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Microstructure and pore structure of concrete mixed with superfine phosphorous slag and superplasticizer
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Construction and Building
MATERIALS
Construction and Building Materials 22 (2008) 837–840
www.elsevier.com/locate/conbuildmat
Microstructure and pore structure of concrete mixed with superfine phosphorous slag and superplasticizer Gao Peiwei *, Lu Xiaolin, Yang Chuanxi, Li Xiaoyan, Shi Nannan, Jin Shaochun Department of Civil Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China Received 20 August 2006; received in revised form 10 December 2006; accepted 27 December 2006 Available online 16 February 2007
Abstract The hydration products, microstructures and pore structures of paste with part of Portland cement replaced by superfine phosphoric slag were investigated by XRD, DTA and SEM in this article. The results shown that the appropriate replacement of Portland cement by superfine phosphoric slag can decrease the amount of portlandite, increase the amount of C–S–H gel, reduce the harmful pores (larger than 100 nm), make the structure denser, and improve the microstructure and durability of concrete. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Concrete; Microstructure; Pore structure; Superfine phosphoric slag; Superplasticizer; C–S–H gel
1. Introduction In the 20th century, a lot of reinforced concrete structures that are constructed with Portland cement (PC) were quickly deteriorating, while some 2000-year-old unreinforced concrete structures, such as the Great Wall in China, made of slow-hardening, lime-pozzolanic materials, are in excellent condition. When these modern concrete structures were exposed to corrosive environments, serious durability problems emerged in parking garages, bridges, undersea tunnels, and the evidence of premature deterioration in recent structures has increased [1,2]. In recent years, the concrete construction industry has faced a heavy challenge that is the demand for housing and infrastructure constructed in an environmentally sustainable and cost-effective manner, as the primary greenhouse gas – carbon dioxide concentration in the environment had risen by 50% during the 20th century [3], which is one of the major by-products of Portland cement and steel plant. Therefore, a new material that can replace part of cement in concrete is required, which *
Corresponding author. Tel.: +86 25 837 864 73. E-mail address: [email protected] (P. Gao).
0950-0618/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2006.12.015
can not consume the large amount of resource and energy, cause the environment pollution and decrease the durability of construction. Since 1974 the cement and concrete mixtures with relatively high strength at early ages, the durability problems accelerate. Neville has also stated that the deterioration of concrete increased because cement specifications have no limits on fineness, C3S and early strength [4,5]. A high C3S and C3A content, and a high Blain fineness are not needed anymore to make a high strength concrete, and it is simply necessary to lower the water/cement (or water/ binder) ratio [6]. Moreover, up to now too much emphasis has been placed on 28 day compressive strength and not on concrete durability. It is very important to design concrete mixtures that keep their 28-day compressive strength over the life of the structure under its peculiar environmental conditions. More mineral components will be used, and water/cement (or water/binder) ratios will be lowered. Owing to the consideration of environmental protection and durability of concrete, using superfine phosphoric slag (SFPS) to replace part of cement can reduce the demand for cement, decrease hydrate heat and early strength of concrete, and improve the durability of concrete and decrease the environment pollution [7,8].
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P. Gao et al. / Construction and Building Materials 22 (2008) 837–840
In previous researches, the mechanical properties and durability, including the resistance to freezing and thawing, salt attack and diffusion coefficient of chloride of concrete with part of superfine phosphoric slag have been published. The results shown that the concrete mixed with superfine phosphoric slag had a higher compressive strengths, lower chloride diffusion coefficient and better durability [9,10]. Yet the mechanism of these changes has not been introduced until now. In this paper, the microstructures and pore structures of specimens mixed with superfine phosphoric slag are described. 2. Experiments 2.1. Materials 42.5 MPa Portland cement (ISO) and the superfine powder of phosphoric slag (SFPS) are used their compositions of which are shown in Table 1. The fineness of SFPS is about 600–800 m2/kg. Superplasticizer (NF) was from Tsinghua University. 2.2. Methods Ten centimeters of cube concrete specimens were used in the compressive strength test. Demoulded at one day, the specimens were cured in water at 20 ± 3 °C until 7 and 28 day age, then tested. The paste specimens for microstructure and pore structure analysis are made in accordance with Table 2. After cured in water at 20 ± 2 °C for 7 day or 28 day age, the specimens were placed into alcohol, 3 day later dried at 60 ± 2 °C, then tested. The pore structure analysis was tested by Auto-60, mercury scanning porosimeter from Quanfachrom of USA. 3. Results and discussion 3.1. Effect of SFPS on the slumps and compressive strengths of concrete The results of slump and strength of concrete mixed with the superfine phosphoric slag and superplasticizer are shown in Table 3. For the same water–binder ratio, the slump results show that concrete mixed with the superfine phosphoric slag which replaces about 20% of Portland cement had higher slump value than the control specimens made with only Portland cement. Added with the superfine phosphoric slag only, the slumps of concrete increased 35 mm; and with the
Table 2 Mix proportions of cement paste specimens (mass %) Item
Portland cement
SFPS
NF
Water
A B C D
100 100 60 60
0 0 40 40
0.3 0.6 0.3 0.6
30 30 30 30
superfine phosphoric slag and superplasticizer, the slumps increased 50 mm. The results mean that superfine phosphoric slag plays a role of dispersing agent which can improve the fluidity of concrete. When 20% cement replaced by the superfine phosphoric slag, the compressive strength of concrete (see Table 3, No. 2) decreases 15.4% in 7 day, increases 29.7% in 28 day (No. 1). With the superfine phosphoric slag and superplasticizer, the compressive strength of concrete (No. 4) decreases 3.4% in 7 day, increases 4.5% in 28 day (No. 3). The results show that the part replacement of Portland cement by the superfine phosphoric slag in concrete can decrease the early strength and increase the later strength. When the superplasticizer was added, the compressive strength of concrete (No. 3) increased 55.4% and 92.5% for 7 and 28 day curing period (No. 1). When the superfine phosphoric slag and superplasticizer were added, the compressive strength of concrete (No. 4) increased 74.8% and 55.1% for 7 and 28 day curing period (No. 2). It is shown that the superplasticizer and superfine phosphoric slag can obviously improve the strength of concrete. 3.2. Evidence from X-ray diffraction Fig. 1 shows X-ray diffraction patterns of paste specimens cured in water for 7 day and 28 day. From Fig. 1, it is can be observed that the peaks of Ca(OH)2 in 7 day and 28 day are dominant in the XRD pattern. A diffuse band at 0.26–0.31 nm and a sharper one at 0.49 nm develop more slowly, the disparity is not prominent. The intensities of Ca(OH)2 peaks for specimens C and D (in 28 day) are lower than those of A and B slightly, but the differences between A and C, or B and D in 7 day cannot be observed. This is due to some of Portland cement replaced by the superfine phosphoric slag could reduce the intensities of Ca(OH)2 peaks associated with the pozzolanic reaction of superfine phosphoric slag, and the pozzolanic reaction of the superfine phosphoric slag with Ca(OH)2 is very slow in 7 day. It is indicated that the superfine phosphoric slag in cement paste possess cementitious properties, and has little cementing action in
Table 1 Chemical composition of Portland cement and SFPS (mass %) Item
CaO
SiO2
Al2O3
Fe2O3
MgO
Na2O
K2O
SO3
P2O5
L.O.I
Portland cement SFPS
65.11 44.40
21.82 36.88
5.43 3.92
3.72 2.93
1.55 1.56
0.15 0.58
1.20 0.74
0.67 0.18
0.10 0.60
0.27 2.80
P. Gao et al. / Construction and Building Materials 22 (2008) 837–840
839
Table 3 Mix proportions, slumps and strengths of concrete No.
Cement (kg/ m3)
SFPS (kg/ m3)
Water (kg/ m3)
NF (kg/ m3)
Sand (kg/ m3)
Gravel (kg/ m3)
Slump (mm)
Compressive strength (MPa) 7 day
28 day
1 2 3 4
550 440 550 440
– 110 – 110
175 175 175 175
0 0 5.5 5.5
640 640 640 640
1080 1080 1080 1080
20 55 130 180
38.3 32.9 59.5 57.5
42.4 55.0 81.6 85.3
7 day, but if more suitable Ca(OH)2 are present, the superfine phosphoric slag shows cementitious properties in 28 day. When the superplasticizer content is 0.3% or 0.6%, Ca(OH)2 in specimens is not different obviously for the cases with or without the superfine phosphoric slag. It shows that the superplasticizer content has little influence on pozzolanic reaction.
thermal analysis (DTA) – thermogravimetry (TG) was used by determination of Ca(OH)2 content in cement paste, which was probably the most satisfactory method. Table 4 shows the content of Ca(OH)2 in cement paste. By analyzing the portlandite and unhydrated clinker minerals in the interfacial zone from Table 4, we can observe that the Ca(OH)2 content in paste mixed with the superfine phosphoric slag (C and D) was less than that of
3.3. SEM observation From Figs. 1 and 2, it can be seen that the main constituents in the interfacial zone of the cement paste are C–S–H gel, portlandite, ettringite and unhydrated cement. When part of Portland cement is replaced by the superfine phosphoric slag, well crystallized hexagonal portlandite is hardly found under SEM, and more C–S–H gel and finer ettringite can be found (C and D in Fig. 2). In the interface zone (A and B in Fig. 2), some micro-cracks can be observed, but the authors almost do not find micro-cracks in the pastes that contain the superfine phosphoric slag. When part of Portland cement replaced by the superfine phosphoric slag, C–S–H gel greatly increase, less and finer ettringite that embedded in the matrix of hydration products is observed (C and D in Fig. 2), which is harmless to the concrete [11,12]. Therefore, the microstructures of concrete have been improved, due to the reaction of the active superfine phosphoric slag with portlandite in the cement paste. 3.4. Differential thermal analysis In order to determine the potential activity of the superfine phosphoric slag in cement paste, the differential
Fig. 2. SEM photographs of specimens cured in water for 28 day.
Fig. 1. XRD analysis of paste cured in water for 7 day and 28 day.
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P. Gao et al. / Construction and Building Materials 22 (2008) 837–840
Table 4 The content of Ca(OH)2 in cement paste specimens Item
A (%)
B (%)
C (%)
D (%)
7 day 28 day
9.33 11.43
9.14 11.36
8.29 9.00
8.06 8.67
Table 5 Volume of pores control and paste with SFPS Pore radius
Larger than 100 nm/%
Age
3 day
7 day
28 day
Control (A) Paste (B)
4.92 3.82
3.86 2.63
2.91 2.29
paste without the superfine phosphoric slag (A and B) curing in water for 7 day and 28 day. The results illustrate that superfine phosphoric slag can react with Ca(OH)2 in paste and form calcium silicate hydrate and hexagonal aluminates. There are no obviously differences between A and B, C and D, respectively. It illustrates that the superfine phosphoric slag and superplasticizer content (from 0.3% to 0.6%) has little influence on pozzolanic reaction. 3.5. Pore structure analysis The pore structures of paste mixed with the superfine phosphoric slag and control paste are shown in Table 5. The amount of pore with radius larger than 100 nm in cement/stone specimen decreases with the hydration time. When some Portland cements were replaced by the superfine phosphoric slag, the pore radiuses larger than 100 nm of cement products were decreased. Because the superfine phosphoric slag fills up the free space of cement stone and increases the density of the structure, the harmful pores (larger than 100 nm) have decreased, the pore structures and durability can been improved slightly [13]. 4. Conclusion The appropriate replacement of Portland cement by the superfine phosphoric slag can change the quantity and shape of the hydrated products of cement. By the XRD, DTA and SEM, we can observe that the Ca(OH)2 content is reduced, the amount of C–S–H gel is increased, and ettringite becomes finer and fewer. The superfine phosphoric slag with small particle size and high chemical activity fills up the pore of cement prod-
ucts, which increases the density of the paste and improves the structure of paste, and reduces the porosity of harmful pore and the micro-cracks of concrete. The superfine phosphoric slag in cement paste possesses cementitious properties, and has little cementing action in 7 day, but shows marked cementitious properties in 28 day. The superfine phosphoric slag replacing some of Portland cement is not only economical but also beneficial for the improvement of microstructure and the durability of concrete. Acknowledgements The authors are thankful to the Natural Sciences Foundation Council of China (60672166) and Nanhang Research Funds for their financial support for this project. References [1] Tang MS. Improving the durability of major concrete engineering. China Chem Eng 1996;3:34–7. [2] Gao Peiwei, Wu Zhong-ru, Tang Mingshu. The characteristics of air void and frost resistance of RCC with fly ash and expansive agent. Constr Build Mater 2006;20(8):586–90. [3] Mehta PK, Burrows RW. Building durable structures in the 21st century. Concrete Int 2001;23:57–63. [4] Neville A. Why we have concrete durability problems SP 100-3. In: Concrete durability. Katharine and Bryant Mather international conference. Farmington Hills (MI): American Concrete Institute; 1987. p. 21–30. [5] Diamond S. Delayed ettringite formation – processes and problems. Cem Concrete Compos 1996;18:205–15. [6] Pierre-Claude A. Cements of yesterday and today concrete of tomorrow. Cem Concrete Res 2000;30:1349–59. [7] Feng Naiqian. The structure, performance and powder effect of HPC. Concrete Cem Prod 1996;2:6–13. [8] Cheng L. Mechanical properties and microstructures of alkali– activated phosphorous slag cement. J Chinese Ceram Soc 2006;5:604–9. [9] Gao Peiwei, Deng M, Feng N. The influence of superplasticizer and superfine mineral powder on the flexibility, strength and durability of HPC. Cem Concrete Res 2001;31:703–6. [10] Gao Peiwei. Influence of superfine powder of phosphoric slag on strength and durability of high performance concrete. J Shandong Univ Build Mater 1998;12:130–4. [11] Gao Peiwei. Effect of different expansive agents on mass concrete deformation property. J Nanjing Univ Aeronaut Astronaut 2006;2:251–5. [12] Shi CJ, Li YY. Investigation on some factors affecting the characteristics of alkali–phosphorous slag cement. Cem Concrete Res 1989;4:527–33. [13] Mehta PK. In: Proceedings of the 7th international congress on the chemistry of Cement 1980;3:1–5.
Construction and Building
MATERIALS
Construction and Building Materials 22 (2008) 837–840
www.elsevier.com/locate/conbuildmat
Microstructure and pore structure of concrete mixed with superfine phosphorous slag and superplasticizer Gao Peiwei *, Lu Xiaolin, Yang Chuanxi, Li Xiaoyan, Shi Nannan, Jin Shaochun Department of Civil Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China Received 20 August 2006; received in revised form 10 December 2006; accepted 27 December 2006 Available online 16 February 2007
Abstract The hydration products, microstructures and pore structures of paste with part of Portland cement replaced by superfine phosphoric slag were investigated by XRD, DTA and SEM in this article. The results shown that the appropriate replacement of Portland cement by superfine phosphoric slag can decrease the amount of portlandite, increase the amount of C–S–H gel, reduce the harmful pores (larger than 100 nm), make the structure denser, and improve the microstructure and durability of concrete. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Concrete; Microstructure; Pore structure; Superfine phosphoric slag; Superplasticizer; C–S–H gel
1. Introduction In the 20th century, a lot of reinforced concrete structures that are constructed with Portland cement (PC) were quickly deteriorating, while some 2000-year-old unreinforced concrete structures, such as the Great Wall in China, made of slow-hardening, lime-pozzolanic materials, are in excellent condition. When these modern concrete structures were exposed to corrosive environments, serious durability problems emerged in parking garages, bridges, undersea tunnels, and the evidence of premature deterioration in recent structures has increased [1,2]. In recent years, the concrete construction industry has faced a heavy challenge that is the demand for housing and infrastructure constructed in an environmentally sustainable and cost-effective manner, as the primary greenhouse gas – carbon dioxide concentration in the environment had risen by 50% during the 20th century [3], which is one of the major by-products of Portland cement and steel plant. Therefore, a new material that can replace part of cement in concrete is required, which *
Corresponding author. Tel.: +86 25 837 864 73. E-mail address: [email protected] (P. Gao).
0950-0618/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2006.12.015
can not consume the large amount of resource and energy, cause the environment pollution and decrease the durability of construction. Since 1974 the cement and concrete mixtures with relatively high strength at early ages, the durability problems accelerate. Neville has also stated that the deterioration of concrete increased because cement specifications have no limits on fineness, C3S and early strength [4,5]. A high C3S and C3A content, and a high Blain fineness are not needed anymore to make a high strength concrete, and it is simply necessary to lower the water/cement (or water/ binder) ratio [6]. Moreover, up to now too much emphasis has been placed on 28 day compressive strength and not on concrete durability. It is very important to design concrete mixtures that keep their 28-day compressive strength over the life of the structure under its peculiar environmental conditions. More mineral components will be used, and water/cement (or water/binder) ratios will be lowered. Owing to the consideration of environmental protection and durability of concrete, using superfine phosphoric slag (SFPS) to replace part of cement can reduce the demand for cement, decrease hydrate heat and early strength of concrete, and improve the durability of concrete and decrease the environment pollution [7,8].
838
P. Gao et al. / Construction and Building Materials 22 (2008) 837–840
In previous researches, the mechanical properties and durability, including the resistance to freezing and thawing, salt attack and diffusion coefficient of chloride of concrete with part of superfine phosphoric slag have been published. The results shown that the concrete mixed with superfine phosphoric slag had a higher compressive strengths, lower chloride diffusion coefficient and better durability [9,10]. Yet the mechanism of these changes has not been introduced until now. In this paper, the microstructures and pore structures of specimens mixed with superfine phosphoric slag are described. 2. Experiments 2.1. Materials 42.5 MPa Portland cement (ISO) and the superfine powder of phosphoric slag (SFPS) are used their compositions of which are shown in Table 1. The fineness of SFPS is about 600–800 m2/kg. Superplasticizer (NF) was from Tsinghua University. 2.2. Methods Ten centimeters of cube concrete specimens were used in the compressive strength test. Demoulded at one day, the specimens were cured in water at 20 ± 3 °C until 7 and 28 day age, then tested. The paste specimens for microstructure and pore structure analysis are made in accordance with Table 2. After cured in water at 20 ± 2 °C for 7 day or 28 day age, the specimens were placed into alcohol, 3 day later dried at 60 ± 2 °C, then tested. The pore structure analysis was tested by Auto-60, mercury scanning porosimeter from Quanfachrom of USA. 3. Results and discussion 3.1. Effect of SFPS on the slumps and compressive strengths of concrete The results of slump and strength of concrete mixed with the superfine phosphoric slag and superplasticizer are shown in Table 3. For the same water–binder ratio, the slump results show that concrete mixed with the superfine phosphoric slag which replaces about 20% of Portland cement had higher slump value than the control specimens made with only Portland cement. Added with the superfine phosphoric slag only, the slumps of concrete increased 35 mm; and with the
Table 2 Mix proportions of cement paste specimens (mass %) Item
Portland cement
SFPS
NF
Water
A B C D
100 100 60 60
0 0 40 40
0.3 0.6 0.3 0.6
30 30 30 30
superfine phosphoric slag and superplasticizer, the slumps increased 50 mm. The results mean that superfine phosphoric slag plays a role of dispersing agent which can improve the fluidity of concrete. When 20% cement replaced by the superfine phosphoric slag, the compressive strength of concrete (see Table 3, No. 2) decreases 15.4% in 7 day, increases 29.7% in 28 day (No. 1). With the superfine phosphoric slag and superplasticizer, the compressive strength of concrete (No. 4) decreases 3.4% in 7 day, increases 4.5% in 28 day (No. 3). The results show that the part replacement of Portland cement by the superfine phosphoric slag in concrete can decrease the early strength and increase the later strength. When the superplasticizer was added, the compressive strength of concrete (No. 3) increased 55.4% and 92.5% for 7 and 28 day curing period (No. 1). When the superfine phosphoric slag and superplasticizer were added, the compressive strength of concrete (No. 4) increased 74.8% and 55.1% for 7 and 28 day curing period (No. 2). It is shown that the superplasticizer and superfine phosphoric slag can obviously improve the strength of concrete. 3.2. Evidence from X-ray diffraction Fig. 1 shows X-ray diffraction patterns of paste specimens cured in water for 7 day and 28 day. From Fig. 1, it is can be observed that the peaks of Ca(OH)2 in 7 day and 28 day are dominant in the XRD pattern. A diffuse band at 0.26–0.31 nm and a sharper one at 0.49 nm develop more slowly, the disparity is not prominent. The intensities of Ca(OH)2 peaks for specimens C and D (in 28 day) are lower than those of A and B slightly, but the differences between A and C, or B and D in 7 day cannot be observed. This is due to some of Portland cement replaced by the superfine phosphoric slag could reduce the intensities of Ca(OH)2 peaks associated with the pozzolanic reaction of superfine phosphoric slag, and the pozzolanic reaction of the superfine phosphoric slag with Ca(OH)2 is very slow in 7 day. It is indicated that the superfine phosphoric slag in cement paste possess cementitious properties, and has little cementing action in
Table 1 Chemical composition of Portland cement and SFPS (mass %) Item
CaO
SiO2
Al2O3
Fe2O3
MgO
Na2O
K2O
SO3
P2O5
L.O.I
Portland cement SFPS
65.11 44.40
21.82 36.88
5.43 3.92
3.72 2.93
1.55 1.56
0.15 0.58
1.20 0.74
0.67 0.18
0.10 0.60
0.27 2.80
P. Gao et al. / Construction and Building Materials 22 (2008) 837–840
839
Table 3 Mix proportions, slumps and strengths of concrete No.
Cement (kg/ m3)
SFPS (kg/ m3)
Water (kg/ m3)
NF (kg/ m3)
Sand (kg/ m3)
Gravel (kg/ m3)
Slump (mm)
Compressive strength (MPa) 7 day
28 day
1 2 3 4
550 440 550 440
– 110 – 110
175 175 175 175
0 0 5.5 5.5
640 640 640 640
1080 1080 1080 1080
20 55 130 180
38.3 32.9 59.5 57.5
42.4 55.0 81.6 85.3
7 day, but if more suitable Ca(OH)2 are present, the superfine phosphoric slag shows cementitious properties in 28 day. When the superplasticizer content is 0.3% or 0.6%, Ca(OH)2 in specimens is not different obviously for the cases with or without the superfine phosphoric slag. It shows that the superplasticizer content has little influence on pozzolanic reaction.
thermal analysis (DTA) – thermogravimetry (TG) was used by determination of Ca(OH)2 content in cement paste, which was probably the most satisfactory method. Table 4 shows the content of Ca(OH)2 in cement paste. By analyzing the portlandite and unhydrated clinker minerals in the interfacial zone from Table 4, we can observe that the Ca(OH)2 content in paste mixed with the superfine phosphoric slag (C and D) was less than that of
3.3. SEM observation From Figs. 1 and 2, it can be seen that the main constituents in the interfacial zone of the cement paste are C–S–H gel, portlandite, ettringite and unhydrated cement. When part of Portland cement is replaced by the superfine phosphoric slag, well crystallized hexagonal portlandite is hardly found under SEM, and more C–S–H gel and finer ettringite can be found (C and D in Fig. 2). In the interface zone (A and B in Fig. 2), some micro-cracks can be observed, but the authors almost do not find micro-cracks in the pastes that contain the superfine phosphoric slag. When part of Portland cement replaced by the superfine phosphoric slag, C–S–H gel greatly increase, less and finer ettringite that embedded in the matrix of hydration products is observed (C and D in Fig. 2), which is harmless to the concrete [11,12]. Therefore, the microstructures of concrete have been improved, due to the reaction of the active superfine phosphoric slag with portlandite in the cement paste. 3.4. Differential thermal analysis In order to determine the potential activity of the superfine phosphoric slag in cement paste, the differential
Fig. 2. SEM photographs of specimens cured in water for 28 day.
Fig. 1. XRD analysis of paste cured in water for 7 day and 28 day.
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P. Gao et al. / Construction and Building Materials 22 (2008) 837–840
Table 4 The content of Ca(OH)2 in cement paste specimens Item
A (%)
B (%)
C (%)
D (%)
7 day 28 day
9.33 11.43
9.14 11.36
8.29 9.00
8.06 8.67
Table 5 Volume of pores control and paste with SFPS Pore radius
Larger than 100 nm/%
Age
3 day
7 day
28 day
Control (A) Paste (B)
4.92 3.82
3.86 2.63
2.91 2.29
paste without the superfine phosphoric slag (A and B) curing in water for 7 day and 28 day. The results illustrate that superfine phosphoric slag can react with Ca(OH)2 in paste and form calcium silicate hydrate and hexagonal aluminates. There are no obviously differences between A and B, C and D, respectively. It illustrates that the superfine phosphoric slag and superplasticizer content (from 0.3% to 0.6%) has little influence on pozzolanic reaction. 3.5. Pore structure analysis The pore structures of paste mixed with the superfine phosphoric slag and control paste are shown in Table 5. The amount of pore with radius larger than 100 nm in cement/stone specimen decreases with the hydration time. When some Portland cements were replaced by the superfine phosphoric slag, the pore radiuses larger than 100 nm of cement products were decreased. Because the superfine phosphoric slag fills up the free space of cement stone and increases the density of the structure, the harmful pores (larger than 100 nm) have decreased, the pore structures and durability can been improved slightly [13]. 4. Conclusion The appropriate replacement of Portland cement by the superfine phosphoric slag can change the quantity and shape of the hydrated products of cement. By the XRD, DTA and SEM, we can observe that the Ca(OH)2 content is reduced, the amount of C–S–H gel is increased, and ettringite becomes finer and fewer. The superfine phosphoric slag with small particle size and high chemical activity fills up the pore of cement prod-
ucts, which increases the density of the paste and improves the structure of paste, and reduces the porosity of harmful pore and the micro-cracks of concrete. The superfine phosphoric slag in cement paste possesses cementitious properties, and has little cementing action in 7 day, but shows marked cementitious properties in 28 day. The superfine phosphoric slag replacing some of Portland cement is not only economical but also beneficial for the improvement of microstructure and the durability of concrete. Acknowledgements The authors are thankful to the Natural Sciences Foundation Council of China (60672166) and Nanhang Research Funds for their financial support for this project. References [1] Tang MS. Improving the durability of major concrete engineering. China Chem Eng 1996;3:34–7. [2] Gao Peiwei, Wu Zhong-ru, Tang Mingshu. The characteristics of air void and frost resistance of RCC with fly ash and expansive agent. Constr Build Mater 2006;20(8):586–90. [3] Mehta PK, Burrows RW. Building durable structures in the 21st century. Concrete Int 2001;23:57–63. [4] Neville A. Why we have concrete durability problems SP 100-3. In: Concrete durability. Katharine and Bryant Mather international conference. Farmington Hills (MI): American Concrete Institute; 1987. p. 21–30. [5] Diamond S. Delayed ettringite formation – processes and problems. Cem Concrete Compos 1996;18:205–15. [6] Pierre-Claude A. Cements of yesterday and today concrete of tomorrow. Cem Concrete Res 2000;30:1349–59. [7] Feng Naiqian. The structure, performance and powder effect of HPC. Concrete Cem Prod 1996;2:6–13. [8] Cheng L. Mechanical properties and microstructures of alkali– activated phosphorous slag cement. J Chinese Ceram Soc 2006;5:604–9. [9] Gao Peiwei, Deng M, Feng N. The influence of superplasticizer and superfine mineral powder on the flexibility, strength and durability of HPC. Cem Concrete Res 2001;31:703–6. [10] Gao Peiwei. Influence of superfine powder of phosphoric slag on strength and durability of high performance concrete. J Shandong Univ Build Mater 1998;12:130–4. [11] Gao Peiwei. Effect of different expansive agents on mass concrete deformation property. J Nanjing Univ Aeronaut Astronaut 2006;2:251–5. [12] Shi CJ, Li YY. Investigation on some factors affecting the characteristics of alkali–phosphorous slag cement. Cem Concrete Res 1989;4:527–33. [13] Mehta PK. In: Proceedings of the 7th international congress on the chemistry of Cement 1980;3:1–5.