Reactive solids embedded in cementitious composites can leverage both packing and binding, which act distinctly in determining the composite strength and toughness. Here, the individual and coupled behaviors of particle packing versus binding are investigated in hardened cementitious pastes (HCP), aiming to understand their physiochemical interplays to inform rational use of filler materials. The actions of cement hydration, pozzolanic reaction, and pore densification were examined separately, and the resultant influences on the compressive strength (f_c), tensile strength (f_t), and mode I fracture toughness (K_IC) were characterized. Results show that inert packing (up to 30 vol%) promoted f_c, f_t, and K_IC, while the improvement in K_IC became increasingly limited at high packing densities. Contrastingly, reactive particles produced consistently higher K_IC by enhancing particle binding. Our discussion highlights the following toughening mechanisms associated with filler effect: 1) a narrow particle spacing induced by a large filler volume makes it easier to bind adjacent particles without raising total cementitious content; 2) pozzolanic reaction reduces weak portlandite interfacial planes, demonstrating a greater toughening efficiency than cement hydration. These findings shed light on effective strategies for controlling HCP’s strength-toughness relationship and offer references for selecting emerging natural and recycled mineral resources for value-added construction applications.

To explore the anti-explosion protection effect of reinforced concrete slab structures strengthened with different material protective layers on the blast-facing side under contact explosion, this study investigates the shock wave propagation characteristics, energy absorption efficiency, and damage mechanism of the protected structures reinforced with protective layers of various materials (aluminum foam, polyurea, steel plate, UHPC, POZD, and CFRP). The protection effects under different protection schemes are evaluated using relevant assessment indicators. The research results demonstrate that all reinforcement schemes with different material protective layers can effectively mitigate the damage degree of the protected structures. Specifically, the energy absorption and impact strength of the structure reinforced with the aluminum foam protective layer are 285.5 kJ and 494.6 kJ/m², respectively, which are 97.7% and 74.3% higher than those of the schemes using other material protective layers. Additionally, the maximum support rotation of the aluminum foam protective layer reinforcement scheme (0.59°) is 76.8% lower than that of the unprotected scheme, with a significant reduction in damage grade. Therefore, it is recommended to adopt the aluminum foam protective layer for reinforcing protected concrete structures. The research findings can lay a theoretical foundation for the application of reinforced concrete structures in the field of anti-explosion protection.

Phosphogypsum (PG), as a byproduct, holds significant potential for reducing alkali concentration in sulfoaluminate cement systems and stabilizing corrosive ions in seawater. However, its application is hindered by several challenges, including excessive fluidity, accelerated setting time, and reduced strength, which limit its practical dosage. To overcome these challenges, this study proposes a ternary system, wherein phosphogypsum composite cement was evaluated with and without the addition of polyaluminum sulfate (AS). The findings revealed that the rheological properties, such as yield stress and plastic viscosity, exhibit a strong correlation with compressive strength development, where lower values of these properties are associated with improved strength gain. The incorporation of AS not only reduces the alkalinity and extends the setting time but also mitigates the strength degradation typically observed with higher PG content. Microstructural analysis showed that the ternary system demonstrated an enhanced degree of reaction, forming a dense and stable matrix that effectively encapsulated PG. Subsequent hydration processes reduced the formation of Friedel's salt and refined the pore structure. This study highlights that a well designed ternary system provides a viable, environmentally sustainable, and engineered solution for the efficient utilization of PG in cementitious materials.

To promote sustainable 3D concrete printing (3DCP), this study develops an extrusion-based 3D printable geopolymer concrete and evaluates the effectiveness of four chemical retarders, namely tartaric acid, sucrose, sodium tripolyphosphate, and barium chloride, in improving its printability. Based on a comparative assessment of time-dependent flowability and compressive strength of the mixture, the barium chloride demonstrated the most favorable overall performance among the four retarders and was therefore selected for further investigation. When the barium chloride dosage exceeded 2.5%, the mixtures satisfied the early-age strength requirements for printing and demonstrated stable extrudability and good buildability. For mixtures containing 2.5% and 3.5% barium chloride, the open time reached approximately 30 minutes and 60 minutes, respectively. A higher dosage (3.5%) shows better printing quality, resulting in printed structures with compressive and tensile strengths surpassing those of mold-cast specimens and exhibiting reduced mechanical anisotropy. Furthermore, the printed concrete showed the highest compressive strength along with the printing direction, whereas its tensile strength in this direction was lower due to the influence of interlayer interfaces. Overall, a dosage of 3.5% barium chloride provided superior flowability and extended open time, achieving an optimal balance between printability and mechanical performance. This formulation offers a promising retarder strategy for extrusion-based geopolymer concrete 3D printing.