/EDS and XRD analysis and consistent with previous studies on fly-ash/GGBFS binders and mortars [57,58,60]. Ternary mixes obtained 7% to 14% lower strengths compared to binary binders at 28 days.…”

“…The dominant binding gel in a high calcium system was observed to be crystalline C-A-S-H under X-ray diffraction (XRD), which validates the workability parameters' results. Similar observations were made in a previous study on GGBFS-based binders and mortars incorporating CaO as a reagent [57,58,63]. The workability of the binders was not significantly affected by the incorporation of PVA fibers as normally observed in cement-based materials.…”

“…The compressive strength development was associated with the formation of both binding phases (C-A-S-H and N-A-S-H) in FA/GGBFS binders, as observed from the matrix microstructure through SEM/EDS and XRD analyses. The formation of such binding phases was also observed in previous research studies on FA/GGBFS-based alkali-activated mortars and composites [9,[55][56][57][58]. The binders (without fibers) incorporating reagent 2 exhibited 2% to 18% higher 28 day compressive strength (between 39 and 57 MPa) than their reagent 1 counterparts (Figure 4a,b and Table 2) due to the formation of additional C-S-H gel formation associated with high calcium content in the system.…”

“…These results are also consistent with the findings of previous research works. The formation of the C-A-S-H binding phase produces high early and ultimate strength in high calcium binder and mortar systems [57][58][59]. The binary binders without fibers obtained compressive strengths ranging from 41 to 57 MPa at 28 days, as shown in Figure 4a,b.…”

“…These results are also consistent with the findings of previous research works. The formation of the C-A-S-H binding phase produces high early and ultimate strength in high calcium binder and mortar systems [57][58][59].…”