Grinding aids are mostly organic compounds that are added to the clinker during grinding in the cement mill for increasing the grind ability of the cement clinker and therefore reduce the energy required to grind the clinker into a given fineness. As a consequence, the presence of grinding aids increases the efficiency of the cement mill. Grinding aids have been used for more than 50 years and the most common additives can be divided into groups according to their structure as glycols, alkanols, amines, and phenol type compounds. In addition, to increase the efficiency of the mill, some grinding aids also provide important positive effects on the final cement such as increasing rheology of the fresh cement paste or concrete and improved the strength development. Grinding aids which provides these “extra” properties are called “quality improvers” or “performance enhancer” (Engelsen, C.J. 2008).
The addition of grinding aids to the clinker in cement production is indeed important for several reasons. Firstly, the fineness (Blaine specific surface) of the finished cement is one of the main factors that are affecting the development of early strength. Besides, the addition of grinding aids provides an energy consumption which consumed during cement grinding. In general, the energy consumption is linear up to a fineness of approximately 3000 cm2/g (Hewlett, 1998). Above this level, the energy consumption increases progressively due to agglomeration in the cement mill and a higher amount of energy is lost in heat. This is usually taken care of by the use of grinding aids and also by optimizing the cement mill technology. Reducing energy consumption is, therefore, another main driving force in the field of cement mill technology. Approximately 35-40 kWh/t is required to produce Portland cement with a Blaine in the area of 3000-3400 cm2/g (Hewlett, 1998).
When the grinding agents are used, the energy is reduced or the production rate is increased at the same energy consumption level. Moreover, energy saving is indeed important when producing blended cement. In the production of blast furnace slag cements the pure cement clinker is far more easy to size reduce compared to the slag. Trenkwalder and Ludwig, 2001 estimated the power consumption to be 43 and 68 kWh/t for producing cement and slag meal (separate processes) with a Blaine of 3000 and 4000 cm2/g respectively. Together with optimized mill technology they achieved a reduction in the electric power consumption of 7% without the use of grinding aid. Including a suitable grinding aids in such processes, the possibilities for the further increase in the grinding efficiency are most likely present. In summary, conventional grinding aids are usually used to increase the production rate in the cement mill. If such additions give beneficial chemical effects during hydration of the final cement (e.g. increased strength, improved workability etc.) the grinding aid will be regarded as quality improver or performance enhancer. Several conventional grinding aids today are also claimed to give beneficial chemical effects to a certain extent (Engelsen; C.J., 2008).
The main goals of GAs addition are to reduce the energy required to grind clinker in this subtlety and therefore increase the efficiency of cement mill, furthermore, to increase performance efficiency of the mill. Grinding aids provide some important positive effects on the final stage of grinding, such as cement, cement paste rheology of fresh or enhanced strength development of concrete (Gan, 1997). Milling is carried out at present time mainly in drum-ball mill.
Because of disadvantages: the high specific energy consumption and metal in recent times beginning to apply also other mill types, but for fine grinding of cement clinker, drum mill still indispensable (Shevchenko et al., 2008). It is clear that with increasing dispersion of cement is growing its activity, that allowing reduces specific consumption of cement at manufacturing concrete specified strength (Luginina, 2004). Moreover, the intensification of cement grinding helps save energy and improves the performance of existing equipment (Katsioti, 2009). Also, reducing wear metal grinding bodies and reduces gild expenses. Various grinding aids have different effects on the grind ability of clinker (Bravo, 2003). At present time, it is expedient study the effect of different, complex compositions intensifiers on grind ability and clinker quality of different mineralogical composition (Domone, 2010).
Grinding aids are preventing agglomeration and coating on ball and mill lining, this is due to its nature as organic substances that are strongly adsorbed on the surface of ground particles. Besides their dispersing effect, grinding aids also increase the efficiency of air separators because the finest particles are not carried along with the largest. The result is a reduction of circulating load and an improvement of particle size distribution (Sottili, 2001) (Emeish, S., 2013). The grinding process of cement absorbs 60-70% of the total energy employed. Finish grinding accounts for about 38% of specific electric power consumption. The quantity of energy required by the process to obtain the correct fineness is only partially employed for the creation of new surface: in fact, most of the total energy is lost as heat. Grinding efficiency rapidly decreases as fineness increases, mainly due to the agglomeration between the finest particles (Heren, 1996).
The advantages obtained by the use of grinding aids are significant mill output increase at the same fineness, the increase in production that can be used to reduce production costs or to cover market demand and fineness increase at an equal output, or both effects. In some cases very high fineness may only be obtained by using grinding aids and improved particle size distribution at equal fineness can be observed. It is well known that the particle size fraction between 3 and 30 µm affects the strengths development while the fractions below 3 µm contribute in the early strengths enhancement. The use of grinding aids allows higher mechanical strengths to be obtained and have a positive influence on particle size distribution. Such as higher separator efficiency, improved flow characteristics of the cement during transport, silo storage and during loading/unloading operations (Cella, 2001). In summary, conventional grinding aids are used to increase the production rate in the cement mill. If such additions give beneficial chemical effects during hydration of the final cement (e.g. increased strength, improved workability etc.) the grinding aid is regarded as quality improver or performance enhancer. It is emphasized that several conventional grinding aids today are also claimed to give beneficial chemical effects to a certain extent (Emeish, S., 2013).
In the mid-1930s, cement plants started to use cement additives to increase the production volume of cement. Since then, the use of additives became related to cement production. Until the 1960s, the basic materials used as grinding aids were amines, amino-acetates, phosphate, lingo sulfonate, acetic acid, glycols, and gluconates. Developed countries, such as Japan, United States, Germany, and Russia, conducted intensive research. The application of grinding aids became more diversified; the limitations of research on grinding aids remain an obstacle to the development of grinding aids technology. Most of the studies conducted on the hydration processes, interaction mechanisms of the main cement compounds, and the comparison of the effect between higher dosage (0.1% to 1% of the cement weight) and common dosage (0.02% to 0.05%) of grinding aids did not focus on grinding efficiency (Ramachandran; 1976; Heren and Olmez, 1996; Aggounet et al, 2008). Although China is the world’s largest cement producer, its development of grinding aids was slow and only started by the end of the1950s. Experiments used pulp waste and detergent waste. Results showed poor performance of these aids. At the beginning of the 1970s, the use of amine glycol and glycol base as grinding aids demonstrated better performance. However, the high cost of materials, limited availability, and uncertain quality limited further development. In the last decades, the main focus was to find new and more efficient amines to enhance grind ability and hydration (Lai; F.C.et al 2013).
Sika was the first construction chemicals supplier that launched sustainable construction. The company patented the Sika Grind 800 series, a new technology for green products based on polycarboxylic ether polymers. Sika Grind 800 series shows superior performance in the grind ability, strength enhancement, and flowability compared with the conventional glycol and amine-glycol-based grinding aids. To reduce agglomeration during the grinding of clinker, grinding aids are usually added in the range of 0.02% to 0.1% of the manufactured cement weight. Chemical basis of the grinding aids includes ethanolamines, such as triethanolamine, monoethanolamine, and tri isopropanolamine, as well as glycols, such as ethylene glycol and propylene glycol. The high polarity in their chemical functioning groups of -OH, -NH, etc. causes the tendency to adsorb on electrostatic surfaces from fractured covalent bonds of Ca-O, Al-O, and Si–O, and to resist the rebinding phenomenon, greatly assisting the formation of further cracks in the grinding process (Jeknavorian et al, 1998).
Eventually, 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 a recent study of Teoreanu and Guslicov, (1999), a specific power consumption value of 25 kWh/t to100 kWh/t was used in cement manufacturing plants for conventional amine-glycol and glycol bases. However, no superior performance was observed or no new polymer grinding aids for grind ability and strength enhancement in grinding cement was formulated. Majority of the studies in literature (Teoreanu and Guslicov, 1999 and Joseph and Salim, 2011) are focused on the laboratory-scale milling control system and may not represent the actual production scale results. Therefore, a comparative study was carried out to evaluate the performance of polycarboxylic ether (PCE)-based new polymer grinding aids (SikaGrind 874MY) and the conventional amine-glycol- and glycol-based grinding aids (conventional/existing strength enhancer (SE)). The parameters used were production output, fineness, and mortar compressive strength. There are correspondingly many hypotheses in the scientific literature as well as in industrial practice. Starting from basic physical and chemical background, the laboratory screening process covering several hundred compounds and mixtures as well as extensive computer simulations (molecular modeling) (Mishra; R.K.,2012) provide a better understanding of grinding aids(Mishra; R.K.,2013). This makes it possible to design new, more efficient, customized additives. A wide variety of chemical admixtures (CAs) is added in cement and concrete with some dedicated function to enhance various performances of cement and concrete, such as 1) grinding aids for grinding energy-saving in cement production; 2) superplasticizers for improvement of workability of concrete and reduction of water amount required; 3) thickeners for mitigation of the bleeding problem of fresh concrete; 4) retarders and accelerators for adjustment of the setting behavior of fresh concrete and early strength, and 5) air-entraining agent for control of the air content in concrete, anti-foaming agent, shrinkage reducers, anti-corrosion agent, waterproofers (Kuei Suan Jen Hsueh Pao;2012). Besides their expected functions, those grinding aids will usually make noticeable impacts on other properties of concrete like rheological properties, setting behavior, strength development, shrinkage and cracking behaviors (Qiu, X., 2011). Inorganic ingredients, mainly including some inorganic salts and a wide range of organic substances like alcohol, carboxylate acid, ammonia, sugars, surfactants, and polymers, are frequently used in production as GAs alone or in the mixture. The conventional cement and concrete chemistry has comprehensively represented the process of cement hydration, kinetics, microstructural development, the relationship between microstructure and macroscopic properties of cement and concrete.
However, conventional chemistry with inorganic ingredients could not analyze the aspects and the processes for the cases of the addition of various organic substances in modern cement and concrete. The organic substances could change the kinetics of cement hydration by adsorption and complexation, participate into the microstructure of cement hydrates of calcium hydroxide and calcium silicate hydrate that consequently affect the rheological properties, setting behavior, strong growth and even shrinkage and cracking behavior (Shatish Chandra and Per Flodin,1987). The admixture of grinding aids in cement grinding process can save mechanical energy and affect the variations of ?neness characteristics; the action of grinding aids is governed by mechanochemical activation that has been discussed. Grinding is an important operation used widely in various industries, but it is also one of the most inefficient unit operations. In the cement industry where huge amounts of clinker are dry-ground, a large number of studies have been carried out on the grinding aids to improve grinding efficiency of the cement clinker; beneficial observations have been obtained for several decades (Iwabuchi, T. and Res. Assoc, J., 1970).
However, the action mechanism of grinding aids, which improves the efficiency of the grinding remarkably with a small amount of the addition, has not been understood perfectly despite a large number of useful information (El-Shall H. and Somasundaran, 1984)( Fuerstenau, D.W,1995). If we actually use a grinding aid at the present technical level, we must empirically determine the variety and quantity of the grinding aid based on experimental data. In most of the studies (Tanaka, T. and Kawai, S., 1962) (Sureshan, M.K., and Ahluwalia, S.C., 1992) on grinding aids, the effects of grinding aids have been discussed to get the fine powders of micron sizes, but there are only a few reports (Ikekawa, A. et al, 1991). Grinding aids should even more positively be applied to ultrafine grinding operations with higher energy consumption. However, not only in ultrafine grinding operations but also in fine ones in fields other than the cement industry, grinding aids have scarcely been utilized because the undesirable contamination of the product occurs with the use of the aids (Takahashi, J. et al, 1991).
We must select an appropriate one of grinding aids that has no detrimental effect on downstream processing on the final product. Generally, various chemical additives and treating agents are used in several processes after the grinding process for the purposes, such as the prevention of agglomeration between particles, the stabilization of powder surface and the aid of formation of ceramic powder, etc.
I.3.1 Classification of Grinding Aids
Several of materials have been used as grinding aids which can be classi?ed as liquid and solid grinding aids.
a. Liquid grinding aids such as aliphatic amines, amino alcohols and glycol compounds, including triethylenetetramine (TETA), Tetraethylenepentamine (TEPA), triethanolamine (TEA) and tri isopropanol amine (TIPA).
b. Solid grinding aids such as metaphosphoric and sulfonate, including sodium tripolyphosphate (STPP), dodecyl benzene sulfonic acid (DBA) and calcium lignosulfonate (CL), etc. The plurality of functional groups in grinding aids made them has a strong adsorption that can be adsorbed on the surface and micro-crack of cement particle, forming absorption ?lm, reducing the free energy of particle surface and promoting the propagation of cracks.
TEA is one of the most commonly used grinding aids that has an obvious effect on strength enhancement. Worsfold and Yan 1991 reported that there are different grinding aids have been used and their formulas. There are several types of aliphatic amines such as triethylenetetramine (TETA), tetraethylenepentamine (TEPA) and amino alcohols such as diethanolamine (DEA), triethanolamine (TEA), tri-isopropanol amine (TIPA). Glycol compounds are included such as ethylene glycol (EG), diethylene glycol (DEG). In addition, there are more complex compounds such as aminoethyl ethanolamine (AEEA) and hydroxyethyl diethylenetriamine (HEDETA).
Phenol and phenol-derivates are also used as grinding aids. Other compounds, mentioned in the product data sheets, such as amine acetate, higher polyamines, and their hydroxyethyl derivates, are used, but these are undefined in data sheets. Therefore, these compounds are not considered here, only the listed compounds are reviewed and discussed in more detailed