EFFECT OF FLY ASH GRADATION ON WORKABILITY, STRENGTH AND DURABILITY OF PORTLAND CEMENT FLY ASH MORTARS

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1 EFFECT OF FLY ASH GRADATION ON WORKABILITY, STRENGTH AND DURABILITY OF PORTLAND CEMENT FLY ASH MORTARS Xian P. Liu, Pei M. Wang and Jie Sun Key Laboratory of Advanced Civil Engineering Materials (Tongji University), Ministry of Education, Shanghai, China Abstract Four graded fly ashes were incorporated respectively with Portland cement by 50% (w/w) to investigate the effect of fly ash gradation on the workability, strength and durability of Portland cement fly ash mortars. The results indicated that appropriate gradation of fly ash decreased the void content and increased the flowability of fresh mortar, as well as improved strength and durability of hardened mortar due to the modification of microstructure. Based on the grey correlation theory, the particle size of fly ash under 8.1 µm, especially ranged between 3.9 and 8.1 µm, was beneficial to the increase of compressive strength, impermeability and carbonation resistance, but unfavorable to the decrease of shrinkage of fly ash incorporated mortars. However, the particle size of fly ash above 8.1 µm, especially ranged between 8.1 and 15.7 µm, was on the contrary. With the increase of specific surface area, the pozzolanic activity of fly ash was improved due to more amorphous phase and calcium oxide, which resulted in more non-evaporable water and less calcium hydroxide in hardened cement paste. Fly ash particles with different chemical compositions had different reaction activity, hence played different roles in the microstructure of hardened mortar. 1 INTRODUCTION The use of fly ash as supplementary cementing material in cement or concrete is increasing in popularity. These blends offer several advantages over pure cement systems, providing better properties, reducing environmental impact, reducing production energy and CO 2 emissions. The morphological, micro-aggregate and active effects of fly ash contribute to the excellent performance of fly ash incorporated mortar or concrete, such as high in flowability and impermeability, low in hydration heat and dry shrinkage, inhibiting alkali-aggregate reaction, except for degradation in early strength and carbonation resistance [1,2]. However, the effect of different fly ashes are distinct due to the variation of particle size gradation and chemical composition [3-6]. Qiao compared the particle size distributions of Chinese cements with the Fuller curve, and found that there were less fine and more coarse particles in those cements than in theoretical dense packing particle group [7]. This conclusion also conformed to Portland cement incorporating 50% (w/w) Chinese fly ashes from different districts (Figure

2 1). Therefore in this study, four graded fly ashes were prepared with one of the typically used fly ash in China to simulate the gradation of fly ash incorporated Portland cement (Figure 2) on the basis of minimum influence of chemical composition, and the effect of fly ash gradation on workability, strength and durability of Portland cement fly ash mortars were investigated. Figure 1: Particle size distributions of cement Figure 2: Particle size distributions of incorporating 50% (w/w) Chinese fly ash from cement incorporating 50% (w/w) graded different districts and the theoretical Fuller curve fly ash and the theoretical Fuller curve 2 MATERIALS AND METHODS 2.1 Materials Fly ash Fly ash Ⅱ was used to prepare four graded fly ashes (F1, F2, F3 and F4) through grinding, air classification and mixing. The chemical compositions, particle size distributions and physical properties of four graded fly ashes are shown in Table 1, Table 2 and Table 3. Table 1: Chemical compositions of four graded fly ashes, % (w/w) CaO SiO 2 Al 2 O 3 Fe 2 O 3 MgO SO 3 Na 2 O K 2 O TiO 2 LOI Amorphous phase F F F F Table 2: Particle size distributions of four graded fly ashes, % (v/v) <3.9 (µm) (µm) (µm) (µm) >30 (µm) Mean size F * F * F * F * *: Mean size in μm.

3 Table 3: Physical properties of four graded fly ashes F1 F2 F3 F4 Density (kg/m 3 ) Specific surface area (m 2 /kg) Cement The selected cement was P.I Portland cement grade 52.5, whose physical and chemical characters are shown in Table 4 and Table 5. Table 4: Physical properties of cement Density Specific Setting time Water Compressive Flexural (kg/m 3 surface area (min) requirement strength (MPa) strength (MPa) ) (m 2 /kg) Initial Final (%, w/w) 3d 7d 28d 3d 7d 28d Table 5: Chemical properties of cement, % (w/w) Chemical composition Mineral composition CaO SiO 2 Al 2 O 3 Fe 2 O 3 MgO SO 3 K 2 O TiO 2 MnO SrO LOI C 3 S C 2 S C 3 A C 4 AF Sand The quality of sand conforms to Chinese Standard GB/T The density of the sand is 2670 kg/m Methods Besides the control sample prepared with pure Portland cement (CC for cement paste and CM for mortar), fly ash incorporated cement paste and mortar samples were prepared with a 50% (w/w) substitution of each of the four graded fly ashes for Portland cement (F1C, F2C, F3C and F4C for cement pastes, and F1M, F2M, F3M and F4M for mortars). The particle size distributions of the cementitious materials and the theoretical Fuller curve of particles having size range 1~125 µm are shown in Figure 2. For both cement pastes and mortars, the water to binder ratio was 0.5 and the curing temperature was (20±1). Packing density and flowability of freshly mixed mortars were tested according to Chinese Standards JGJ/T and GB/T , respectively. Mortar samples of 40mm 40mm 160mm were prepared to test compressive strength, carbonation depth and shrinkage according to Chinese Standards GB/T , GBJ and JC/T Mortar samples of 30mm in thickness, 70mm in top diameter and 80mm in bottom diameter were prepared to test impermeability factor according to Chinese Standards DL/T and GBJ When formed, samples were put into water till the preset time except for shrinkage testing samples, which were cured at a relative humidity of (60±5) %. Cement paste samples were prepared to determine the non-evaporable water (NEW) content by calcination and the content of calcium hydroxide with DSC. Dried pieces of 90 d-

4 old cement pastes were embedded in resin and polished to make BSE images and determine the chemical composition of various phases using an ESEM equipped with EDS. 3 RESULTS AND DISCUSSION 3.1 Freshly mixed mortars Void content The void content of freshly mixed mortar is calculated with the following equation: Where - void content of freshly mixed mortar, %; 1 - packing density of freshly mixed mortar, kg/m 3 ; 2 - density of freshly mixed mortar, kg/m 3. The density of freshly mixed mortar is calculated with the following equation: ( ma mb ) A B 2 m m B A A B Where A - density of particle group A, kg/m 3 ; B - density of particle group B, kg/m 3 ; ma - mass of particle group A, kg; mb - mass of particle group B, kg. The packing density, density and void content of mortars are shown in Table 6. With the increase of specific surface area of fly ash, the void content of mortars decreased, but slightly increased when the specific surface area was above 630 m 2 /kg. The void content of fly ash incorporated mortars were far below that of pure cement mortar, indicating that the morphological effect and micro-aggregate effect of fly ash benefit the degree of dense packing of fresh mortar. But fly ash particles with very high specific surface area conglomerated and have no more advantages of improving the degree of dense packing. Flowability The flowability of mortars is shown in Table 6. With the increase of specific surface area of fly ash, the flowability of fly ash incorporated mortars increased, but decreased rapidly when the specific surface area was very high. In a fresh mortar packing system, a part of water fill in the void, and the rest part of water forms a lubricating membrane which wraps the particles and offers flowability. As shown in Table 1, the higher the specific surface area of fly ash was, the less void formed in the fresh mortar, hence more water worked as lubricating membrane and the flowability increased. When the specific surface area reached 700 m 2 /kg, conglomerating of small particles enclosed some free water and excessively grinding destroyed the spherical morphology of fly ash, the flowability decreased. Therefore an.optimum fineness of fly ash brings forth the best workability of fresh mortar. Table 6: The packing density, density, void content and flowability of freshly mixed mortars (1) (2)

5 Specific surface area of Packing density Density Void content Flowability graded fly ash (m 2 /kg) (kg/m 3 ) (kg/m 3 ) (%) (mm) CM F1M F2M F3M F4M Hardened cement mortars Compressive strength The compressive strength results of mortars are in Table 7. It can be observed that although the compressive strength of fly ash incorporated mortars was lower than that of pure cement mortars, it improved with the increase of specific surface area of fly ash. The compressive strength of mortar is determined by the initial packing state and the hydration degree. As shown in Table 6, the high specific surface area of fly ash tended to form initial closing packing state, hence the early strength of fly ash incorporated mortar increased mainly due to the micro-aggregate effect of fly ash. Besides, the grinding process destroyed the dense vitreous shell and added active spots on the surface of fly ash particles, and the water enclosed in the conglomeration of small particles took part in the reaction process at later period, hence the active effect of fly ash exerted at full stream. As a result, the increase of compressive strength of mortars incorporating fly ash with high specific surface area at later period is faster than that of mortars incorporating fly ash with low specific surface area. Impermeability The impermeability factor results of mortars are in Table 7. It can be observed that incorporating fly ash into mortar greatly decreased the impermeability at early period due to the dilution effect of the admixture. However, at later period, the pozzolanic reaction of fly ash occurred which modified the pore structure of mortars and improved the impermeability. 630 m 2 /kg was an optimum specific surface area of fly ash to obtain the highest impermeability of mortar both at 28d and 90d, since the fly ash particles dispersed well in the void, and the micro-aggregate and active effects exerted to a maximum extent. Carbonation resistance The carbonation depth results of mortars are in Table 7. Compared with pure cement mortars, the carbonation resistance of fly ash incorporated mortars greatly deteriorated due to the dilution effect of the admixture. However, with the increase of the specific surface area of fly ash, the carbonation resistance improved until to a highest value at the specific surface area of 630 m 2 /kg. It was also related to the micro-aggregate and active effects of fly ash. Shrinkage The shrinkage results of mortars are shown in Table 7. Due to the micro-aggregate effect of fly ash and the reduction of cement content, the shrinkage of fly ash incorporated mortars was smaller than that of pure cement mortars, but it increased when the specific surface area of fly ash increased from 300 m 2 /kg to above 480 m 2 /kg. The incorporation of fly ash with high specific surface area into mortar effectively separated cement particles and increased the contact surface between cement and water, which promoted cement hydration at early period. Besides, the active effect of fly ash with high specific surface area also improved, which promoted the pozzolanic reaction of fly ash at later period. Hence, with the increase of specific surface area of fly ash, the content of hydrates was high and the shrinkage was large.

6 Grey correlation analysis Grey correlation theory was used to analyze the degree of the sensitivity of the influence of fly ash gradation on the properties of mortars. The correlation coefficient results between particle size intervals of fly ash and properties of mortars are in Table 8. The higher the value, the higher the correlation degree between particle size interval and the property is. A positive or negative value means the particle size interval promotes or weakens the property. From a long-term point of view, the particle size of fly ash under 8.1 µm, especially ranged between 3.9 and 8.1 µm, was beneficial to the compressive strength development, impermeability and carbonation resistance of fly ash incorporated mortars, but unfavorable to the decrease of shrinkage, however the particle size of fly ash above 8.1µm, especially ranged between 8.1 and 15.7 µm, was on the contrary. Table 7: Properties of hardened cement mortars Properties Hydration time (d) CM F1M F2M F3M F4M Compressive strength (MPa) Impermeability factor (MPa h) Carbonation depth (mm) Shrinkage (mm) Table 8: Correlation coefficient between properties of hardened cement mortars and particle size intervals of fly ash Hydration < >30 Properties Compressive strength (MPa) Impermeability factor (MPa h) time (d) (µm) (µm) (µm) (µm) (µm)

7 Carbonation depth (mm) Shrinkage (mm) Hardened cement pastes Non-evaporable water (NEW) content The NEW content results of cement pastes are in Table 9. Although the value of fly ash incorporated cement pastes was lower than that of pure cement pastes, it improved with the increase of specific surface area of fly ash, especially during the early period. After 28d, the NEW content of cement pastes incorporating fly ash with the specific surface area of 300 m 2 /kg was lowest among all the pastes. Moreover, the difference in NEW content of cement pastes incorporating fly ash with the specific surface area above 480 m 2 /kg tended to be small. Content of calcium hydroxide The content of calcium hydroxide results of cement pastes are in Table 9. The value of fly ash incorporated cement pastes was lower than that of pure cement pastes. At 3d and 7d, it increased with the specific surface area of fly ash due to the dilution effect, but slightly decreased at 700 m 2 /kg since the vitreous shell abound the fly ash was destroyed greatly and the pozzolanic reaction of fly ash exerted much early. At 90d, with the increase of active effect of fly ash, it decreased with the increase of specific surface area of fly ash. Table 9: Content of NEW and calcium hydroxide of hardened cement pastes, % (w/w) NEW content Content of calcium hydroxide 3d 7d 28d 90d 3d 7d 28d 90d CC F1C F2C F3C F4C Microstructure Figure 3 shows the BSE images and EDS analysis of 90d-old hardened cement paste incorporating graded fly ash F3. It can be observed from Figure 3(a) that the particles of graded fly ash F3 distributed evenly in the cement matrix and worked well as microaggregate. The pozzolanic reaction of fly ash occurred and the residue of fly ash particles became small. Due to the variation of chemical compositions of fly ash particles, the reaction process was different and distinct interfacial structure between residue and matrix was formed. EDS analysis from dot 1 to dot 5 (Figure 3(b)) shows that with the decrease of atomic percentage of Ca and the increase of atomic percentages of Si and Al in fly ash particle, the gap between the residue and the matrix narrows, which indicates the decrease of active effect

8 of fly ash. Based on the chemical composition analysis of graded fly ash F1, F2, F3 and F4 (Table 1), the active effect of fly ash improved accordingly, hence the content of hydrates increased, resulting in an improvement of compressive strength and shrinkage. Fly ash with high activity tends to consume more calcium hydroxide and forms wide gap between the residue and the matrix, which affects the impermeability and carbonation resistance of mortars due to the characteristic of the hydrates layer. If the hydrates layer is composed of C- S-H, the gap does no harm to the impermeability and carbonation resistance since it is not connected to the outside capillary pores. But if the hydrates layer is composed of ettringite, the gap is connected to the outside capillary pores through the void between the ettringite, which is unfavorable to the above properties. Hence the permeability and carbonation resistance of F4M was not the best among fly ash incorporated mortars although the active effect of fly ash F4 was the highest Ca Si Al Fe (At%) (At%) (At%) (At%) (a) BSE images (b) EDS analysis of spots 1 to 5 Figure 3: BSE images of 90d-old F3C cement paste and EDS analysis of spots 1 to 5 4 CONCLUSIONS 1. There were combined effects of physical and chemical compositions of graded fly ash on the properties of fly ash incorporated mortars. 2. An appropriate fly ash gradation decreased void content and increased flowability of fresh mortars, as well as improved strength and durability of hardened mortars due to dense packing and fly ash effect induced modification of microstructure. 3. Particle size of fly ash under 8.1 µm, especially ranged between 3.9 and 8.1 µm, was beneficial to the increase of compressive strength, impermeability and carbonation resistance, but unfavorable to the decrease of shrinkage. However, particle size of fly ash above 8.1 µm, especially ranged between 8.1 and 15.7 µm, was on the contrary. ACNOWLEDGEMENTS The authors gratefully acknowledge the financial support of the National Basic Research Program of China (Grant No. 2009CB623104). REFERENCES [1] Isaia, G.C., Gastaldini,A.L.G. and Moraes R., 'Physical and pozzolanic action of mineral additions on the mechanical strength of high-performance concrete', cem. Conr. Comp. 25 (1) (2003) [2] Ferrarisa, C. F., Obla, K. H. and Hill, R., 'The influence of mineral admixtures on the theology of cement paste and concrete', Cem. Conr. Res. 31 (2) (2001)

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