Stiffness deficiency of recycled coarse aggregate (RCA) is largely governed by the porous old mortar and mechanically weak old interfacial transition zone (old ITZ), where stiffness discontinuities disrupt stress transfer. Inspired by natural enamel, this study employed organic-template-assisted in situ hydroxyapatite (HAp) mineralization to repair this coupled defect system. Mineralization temperatures and durations were varied to regulate HAp deposition, while mineral-layer stability, phase assemblage, pore structure, and old ITZ micromechanics were characterized. Mineralization at 60 °C for 24 h provided the most favorable reaction window, reducing the crushing value, water absorption, and ultrasonic mass loss by 15.33%, 53.9% and 31.3%, respectively. Under this condition, relatively uniform Ca-P deposits with an apparent Ca/P ratio of 1.63 formed and confirmed the presence of HAp-containing mineral phases. Mineralization also reduced the accessible mesopore volume and suppressed the contribution of larger mesopores, suggesting partial pore filling and disruption of interconnected pore pathways. These structural changes were accompanied by pronounced micromechanical recovery of the old ITZ. The low-hardness band across the old ITZ was attenuated, the interfacial matching coefficient increased from 0.494 to 0.576, local mechanical populations shifted toward higher modulus ranges, and the plastic-energy fraction decreased from approximately 71% to 63%. These results demonstrate that effective micromechanical stiffening depends not on maximizing mineral deposition, but on balancing HAp formation and layer stability with pore-network modification and reduced local stiffness discontinuities. This reaction-regulated mechanism establishes a direct link between HAp deposition and micromechanical recovery of the old ITZ, providing a stiffness-oriented strategy for RCA modification.
This study proposes a novel 3D auxetic aggregate designed to improve mechanical performance and mitigate lateral dilation of cementitious composites through its hollow architecture and coordinated deformation with the matrix. Two types of auxetic aggregates with distinct strengths and stiffnesses, made of standard photopolymer resin and 316L stainless steel (SS), were fabricated by 3D printing and added to the matrix to form auxetic aggregate cementitious composites (AACCs). Uniaxial compression tests and digital image correlation were conducted to evaluate the mechanical response and failure patterns. A three-phase mesoscale model with randomly distributed aggregates was developed, validated against the experimental results, and subsequently used to reveal the internal damage evolution. The results show that 316L SS aggregates effectively reduced the apparent Poisson's ratio of AACCs. Compared with the pure matrix, the AACCs with an SS aggregate volume fraction of 20% exhibited increases of 37.9% in peak strength, 14% and 66% in ductility indices μ1 and μ2, and 98% and 176% in toughness indices W0.85 and W0.5, respectively. However, resin was found unsuitable as the aggregate base material due to low strength and deformation incompatibility with the matrix. This study demonstrates the potential of integrating auxetic aggregates into cementitious composites and emphasizes that deformation compatibility should be a key consideration in the design of AACCs. This work provides a novel supplement to existing reinforcement approaches for cementitious composites.
Seawater exposure is generally associated with progressive degradation of cementitious materials; however, its mechanistic impact in multi-ionic environments remains inadequately resolved, particularly for low-carbon binders. Here, we demonstrate that a fully-solid-waste cementitious material (FSCM) exhibits a fundamentally different response, characterized by surface-confined alteration rather than bulk deterioration. After 28 d of seawater exposure, the compressive strength was reduced by only 2.35% relative to the pre-exposure value, although it remained 27.65% lower than that of the continuously cured 56 d control, indicating suppressed strength development but limited additional degradation. A pronounced depth-dependent architecture was established, consisting of a carbonate- and sulfate-enriched surface layer, a transition zone, and a gel-dominated inner core that remained comparatively intact. Concurrently, hardening of the low-density C-(A)-S-H gel was observed, while a mechanically stable high-density gel fraction was preserved, suggesting that chemical alteration was coupled with internal structural reorganization rather than progressive damage accumulation. A carbonate-mediated confinement mechanism is proposed, whereby near-surface precipitation reduces transport pathways and constrains further ion ingress, while decalcification is redirected toward a reorganization-driven process within the confined core. These findings may provide useful insights for the future design and optimization of low-carbon cementitious materials with improved seawater resistance.
Concrete's embodied emissions are dominated by clinker, with additional consumption from producer-side strength overdesign: target average compressive strengths set above specifications to avoid noncompliance under batch-to-batch variability. We quantify the carbon and cost penalties of overdesign and batch-to-batch variability and evaluate two operations-first mitigation levers without specification changes: (i) overdesign optimization, which removes excess overdesign by resetting mean strength to the minimum for compliance, and (ii) variability control, which tightens the strength's coefficient of variation to 3.5%. Regression analysis of >30,000 ready-mixed batch records from five producers relates mixture proportions, 28-day strength, embodied CO2-eq, and material cost. Machine learning with interventional Shapley additive explanations decomposes strength variability into batching, material quality, testing laboratory, ready-mix plant, and ambient mixing conditions. Overdesign is highest in commodity-grade mixes (<4000 psi) and declines with specified strength. Strength variability is governed by intrinsic material controls (cement content, cementitious chemistry, admixture dosages causing >700–1000 psi shifts) and extrinsic operational context (plant, project, laboratory effects of ∼550–600 psi), showing that production and testing practices both drive variability. Across specified-strength bins, overdesign optimization reduces embodied CO2-eq by 1.7–6.5% and material costs by 1.3–4.5% on average. Variability control, evaluated as an upper-bound best-practice scenario, yields average 9.8–12.9% CO2-eq and 6.8–8.9% material-cost reductions. Scaled to U.S. cement and specified-strength frequencies, national cement-related savings reach 5.6 Mt CO2-eq (5.9%) and $1.0B (4.2%) annually for overdesign optimization, and 11.6 Mt CO2-eq (12.2%) and $1.9B (8.3%) for variability control. These near-term levers deliver system-wide carbon reductions within existing production constraints, creating headroom to accelerate the deployment of capital-intensive, carbon-neutral technologies.
