The Sweet Spot for Geopolymers: Why 12 Molar Sodium Hydroxide Makes Fly Ash Mortar Stronger

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The distinction between Class C and Class F fly ash plays a central role in understanding why the mortar mixes in this study could achieve meaningful strength without any thermal curing step. Class C fly ash, typically produced from sub-bituminous or lignite coals, carries a substantial calcium content, and that calcium participates directly in the reaction chemistry alongside the aluminosilicate dissolution driven by the alkaline activators. When sodium hydroxide attacks the glassy phases of the fly ash particles, silica and alumina species are released into solution, while the calcium fraction can form additional gel phases that coexist with the sodium-based aluminosilicate network. This dual-gel chemistry is one reason calcium-rich precursors are frequently favored for ambient-cured geopolymer systems, since low-calcium precursors generally require elevated temperatures to develop strength within practical timeframes.

The role of manufactured sand, or M-sand, in the mortar mixes deserves attention as well. Produced by crushing rock to fine gradation, M-sand offers angular particle shapes and a rougher surface texture compared with river sand, which can improve mechanical interlocking at the binder-aggregate interface. The study’s observation that the 12 M mix produced an improved bond between the binder and fine aggregate is consistent with this interfacial behavior: a denser geopolymer gel filling the interfacial transition zone translates directly into better load transfer across the paste-aggregate boundary. Because mortar properties are strongly governed by this transition zone, the choice of fine aggregate is not a trivial variable but an active contributor to the measured compressive, flexural, and tensile results.

The pattern of strength gain across molarities reflects a well-established trade-off in geopolymer chemistry. At 8 M, the hydroxide concentration is too dilute to dissolve sufficient aluminosilicate material, leaving much of the fly ash as unreacted filler rather than reactive precursor. Raising the concentration to 10 M and then 12 M increases the rate and extent of dissolution, supplying more monomeric silica and aluminate species to the pore solution where they condense into the characteristic Si-O-Al and Si-O-Si networks. However, the behavior at 14 M demonstrates that the relationship is not monotonic. Excessively concentrated hydroxide can promote the formation of crystalline phases and can also lead to premature precipitation or flocculation of dissolved species, reducing the fraction of material available for the amorphous gel that gives geopolymers their strength. The absence of a significant strength improvement at 14 M, despite the harsher chemical environment, illustrates this diminishing and eventually counterproductive return.

The silicate-to-hydroxide ratio acts on a different lever of the same chemistry. Sodium silicate contributes dissolved reactive silica and, importantly, raises the overall alkalinity of the activating solution while moderating the aggressiveness of the hydroxide alone. A ratio of 2.5:1 supplies abundant silicate species that can bridge the dissolved aluminate and silica units into a more highly cross-linked three-dimensional network. The elemental ratios reported from EDAX analysis at the optimum condition, with Si/Al at 4.46 and Na/Al at 1.18, sit within ranges generally associated with well-formed sodium aluminosilicate hydrate gels, in which charge balancing of tetrahedral aluminum by sodium ions is complete enough to stabilize the framework without leaving excessive unreacted alkali. The Ca/Si ratio of 0.18 further suggests a balanced contribution from the calcium-bearing phases relative to the dominant aluminosilicate gel.

Spectroscopic and diffraction evidence in the study aligns with this mechanistic picture. The strong amorphous hump observed in X-ray diffraction patterns, together with the baseline shift noted at 12 M, is a hallmark of a glass-like disordered network, which is precisely the structural feature responsible for the binding capacity of geopolymer materials. Crystalline peaks, by contrast, typically correspond to unreacted fly ash minerals or secondary phases that contribute little to cohesion. The FTIR confirmation of Si-O-T bonding, where T denotes either silicon or aluminum, provides direct molecular-level evidence that condensation reactions have substituted aluminum into silicate chains, a necessary step for the formation of a genuine geopolymer gel rather than a merely dense sodium silicate paste.

The relative magnitudes of the strength differences reported between the optimum mix and its neighbors are informative about sensitivity. The 15 percent compressive strength advantage over the 8 M mix is substantially larger than the 5 percent advantage over 10 M and the 9.5 percent advantage over 14 M, indicating that insufficient dissolution is a more severe failure mode than excessive molarity. Flexural and tensile strengths follow the same ordering but with smaller percentage differences, which is a common observation in cementitious systems: flexural and tensile behavior are more sensitive to flaws, porosity, and interfacial defects than to incremental improvements in matrix density, so relative gains tend to compress at the upper end of the strength spectrum.

Ambient curing itself carries practical significance beyond convenience. Avoiding thermal treatment reduces energy demand on site, permits in-situ casting and conventional formwork practice, and removes a logistical barrier that has historically limited geopolymer adoption outside precast applications. The demonstration that a Class C fly ash system can reach 40.50 MPa at 28 days without heat suggests that calcium-rich precursor chemistry can substitute for thermal activation, at least in mortar-scale specimens. The binder-to-aggregate ratio was also varied in the experimental program, and its influence on early-age properties underscores that paste volume and aggregate packing interact with activator chemistry in determining the final microstructure; a leaner paste may starve the interfacial zone, while an overly rich paste can increase shrinkage and heat evolution.

From a materials-design standpoint, the study’s multi-technique characterization approach illustrates why mechanical testing alone is insufficient for optimizing activator chemistry. Compressive strength provides an integrated outcome measure, but the XRD, FTIR, SEM, and EDAX results together reveal whether strength differences arise from incomplete dissolution, poor gel formation, excessive crystallinity, or interfacial weakness. This mechanistic resolution matters because the same strength deficit can have different causes at different molarities, and remedies that work for one regime may be ineffective or harmful in another. The convergence of elemental ratio targets, spectral signatures, and imaging evidence at 12 M provides a coherent template for mix design that other researchers working with similar Class C precursors can adapt and test.

The findings also sit within a broader effort to valorize fly ash streams that might otherwise be landfilled. Class C fly ash availability varies regionally with coal source, and its higher calcium content makes it chemically distinct from the Class F ashes more commonly studied in geopolymer literature. Demonstrating robust ambient-cured performance with a Class C precursor and manufactured sand broadens the palette of locally available materials that can be considered for alkali-activated construction, particularly in regions where natural sand extraction is constrained and coal combustion products remain abundant. The quantitative benchmarks established here, including the specific strength values and the elemental ratio fingerprints at the optimum condition, offer a reference point against which future formulations incorporating supplementary precursors, alternative activators, or admixture modifications can be compared.

Subject of Research: …

Article Title: Influence of alkaline activator concentration on the mechanical, microstructural properties of geopolymer mortar using Class C fly ash and M-sand

Article References: Deeba, S., & Arun Kumar, A. (2026). Influence of alkaline activator concentration on the mechanical, microstructural properties of geopolymer mortar using Class C fly ash and M-sand. Scientific Reports. https://doi.org/10.1038/s41598-026-70102-7

Image Credits: AI Generated

DOI: 10.1038/s41598-026-70102-7

Keywords: geopolymer mortar, Class C fly ash, alkaline activator, sodium hydroxide molarity, sodium silicate, M-sand, ambient curing, compressive strength, microstructure, aluminosilicate gel, Influence, alkaline