As a predominant construction material in modern civil infrastructure (i.e., roads, bridges, ports, and airports), concrete is susceptible to adversing actions such as external loading, freeze–thaw cycling, and salt ingress. These factors readily induce surface spalling and scaling, accelerating service-life deterioration and posing significant safety risks. The existing polymer-modified mortars and epoxy-based systems are commonly employed for the repair and strengthening of concrete structures. An organically modified belite-calcium sulfoaluminate (HB-CSA) cement mortar is adopted in concrete repair projects due to its rapid setting and early strength. However, the performance requirements differ markedly across construction scenarios in low-flow applications (e.g., vertical or overhead placement), the mortar must maintain a low flowability to prevent sagging/run-off during placement, and mechanical performance depends primarily on bond strength, with relatively relaxed demands on compressive and flexural strengths. By contrast, high-flow applications (e.g., pavement repair or large-area casting) require a high flowability to ensure adequate spreading and filling workability. The compressive strength is a principal mechanical target, while the flexural and bond performance must be also satisfied. These divergent demands make it difficult for a single HB-CSA mixture to meet the performance needs of multiple repair scenarios, underscoring the practical importance of multi-scenario mixture-design optimization for HB-CSA repair mortars.

This study was to investigate an organically modified belite-calcium sulfoaluminate (HB-CSA) cement mortar system. Flow spread was employed as a constraint to distinguish high-and low-flow-state scenarios, while 7-d compressive, 7-d flexural, and 7-d bond strengths were taken as the optimization objectives. To address mixture optimization for both flow states, we introduced (i.e., pioneering its use in the cement materials field) a variational-autoencoder-based generative optimization algorithm (VAE-GOA). The method could leverage a generative model to progressively estimate the probability distribution of optimal mixtures over the global design space. In parallel, an adaptive global-exploration strategy prioritized high-potential regions to mitigate premature convergence to local optima, and an iterative optimization scheme further drived the search toward superior solutions. The approach delineated the optimal composition windows and corresponding performance of HB-CSA repair mortars under different flow-state scenarios via visualizing the VAE-GOA–estimated distribution of optimal mixtures. In addition, backscattered electron (BSE) imaging of the interfacial transition zone (ITZ) was also employed to elucidate the mechanisms underlying the improvement in bond performance.

At the outset, the VAE-GOA conducts broad, globally exploratory sampling, while unavoidably covering some low-performing regions, effectively uncovering previously unexplored high-potential areas. As iterations proceed, probability mass progressively concentrates in high-performance regions. The search distribution transitions smoothly from global exploration to local exploitation, reduces attention to low-performing zones, and converges toward a compact subspace containing the best-performing mixtures. These dynamics substantiate the algorithm's intended explore-then-exploit behavior in a multi-constraint design space and effectively mitigate premature convergence to local optima.

Based on two optimization generations and despite strict flowability constraints and limited sampling, the VAE-GOA delivers both macro-level performance gains and a marked expansion of feasible design space. The optimum weighted comprehensive performance is improved by about 20% in the high-flow state group and by >25% in the low-flow state group. The candidates with compressive strength >50 MPa increase from 2 to 6, and those with bond strength >6 MPa increase from 4 to 12, substantially broadening mixture options that meet key targets. For low-flow state scenario, increasing the USCMs is accompanied by a nonlinear decrease in the HPMC, and higher USCMs generally require a lower water–binder ratio (w/b). A practical window of 15%–30% USCMs, 2%–6% Wacker 328, and about 0.1% HPMC at a w/b ratio of 0.22 achieves a low flowability witha high interfacial performance (i.e., mixture L8 attains 48.6 MPa (compressive), 9.2 MPa (flexural), and 6.5 MPa (bond)). For high-flow state scenario, the strength-oriented optimum occurs near about 5% USCMs + 1% Wacker 328 at a w/b ratio of 0.25 (i.e., H7 reaches 63.4 MPa (compressive), 8.3 MPa (flexural), and 4.5 MPa (bond)). For cost-oriented deployment, increasing the USCMs to ~30% with 5% Wacker 328 at a w/b ratio of 0.20 yields a balanced, economical option (i.e., ≈50.7/7.4/6.6 MPa for compressive/flexural/bond). Overall, substituting 15%–30% the USCMs for HB-CSA regulates a flowability and enhances a bond strength as well as reduces production costs by 13%–27%.

Holding the HPMC and w/b ratio roughly fixed while pushing the USCMs beyond 30% (H8 vs. H1) reduces the compressive strength by 14.7 MPa but elevates the bond strength to 6.9 MPa, highlighting a deliberate bond-first Pareto choice. The backscattered electron imaging corroborates this pheoneman. The interfacial transition zone (ITZ) in H8 exhibits a significantly lower porosity and a denser microstructure at the old–new interface, explaining the observed bond enhancement.

In this study, the VAE-GOA could expedite the discovery of high-potential HB-CSA mixtures under strict flow constraints and limited sampling. The comprehensive performance of high-flow state mortars was increased by 20%, and that of low-flow state mortars by >25%. Beyond single best points, the method could yield scenario-specific composition windows and a diverse portfolio choices that practitioners could select from according to the strength, bond, and cost priorities. In particular, low-flow applications were supported by a stable window (e.g., 15%–30% USCMs with 0.1% HPMC and w/b ≈ 0.22), while high-flow placement admited both a strength-oriented option (~5% USCMs + 1% Wacker 328 at w/b = 0.25) and a cost-oriented alternative (30% USCMs + 5% Wacker 328 at w/b = 0.20). The VAE-GOA could be sample-efficient, resilient to local optima, and readily extensible to multi-objective constraints. Looking ahead, coupling the framework with durability targets (e.g., freeze–thaw and chloride ingress), long-term field validation, and uncertainty-aware priors could broaden its applicability to scenario-aware specification and lifecycle-optimized repair design.

Cement concrete materials are one of the most widely used building materials in modern infrastructure, but their production process is associated with high energy consumption and significant carbon emissions, imposing substantial environmental and resource pressures. While traditional mineral admixtures (such as fly ash and slag) can improve concrete workability, enhance mechanical properties, reduce hydration heat, and improve durability by partially replacing cement, they suffer from low early-stage activity. The application of advanced grinding or sorting technologies to refine mineral admixtures into ultrafine particles has proven effective. This process increases the specific surface area of the admixtures, thereby enhancing particle surface energy and reactivity, which compensates for the deficient early-stage activity of conventional mineral admixtures. Ultrafine mineral admixtures demonstrate remarkable potential in improving concrete workability, early-stage mechanical performance, and durability. In recent years, this approach has garnered widespread attention in academic and industrial research.

This paper first elucidates the characteristics of fly ash microsphere, ultrafine fly ash, and ultrafine slag powder. Subsequently, it systematically investigates the impacts of these three typical ultrafine mineral admixtures on various properties of cement and concrete, accompanied by comparative analyses of their performance differences compared with conventional mineral admixtures. Furthermore, the intrinsic properties, compatibility design principles, and comprehensive effects of ultrafine composite mineral admixtures on cement concrete performance are expounded. Finally, the current application status of ultrafine mineral admixtures in cement concrete is summarized. Ultrafine mineral admixtures exert multiple beneficial effects in cementitious systems, including filling effect, morphological effect, nucleation effect, pozzolanic effect, density effect, dispersion effect, specific surface area effect, and interfacial effect. Their dosage and fineness significantly influence critical parameters such as water demand for standard cement consistency, setting time, rheological properties, and hydration heat release. Appropriately formulated fly ash microsphere, ultrafine fly ash, or ultrafine slag powder with optimized fineness can enhance concrete workability, improve durability, inhibit shrinkage, and suppress alkali-aggregate reactions, though potentially compromising carbonation resistance. These materials also demonstrate pore structure refinement, microstructural optimization, and mechanical performance enhancement. The primary distinction between ultrafine and conventional mineral admixtures (e.g., fly ash and slag) lies in particle fineness, which yields differential performance outcomes despite sharing identical chemical reaction mechanisms. Conventional admixtures typically enhance concrete workability, long-term strength, and durability at the expense of early-age strength reduction. In contrast, ultrafine variants leverage superior pozzolanic reactivity and filling capability, where the increased specific surface area amplifies nucleation effects, leading to significant improvements in early-age strength development and workability. Their micro-aggregate effect and enhanced pozzolanic activity further contribute to more pronounced durability enhancement. Compared with single-type ultrafine admixtures, ultrafine composite mineral admixtures employ “gradient hydration” and “functional complementarity” mechanisms to synergistically accelerate hydration processes. This strategy effectively increases amorphous C-S-H gel formation, optimizes pore structure of hardened paste, and enhances matrix compactness through multi-scale interactions. In the concrete mix design, the dosage of ultrafine mineral admixtures is recommended to be controlled between 20% and 35%, which can significantly improve the workability and mechanical properties of concrete. However, excessive dosage may trigger a significant dilution effect, which is detrimental to the overall performance of concrete. Ultrafine mineral admixtures have shown great application potential in enhancing the workability of cement-based repair materials, manufacturing cement-based refractory materials, producing high-performance insulation materials, enabling steam-free curing of prefabricated components, and improving the comprehensive performance of ultra-high-performance concrete (UHPC). Currently, the application of ultrafine mineral admixtures mainly faces two major challenges: First, the relevant standard and specification system is still incomplete. Second, it is challenging to produce ultrafine powders that meet the standard requirements using diverse and complex raw materials. Due to the complex sources of mineral admixtures, the performance of ultrafine mineral admixtures can vary significantly, and improper dosage control may adversely affect the performance of concrete. Therefore, it is urgent to improve the standard specifications, enhance the preparation processes and equipment, reduce energy consumption and pollution, and further investigate their effects on the hydration mechanisms of cementitious materials to promote their wider application.

Compared with traditional mineral admixtures such as fly ash and slag, ultrafine mineral admixtures, characterized by higher specific surface area and pozzolanic reactivity, have shown significant advantages in improving the workability, mechanical properties, and durability of cementitious materials. Against the backdrop of green and low-carbon transformation in the cement and concrete industry, significant progress has been made in the application research of ultrafine mineral admixtures. By reducing the clinker factor and decreasing the cement content per unit of concrete, they provide an effective pathway for achieving sustainable development in building materials. Future research should focus on the following key areas: First, improving existing grinding equipment and processes to achieve rational composite grinding of mineral admixtures, thereby enhancing quality and reducing costs. Second, leveraging artificial intelligence technology to accurately predict the performance of ultrafine mineral admixtures, significantly improving design efficiency. Third, conducting in-depth studies on the hydration synergistic effects and microstructural evolution mechanisms of different ultrafine mineral admixtures. Fourth, refining technical standards and specifications to promote product quality improvement and the expansion of application fields. With technological advancements and increasing environmental demands, ultrafine mineral admixtures will play a more important role in enhancing the performance of cement concrete, reducing costs, and driving the development of green buildings.