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Using a new composite expansive material to decrease deformation and fracture of concrete
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Materials Letters 62 (2008) 106 – 108 www.elsevier.com/locate/matlet
Using a new composite expansive material to decrease deformation and fracture of concrete Gao Peiwei ⁎, Lu Xiaolin, Jin Shaochun, Zhang Hui, Guo Chunxing College of Civil Engineering, Nanjing University of Aeronautics & Astronautics, Nanjin, 210016, China Received 21 November 2006; accepted 24 April 2007 Available online 27 April 2007
Abstract Using minerals of dolomite, serpentine and magnesite produce a new composite material which provides an expansive stress to decrease deformation and fracture of hydraulic concrete. The compositions of the expansive material are mainly MgO, CaO and C2S by X-ray techniques. Adding the expansive material to concrete, the shrinkage of concrete may be compensated by the hydration of CaO to Ca(OH)2 in early ages and the hydration of MgO to Mg(OH)2 in later ages respectively, so decrease deformation and fracture of concrete. © 2007 Elsevier B.V. All rights reserved. Keywords: Composite material; Minerals; Deformation and fracture; X-ray techniques; Hydraulic concrete
1. Introduction There is a lot of large-scale hydropower stations have been or will be set up in China. Although the material qualities and temperature differences of concrete were controlled strictly, superficial cracks were still found in the Three Gorges Project dam [1]. Cracks may reduce the service-ability and durability the of concrete dam. Using an expansive stress to compensate for the shrinkage stress is a convenient and effective method [2]. Expansive stress has successfully compensated for the shrinkage of concrete in the period of decreasing temperature by using MgO-type expansive material in China [3]. As magnesite minerals lack in distribution, the application of MgO-type expansive materials in hydraulic concrete are limited to China [4]. In recent years, with the growing demand of fertilizers or building materials in China, the accumulated amounts of inorganic solid wastes such as the residue of dolomite, serpentine and magnesite have increased sharply. About 45%–65% of these solid materials are discarded as wastes [5]. To enhance the utilization of them is an urgent task in China. In this paper, a new type of expansive material from mineral wastes of dolomite, serpentine and a few
⁎ Corresponding author. Tel./fax: +86 25 8378 6473. E-mail address: [email protected] (G. Peiwei). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.04.091
of magnesite burned at a given temperature in particle or powder are produced and used in dam concrete to decrease shrinkage deformation and fracture. 2. Materials and methods 2.1. Materials Cement used is China Type 42.5 medium heat Portland cement. MgO-type expansive material is prepared with solid wastes of dolomite, serpentine and magnesite which were ground and mixed according to the designed proportion, so burned at 1150–1200 °C for 1–1.5 h in an electric furnace, then cooled down in air, after were ground. The chemical compositions of these materials are shown in Table 1.
Table 1 Chemical composition of materials /% Items
SiO2
Al2O3 Fe2O3 CaO
Cement 20.90 4.57 MgO-type 14.29 2.54 expansive material (EA)
5.35 1.05
MgO SO3 L.O.I Σ
60.91 3.81 2.59 1.48 39.46 38.31 0.05 3.01
99.61 98.72
G. Peiwei et al. / Materials Letters 62 (2008) 106–108
107
3. Results and discussions 3.1. Hydration of MgO-type expansive material in water The main phases of MgO-type expansive material which burned with by-products are MgO, C2S and CaO as identified by X-ray diffraction analysis (in Fig. 1). In the 28 day curing period at 20 °C, the formation of brucite would be found around the MgO-type expansive material particles, which was about 0.5–1 μm in length [6]. Curing at 20 °C, the hydration of MgOtype expansive material is very slow, and about 57% MgO formed brucite after curing 180 days by DSC analysis (Fig. 2). 3.2. Effects of MgO-type expansive material on deformation of concrete Fig. 1. XRD pattern of expansive material.
Fig. 2. Hydration degree of MgO in EA.
Two types of coarse aggregates with the size grading of: large size stone (10–20 mm) and small size stone (2.5–10 mm) = 6:4 were used. A retarded naphthalene water reducing agent containing air-entraining components was used.
The amounts of MgO-type expansive material were 0 and 8% (by weight of cement). The specimens were cured under adiabatic condition. The deformation of concrete without and with MgO-type expansive material is presented in Fig. 3. In the early age (3 days), the shrinkage deformation of concrete specimens occurred without MgO-type expansive material, and the value of shrinkage of concrete specimens in 28 days is 20 × 10− 6 when the cracks would be produced. In about 100 days, the value of shrinkage deformation of concrete specimens is about 40 × 10− 6. A higher change in the ratio tends to cause the fracture of concrete at the early ages, decreasing the strength and durability of concrete [7]. When MgO-type expansive material was used, the early shrinkages of concrete specimens rarely occurred in the early age due to CaO hydration. The CaO and MgO in the MgO-based expansive material reacted with water to form Ca(OH)2 and Mg(OH)2 that caused expansion and compensated for the shrinkage of concrete at the early and later ages [8]. The content and mix of MgO-type expansive material in concrete must be restricted and controlled, which may cause local expansive cracking, and reduce the strength and durability of concrete.
4. Conclusions 2.2. Methods The shrinkage and expansive strain of concrete specimens were φ150 mm × 400 mm, the mix proportions as follow: Cement: 200 kg/m3; water: 96 kg/m3; expansive material: 0, 16 kg/m3; aggregate: 635 kg/m3 (2.5–10 mm), 803 kg/m3 (10–20 mm); sand: 720 kg/m3; superplasticizer: 1.4 kg/m3. The specimens were moulded in a barrel with an outer layer made of galvanized tube and an intermediate insulation and inner layer made with a stainless steel tube, then measuring their shrinkage strain and expansive strain.
The main phases of the material are MgO, CaO and C2S. The expansions are mainly generated by the hydrations of CaO and MgO in the expansive material. MgO hydrates to Mg(OH)2 very slowly, and about 57% MgO forms brucite when the specimens are cured in 20 °C water for 180 days, which generates a delayed expansion. Using MgO-type expansive material from mineral wastes of dolomite, serpentine and magnesite to decrease the shrinkage deformation of concrete dams is possible, but there is a need to control the content of MgO.
Fig. 3. Deformation of concrete with or without EA.
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G. Peiwei et al. / Materials Letters 62 (2008) 106–108
Acknowledgments The authors are thankful to the Natural Sciences Foundation Council of China (60672166) and the Nanhang Foundation for their financial support of this project. References [1] C.G. Shen, Electricity 4 (2000) 26−36. [2] X.H. Cui, J. Nanjing Uni. Chem. Tech. 2 (1987) 30−37.
[3] [4] [5] [6] [7] [8]
S. Chatterji, Cem. Concr. Res. 25 (1995) 51−56. E.K. Mako, A.Z. Juhasz, Thermochim. Acta 342 (1999) 105−114. Gao Pei-wei, Constr. Build. Mater. 20 (2006) 586−590. Gao Pei-wei, J. Nanjing Uni. Aero. Astro. 23 (2006) 120−124. Gao Pei-wei, Fules 86 (2007) 1208−1211. Gao Pei-wei, Constr. Build. Mater. 21 (2007) 132−138.
Materials Letters 62 (2008) 106 – 108 www.elsevier.com/locate/matlet
Using a new composite expansive material to decrease deformation and fracture of concrete Gao Peiwei ⁎, Lu Xiaolin, Jin Shaochun, Zhang Hui, Guo Chunxing College of Civil Engineering, Nanjing University of Aeronautics & Astronautics, Nanjin, 210016, China Received 21 November 2006; accepted 24 April 2007 Available online 27 April 2007
Abstract Using minerals of dolomite, serpentine and magnesite produce a new composite material which provides an expansive stress to decrease deformation and fracture of hydraulic concrete. The compositions of the expansive material are mainly MgO, CaO and C2S by X-ray techniques. Adding the expansive material to concrete, the shrinkage of concrete may be compensated by the hydration of CaO to Ca(OH)2 in early ages and the hydration of MgO to Mg(OH)2 in later ages respectively, so decrease deformation and fracture of concrete. © 2007 Elsevier B.V. All rights reserved. Keywords: Composite material; Minerals; Deformation and fracture; X-ray techniques; Hydraulic concrete
1. Introduction There is a lot of large-scale hydropower stations have been or will be set up in China. Although the material qualities and temperature differences of concrete were controlled strictly, superficial cracks were still found in the Three Gorges Project dam [1]. Cracks may reduce the service-ability and durability the of concrete dam. Using an expansive stress to compensate for the shrinkage stress is a convenient and effective method [2]. Expansive stress has successfully compensated for the shrinkage of concrete in the period of decreasing temperature by using MgO-type expansive material in China [3]. As magnesite minerals lack in distribution, the application of MgO-type expansive materials in hydraulic concrete are limited to China [4]. In recent years, with the growing demand of fertilizers or building materials in China, the accumulated amounts of inorganic solid wastes such as the residue of dolomite, serpentine and magnesite have increased sharply. About 45%–65% of these solid materials are discarded as wastes [5]. To enhance the utilization of them is an urgent task in China. In this paper, a new type of expansive material from mineral wastes of dolomite, serpentine and a few
⁎ Corresponding author. Tel./fax: +86 25 8378 6473. E-mail address: [email protected] (G. Peiwei). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.04.091
of magnesite burned at a given temperature in particle or powder are produced and used in dam concrete to decrease shrinkage deformation and fracture. 2. Materials and methods 2.1. Materials Cement used is China Type 42.5 medium heat Portland cement. MgO-type expansive material is prepared with solid wastes of dolomite, serpentine and magnesite which were ground and mixed according to the designed proportion, so burned at 1150–1200 °C for 1–1.5 h in an electric furnace, then cooled down in air, after were ground. The chemical compositions of these materials are shown in Table 1.
Table 1 Chemical composition of materials /% Items
SiO2
Al2O3 Fe2O3 CaO
Cement 20.90 4.57 MgO-type 14.29 2.54 expansive material (EA)
5.35 1.05
MgO SO3 L.O.I Σ
60.91 3.81 2.59 1.48 39.46 38.31 0.05 3.01
99.61 98.72
G. Peiwei et al. / Materials Letters 62 (2008) 106–108
107
3. Results and discussions 3.1. Hydration of MgO-type expansive material in water The main phases of MgO-type expansive material which burned with by-products are MgO, C2S and CaO as identified by X-ray diffraction analysis (in Fig. 1). In the 28 day curing period at 20 °C, the formation of brucite would be found around the MgO-type expansive material particles, which was about 0.5–1 μm in length [6]. Curing at 20 °C, the hydration of MgOtype expansive material is very slow, and about 57% MgO formed brucite after curing 180 days by DSC analysis (Fig. 2). 3.2. Effects of MgO-type expansive material on deformation of concrete Fig. 1. XRD pattern of expansive material.
Fig. 2. Hydration degree of MgO in EA.
Two types of coarse aggregates with the size grading of: large size stone (10–20 mm) and small size stone (2.5–10 mm) = 6:4 were used. A retarded naphthalene water reducing agent containing air-entraining components was used.
The amounts of MgO-type expansive material were 0 and 8% (by weight of cement). The specimens were cured under adiabatic condition. The deformation of concrete without and with MgO-type expansive material is presented in Fig. 3. In the early age (3 days), the shrinkage deformation of concrete specimens occurred without MgO-type expansive material, and the value of shrinkage of concrete specimens in 28 days is 20 × 10− 6 when the cracks would be produced. In about 100 days, the value of shrinkage deformation of concrete specimens is about 40 × 10− 6. A higher change in the ratio tends to cause the fracture of concrete at the early ages, decreasing the strength and durability of concrete [7]. When MgO-type expansive material was used, the early shrinkages of concrete specimens rarely occurred in the early age due to CaO hydration. The CaO and MgO in the MgO-based expansive material reacted with water to form Ca(OH)2 and Mg(OH)2 that caused expansion and compensated for the shrinkage of concrete at the early and later ages [8]. The content and mix of MgO-type expansive material in concrete must be restricted and controlled, which may cause local expansive cracking, and reduce the strength and durability of concrete.
4. Conclusions 2.2. Methods The shrinkage and expansive strain of concrete specimens were φ150 mm × 400 mm, the mix proportions as follow: Cement: 200 kg/m3; water: 96 kg/m3; expansive material: 0, 16 kg/m3; aggregate: 635 kg/m3 (2.5–10 mm), 803 kg/m3 (10–20 mm); sand: 720 kg/m3; superplasticizer: 1.4 kg/m3. The specimens were moulded in a barrel with an outer layer made of galvanized tube and an intermediate insulation and inner layer made with a stainless steel tube, then measuring their shrinkage strain and expansive strain.
The main phases of the material are MgO, CaO and C2S. The expansions are mainly generated by the hydrations of CaO and MgO in the expansive material. MgO hydrates to Mg(OH)2 very slowly, and about 57% MgO forms brucite when the specimens are cured in 20 °C water for 180 days, which generates a delayed expansion. Using MgO-type expansive material from mineral wastes of dolomite, serpentine and magnesite to decrease the shrinkage deformation of concrete dams is possible, but there is a need to control the content of MgO.
Fig. 3. Deformation of concrete with or without EA.
108
G. Peiwei et al. / Materials Letters 62 (2008) 106–108
Acknowledgments The authors are thankful to the Natural Sciences Foundation Council of China (60672166) and the Nanhang Foundation for their financial support of this project. References [1] C.G. Shen, Electricity 4 (2000) 26−36. [2] X.H. Cui, J. Nanjing Uni. Chem. Tech. 2 (1987) 30−37.
[3] [4] [5] [6] [7] [8]
S. Chatterji, Cem. Concr. Res. 25 (1995) 51−56. E.K. Mako, A.Z. Juhasz, Thermochim. Acta 342 (1999) 105−114. Gao Pei-wei, Constr. Build. Mater. 20 (2006) 586−590. Gao Pei-wei, J. Nanjing Uni. Aero. Astro. 23 (2006) 120−124. Gao Pei-wei, Fules 86 (2007) 1208−1211. Gao Pei-wei, Constr. Build. Mater. 21 (2007) 132−138.