The utilization of alkali-free accelerators, primarily based on aluminum sulfate, in shotcrete often leads to significant shrinkage and cracking, jeopardizing long-term durability. This study aims to mitigate this issue by investigating the efficacy of silica fume (SF) and fly ash (FA), individually and combined, in controlling the shrinkage deformation of shotcrete.

Shotcrete mixtures were prepared with cement partially replaced by SF, FA, or their combination (SF-FA). Aluminum sulfate was used as an accelerator. The shrinkage behavior under sealed and dry conditions was monitored. The underlying mechanisms were elucidated through hardened air-void analysis, mercury intrusion porosimetry (MIP), and internal humidity tracking.

Contrary to some previous studies, both individual and combined incorporation of mineral admixtures reduced the 28-day shrinkage. The SF-FA composite exhibited the most substantial reduction (23.72% sealed, 17.76% dry), followed by SF alone (18.11% sealed) and FA alone (11.35% sealed). Mechanism analysis revealed that the admixtures refined the pore structure, reduced the volume of harmful pores, and, crucially, optimized the air-void system by increasing the population of fine bubbles. This created an internal buffering effect that alleviates capillary stress. The synergistic effect in the SF-FA group is attributed to complementary pore-filling at dual scales.

This work demonstrates that, within an aluminum sulfate-accelerated shotcrete system, silica fume can effectively reduce shrinkage when its pore-refining effect is counterbalanced by a well-optimized micro-bubble network. It provides the first comprehensive evidence of the synergistic shrinkage-reducing mechanism of SF and FA in such systems, offering a practical strategy for mix design optimization to enhance the volume stability of shotcrete.

The cracking performance of concrete is one of the key teaching contents in concrete courses in the field of engineering. Due to the varying cracking factors and distinct material properties, the cracking state of concrete yields characteristics such as diversity and complexity. Describing and characterizing the crack morphology of concrete are challenging in the research and experimental teaching design of concrete materials. In recent years, there have been significant advances in crack characterization technology. However, the existing methods have faced practical issues such as high requirements for equipment and complex operations for the concrete experiment teaching course. In addition, the evolution of concrete cracking runs successfully for its entire life cycle, and each process is closely linked to the other. In practice, however, the complete cycle chain is often separated into different courses and independently taught to students. To address this issue, this paper proposes the integration of the digital image processing (DIP) method into the experimental teaching of concrete courses.

First, the DIP method is explained in detail, including image acquisition and digitization, image coding, and image analysis. Then, the DIP method is utilized to analyze three teaching cases of concrete cracking, including early-age cracking (plastic state), multiple cracking (solid state), and crack self-healing (regain). This research analyzes the feasibility of the DIP method at different stages of the concrete life cycle. Further, the obtained crack morphology parameters are processed, and different evaluation methods are selected for parametric analysis and assessment of crack morphology.

The results show that the DIP method can record the morphology and distribution of cracks and accurately extract the morphology parameters, which aids the parametric analysis of cracks. Further, it can connect the full life cycle of concrete from plastic to solid, from early-age cracking to multiple cracking and then to crack healing. Compared with traditional experiments, this method can shorten the experiment duration and decrease the laboratory occupation. Another significant advantage of this method is that it does not bring any interference to the concrete cracking process, while efficiently obtaining a large volume of crack data. Furthermore, it provides an accurate criterion for cracking. For students, this method allows a more intuitive observation of the formation and development of concrete cracks, and it provides a better understanding of the theoretical knowledge.

The incorporation of DIP technology into traditional engineering experiment teaching makes the concrete experiment process simple and fast, and thus, it yields vivid and intuitive results. It enables students to possess a deeper understanding of the evolution of concrete structure and performance, and it enhances their ability to analyze and solve practical problems. It is hugely significant to deepen students' understanding of professional knowledge and stimulate their enthusiasm for scientific research. Under the background of further promoting the reform of new engineering experiment teaching, the integration of the DIP method completely connects the full life cycle of concrete, enhances the correlation and cross between different courses, and facilitates the integration and development of different disciplines.