Guoqing Geng

Elevated-temperature curing is known to accelerate early hydration of Portland cement (PC), but this advantage would diminish as hydration proceeds. Yet bulk measurements combine the reactions of particles of different sizes and therefore cannot show whether all these particles respond similarly to temperatures. Here, bulk PC was separated into fine, medium, and coarse fractions, hydrated at 20–80 °C, and compared using the equivalent inward hydration depth, h. At small h, hydration of the medium and coarse fractions was largely governed by interfacial reactions. Their hydration rates (dh/dt) were similar at a given h and temperature, and followed an Arrhenius-type temperature dependence. The fine fraction deviated because of surface passivation at elevated temperatures and showed weak temperature sensitivity over 60–80 °C. When h reached the critical value hc, dh/dt peaked and hydration shifted from acceleration to deceleration. For each fraction, hc was nearly temperature-independent but determined by the particle-size-associated precipitation space (Lwater). Beyond hc, hydration rates declined differently among fractions as higher temperature and smaller Lwater hastened the buildup of transport resistance. As a result, raising the initial curing temperature from 20 to 80 °C reduced 28 d hydration in the fine fraction, left the medium fraction nearly unchanged, and increased hydration in the coarse fraction. These opposing responses help explain the weak later-age temperature dependence of bulk PC and inform the coordination of particle-size distribution with steam curing.

There is a growing trend of exploring solid waste as sustainable construction materials. However, solid waste varies greatly in their chemistry, structure, and reaction mechanisms, which makes accurate reactivity measurement difficult. This review evaluates reactivity test methods for waste-based materials in multiple cementitious systems. First, solid wastes are categorized according to their source and reaction type, and physical or chemical activations for enhancing their activity are described. Subsequently, common test methods including the Frattini test, strength activity index, and thermogravimetric analysis are explained along with their respective capabilities and limitations. New techniques including the rapid, relevant and reliable (R3) test and ultra-rapid reactivity (UR2) test are also discussed. The review highlights that traditional methods are often time-consuming and unsuitable for testing high-volume and highly varied samples. To address this challenge, a new paradigm integrating high-throughput testing with machine learning is emerging. This approach enables the development of intelligent models that predict long-term performance from early-age signals to shift the methodology from empirical screening to predictive and data-driven design. Future efforts should focus on establishing standardized testing procedures tailored to different waste categories and developing robust data-driven models.

Recent studies suggest that marine clay has the potential to substitute cement after calcination, although its reactivity is limited. This paper evaluates the purification of Singapore marine clay, which mainly consists of low kaolinite and high quartz, for use as a cement substitute. The study explores wet sedimentation and surfactant-assisted sedimentation to enhance reactivity in a typical LC³ recipe by reducing quartz content. The findings indicate that while wet sedimentation effectively removes quartz, it is less effective for silt due to its smaller size. Surfactant-assisted sedimentation further purifies the clay, with material strength depending on both composition and particle size. Specifically, clay filtered through a 22 µm sieve exhibited improved strength. The study endorses the 22-WS-CC method, highlighting its cost-efficiency and feasibility for large-scale implementation without chemical reagents. Overall, this research contributes to the sustainable development of the construction industry by providing insights into the effective utilization of low-grade, impure clays in cement production.