Grinding aid

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Grinding aids (also: grinding aids ) are substances or mixtures of substances that are used in the grinding process in the manufacture of cement.

Application and mode of action

Grinding aids are additives that are used during the dry grinding of cement clinker in a ball mill in order to increase throughput, improve energy efficiency and to modify the cement quality. By using grinding aids, the performance of a cement mill can be increased by up to 40%. They counteract (re) agglomeration of the cement particles and thus help to reduce buildup in the mill, increase the throughput rate, increase the fineness of the grind and thus the specific surface and reactivity of the ground material, and improve the flowability of the dry cement. Grinding aids interact with the surface of the cement particles and neutralize electrical surface charges (unsaturated valences on fresh fracture surfaces) and / or generate unidirectional charges on the surface, which reduces or eliminates the attraction between the individual particles.

It is usually added together with the cement clinker at the entrance to the ball mill with a dosage of 0.01-0.2% by weight.

composition

Raw materials that are used in grinding aids are z. B .:

Depending on the chemical composition of the grinding aids, they can influence the properties of the cement during hydration or in the hardened state. Sugar, glycols or carboxylic acids (with the exception of formic acid) delay the development of strength, while alkanolamines tend to accelerate it. Delayed strength development often leads to increased ultimate strength due to the slower crystal growth and vice versa. Triisopropanolyamine is not intended to improve the early strength, but to increase the final strength of Portland cement, in particular with C 4 AF contents of> 4%. Surfactant raw materials can lead to increased air entry into the cement or the mortar or concrete made from it, which results in a lower early and final strength.

ecology

It is reported that grinding aids made from triethanolamine and 1,2-propanediol , which are added in a dosage of 0.10-0.52 kg / t cement, are first chemisorbed onto the Portland cement clinker and later chemically bonded. It was observed that between 87 and 98% by weight of the grinding aid is bound to the clinker, while the rest of the substances volatile at the mill temperature is emitted during the grinding process. Due to the mechanism described, a subsequent release of the organic constituents of the grinding aid from the hydrated cement into the air or into (drinking) water is very low.

standardization

The use of grinding aids as additives to cement is regulated by the corresponding cement standard EN 197-1. According to this, additives are components that are not added to the cement as main components. Additions may not exceed 5% by weight, organic additives may not exceed 0.5% by weight. They must not influence the processing properties of the cement or the concrete made from it and must not promote a corrosive attack on the reinforcing steel.

Web links

Individual evidence

  1. ↑ HM Seebach: The effect of vapors from organic liquids in the crushing of cement clinker in drum mills
  2. ↑ 41st DAfStb Research Colloquium on July 11 and 12, 2002, (pdf, 121 kB)
  3. ↑ a b Gerhard Spanka and Gerd Thielen: Release of volatile substances from cement-bound building products (pdf, 238 kB)
  4. ↑ Patent DE69514167T2 : Grinding agent for cement. Published on August 24, 2000 . ‌
  5. ↑ Patent EP0976695A1 : Grinding agent for cement. Published February 2, 2000 . ‌
  6. ↑ Patent DE69832999T2 : Use of hydroxylamines to improve the strength of Portland cement compositions. Published on August 17, 2006 . ‌
  7. ^ Roland Benedix: Construction chemistry: Introduction to chemistry for civil engineers and architects . Vieweg + Teubner Verlag, 2011, ISBN 978-3-8348-1348-0 ( page 322 in the Google book search).