New Generation Amphoteric Comb-like Copolymer Superplasticizer and Its Properties

Transcription

1 New Generation Amphoteric Comb-like Copolymer Superplasticizer and Its Properties New Generation Amphoteric Comb-like Copolymer Superplasticizer and Its Properties Changwen Miao,2, Qianping Ran,2*, Jiaping Liu,2, Yonglin Mao 2, Yan Shang 2 and Jianfang Sha 2 Research Centre for Aqueous Polymer Building Materials & Engineering Technology, Jiangsu Provincial Institute of Building Science, Nanjing, 20008, People s Republic of China 2 Jiangsu Bote New Materials Co. Ltd, Nanjing, 20008, People s Republic of China Received: 2 August 2009, Accepted: 3 August 200 Summary The incorporation of high performance superplasticizers in concrete is one of the most effective and economic technological methods for achieving sustainable development in the concrete industry. Conventional polyelectrolytetype superplasticizers have some defects, e.g. poor dispersibility, large slump loss, increasing the shrinkage of concrete and pollution when being manufactured. Although the new generation of comb-like polycarboxylate superplasticizers has remarkable benefits in practice, it also has the shortcoming of low saturation dosage. In this study, based on adsorption-dispersion mechanism of dispersant and polyampholyte solution theory, a new class of amphoteric comb-like copolymer superplasticizer (PACP) was designed and synthesized. The effects of PACP on water reduction, setting, compressive strength, hydration and pore structure were investigated systematically. Results show that PACP can be dosed in small amounts to obtain a water reduction up to 45%. The addition of PACP also does not retard cement hydration and significantly increases the compressive strength of hardened concrete at early and ultimate stages. Moreover, PACP can further improve the rate of heat evolution, delays the time to reach the highest hydration temperature and refines the pore structure of hardened cement paste markedly, leading to decreases of porosity and average pore size.. INTRODUCTION The incorporating of superplasticizers or high-range water reducers in concrete is one of the most effective and economic technological methods for achieving sustainable development in the construction industry because when added in small amounts to concrete, they allow great reductions in the use of cement, increase the durability of concrete structures and also allow efficient incorporation of industrial by-products such as silica fume, fly-ash or slag and recycled concrete. Thus, the use of superplasticizers represents a major environmental issue because the existing technology of production of cement clinker is ecologically unfriendly: it consumes much energy and natural resources, and emits a number of undesirable air pollutants, especially large amounts of CO 2-2. However, conventional polyelectrolyte-type superplasticizers, such as sulphonated naphthalene formaldehyde condensates, sulphonated melamine formaldehyde condensates, have some defects, e.g. poor dispersibility, large slump loss, increased shrinkage of concrete and pollution when being manufactured 3-4. So a new type of highly efficient superplasticizer containing comb-like copolymers with a grafted pendant group of polyethylene oxide has been developed, which has the advantages of outstanding water reduction ratio, high workability retention at small dosages, excellent compatible with higher dosage rates of industrial by-products and manifold variations of the molecular structures of these polymers -2, 4-6. Although this new generation comb-like superplasticizer has such remarkable benefits in practice, anionic comb-like polymer admixtures have the shortcoming of low saturation dosage. Above the saturation point, the dispersing effect or water reducing rate cannot be improved 7. Hence, this impedes the development of ultra high-strength concrete with low water to binder ratio and concrete made with low cement or high industrial by-product content as a replacement of cement. * Corresponding author. address: qianpingran@gmail.com Smithers Rapra Technology, 20 Many researches have proved that the adsorption of such comb-like copolymers on the cement surfaces, controlled by COO - content in polymer Polymers & Polymer Composites, Vol. 9, No., 20

2 Changwen Miao, Qianping Ran, Jiaping Liu, Yonglin Mao, Yan Shang and Jianfang Sha backbones, dominates the dispersing effect and rheological behaviour in cementitious systems 8-9. Portland cement mainly consists of four minerals, namely Ca 3 SiO 5, Ca 2 SiO 4, Ca 3 Al 2 O 6 and Ca 4 Al 2 Fe 2 O 0, which are abbreviated to C 3 S, C 2 S, C 3 A, C 4 AF and gypsum. According to recent research, it is widely accepted that the surfaces of silicate phases (C 3 S, C 2 S) and calcium silicate hydrates (C S H) are negatively charged, whereas the surfaces of the aluminate phases (C 3 A, C 4 AF) are positively charged 6-9. Hence, for these cement component minerals, superplasticizers are not expected to adsorb uniformly. Some component minerals might adsorb much larger amount than the others. Yoshioka et al. 6 reported that the maximum adsorption value of the comb-like superplasticizer on C 3 A was approximately 6-8 times that of C 3 S. Z.M. Wang 9 reported the maximum adsorption value of the comb-like superplasticizer on C 3 A was approximately 5-8 times that of C 2 S; whereas the adsorbed amounts on C 3 S and C 4 AF are negligible. Therefore, there is still much room for improvement of the comb-like superplasticizers by changing the ionic group type in the backbone. In this paper, based on adsorptiondispersion mechanism of dispersant and polyampholyte solution theory, a new class of amphoteric polycarboxylate comb copolymer superplasticizer (hereinafter called PACP) with high water-reduction performance was designed and synthesized. The effects of PACP on water reduction, setting, compressive strength, hydration and pore structure were investigated systematically. 2. EXPERIMENTAL 2. Materials Polycarboxylate Superplasticizer The amphoteric polycarboxylate superplasticizer (PACP) was prepared according to a Chinese patent in our laboratory 20. The anionic polycarboxylate superplasticizer (hereinafter called PCA) was produced by Jiangsu Bote New Materials Co. Ltd. The molecular weight of the superplasticizer was determined using a Wyatt Technology minidawn static three-angle laser light scattering detector (MALLS) equipped with TSK-GELSW (TOSOH) columns. The formulas and molecular characteristics of the superplasticizers used in this study are listed in Table and Figure. Cement Type II Portland cement with 42.5 grades from Nanjing Jiangnan cement plant was used in this study. The cement composition was determined by x-ray fluorescence and Bogue analysis. The Blain surface area was 308 m 2 /kg. The chemical and physical properties of the cement are given in Table 2. Coarse Aggregate 5-25 mm continuous graded crushed basalt. Its density was 2840 kg/m 3. Figure. Schematic illustration of molecular structure of comb-like copolymer superplasticizer Molar composition /% M Table. Molecular characteristics of PACP and PCA superplasticizers Superplasticizer Side chain b M c w length /n a x y z n PACP PCA a Calculated by H-NMR, b M n =number average molecular weight, c M w =weight average molecular weight Table 2. Chemical composition and physical properties of cement Chemical composition (%) Density (kg/m 3 ) Surface area (m 2 /kg) SiO 2 Al 2 O 3 CaO SO 3 MgO Fe 2 O 3 K 2 O Na 2 O Polymers & Polymer Composites, Vol. 9, No., 20

3 New Generation Amphoteric Comb-like Copolymer Superplasticizer and Its Properties Fine Aggregate Natural river sand was used, with a fineness modulus of 2.65 and density of 2620 kg/m Methods FT-IR Analysis The FT-IR analysis was carried out using an EQUINOX55 Fourier transform infrared spectrometer (Bruker). The sample was scanned from 4000 cm - to 400 cm - with a resolution of 2 cm -. H-NMR Analysis H-NMR analysis was performed on BRUKER DRX-500 nuclear magnetic resonance instrument (500 MHz) using deuterated water (D 2 O) as solvent. The composition of the superplasticizer has been estimated classically by H-NMR spectroscopy. Properties of Freshly Mixed Concrete and Hardened Concrete The concrete specimens were prepared and stored according to Chinese standard GB All the concrete mixes contained 390 kg/ m 3 of cement, 840 kg/m 3 of sand and 070 kg/m 3 of coarse aggregate. The water was adjusted to achieve an initial slump of 80±0 mm after mixing (at 3 minutes) at different superplasticizer dosages. Concrete slump, air content, water-reduction rate and setting time were measured in accordance with GB The setting time was measured by the penetration resistance of the mortar sieved from the concrete. Compressive strengths were tested at 24 h, 3, 7, 28 and 60 days according to GB/T The values given are the average of the results obtained on three specimens. The slump loss of fresh concrete containing different superplasticizer dosages were measured in accordance with JC Adsorption Measurements The amount of superplasticizer adsorbed was determined by means of a total organic carbon analysis (TOC), Multi N/C300 (Analytikjene AG, Germany). 20 g of solution containing various amounts of superplasticizers and 0 g of cement were mixed by a magnetic stirrer at 250 rpm for 5 minutes at 20 C. The sample solution was separated by suction filter. The aqueous phase was separated by centrifuging at 3,000 rpm for 5 minutes using a centrifuge. The supernatant was immediately decanted and dilute with deionized water for TOC. Several aliquots of each sample were measured. The difference in the concentration before and after contact with the cement was assumed to be adsorbed polymer. Heat Evolution During Cement Hydration The development of the heat of hydration of cement paste containing the different superplasticizers was recorded by measurements of the temperature of fresh cement paste according to GB/T The temperature probe was dipped into the cement paste immediately after mixing the cement with water and the temperature variations were obtained using a HX-48 hydration heat analyzer manufactured by Huxin Cement Co. Ltd. Porosity Measurement Cement pastes were prepared with a water-to-cement (W/C) ratio of 0.29 by adding tap water with PACP (0.30% as dry polymers by mass of cement). Each paste was blended by a machinecontrolled mixer for 3 min, placed in a mm steel mould and consolidated by an exterior vibrating table. Just after demoulding at the age of day, specimens were cured in a standard curing room maintained at T=20±3 C and RH>90% for defined periods. The specimen fragments with different curing times used for this investigation were taken from the cores of specimens. Then they were immediately plunged into absolute acetone for 6 h to stop the hydration of the cement, and then dried at 05 C using a vacuum oven for 2 h before being tested. After drying, the porosity of fragments was measured by a mercury intrusion porosimetry device made by Quantachrome Corporation. 3. RESULTS AND DISCUSSION 3. Analysis of PACP Structure The FT-IR spectrum of the PACP comb-like copolymer is shown in Figure 2a. The absorption peaks at around 3450 cm - and 2875 cm - were ascribed to stretching vibrations of O H and C-H, respectively. The absorption peak at around 730 cm - was attributed to a stretching vibration of C=O in an ester group. The two peaks at 560 cm - and 408 cm - correspond to the asymmetric and symmetric stretching vibration of COO -, 5 respectively. The absorption peaks at 457 cm - and 352 cm - were attributed to a bending vibration of CH 2 adjacent to the O atom and a scissoring in-plane vibration of CH 2 adjacent to the O atom 5, respectively. The absorption peaks at around 249 cm - and cm - were attributed to stretching vibration of C-O in the ester group and stretching vibrations of C-O-C ether group 5,2-22, respectively. The absorption peak at 037 cm - was attributed to a stretching vibration of C=N. The absorption peaks at around 949 cm - and 845 cm - were related to rocking in-plane vibrations of CH 3 adjacent to the C atom and the O or N atom 5, respectively. Figure 2b is H-NMR spectrum of PACP. The signals at.05 ppm (b ) was assigned to CH 3 protons in the PACP backbone. The signals at.2-.8 ppm (a, a, a ) and.9-2.5ppm (b, b ) were attributed to CH 2 protons and CHCOOR proton in the copolymer backbone 5,22, respectively. The peaks at ppm (e) and 4.40 ppm (g) correspond to the CH 3 protons and CH 2 protons in-n + -(CH 3 ) 3. The peak at 3.29 ppm (d) is the resonance peak of the terminal methoxy protons. The peak Polymers & Polymer Composites, Vol. 9, No., 20 3

4 Changwen Miao, Qianping Ran, Jiaping Liu, Yonglin Mao, Yan Shang and Jianfang Sha Figure 2. Schematic illustration of molecular structure of comb-like copolymer superplasticizer Figure 3. Influence of superplasticizer dosages on water reducing rates compared to concrete without admixtures active component. From the results it is clear that the dosages of PACP and PCA affect the water reduction in different manners. PACP can be dosed at 0.2% to obtain a water reduction of 32% and can achieve a water reduction up to 45% at high dosage rates of 0.40%, as compared to the reference mix. In addition, the water reduction increases with increasing dosages when at higher dosage than 0.4%. In contrast, the anionic polycarboxylate superplasticizer (PCA) did not increase the water reduction further when dosed above 0.3%. at ppm (c, c ) corresponds to the CH 2 protons in O-CH 2 -CH 2 -repeating units. In addition, it is worth noting that a small and broad peak at 4.7 ppm (f, f ) is attributed to the CH 2 protons that are attached to the ester group. All the characteristic resonance peaks of three motifs were present in the above FT-IR and H-NMR spectra, and it thus was demonstrated that the preparation of PACP comb-like copolymer dispersants was successful. The composition of PACP listed in Table was also determined from the H-NMR spectrum. 3.2 Properties of Freshly Mixed Concrete The effects of dosage of different type superplasticizer on water reduction are presented in Figure 3. The dosage was expressed in terms of the weight of the The source of water reduction can be related with their different chemical structure and further related with their adsorption behaviour. Therefore, it is very useful to compare their adsorption behaviour. Figure 4 shows the relation between the dosage of each superplasticizer and the adsorbed amount on the cement particles. In all superplasticizers, their initial adsorbed amount increased rapidly as the dosages added increased from ~.2.0 mg.g - ; The adsorption amount continued to increase at higher dosages than 3 mg.g - for PACP containing cationic and anionic group in main chain; however, there was no further increase for PCA. Thus, it is considered 4 Polymers & Polymer Composites, Vol. 9, No., 20

5 New Generation Amphoteric Comb-like Copolymer Superplasticizer and Its Properties that there is dramatically different water reduction performance between the two different superplasticizers at higher dosages. Figure 4. Adsorbed amount of different type superplasticizers on cement The workability loss is another important property for ready-mixed concrete which can be placed into intricate structures or pumped over long distances. The slump changes over time of fresh concrete containing different PACP dosage are presented in Figure 5. As can be expected, the slump flow ability improved with the increase of PACP dosages. This is thought to correlate to the higher amount of superplasticizer remaining in the aqueous phase 23. The effects of PACP dosage on the setting times of the mortar are shown in Figure 6. Initial setting and final setting were almost unaffected by PACP at lower dosages than 0.3%, but were delayed with further addition of PACP, especially at excessive dosages. The delay of setting for high PACP content is thought to relate to the concentration of acidic groups in the aqueous phase 0. Figure 5. The slump change over time of fresh concrete containing different dosage of PACP 3.3 Compressive Strength Developments of Hardened Concrete The most important function of superplasticizer is the enhancing concrete strength development, which is a function of water reduction. The compressive strength versus age for concrete mixes containing different PACP dosages are plotted in Figure 7. It can be observed that increasing the dosage of PACP from 0.6% to 0.4% increased not only the early age strength but also the late age strength due to the high initial water-reducing capacity of this admixture, and at the same time the W/C ratio decreased. In comparison with the reference concrete, the 3-day compressive strength of concrete containing different PACP dosages was increased by about 40~260% and the 60-day strength was increased by about 80~40%. Furthermore, as can be expected the enhanced strength Figure 6. Setting times of mortars incorporating different amounts of PACP Polymers & Polymer Composites, Vol. 9, No., 20 5

6 Changwen Miao, Qianping Ran, Jiaping Liu, Yonglin Mao, Yan Shang and Jianfang Sha Figure 7. Compressive strength development of PACP incorporated concretes at different dosages Figure 8. Hydration temperature vs time for cement pastes with different PACP dosages to the reference cement paste. But the results presented in Figures 8-9 are not consistent with previous observations about the setting times of mortar in the presence of PACP (Figure 6). These phenomena are not fully understood; they might be related to the cationic group in PACP which can adsorb onto C 2 S or C 3 S phases, and different adsorption behaviour thus changes the relative hydration degree of C 2 S, C 3 S, C 3 A and C 4 AF phase in cement. Further research is needed to prove such hypotheses. 3.5 Pore Structure of Hardened Cement Paste Generally, the incorporation of a superplasticizer is known to affect both the hydration and the pore size distribution of hardened cement pastes, and it thus influences the strength of concrete. Complementary porosity analyses are being done to help to understand the strength-enhancing effect of PACP. effect increased with the increase of PACP dosage. In addition to high early strength and shorter setting times (Figure 6), the high water-reducing capacity of PACP, makes this admixture suitable for the precast industry. 3.4 Heat Evolution During Cement Hydration The superplasticizer not only can influence the strength development, but also change the heat evolution process of cement paste. The relative influence of PACP dosage on the hydration reactions of cement paste was investigated, and typical results are illustrated in Figures 8-9. It is obvious that the PACP reduced the heat amount of early hydration of cement paste, and delayed the time to reach highest hydration temperature. The addition of 0.3% PACP reduced the heat amount of d hydration by 83%, and delayed the time to reach the highest hydration temperature by about 20 h, compared The variations of the porosity distribution of cement pastes with and without PACP cured at 3 days and 28 days are illustrated in Figure 0. The pore structure can be represented by the amount of harmful pores (with diameters in the range of 00~200 nm), minor-harmful pores (with diameters in the range of 20~00 nm) and nonharmful pores (with diameters below 20 nm). It can be observed that the paste incorporated with PACP had the least amount of harmful as well as minor-harmful pores and the highest amount of harm-free pores. At the age of 3 days, the porosity relative to the pores below 00 nm was 88.7% for hardened cement with PACP, and only 47.% for hardened pure cement paste. As the curing time progressed up to 28 days, the porosity relative to the pores below 00 nm was 92.3% for PACP; in contrast, for pure cement paste, the porosity relative to the pores below 00 nm was just 62.7%. The pore structure of the cement paste with PACP underwent further densification, 6 Polymers & Polymer Composites, Vol. 9, No., 20

7 New Generation Amphoteric Comb-like Copolymer Superplasticizer and Its Properties Figure 9. Cumulated heat curves vs time for cement pastes with different PACP dosages ultimately stages. Hence this type of superplasticizer is very suitable for the precast concrete industry due to its higher early strength and shorter setting times, and the high water-reducing capacity of PACP. Moreover, PACP can further improve the heat evolution process and delays the time to reach the highest hydration temperature. This property makes it very suitable for massive concrete. Finally, PACP refines the pore structure of hardened cement paste markedly, which helps to improve the durability of concrete. But more effort needs to be devoted to elucidating its action mechanism, which could open new scenarios in the research of new superplasticizers. Figure 0. Variations of porosity distribution of cement pastes with and without PACP cured at 3 days and 28 days ACKNOWLEDGMENTS This study of this paper is financially supported by Jiangsu Provincial Fund for Natural Sciences (Grant No.BK ) and National Basic Research Program of China (973 Program) (Grant No.2009CB623200). and almost no harmful pores were observed after 28 days of standard curing, which helped to enhance strength, improve durability, especially the impermeability of concrete. 4. CONCLUSIONS The effects of PACP on water reduction, setting, compressive strength, hydration and pore structure were investigated systematically. The results show that this new type of amphoteric comb-like copolymer superplasticizer (PACP) demonstrates significant performance benefits and technological advantages over conventional condensate admixtures and anionic polycarboxylate superplasticizers. PACP can be dosed in small amounts to obtain water reductions up to 45%. The addition of PACP also does not retard the cement setting time, and it significantly increases the compressive strength of hardened concrete at early and REFERENCES. Houst Y.F., Bowen P. Perche F., et al., Cem. Concr. Res. 38 (2008) Collepardi M. and Valente M., ACI SP-239, (2006). 3. Kilinckale F.M. and Dogan G.G., J Appl. Polym. Sci., 03 (2007) Ran Q.P., Somasundaran P., Miao C.W., Liu J. P., Wu S. S. and Shen J., J. Colloid Interface Sci., 336 (2009) Luo Y.L., Ran Q.P., Wu S.S. and Shen J., J Appl. Polym. Sci., 09 (2008) Schober I. and Flat R.J., ACI SP-239, (2000), Ran Q.P., Liu J.P., Miao C.W. and Shen J., J. New Building Materials (In Chinese), 2 (2005) Uchikawa H., Hanehara S. and Sawaki D., Cem. Concr. Res., 27 (997) Kinoshita M., ACI SP-95, (2000), Yamada K., Hanehara S. and Honma K., Cem. Concr. Res., 30 (2000) 97. Polymers & Polymer Composites, Vol. 9, No., 20 7

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