Carbon Quantum Dots Turn Silica Fume Into a Stronger, Cheaper Cement Ingredient

Cement is the invisible backbone of modern civilization, and the race to make it stronger, cheaper, and greener just took a dramatic turn. Researchers in Thailand have shown that a pinch of carbon quantum dots (CQDs) — nanoscale carbon particles roughly 1 to 10 nanometers across — can dramatically improve how efficiently silica fume works inside cement composites. The findings, published in Results in Engineering, reveal that an ultralow CQD dosage of just 0.04 weight percent of the binder can boost compressive strength by up to 83.7% in cement systems containing reduced silica fume contents, all while improving workability and cutting material costs per unit of strength.

Silica fume is a byproduct of the silicon and ferrosilicon alloy industry, prized for its ultrafine particle size and high pozzolanic reactivity. When blended into cement, its amorphous silica reacts with calcium hydroxide (CH) generated during cement hydration to form additional calcium silicate hydrate (C-S-H), the glue-like phase that gives hardened cement its strength. This reaction refines pores and densifies the matrix, making silica fume a staple ingredient in high-strength concrete and ultra-high performance concrete (UHPC). But silica fume is expensive — often several times the price of ordinary Portland cement — because of limited supply, specialized baghouse filtration needed to capture its ultrafine particles, and growing demand from the high-performance concrete sector. Above roughly 20 weight percent replacement, it also makes fresh mixes noticeably less workable due to its enormous specific surface area, which drives up water demand and superplasticizer requirements.

Most practical mixes therefore keep silica fume below 5 to 10 weight percent, yet contents of 20 to 30 weight percent are common in UHPC, where the strength and durability gains are judged to outweigh the high unit price. This tension is exactly why the research team targeted a high silica fume window of 17.5 to 25 weight percent — the regime where the penalties are worst, and thus the most demanding test of whether a nano-additive can recover the lost performance. Their hypothesis was that carbon quantum dots could act as nanoscale organizers of the local hydration environment, adsorbing calcium ions and providing nucleation sites for C-S-H formation, thereby promoting more effective utilization of the silica fume already present in the system.

What sets carbon quantum dots apart from other carbon-based nanomaterials like carbon nanotubes (CNTs) and graphene oxide (GO) is their exceptional water dispersibility and surface chemistry. Structurally, CQDs consist of nanoscale carbon cores containing sp²-hybridized aromatic domains, with surfaces functionalized by hydroxyl and carboxylic acid groups. These oxygen-containing functionalities allow CQDs to disperse uniformly in water without sonication or surfactants — a major practical advantage over CNTs and GO, which typically require dispersing agents and are used at dosages of 0.05 to 0.5 weight percent, one to two orders of magnitude higher than the 0.02 to 0.10 weight percent employed in this study.

The experimental program was rigorous. The team used ordinary Portland cement Type I conforming to ASTM C150, silica fume with 98.36 weight percent SiO₂, and natural river sand. All mixtures maintained a constant water-to-binder ratio of 0.40, sand-to-binder ratio of 1.1, and superplasticizer dosage of 0.25 weight percent of binder. Three experimental groups were designed: one to investigate the influence of CQD dosage on composites containing 25 weight percent silica fume, one to assess whether CQDs could compensate for reduced silica fume contents of 20 and 17.5 weight percent, and one to evaluate the individual and combined effects of CQDs and silica fume using a factorial mixture design. Specimens were cast into 50 mm cube molds, cured at 20°C and above 95% relative humidity, and tested at 3, 7, and 28 days for compressive strength, workability, porosity, and water absorption.

The results were striking. In the 25 weight percent silica fume system, the optimal 0.04 weight percent CQD dosage increased 28-day compressive strength by 23.7% relative to the control, reaching 68.7 MPa. Workability improved simultaneously, with flow diameter increasing by 34.9% at the same dosage — a rare combination, since most nanocarbon additives improve strength at the expense of fresh-state performance. X-ray diffraction (XRD) and thermogravimetric analysis (TGA/DTG) confirmed that CQDs promoted C-S-H formation without altering the type of hydration products, while scanning electron microscopy (SEM) revealed progressively denser microstructures with fewer visible voids at the optimal dosage. The chemically bound water content increased from 8.41% in the control to 9.21% with 0.04 weight percent CQDs, indicating enhanced hydration-product development.

Even more remarkable was the performance at reduced silica fume contents. When the silica fume replacement level was lowered to 20 weight percent, the 0.04 weight percent CQD system achieved a 28-day compressive strength of 63.4 MPa — a 58.5% improvement over its CQD-free counterpart. At 17.5 weight percent silica fume, the enhancement reached 83.7%, with the modified composite achieving 59.1 MPa. These gains suggest that CQDs can partially compensate for the strength loss associated with lower silica fume contents by improving the efficiency of hydration-product formation from the available calcium ions, water molecules, and silicate species. Rather than acting as a direct replacement for silica fume as a SiO₂ source, CQDs function as local hydration-environment organizers that promote the precipitation and growth of hydration products, while the remaining silica fume contributes as a reactive silicate reservoir for secondary C-S-H formation.

The mechanistic investigation revealed how this works at the nanoscale. Zeta-potential measurements showed that CQDs in water carry a strongly negative surface charge of -36.26 mV, attributed to deprotonated carboxylate groups. In saturated calcium hydroxide solution, this charge shifted to -3.28 mV, indicating substantial adsorption of Ca²⁺ onto the CQD surface. Silica fume particles, which carry a negative charge of -27.86 mV in water, reversed to +24.53 mV in saturated calcium hydroxide solution due to Ca²⁺ adsorption onto negatively charged silanol surface sites. When CQDs were added to the ternary CQD-Ca²⁺-silica fume system, the zeta potential shifted to -28.27 mV and the hydrodynamic cluster size increased from 1,261 to 1,906 nanometers — evidence of Ca²⁺-mediated cation bridging between the negatively charged CQDs and silica fume surfaces. X-ray photoelectron spectroscopy (XPS) confirmed that carbon content on the silica fume surface increased from 35.59% to 47.73% after CQD modification, with enhanced C-O and HO-C=O functionalities indicating interfacial interactions between the two materials.

This interfacial organization brings Ca²⁺, reactive silicate species, and interfacial water into nanoscale proximity within the highly alkaline pore solution. Because nucleation constitutes an important kinetic barrier during C-S-H precipitation, this spatial organization provides a favorable chemical environment for the formation of incipient C-S-H domains, effectively lowering the energy barrier for nucleation and accelerating subsequent hydrate growth. The proposed mechanism suggests that CQDs do not merely act as passive nucleation substrates but function as interfacial organizers that facilitate the assembly of species required for C-S-H formation. At excessive dosages, increased CQD-CQD association or aggregation reduces the number of accessible and spatially separated functional sites, diminishing their effectiveness — which explains why 0.04 weight percent emerged as the optimum rather than higher loadings.

A two-way factorial analysis of variance (ANOVA) on 28-day compressive strength revealed that CQDs accounted for 60.2% of the total variation, compared with 25.3% for silica fume, while the CQD × silica fume interaction was not statistically significant (p = 0.211). This indicates that the performance enhancement arises primarily from the hydration-promoting effect of CQDs rather than from a strong synergistic interaction between the two additives. The combined incorporation of 0.04 weight percent CQDs and 25 weight percent silica fume achieved a 28-day compressive strength of approximately 69 MPa — a 36% improvement over the control — with the dual-additive system exceeding the sum of individual effects by roughly 10 percentage points, consistent with complementary rather than strongly synergistic mechanisms.

The economic analysis added a compelling dimension. Strength-normalized material cost (C_SN) — the total raw material cost per cubic meter divided by 28-day compressive strength — was reduced by 18.37% in the 25 weight percent silica fume system with 0.04 weight percent CQDs relative to the CQD-free control. The 20 weight percent and 17.5 weight percent silica fume systems with CQDs reduced C_SN by 8.80% and 12.33%, respectively, relative to the 25 weight percent silica fume control. A one-way sensitivity analysis showed that doubling the CQD price increased C_SN by only 0.96%, while the cost performance was governed primarily by the silica fume price due to its high replacement level. The addition of 0.04 weight percent CQDs transformed the 25 weight percent silica fume system from having a C_SN 21.86% higher than the non-additive cement composite to having a C_SN 0.53% lower, while still achieving a compressive strength of 69 MPa — a high-strength cementitious composite by any standard.

Compared with other carbonaceous additives reported in the literature, CQDs exhibit a prominent advantage. Carbon nanotube-modified systems generally achieve compressive strength enhancements of 20 to 36% at dosages of 0.02 to 0.40 weight percent, with one study reporting only a 6.15% improvement at 0.5 weight percent CNTs accompanied by a 42.1% reduction in workability. Graphene, graphene oxide, and graphene nanoplatelet systems exhibit strength enhancements ranging from 6 to 33%, with some requiring dosages as high as 1.0 to 1.5 weight percent. In comparison, the CQD-modified mixtures in this study achieved 83.7% and 58.5% strength enhancements at only 0.04 weight percent for the 17.5 and 20 weight percent silica fume systems, respectively, while also providing a 34.9% workability improvement in the 25 weight percent system — all without sonication or dispersing agents.

The implications for engineering applications are significant. High-silica fume binders are widely specified for ultra-high-performance concrete, marine structures, bridge decks, and repair mortars, but the high unit price of silica fume and its adverse effect on workability above 20 weight percent replacement have limited broader adoption. The results indicate that an ultralow CQD dosage of 0.04 weight percent addresses both constraints without requiring changes to conventional mixing or placement practice. Where the full 25 weight percent silica fume content is retained, the same dosage delivered a 34.9% increase in flow alongside a 23.7% strength gain, suggesting that superplasticizer demand may be reduced or a lower water-to-binder ratio adopted at equal flow. Because CQDs are water-dispersible and can be introduced through the mixing water using existing batching infrastructure, the approach is compatible with field conditions.

The researchers caution that the results were obtained on mortar specimens under controlled laboratory curing, and that validation at concrete scale, compatibility testing with commercial chemical admixtures, and durability assessment under chloride, sulfate, carbonation, and freeze-thaw exposure are required before the approach can be specified in practice. The economic analysis presented is a raw-material cost comparison and does not constitute a life-cycle assessment. Future work should extend CQD incorporation to concrete-scale mixtures containing coarse aggregates, evaluate long-term durability, and conduct comprehensive techno-economic and carbon-footprint assessments to determine whether the reduction in silica fume demand and improvement in strength-normalized binder efficiency can offset the cost and environmental impact of CQD production.

Nevertheless, the study demonstrates that carbon quantum dots function as local hydration organizers rather than solely as conventional fillers or passive nucleation sites. By modifying the local Ca²⁺-silicate hydration environment, CQDs can promote hydration-product development and improve the mechanical and economic efficiency of silica fume cement composites at very low dosages. The proposed hydration-organizing mechanism may not be restricted to silica fume — ordinary Portland cement inherently contains calcium silicate phases that release calcium and silicate species during hydration, suggesting that the local hydration-organization pathway could represent a more general route through which CQDs influence cement hydration. If validated at scale, this nanoscale strategy could reshape how the construction industry formulates high-performance binders, turning an expensive industrial byproduct into a more efficient and cost-effective ingredient for the sustainable concrete of tomorrow.

Subject of Research: Materials Science

Article Title: Carbon quantum dots enable efficient silica fume utilization by organizing the local hydration environment in cement composites

Article References: Charuchit, A., Nganglumpoon, R., Waiyaka, T., Jongrungrotbaworn, T., Wisuthseriwong, T., Jongvivatsakul, P., Kunawisarut, A., & Panpranot, J. (2026). Carbon quantum dots enable efficient silica fume utilization by organizing the local hydration environment in cement composites. Results in Engineering, 32, Article 113500. https://doi.org/10.1016/j.rineng.2026.113500

Image Credits: AI Generated

DOI: 10.1016/j.rineng.2026.113500

Keywords: carbon quantum dots, silica fume, cement, concrete, nanotechnology, construction materials, sustainability, Carbon, quantum, dots, enable, efficient