Effects of Polycarboxylate-based Grinding Aid on the Performance of Grinded Cement

HE Yan, ZHANG Xiong, WU Genhuo, SHI Pengcheng

(1. School of Civil Engineering, Suzhou University of Science and Technology, Suzhou 215011, China; 2. Key Laboratory of Advanced Civil Engineering Materials of Education Ministry, Tongji University, Shanghai 201804, China; 3. Jiangsu Technology Industrialization and Research Center of Ecological Road Engineering, Suzhou 215011, China)

Abstract: A molecular modified PCE-type GA was self-synthesized, and the grindability concerning grinding duration and particle size distribution of ground cement was evaluated. Setting time, flowability,hydration heat, mechanical properties and microstructure of the ground cement were also investigated. The results show that: PCE-type GA is effective in cement grinding. With 0.03% PCE-type GA added into cement clinker, the Blain surface area reaches to the highest at every ten minutes, and the volume percent of cement particles with size of (0-32 μm) reaches the highest. Excessive amount of PCE-type GA seems to impair the grinding efficiency. The grinding efficiency promotes cement fineness and hence contributes to the cement hydration degree as well as improves the mechanical properties.

Key words: PCE-type GA; cement; grindability; fineness; hydration properties

1 Introduction

Cement and concrete are the most widely-used construction materials. However, they are regarded as energy/resource-consumed and environmentalunfriendly materials, in which cement grinding is the highest energy consuming process[1-4]. In this process,approximately one-third of the total energy consumed in cement production is required to grind cement to a specific surface area[5-7]. This energy consumption is frequently related to the cement clinker grindability and grinding mill. The grindability is the measure of the cement clinker’s ability to resist grinding forces and can be used as an indicator to estimate the energy required to grind the clinker to a given Blain surface area.Grindability provides the relationship between the mill revolution and the given Blain specific surface area[8].To save energy, application of grinding aids (GAs)during this process is vitally important in addition to the other aspects of grinding technology, such as the use of a roller press[9]. GAs are usually vital to decrease electrostatic forces and cripple agglomeration among cement particles, allowing the production of fine particles with desirable properties[1,2,10-12], and GAs are commonly introduced into the material by dropping the chemical either onto the feed conveyor belt or directly into the milling chamber[9,10,13-15], which has achieved significant economic effect as well as environmental benefit[16,17].

Recently, a wide range of studies have been conducted in the field of GAs, and many compounds have been adopted as GAs, including amines, glycols,phenols, and alkanolamines[3,8,9,18-20]. GAs could reduce the solid surface energy while the new surfaces were generated following the grinding action[9]. The surfaces of the particles were neutralized hence the probability of forming agglomerates and coatings was reduced that ultimately provided more efficient grinding environment[21]. Paramasivam and Vedaraman[22,23]studied the effect of the physical properties of liquid additives on dry grinding and found that additives increased the fineness of the ground particles and the powder flow. Choiet al[22,24]discovered that the penetration of grinding aids into the cracks within the particles promoted crack propagation, resulting in easier breakage of the particle. R Green-woodet al[25]found that, with the help of an optimal GA, the typical maximum percentage of solids by weight increased from 50% to 80%. However, the additionof organic substances comprehensively influences the process of cement hydration, kinetics, microstructural development, and relationship between microstructure and macroscopic properties of cement and concrete.For example, glycerin can bring poor compatibility between cement and superplasticizer, which causes serious difficulties to construction[18]. Triethanolamine(TEA) is very sensitive to its dosage, exhibiting a great level of robustness, sometimes acting as accelerator to cement hydration and sometimes acting as retarder[26]. Triisopropanolamine (TIPA) usually causes poor compatibility between cement and chemical admixtures, which is unbeneficial to the workability of fresh binder. And a high amount of inorganic salts adopted as GAs can remarkably impact the durability and volume stability of hardened concrete[7]. So generally, traditional GAs are restricted in grinding process of cement, but make incoercible effects on the mechanical performance as well as durability of hardened cement mortar or concrete[26-28].

Based on the above issues, increasing researches pay attention on the grinding performance of polycarboxylate-based (PCE-based) polymers due to its strong structural designability and functional diversity[22,26,29,30]. The possibility of PCE-based polymers to be applied as GA has been evaluated[7,26,31].As a kind of macromolecule polymer, PCE-based polymers had the stabilizing effect of the electrostatic repulsion and steric stabilization, which were more effective than small organic molecules[22]. According to our previous discovery, polyhydroxyl substances possess high grinding performance. The hydroxyl group and the amidocyanogen groups in the PCE-type polymer contribute to the grinding effects. Through synergistic reactions among hydroxyl group, carboxylic group and polyether segments, the functional recombination and performance improvement of PCEtype GA can be realized.

In this research, a molecular modified PCEtype GA was self-synthesized, and the grindability concerning grinding duration and particle size distribution of ground cement was evaluated. Setting time, flowability, hydration heat, mechanical properties and microstructure of the ground cement was also investigated. The aim was to provide theoretical information for the development of innovative GAs.

2 Experimental

2.1 Materials

Portland cement clinker and gypsum were provided by Wuhu Hailuo Cement Co., Ltd in Anhui Province, China, and their chemical composition are shown in Tables 1-3. Sand, China ISO standard sand,was produced by Xiamen ISO Standard Sand Co., Ltd.

Table 1 Chemical composition of cement clinker

Table 2 Mineral composition of cement clinker

Table 3 Chemical composition of gypsum

Methacrylic acid (MAA), allyl polyoxyethylene ether (APEG,Mn-2 400 g/mol-1, average polymerization degree-28), triethanolamine (TEA),and sodium methallyl sulfonate (SMAS) were adopted as monomers to polymerize polycarboxylate-type GA, and these monomers were provided by Aoke Polyurethane Company. Ammonium persulfate (APS),hydrogen peroxide, and L-ascorbic acid were applied as initiators for the polymerization process, and 3-mercaptopropionic acid was adopted as chain transfer agent.

2.2 Methods

2.2.1 Synthesis and characterization of polycarboxylate-based GA

Self-synthesized polycarboxylate-based GA was employed in the experiment. The polycarboxylatebased GA was a comb-like copolymer of methacrylic acid (MAA), allyl polyoxyethylene ether (APEG,Mn-2 400 g/mol-1, average polymerization degree-28),triethanolamine (TEA), and sodium methallyl sulfonate (SMAS). Ammonium persulfate (APS),hydrogen peroxide, and L-ascorbic acid were used as initiator, with a molar ratio of 1:0.7:1.4, and 3-mercaptopropionic acid as chain transfer agent. Thecontent of initiator was 2.45% of the total monomer weight, while the content of chain transfer agent was 0.56% of the total monomer weight. Before synthesis,80 g APEG was dissolved with 100 g deionized water in a bottle flask. Then APS as well as hydrogen peroxide were placed into the synthesis system,and stirred until dissolution. Methacrylic acid and 3-mercaptopropionic acid were mixed together with 50 g deionized water. The extra monomers were mixed with L-ascorbic acid using 50 g deionized water. Water bath was used to provide stable heat for the synthesis system at 60 ℃. Peristaltic pump was used to titrate the above dissolved solutions for 2.5-3 h. After that, the copolymers were cooled to 30 ℃ and then neutralized to pH=6-7 using NaOH solution.

The objective molecular structure of the polycarboxylate-based GA was shown in Fig.1. And the chemical structure of the polycarboxylate-based GA was characterized by a Thermo-Nicolet Nexus 670 FTIR, in the range of 4 000-400 cm-1with 200 successive scans. The spectra rationed against a potassium bromide (KBr) background.

Fig.1 Molecular structure of the polycarboxylate-based grinding aids

2.2.2 Cement test mill

The control cement mixture containing Portland cement clinker of 95% and gypsum of 5% was mixed and ground in a laboratory ball mill with steel balls as grinding media. Clinker and gypsum were crushed into small particle less than 6-7 mm diameters prior to grinding procedure, and the total feed weight was 5 kg per mill and the grinding time was kept the same as 30 min for each ground cement.

2.2.3 Grinding efficiency test

The fineness of cement was evaluated from the Blaine surface measurement and particle size distribution[5,32,33]. Blaine surface of the blended cement were tested according to the Chinese National Standard GB/T8074-2008 (Testing method for specific surface of cement-Blaine method). Particle size distribution of the blended cement were tested according to the Chinese National Standard JC/T 721-2006 (Test method forparticle size of cement, Laser based methods).

2.2.4 Physical performance test

The measurements of water demand of normal consistency and the setting time of the ground cement were tested according to the Chinese National Standard GB/T1346-2001 (Test method for water requirement of normal consistency, setting time and soundness of the Portland cement).

The normal mortar flow was tested according to the standard method GB/T 2419-1994 (Measurement for fluidity of normal mortar). Mortar prisms (160 mm × 40 mm × 40 mm) were cast using a cement :sand : water weight ratio of 1:3.0:0.5. After 24 h in a moist cabinet, they were removed from the molds and cured in water under standard conditions (20±2℃, 90% relative humidity). At 3 and 28 days, three prismatic specimens were pressed into six pieces, and then, the compressive strength was tested on six prisms according to the GB/T 17671-1999 (Method of testing cements-Determination of strength).

2.2.5 Cement hydration analysis

Early hydration of ground cement was investigated by hydration heat evolution measurement(isothermal calorimetric), X-ray diffraction analysis(XRD) and differential scanning calorimeter analysis(DSC). In addition, microstructure observation of the hydrated binders was conducted through scanning electron microscope (SEM)[34].

The small pieces of hydrated binders were firstly immersed into alcohol for 24 h, and then dried at 30℃ for 24 h. Before SEM test, the samples were gold coated under 20 mA electrical current for 2 min. For XRD as well as DSC-TG analysis, the dried samples were ground to ＜63 μm[34].

3 Results and discussion

3.1 FT-IR characterization of PCE-based GA

Fig.2 shows the FT-IR spectrum of the synthesized PCE-based GA. From the FT-IR spectrum,we can see the stretching vibration peak of hydroxyl group at 3 507.70 cm-1, the stretching vibration peak of saturated hydrocarbon bond at 2 880.81 cm-1,the stretching vibration peaks of carboxyl functional groups at 1 721.54 cm-1, the bending absorption peak of saturated hydrocarbon bond at 1 464.99 cm-1, the stretching vibration peak of C-O bond in menthol and ether structure, the olefin trans vibrational peak at 944.19 cm-1, and the characteristic absorption peak of ether bond at 1 108.15 cm-1. This proves that there are chain segments of polyoxyethylene in the molecular structure of the synthesized PCE-type GA.

Fig.2 Infrared spectrum of the synthesized PCE-based GA

3.2 Grinding efficiency

Portland cement clinker, gypsum and PCE-type GA were grinded together, and the evaluation of grinding efficiency for the PCE-type GA was carried out in terms of grinding duration, particle morphology, particle size distribution, and Blaine surface area, in comparison with the blank sample (grinded without GA).

3.2.1 Grinding duration

The variation of Blain surface area of cement particles in the function of grinding time was analyzed,and the results are shown in Fig.3.

Fig.3 The variation of Blain surface area of cement samples in the function of grinding time

From Fig.3, it is clear that clinker ground under different conditions can reach up to Blain surface area of 370 m2/kg for different grinding durations. Blank sample (clinker ground without GA), reaches up to Blain surface area of 369 m2/kg, after being ground for 60 min. With PCE-type GA added, the grinding efficiency improves significantly. When added with 0.03% PCE-type GA, the Blain surface area of cement clinker reaches 374 m2/kg, after ground for 50 min.Besides, with 0.03% PCE-type GA added into cement clinker, the Blain surface area reaches to the highest at every ten minutes. However, excessive amount of PCEtype GA seems to impair the grinding efficiency. For example, with 0.1% PCE-type GA added into cement clinker, the Blain surface area reaches to 369 m2/kg after ground for 58 min. GA could mitigate the caking and agglomeration phenomena during grinding process.Cement particles with the oppositely charged surfaces are attracted to form agglomerates during grinding.The added GA will partially neutralize the surface charge by adsorption on cement particle surface,thus alleviating the agglomeration and improving the grinding performance[1,2].

3.2.2 Particle size distribution

Table 4 shows the particle size distribution of cement clinker in the function of PCE-type GA. With PCE-type GA added into cement clinker sample,the ground cement particles with size of (0-32 μm)improves significantly, which vitally influences the cement properties. And when 0.03% PCE-type GA is added into cement clinker, the volume percent of cement particles with size of (0-32 μm) reaches the highest. This indicates that at the dosage of 0.03%,PCE-type GA gets to the highest grinding efficiency.Due to the modification by GAs, the nature of cement surface is changed from hydrophilic feature for neat cement to hydrophobic feature. Thus, the rheological behavior of fresh cement paste will be affected somehow. Although, the increased cement fineness leads to a higher water demand, PCE-type GA acts as dispersants itself and releases the entrapped water from the flocculation structures. And the modified surface feature of cement with different GA can lead todifferent adsorption capabilities.

Table 4 Particle size distribution of the cement powder particles

3.3 Setting time, flowability and mechanical properties

The water demand and the setting time of cement paste with normal consistency as well as fluidity and compressive strength at the ages of 3 and 28 days of normal mortar were tested. Results are shown in Table 5.

Table 5 Physical and mechanical performance of the ground cement samples

The water demand of the blank sample is 24.8%and the fluidity of blank cement paste is 141 mm, while the water demand for cement sample ground with PCEtype GA decrease. With the increase of PCE-type GA content, the water demand decreases gradually, and the fluidity increases significantly. This is because PCEtype GA adsorbs onto the cement surface and disperses the cement flocculation in the cement paste due to its dispersion properties. On the other hand, the finer the cement particle is, the more water will be needed.These two reasons equalize each other, and as a result there is a slight decrease in water demand with the increase of PCE-type GA content.

The normal consistency is related with a series of aspects. On one hand, the Blain surface area influences the normal consistency. Higher surface area of cement particles may lead to higher water demand. On the other hand, the adsorbed PCE-type GA onto the surface of cement particles affects the water demand as well as the normal consistency.

However, PCE-type GA increases the Blain surface area of the ground clinker particles, which therefore changes the setting time as well as the hydration degree of the cement particles. So, PCEtype GA on the one hand increases the fineness of the ground cement, and on the other hand disperses the cement particles as well as decomposes the flocculation in the cement paste. The grinding efficiency promotes the increase of cement fineness and hence contributes to the cement hydration degree as well as improves the mechanical properties.

3.4 Hydration properties of the cement samples

3.4.1 Hydration heat

The hydration heat release rate and cumulative heat flow of the cement samples were tested, and the results are shown in Fig.4.

Fig.4 (a) Hydration heat evolution rate and (b) cumulative heat flow of ground cement samples

In comparison with blank cement sample ground without GA, the addition of PCE-type GA tends to prolong the induction period of cement paste,especially at the dosage of 0.10% PCE-type GA.And when 0.03% PCE-type GA is added into clinker sample, the hydration heat release rate increases due to the finer cement particles and dispersion properties of the cement paste. This trend confirms the compressive strength result that clinker ground with 0.03% PCEtype GA at the early age is the highest, due to the finer cement particles and dispersion properties.As an organic dispersant, PCE chemically affect the dissolution process as well as the early cement hydration process.

3.4.2 SEM analysis

After curing under standard conditions for 28 d,hardened cement pastes were selected, their fracture surfaces were observed through SEM, and the SEMimages are given in Fig.5.

Fig.5 SEM images of the 28-day hydrated clinker: (a) without GA; (b) with 0.03% PCE-type GA

From Fig.5, it is clear to analyze the effects of PCE-type GA on the microstructure and morphology of the hydration products. It can be seen from Fig.5 that there are pores in the fracture surfaces of the hardened paste. The pore size in the Blank sample without GA is remarkably larger. Additionally, the blank sample has a sparsest microstructure. The cement with PCE-type GA has a lower water-to-cement ratio requirement to achieve normal consistency. Therefore, the pastes can be more evenly stirred during paste mixing, and denser microstructures are formed at later hydration ages.The lower water-to-cement ratio required to achieve normal consistency is obtained when more PCE-type GA is added during grinding process. Therefore, the cement with 0.1% PCE-type GA results in a denser microstructure compared with the cement with 0.03%or 0.02% PCE-type GA.

4 Conclusions

a) With 0.03% PCE-type GA added into cement clinker, the Blain surface area reaches to the highest at every ten minutes, and the volume percent of cement particles with size of (0-32 μm) reaches the highest.Excessive amount of PCE-type GA seems to impair the grinding efficiency.

b) With the addition of PCE-type GA, the grinding efficiency promotes the increase of cement fineness and hence contributes to the cement hydration degree as well as improves the mechanical properties.Increased content of PCE-type GA shows retardation effects on cement hydration, but promotes cumulative hydration heat flow at 7 days. The microstructure of hardened matrix is more compact with less pores and more amount of hydration products for cement clinker with 0.03% PCE-type GA.