Physical & Chemical Mechanisms of Cement Grinding Aids

Grinding aids are primarily used to reduce energy consumption by increasing grinding efficiency and mill productivity. Better dry powder dispersion of the ground cement increases mill productivity and cement fineness for the same energy consumption, and produces improvement in flow, leading to faster unloading and improved storage volume of bulk cement storage. In addition to increasing the efficiency of the mill, some grinding aids also yield positive effects on the final cement paste pertaining to rheology and improved strength development. Grinding aids that provide these “extra” properties are called quality improvers or performance enhancers.

Chemical and physical processes

The Clinker surface in grinding processes is usually hydroxylated from water incorporated in moist raw materials or from cooling water injection, which reduces the high surface energy of clinker. Water can be considered a weak-grinding aid, contributing to the reduction of grinding energy. Grinding aids have a non-polar hydrocarbon skeleton and a polar functional group. The latter interacts readily with clinker offsetting partially the polarity of the clinker, while the non-polar skeleton shields it. A larger hydrocarbon part reduces surface energy of covered cement particles and yields better grinding. The hydrocarbon parts of glycerin are small di-ethylene glycol moderate and di-isopropanolamine large and thus the addition of the latter as a grinding aid yields higher reduction in surface energy. On the other hand, studies have shown that the adsorption energy does not correlate with grinding performance. Glycerin with higher adsorption energy on C₃S than TEA and TIPA does not yield better grinding performance. Thus, the grinding performance depends on other properties than adsorption energy as no correlation has been identified. Furthermore, The agglomeration energies and rankings in case of tri-calcium aluminate (C₃A) are quite different and thus not all clinker phases behave in a similar manner. Similarly to the forces of attraction between cement particles, the forces of attraction between cement particles and milling media, steel balls, which without the use of grinding aids lead to agglomeration and adhesion to the grinding media, also depend on the surface energy.

Grinding aids work by reducing the surface energy of clinker. They must have a low interfacial energy to adsorb sufficiently strongly on clinker and form separating layers between particles. They should withstand normal grinding conditions to avoid irreversible decomposition. Few organic compounds are stable at temperatures above 200°C, as they break-up irreversibly, forming volatile secondary compounds. Grinding temperatures normally hover between 80°C and 120°C. The majority of commercial grinding aids have boiling points and decomposition temperatures that are significantly above the grinding temperature. Diethylene Glycol, Triethanolamine have boiling temperatures hovering around 100°C to 200°C, and 200°C to 300°C respectively while polypropylene glycol and polycarboxylate ethers have relatively lower boiling temperatures standing at around 100°C to 200°C and 150°C respectively. Their vapor pressures are low but sufficiently high to disperse via both gas phase and surface contact transfer mechanisms.

The major part of the energy consumed during the grinding process of cement is converted into heat, but a small fraction (≤ 0.5%) is retained in the cement as surface energy. Cement particles possess positive and negative charges when ground into smaller particles, which result in the agglomeration of cement particles and adherence to ball mill surface. When grinding clinker, ground particles agglomerate. Cement particles aggregation depends on clinker composition, crystalline structure of the cement phases, cement dispersion, mill type, mill conditions and other factors. Grinding aids are added in concentration ranges of 0.02 to 0.1 % of the manufactured cement weight. According to their structure, grinding aids are classified as aliphatic amines based, glycol-based or phenol-based grinding aids, with, in some instances, more complex compounds, such as Hydroxyl ethyl di-ethylene tri-amine, used in smaller proportions. Because of their high polarity, these cement additives compounds adsorb on clinker surfaces resisting binding on ball mills and particle agglomeration. Thermodynamics first and second law suggest systems tend towards the lowest energy state. The presence of grinding aid leads to different minimum energy states. Milling media (balls) coated by ground particles, represent a state of minimum energy of a system without grinding aids while a clean milling media represent a state of minimum energy of a system with grinding aids. Grinding aids keep the milling media (balls) clean and increase the productivity.

Grinding aids and cement performance enhancers offered include a combination of organic and inorganic compounds. Traditional Grinding aids contribute to the reduction of the polarity of clinker surfaces. In parallel with the reduction of polarity they contribute by reducing surface energy. More importantly, they reduce agglomeration by reducing the forces of attraction between surfaces. As opposed to traditional grinding aids, PCEs contain weakly polar ether groups and strongly polar acid groups and no chemical groups of moderate polarity such as alcohol groups. Their performance as grinding aids is due to steric effects. The branching in the molecular structure, or very long polymer chains overlaps, prevent adsorption of the entire carboxylate molecules, thus creating distances between clinker surfaces twice wider than when using glycerin. In addition, polycarboxylate molecules reduce to a larger extent agglomeration as less easily displaced than molecules of traditional grinding aids. The mechanism and steric effect of polycarboxylates on dry cement is totally different from their effect on concrete paste in wet concrete.

Grinding aids have an effect on the kinetics of cement hydration, morphology of cement hydrates, macroscopic properties such as water demand, rheological properties, and strength development. Triethanolamine is considered a popular grinding aid. Its effects on cement characteristics depend on the type of cement and dosage. An addition of 0.025% of Triethanolamine to Portland cement, acts as a set accelerator, at 0.25% as a mild set retarder, at 0.5% as a severe retarder and at 1% as a strong accelerator. Besides particle size distribution, grinding aids added to cement may alter the kinetics of cement hydration, by accelerating the initial dissolution of ions and early hydration of C₃ and C₄AF, and finally enhance the early strength and even 28-days strength to a visible extent.

Clinker grindability depends on clinker composition. C₃S is the easiest to grind, C₃A is intermediate and C₂S and C₄AF are the hardest. Since the agglomeration energies of dry and hydroxylated C₃A are almost double those measured for C₃S, the effect of grinding aids on C₃A is more important than with C₃S. Thus grinding aids reduce grinding disparities of different clinkers. The effectiveness of grinding aids depends on the mineralogy, i.e. the percentages of different phases. This is one of the reasons grinding aids do not always have the same effect, as dependent on cement characteristics.

The effectiveness of grinding aids also depends on the dosage rate, with the optimal dosage reached when a monolayer of the inorganic material lies between clinker surfaces. At too high dosage rates, capillary forces may favor agglomeration and thus reduce effectiveness.

Holderchem has a longstanding experience in grinding aids, which enables it to design and offer customized products yielding better operating and product performances.. In close coordination with its clients, it is able to optimize the grinding process and cement properties through lab simulations of customized additives combining compounds of different functional groups, types, and molecular weights, and at varying dosage rates. It seeks to help clients identify compounds that offer a good compromise between their different requirements.