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TELTOW - A new innovative technology – the ultrasonic mixing of cement pastes – offers great benefits for precast molding, drycast and concrete producers.
The advantages are shorter initial and final set time, lower dosage of superplasticizer, faster and more complete hydration as well as higher compressive strength.
Conventional concrete mixing technologies provide mostly deficient mixing action to disperse the agglomerates of cement particles and other cementious materials, such as fly ash orsilica. The problem resulting from this, is that the outer particles of such agglomerates are exposed to water, but the inner particle surfaces remain dry. The consequence is a slow and incomplete hydration.
Benefits of Ultrasonic Mixing Technology
Ultrasonic dispersing is the most advanced technology to deagglomerate and disperse micron-size and nano-size materials in liquids. For cement, fly ash, silicas, pigments and carbon nanotubes ultrasonic dispersing advances the particle distribution and the contact of the particles with water. Especially micro- and nanosilica as well as nanotubes are used to improve the compressive strength of high-performance concrete. Based on the cavitational shearforces, ultrasonic mixing of fine size materials is more effective than conventional rotary mixers and rotor-stator mixers.
During the hydration - the reaction of cement with water - C‑S‑H‑phases (calciumsilicahydrate) grow crystalline needle structures. The pictures below show the microstructure in cement paste after 5hrs of hydration. In the sonicated cement paste, the C‑S‑H‑phases are almost 500nm long, while in the unsonicated paste, C‑S‑H‑phases are around 100nm, only.
The ultrasonic cavitational mixing leads to faster growth of C‑S‑H‑phases. There is a direct correlation between the growth of C‑S‑H-phases and the temperature in the cement paste during the hydration period. In the ultrasonically mixed cement paste, the hydration starts approx. one hour earlier. In turn, the earlier hydration correlates with the earlier increase in compressional strength.
In particular for precast and drycast concrete, this leads to significantly shorter time until the cast concrete can be taken from the mold. Another interesting benefit of ultrasonic mixing is the influence on the fluidity. As shown in the table above, the slump increases by approx. 30%. This can allows for reduced dosage of superplasticizers.
Integration of ultrasonic mixing into the process
First, any dry material should be premixed with water in order to form a high concentration - yet pumpable paste. The ultrasonic mixer, deagglomerates and disperses the particles using cavitational shear. In result, the entire surface of each particle is fully exposed to water.
In the case of cement paste, hydration starts after ultrasonic processing. Therefore, the ultrasonic mixer should be used inline, as the cement paste cannot be stored for long periods. The schematic drawing below illustrates the process. In a next step, the aggregate, such as sand or gravel is added and mixed with the cement paste. As the cement particles are already well dispersed at that stage, the cement paste blends well with the aggregate. The concrete is then ready to be filled into precast molds or for transportation. A break-up tank next to the ultrasonic mixer may be used to process more continuously in case of unsteady concrete demand.
Integration of ultrasonic mixer into the concrete production
Hielscher high power ultrasonic mixers for the effective dispersing of cement, silica, fly ash, pigments or CNTs are available for trials at pilot scale in lab just as well for industrial full scale processing. All results, reached at pilot scale can be linear scaled up due to the accurate parameter control and the extraordinary reproducibility. Typically, ultrasonic mixers are installed inline. Pumping the material into the ultrasonic reactor, it is exposed there to intense ultrasonic cavitation. As all particles pass the chamber following a defined path, the inline sonication prevents the by-passing. Therefore, ultrasonication shifts the particle size distribution curve rather than widening it.
For improvements in concrete properties, the mixing is an essential step to produce uniform, high quality concrete. In particular the production of self-compacting concrete (SCC) and ultras high strength concrete (UHPC) needs a longer mixing time or a more intensive, effective mixing technology.
© Hielscher Ultrasonics GmbH
This article is published by Hielscher Ultrasonics GmbH
Hielscher Ultrasonics is your top supplier worldwide for innovative ultrasonic devices from lab homogenizers to industrial processing reactors. Typical applications include, mixing and homogenizing, disintegration and sonochemistry.View company page