Effects of the fineness of densified silica fume on the hydration of Portland cement

As one of the mineral admixtures, silica fume (SF) with high pozzolanic reactivity is generally used to improve the properties of concrete. Most commercial silica fume used in concrete project is dry densified silica fume (DSF). It consists of a good deal of agglomerates of sizes ranging from 10 μm to several millimeters. The size of the undispersed silica fume is usually larger than that of the cement particle. At present, the effect of DSF on the hydration of cement is lack of studied. In this study, the commercial dry DSF is sifted into three sections (>150 μm, 80 ~ 150 μm, 35 ~ 80 μm) by different sieves. The amount of densified silica fume in mixture is 5% by weight of total binder. The used water/binder ratio (w/b) is 0.5. The effect of the fineness of DSF on the setting time of cement paste is firstly discussed. The hydration process of cement incorporated with different fineness of SF is investigated by a semi-adiabatic calorimetric for 3 days at 20 °C and x-ray diffraction (XRD). The obtained results indicate that the setting time of cement paste increases with the decrease of the fineness of DSF. At the w/b ratio of 0.5, the addition of SF has little effect on the hydration period of cement paste. But the hydration rate of cement is decreased with the increase of the size of DSF particles

Density-Based Characterization of Microplastics via Cross-Halbach Magnetic Levitation

The analysis of microplastics poses significant challenges for conventional characterization techniques due to their small size and low concentrations. Magnetic levitation (MagLev), already proven effective for microscale material testing, provides a robust solution for sensitive, accessible, and untethered characterization of such materials. In this paper, we propose a Cross-Halbach magnetic levitation device to measure the densities of microscale plastic materials. Common types of plastic samples, varying in size and concentration, are successfully levitated, and the levitation times are recorded. The samples of common microplastic materials are characterized in less than 180 s. The characterized density values are validated against theoretical results, enabling density-based identification of microplastics. The experimental results demonstrate that the magnetic levitation method is suitable for the characterization of small-sized plastic materials, and the high-speed, low-volume measurement of plastic samples lays the foundation for future applications such as detection, separation, and recycling of ultrafine materials