When cement clinker is ground using grinding aids a narrower particle size range is generated

When cement clinker is ground using grinding aids a narrower particle size range is generated, as the percentage of very fine particles, which only influence the setting time, is reduced. This is why the strengths at equal Blaine values are higher than when grinding without grinding aids. With closed-circuit grinding plants it was also noted that cement ground with heavy circulating loads often contains smaller amounts of both ultra-fine and coarse particles. To some extent, the grinding aids force the mill to work with a higher circulating load (Magistri, M. and Presti, AL., 2007). Generally, the concentration range of grinding aids added is from 50 to 500 ppm. After the grinding process, the additives might not be any longer in their original chemical form. In addition, grinding aid composition might not consist of mixtures of pure compounds, but rather more complex raw materials (Jeknavorian, AA. et al, 1998).

I.3.1.1 Amines

Ethanolamines are used in several industrial applications such as: in the textile industry, in gas purification processes, as solubilizers for pesticides, as dispersing agents in the application of agricultural chemicals, as emulsifying agents, as catalysts in the production of polyurethanes and in the rubber industry. Also, it used as corrosion inhibitors, as pigment dispersants and as chemical intermediates for other chemicals products. Isopropanolamine in water is a medium strong base. Diethanolamine is classified as a hazardous air pollutant. Ethylene amines TETA and TEPA are used as asphalt additives, as corrosion inhibitors, as epoxy curing agents in the hydrocarbon purification, as surfactants, as dispersants, as chelating agents, as catalysts, as textile additives and fuel additives.
The commercial products of TETA and TEPA are often mixtures of alkanol amines and no single, pure compounds. TEPA is completely miscible in water and is not biodegradable. Hydroxyethyl-diethylenetriamine (HEDETA) is very soluble in water. Compounds relating to the class of amines merely modify particle size cement neutralized charges arising at rupture valence bond and catalyze hydration process to increase strength, both in initial and late periods of hardening. Glycol compositions mainly prevent agglomeration of cement particles in grinding process and exert little effect on change in particle size. The most effective influence on the processes of grinding and hardening have intensifiers based on surfactants (Shakhova; L.D. et al, 2014).

Triethanolamine (TEA)

Triethanolamine is used for various reasons in the cement industry. Depending on the amount of TEA it behaves differently in the cement production process. At an addition of 0.02% to Portland cement, TEA acts as a set accelerator, at 0.25% it acts as a mild set retarder, at 0.5% TEA acts as a severe retarder, and at 1% it is a very strong accelerator. The acetate of triethanolamine is also one type of grinding aid (Flatt, R.J. et al, 1998). The mechanism of the action of TEA in cement hydration is not completely understood. TEA is a weak base and in an aqueous phase, it is mostly in the molecular state. TEA has the ability to chelate with certain metallic ions such as Fe 3+ in highly alkaline media (Yilmaz et al, 1993).

In Portland cement pastes, TEA reduced the strength at all ages of hydration and setting characteristics were found to be drastically altered, especially at higher TEA contents. It is important to observe that, the use of TIPA is known to yield a reduction in setting times and significant increases in strength development at early and late ages, regardless of the cement type if it was compared with TEA (Perez et al., 2003; Sandberg and Doncaster, 2004).TEA could accelerate the reaction of C3A with calcium sulfate in Portland cement, but the addition of TEA will lead to a higher cement manufacturing cost and it seems not as effective as expected for the grinding production of blended cement, so the advantages of various grinding aids are needed to be considered in the cement grinding process (Yi Zang et al, 2016).

The evaluation of grinding aid (GA) effect on clinker processing in laboratory grinding mills is relatively simple. Yet, the results obtained cannot be directly transposed to industrial mills, given the fundamentally different operational modes and grinding parameters (Assaad; J.J., 2015). Grinding aids (GAs) are incorporated during clinker processing to reduce electrostatic forces and agglomeration of cement grains. Their chemical basis mostly includes ethanolamines such as triethanolamine (TEA) and tri isopropanol amine (TIPA) as well as glycols such as diethylene glycol (DEG) and propylene glycol (PG) (Teoreanu; I. and Guslicov; G., 1999) (Assaad; J.J., 2015).

The practical results of these additions (Gas) to the cement industry can be divided into two aspects including an increase in cement Blaine fineness and compressive strength for given specific energy consumption (Ec) and/or savings in electrical energy and Ec together with improved mill productivity for given fineness. The former direction is relevant when producing cement possessing increased fineness necessary for high early strength requirements (i.e., ASTM C150 Type III) (ASTMC150, 2012), while the latter is more and more demanded with today’s constraints regarding the reduction of usable energy (Assaad; J.J. et al, 2009).

Gartner, E., and Myers, D., 1993 observed increased iron solubility due to the chelating potential of TEA so more cement iron reacted to form mono and tri sulfates. Depending on the chemical composition of the Portland cement, TEA addition was observed to produce a gain in mortar strength in some cases. (Lee, C.Y et al, 2003) noted accelerated hydration of fly ash cement due to TEA addition but did not consider whether the hydration of Portland cement or fly ash is affected by TEA. Lee, C.Y et al, 2003 observed that overdosage of TEA leads to a decrease in strength. Because of its aluminum chelating potential, TEA was found by (Spencer, B.B., 2005) and (Palmer, D.A., 2003) to be able to extract aluminum phases from sludge. In analytic chemistry, complexometric titration is a standard procedure to quantify cations where unwanted Al and Fe ions are masked by the formation of stable complexes with TEA.

Triisopropanolamine (TIPA) is a tertiary amine. The cement industry uses TIPA as a grinding aid, and it is also used in concrete admixtures. The addition of small amounts of TIPA can result in a significant increase in the strength of cement pastes at different ages (Gartner, E. and Myers, D., 1993). It was proposed that tri isopropanolamine (TIPA) does not improve the mechanical properties of hydrated Portland cement paste, but rather improves mortar and concrete strength by acting on the interfacial transition zone (ITZ) between the Portland cement paste and sand or aggregate(Perez, J. et al, 2003). However, the compressive strength data for 10 Portland cement (with TIPA) tested as cement paste, as well as two different kinds of mortar after 28 days hydration was recently presented (Sandberg, PJ. and Doncaster, F., 2004). The average strength improvement with TIPA was 10% in the hydrated Portland cement paste and 9% in the mortar, clearly showing that the strength enhancement is not dependent on an ITZ mechanism. It is noted that GAs blended with TIPA molecules are known by their capability to promote cement hydration reactions, thereby leading to increased compressive strengths(Perez; J. P. et al,2003)(Sandberg; P.J. and Doncaster; F., 2004) (Assaad; J.J. and Issa; C.A., 2014).

Unlike industrial mills, laboratory grinding mills operated over given time interval do not account for CL, therefore leading to different cement particle size distribution (PSD) curves (Bhatty; J. et al, 2004) (Fidan; B., 2011). This consequently alters cement properties such as water demand, rheology, and hydration processes such as heat release, setting time, volume change, and strength development (Zhang;Y.M.;Napier-Munn;T.J.,1995)(Bentz; D.P.1999).
It is noted that the Blaine measurement can’t effectively represent the entire PSD, given the fact that two cement with different ratios of fine-to-coarse particles and described by different PSD can possess the same Blaine value (Ferraris; C.,2002)(Delagrammatikas; G. and Tsimas; S.,2004). The differences in PSD curves determined under laboratory and industrial mill conditions were noticed by several researchers in the cement and mineral grinding industries(Assaad; J.J. and Issa; C.A., 2014) (Mejeoumov; G.G., 2007) (Fidan; B., 2011); the laboratory tests yielded significantly wider PSD curves than those experienced in practice. ASTM C465 Standard Specification for GAs related such changes to cement flowability and mill retention time (MRT) during grinding (ASTM C465, 2010).

The MRT can be defined as the average time necessary for the bulk material to pass through the tube mill (Sottili; L. and Padovani; D., 2002). Hence, to properly evaluate GA effect on cement properties, the standard recommends the realization of full-scale tests over enough time to ensure reaching equilibrium conditions and stable circulating load (CL) (ASTM C 465, 2010). The CL is defined as the average number of times that the material circulates through the grinding system before becoming the product.
In the literature, limited attempts have been made to quantify the scale effect (i.e. industrial versus laboratory mill) that could result from GA additions on cement fineness and properties.
Grinding is one of the most inefficient unit operations in the cement industries where fine particles are produced by grinding, viz. mineral, cement, pigment, metal powder, etc. The effect of diethanolamine modified lignin on grind ability and cement performance including standard consistency, setting times and compressive strengths was studied. To the best of our knowledge, this is the first report about the usage of diethanolamine modified lignin as cement grinding aids. As known, lignin is one kind of natural macromolecular material; because of the active epoxy group, the diethanolamine modified lignin system has many side reactions such as epoxy ring opening reaction with hydroxyl and methoxy group.

In particular, in the cement industry, huge amounts of clinker, coal and other raw materials are needed for grinding. It has already been proved that grinding aids sometimes referred to as grinding additives are one of the most effective measures to maximize energy saving and improve grinding efficiency in the cement grinding process(Choi, H. et al, 2010)( Hasegawa, M. et al,2001).

Small percentages of grinding aids can effectively improve the performance of the mill, reduce the particle size and increase the specific surface area of the cement under the same grinding condition. Grinding aids are certain to surface active chemicals, a variety of materials have already been used as grinding aids, such as triethanolamine (TEA), tri isopropanolamine (TIPA), glycol, glycerol, organic acetates, and calcium sulfate (Katsioti, M. et al,1954)( Gao, XJ. et al, 2011). Two important mechanisms (Rebounder’s strength reduction theory and Mardulier’s particle dispersion theory) have been suggested in order to explain the action of various grinding aids (Moothedath, SK., and Ahluwalia, SC., 1992). However, the action mechanisms of grinding aids which remarkably improve the grinding efficiency with small adding amount have not been understood perfectly.

Therefore, if actually using some new compounds and chemicals as grinding aids at the present technical level, empirically determining the variety and quantity of the compounds and chemicals based on experimental data is very important. As a renewable material, lignin is a constituent of all plants, including annual plants and wood, and is second in the natural abundance in the organic world after cellulose (Nadif, A. et al, 2002).
Meanwhile, lignin is unusual because of its heterogeneity and lack of a defined primary structure. Lignin is a cross-linked racemic macromolecule with molecular masses in excess of 10,000. It is relatively hydrophobic and aromatic in nature (Del Rio, JC et al, 2001). However, through chemical modification of the active groups (phenolic hydroxyl, carboxyl and hydroxyl) in lignin molecular, a variety of lignin-based high value-added fine chemicals and polymer materials have been produced and widely used in oil well additives, cement and concrete additives, dyestuff dispersants, agricultural chemicals and other industrial binders(Matsushita, Y. and Yasuda, S., 2005)(Singh, NB., 2002).

As one kind of the most important polymeric surfactants, lignin and its derivatives especially the lignosulfonates have been widely used as concrete additives owing to its advantages including easily available, low cost and environmental friendliness (Singh, NB., 2002) (Miyake, N. et al 1995). Lignosulfonates are also used as auxiliary components for cement grinding aids. However, the poor grinding and strength enhancing performance have limited their further application in the cement and concrete field.

In order to make more efficient use of lignin and its derivatives, researchers have done lots of works and mainly concentrated on the physical or chemical modification to improve its water solubility and surface activity (Ai, Q. et al,2009)( Pang, YX. et al, 2008). During the study on lignin modification and cement additives, we found incidentally that by proper chemical modification lignin had excellent grinding performance and strong retarding effect.

Glycol is a dihydric alcohol where two hydroxyl groups are bonded to different carbon atoms; the general formula for glycol is (CH2)n(OH). The most important glycol is the simplest, ethylene glycol (EG). Glycols used as grinding aids and are also used as raw material for the production of plasticizers and polyester resins, humectants, textile lubricants, and coupling agents. Large volumes of ethylene glycol are