Cement as a binding material has played a prominent role in construction since antiquity, and has not lost its importance today. When it comes to the production of cement, mixing and grinding is an essential part. Grinding in particular is an energy-consuming process, and properly managing its extent is an important economic issue. In general, the duration of grinding period can be optimised by monitoring the particle size distribution (PSD). However, different raw components of cement have varying grindabilities, and the PSD is usually broader for softer materials. Paradoxically, the addition of softer components (with broader PSD) leads to a narrowing of the PSD for the harder components, and vice versa. Thus, as grinding is critically influenced by the particle size distribution of the components, its analysis with a particle sizer, such as the Particle Size Analyzer (PSA) by Anton Paar, is recommended at almost every step of the process.
In addition to the economic aspects of cement production, particle size also has a major influence on the properties of the final product. Together with the clinker’s chemical composition and its specific surface area, the PSD is a major factor affecting the hydration curve of cement, as well as the strength of the hardened paste. Specifically, reduction in average particle size leads to decreased setting time and enhanced early strength. In contrast, the importance of coarser particles grows with the aging of the cement. The width of the PSD also determines the cement’s water demand. In brief, it is possible to change the cement properties by modifying the particle size and its distribution. Measuring the PSD in cement can be difficult due to the fact that the particles have a broad size distribution (from less than 1 μm to over 100 μm), and that they exhibit irregular shapes. Additionally, they tend to agglomerate in dry state, and proper dispersion is crucial.
This article will outline the accuracy of dry cement particle size analysis with Anton Paar’s PSA, which exploits the company’s patented Dry Jet Dispersion (DJD) technology (FR2933314) for efficiently dispersing and precisely analysing powder particles. The innovative design features an air pressure regulator that quickly and easily adjusts the air flow in accordance with the sample properties. The shear forces created by the air flow separate agglomerated particles, and in this way the size of each single particle can be detected. The results are then compared to measurements performed using the same instrument in liquid mode, on a non-aqueous cement suspension.
The size range of the cement sample was expected to be between 1 and 100 μm. The measurements were performed with a PSA 1090 L/D Particle Size Analyzer. For data analysis a Mie mathematical model was used and the sample refractive indices were set to 1.680 – 0.100i. For the dry mode, measurements we set the vibration frequency to 50 Hz, the duty cycle to 50%, and the pressure to 500 mbar. For liquid measurements, ethanol was chosen as a carrier liquid, and the alcohol regenerator accessory was used to limit alcohol consumption. No additional dispersant was necessary, but a 60 sec. ultrasound cycle was applied to the liquid dispersion before the measurement.
Results and discussion
An excellent correlation was observed between results returned by the liquid and dry modes. This indicates that the DJD technology enables the accurate measurement of cement particles in dry mode.
An excellent correlation between liquid and dry modes is obtained thanks to the calibration method of the two dispersion modes. In spite of the cohesive properties of cement powder, the DJD technology efficiently disperses cement particles at low pressure (less than 1 bar), which prevents possible particle erosion. Altogether, the PSA proved to be a system well-suited to combining liquid and dry measurements, as both dispersion units are fitted compactly within the same instrument. The robustness of the PSA, due to a permanently aligned and vibration-resistant optical bench, makes it well-suited to cement particle analysis.
Read the article online at: https://www.worldcement.com/europe-cis/05092019/heavy-duty-made-easy/
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