ACI SP-302-2015 Eleventh International Conference on Superplasticizers and Other Chemical Admixtures in Concrete.pdf

1、The impact of clinker grinding aids (GAs) based on amine, glycol, or phenol on static yield stress ( 0 ) of cement pastes is not well understood. Results obtained from this project have shown that GA molecules remain active after the grinding process and provide variations in cement properties, whet

2、her in the fresh or hardened states. Flowability improved and 0decreased when the cement is ground using increased GA concentrations. This was attrib- uted to the adsorption of these molecules onto the cement grains and saturation of surface charges, thus creating repulsive forces between neighborin

3、g particles. The decrease in 0was particularly pronounced when phenol-based GA was used, given the presence of polycarboxylate polymers that help dispersing cement particles upon mixing with water. Keywords: amine; cement; clinker; glycol; grinding aids; phenol; static yield stress. INTRODUCTION Gri

4、nding aids (GAs) are incorporated during comminution of clinker to reduce elec- trostatic forces and minimize agglomeration of cement grains. 1-3Because of their highly organic polar nature, such additives are preferentially adsorbed on surfaces formed by the fracture of electrovalent bonds (Ca-O an

5、d Si-O). For a given cement fineness, this helps in improving mill productivity and/or reducing grinding energy consumption. Typical dosage rates vary from 0.01% to 0.15% of the manufactured cement mass. After the grinding process, GAs may not preserve their original molecular structures; however, t

6、hey do remain adsorbed onto the cement particles to provide variations of cement properties whether in the fresh or hardened states. The setting and hardening properties of cement containing GAs are well documented in literature. For instance, the hydration of C 3 A was accelerated in the presence o

7、f triethanolamine (TEA) due to the rapid formation of hexagonal aluminate hydrate and its transformation to a cubic form. 4Heren and Olmez found that the addition of increased ethanolamine concentrations alters cement hydration and leads to retardation in setting times in the order of TEA diethanola

8、mine (DEA) monoethanolamine (MEA). 5Triisopropanolamine (TIPA), which is an amino-alcohol and belongs to the group of alkanolamines, was found to change hydration reactions and partic- ularly increase cement strengths. Perez et al. reported that TIPA remains in the interstitial paste solution (not a

9、dsorbed to the cement surface, as the TEA) and form iron complexes SP-302-01 Effect of Clinker Grinding Aids on Static Yield Stress of Cement Pastes by Joseph J. Assaad and Salim E. Asseily 11to accelerate hydration of C 3 S and C 4 AF. 6Ichikawa et al. presented evidence that TIPA promotes hydratio

10、n of limestone and densifies the interfacial transition zone (ITZ) between hydrated cement paste and sand or aggregate particles. 7 Limited data exits in literature pertaining to the effect of GAs added during clinker grinding on rheology of cement-based materials. Aiad et al. found that viscosity o

11、f cement pastes is dependent on the type and dosage rate of ethanolamine used, whereby a decrease in viscosity was noticed following the sequence of TEA poly-TEA MEA. 8This was related to the number of O-H groups in the ethanolamine molecules that are adsorbed on the surface of cement grains, causin

12、g different repulsive forces and leading to variations in fluidity levels. However, it is important to note that the tests carried out by Aiad et al. 8cannot be conclusive as the ethanolamines were post-added to the cement (i.e., not added during the grinding process) at concentrations varying from

13、0.1% to 2% of cement weight (i.e., substantially higher than in real situations). Anna et al. compared the Z-potential of clinker containing TEA with others ground with polycarboxylate (PC) or poly-naphtha- lene sulphonate (PNS) concrete superplasticisers. 9They found that the TEA fluidifying mechan

14、ism for the dry cement system lies between the steric hindrance associated with PC polymers and electrostatic interaction of PNS with the positive charges of cement grains. 9While characterizing GAs and their impact on cement performance, Katsioti et al. noted an improvement in workability of cement

15、 pastes containing TIPA. 10This was related to the breaking down of cement agglomerates and balance modification between inter-particle forces. The objective of this paper is to assess the effect of GAs on variations in flow and static yield stress of cement pastes prepared with different water-to-c

16、ement ratios (w/c). Grinding tests were performed by adjusting the specific energy consumption (Ec) in order to main- tain similar Blaine fineness. Amine, glycol, and phenol-based GAs were used at various concentrations. Relevant parameters including flowability, yield stress, water demand, setting

17、time, and compressive strength were evaluated. RESEARCH SIGNIFICANCE Grinding aids are increasingly used during comminution of clinker to prevent cement particle attraction and re-agglomeration, thus resulting in clinker and energy savings that can both lead to reduced carbon dioxide (CO 2 ) emissio

18、ns. Results presented in this paper aim at assessing the effect of such additions on flow and yield stress of cement pastes. Such data can be of particular interest to cement manufacturers and concrete technologists as well as standardization committees dealing with specifications for GAs. EXPERIMEN

19、TAL INVESTIGATION Materials Industrial clinker used for the production of ASTM C150 Type I cement, ground granu- lated blast furnace slag meeting the requirements of ASTM C989 Grade 80, and gypsum materials were employed in this study. The C 3 S, C 2 S, C 3 A, and C 4 AF of clinker were 54.6%, 17.4%

20、, 9.2%, and 13.7%, respectively. The slag activity index with cement at 28 days is 86.4%. The relative hardness of the clinker, slag, and gypsum determined according to the Mohs hardness scale were around 5.5, 6, and 2, respectively. 12 SP-302-01Two commercially available GAs and a third one special

21、ly formulated for this study were tested. First, the amine-based GA is commonly used as a grinding aid and strength enhancer in the cement industry. It had 68% active chemicals when determined by the Karl Fischer method, and specific gravity and pH values of 1.09 and 7.2, respectively. The second GA

22、 is glycol-based composed by diethylene glycol (DEG) and monoethylene glycol (MEG) chemicals. It is commonly referred to as a grinding aid and pack-set inhibitor in the cement industry. Its active chemicals, pH, and specific gravity were 72%, 7.8, and 1.107, respectively. Finally, a specially formul

23、ated phenol-derivative GA with 70% active chemicals, pH of 5.8, and specific gravity of 1.09 was used. This latter GA was blended with 17.5% oleic fatty acids and 30% polycarboxylate ester molecules that are usually used in concrete admixtures to improve cement dispersion and workability during mixi

24、ng. Production of cement used for testingA 50-L (13.25-gal) grinding mill connected to an electric counter for monitoring Ec was used 2,3(Fig. 1). The mills drum diameter, width, and rotational speed were 400 mm (15.7 in.), 400 mm (15.7 in.), and 50 rpm, respectively. It contained a total of 80 kg (

25、176 lb) steel balls among which 36 kg (79.2 lb) have 20-mm (0.7 in.) diameter and 44 kg (96.8 lb) have 30-mm (1.18 in.) diameter. Prior to grinding, the clinker, gypsum, and slag materials were crushed and sieved so that all particles were smaller than 10 mm (0.4 in.). Figure 1Photo of the grinding

26、mill used for testing. Effect of Clinker Grinding Aids on Static Yield Stress of Cement Pastes 13All grinding tests were conducted using 7 kg (15.4 lb) of a mix composed of 90% clinker, 5% gypsum, and 5% slag. First, a mix ground without GA at 42 kWh/ton was tested and considered in this project as

27、being the control cement; its Blaine fineness was 3460 cm 2 /g. Then, GAs were introduced at pre-selected concentrations varying from low to high levels; the Ec was adjusted accordingly in a way to maintain the fineness equivalent to the control cement, i.e. Blaine of 3460 100 cm 2 /g. The high GA l

28、evels were determined following previous studies, 2,3thus ensuring that ASTM C465 requirements for water demand, setting time, and compressive strength are satisfactorily fulfilled. 11The method for cement produc- tion consisted on grinding the materials for a certain elapsed time, stopping the mill

29、, and sampling around 100 grams to check whether the Blaine was close to the targeted value. If not, additional grinding was performed. At the end of grinding, the temperature of the charge was found to increase from ambient (i.e., 23 C (73 F) to around 37 C (99 F). Testing equipment and procedures

30、Tests on powder cementFollowing grinding, the cement fineness was determined using the Blaine apparatus, as per ASTM C204 Test Method, and by mechanical sieving on 106, 90, and 38 m mesh openings. The R-90 and R-38 values given in this paper refer to the individual percentages retained on the 90-m a

31、nd 38-m sieve, respectively. The residues on the 106-m sieve were in the range of 0.2% to 3.5%, depending mostly on the Ec used (note that all particles retained on this later sieve were not included in the cement mix used for subsequent testing). Tests on cement pastesAll pastes were batched with a

32、 laboratory mixer using water cooled to a temperature of around 23 C (73 F). Water was first introduced in the mixer followed gradually by the ground cement over 2 minutes. After a rest period of 30 s, the mixing was resumed for one additional minute. The ambient temperature and relative humidity du

33、ring testing were maintained at 23 2 C (73 36 F) and 55 5%, respectively. The water demand required to achieve normal consistency was determined following ASTM C187 Test Method. Using the same cement paste, the Vicat initial and final setting times were then determined as per ASTM C191 Test Method.

34、The effect of GA types and concentrations on flow and rheological properties was evalu- ated using cement pastes prepared at 0.48 and 0.42 w/c. These w/c were selected in order to produce pastes with different consistency levels ranging from highly flowable to relatively cohesive. The flow was deter

35、mined as the average diameter of the paste after spreading on a horizontal surface using an ASTM C230 mini-slump cone. A rotational viscometer connected to a data logger was used to evaluate static yield stress ( 0 ). The vane used consisted of four blades arranged at equal angles around the main sh

36、aft; it measured 24 mm (0.95 in.) in height and 12 mm (0.47 in.) in diameter. Right after mixing, the cement paste was poured in a cylindrical bowl and allowed to rest for one min prior to measuring the 0value. The testing protocol consisted on subjecting the paste to a very low rotational speed of

37、0.3 rpm and recording the changes in torque as a function of time. The 0was determined in accordance to Nguyen and Boger, 12by considering the maximum torque registered that indicates the initiation of flow. It is important to note that the total elapsed time from the initial contact of cement with

38、water until the flow and 0measurements was around 10 minutes. 14 SP-302-01Tests on mortarsThe compressive strength was determined as per ASTM C109 Test Method. The 50-mm (2-in.) cubes were cured in water until testing at 7 and 28 days. TEST RESULTS AND DISCUSSION The various cement properties determ

39、ined following clinker grinding at fixed Blaine fineness of 3460 100 cm 2 /g using either amine, glycol, or phenol-based GA are summa- rized in Tables 1, 2, and 3, respectively. It is to be noted that several cement mixtures were ground two to three times to evaluate reproducibility of testing. Acce

40、ptable coefficients of variation (COV) were obtained; as these were equal to 3.8%, 5.1%, 4.6%, 7.4%, and 5.7% for the Blaine, R-38, water demand, setting time, and compressive strength, respectively. The COV increased to 8.8% for 0responses, given the variations in Blaine fineness that could lead to

41、 different restructuring rates during the rest period prior to shearing. Also, confinement and yielding of paste within the rotating vane impeller could lead to some Table 1Properties when cement is ground with amine-based GA Ec, kWh/ton 42 40.4 38.6 37.1 35.7 34.8 GA dosage, % of mass 0 0.03 0.06 0

42、.09 0.11 0.13 Blaine, cm 2 /g 3460 3430 3495 3360 3405 3375 R-90, % 8.6 8.1 8.2 9.3 9.8 9.5 R-38, % 28.3 25.8 22 30.1 28.6 32.8 Water demand, % 27.25 27.3 27.45 27.2 27.5 27.05 Final set, min 235 240 235 255 275 290 w/c = 0.48 Flow, mm 195 190 200 195 200 205 0 , Pa 9.2 10 8.8 7.6 7.2 7.6 w/c = 0.42

43、 Flow, mm 150 150 160 155 160 165 0 , Pa 30.4 28.8 29.6 28 28.8 26.4 7-d compression, MPa 35.6 35.9 38.1 38 39 36.9 28-d compression, MPa 47.2 48.8 49.7 49.6 52.1 50.3 Notes: 1 cm 2 /g = 6.8 10 -5in 2 /lb; 1 MPa = 145 psi; 1 in. = 25.4 mm Table 2Properties when cement is ground with glycol-based GA

44、Ec, kWh/ton 42 40.7 39.2 38.4 37.5 GA dosage, % of mass 0 0.03 0.06 0.08 0.1 Blaine, cm 2 /g 3460 3415 3475 3520 3515 R-90, % 8.6 7.8 8.4 9 8.9 R-38, % 28.3 25.2 24 29.1 34.3 Water demand, % 27.25 27.05 27.2 27.1 27.3 Final set, min 235 255 265 260 300 w/c = 0.48 Flow, mm 195 210 210 225 220 0 , Pa

45、9.2 8.8 8 7.6 7.6 w/c = 0.42 Flow, mm 150 145 160 175 170 0 , Pa 30.4 28.8 25.6 21.6 22.4 7-d compression, MPa 35.6 34.8 37 36.7 35.3 28-d compression, MPa 47.2 48 47 49.5 45 Notes: 1 cm 2 /g = 6.8 10 -5in 2 /lb; 1 MPa = 145 psi; 1 in. = 25.4 mm Effect of Clinker Grinding Aids on Static Yield Stress

46、 of Cement Pastes 15discrepancies in rheological measurements. 12The COV is taken as the ratio between stan- dard deviation and mean values, multiplied by 100. Effect of GA on Ec values Regardless of GA type, the addition of increased concentration led to consecutively reduced Ec values. For example

47、, Ec decreased from 42 kWh/ton for the control cement to 37.5 kWh/ton with the use of 0.1% glycol-based GA, corresponding to 10.7% reduction in energy consumption. Such decrease reached 34.8 and 38.5 kWh/ton with the use of 0.13% amine-based GA and 0.09% phenol-based GA, respectively (i.e., 17.1% an

48、d 8.3% energy decrease, respectively). This can normally be related to the GA molecules that are adsorbed onto newly fractured surface grains, thus attenuating electrostatic forces and improving cement fineness. 2 It is to be noted that the decrease in Ec provided during the grinding process led to

49、an increase in R-38 and R-90 values, particularly at high GA concentrations (Tables 1, 2, and 3). Hence, R-38 increased from 28.3% for the control cement to 32.8% and 34.3% when the amine or glycol-based GA were used, respectively, at a rate of 0.13% or 0.1%, respec- tively. This indicates that the sieve residues that are functions of the maximum particle size are directly affected by the amount of grinding energy. 3Nevertheless, data provided by the particle size analysis has shown an increased fraction of particles finer than around 5 m fo