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Early-age hydration characteristics of composite binder containing iron tailing powder
Powder Technology 315 (2017) 322–331
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Powder Technology journal homepage: www.elsevier.com/locate/powtec
Early-age hydration characteristics of composite binder containing iron tailing powder Fanghui Han a,⁎, Li Li b, Shaomin Song b, Juanhong Liu a a b
School of Civil and Resource Engineering, University of Science and Technology Beijing, Beijing, China School of Civil and Transportation Engineering, Beijing University of Civil Engineering and Architecture, Beijing, China
a r t i c l e
i n f o
Article history: Received 10 October 2016 Received in revised form 9 February 2017 Accepted 3 April 2017 Available online 7 April 2017 Keywords: Iron tailing powder Hydration Pore structure Morphology Strength
a b s t r a c t The early-age hydration characteristics of composite binder containing iron tailing powder were investigated by determining the hydration heat, non-evaporable water content, pore structure, and morphology of hardened paste as well as the compressive strength of mortar. The results show that the reaction degree of iron tailing powder is extremely low at early age. Addition of coarse iron tailing powder negatively affects the properties of composite binder paste and mortar. Compared to samples containing coarse iron tailing powder, the samples containing fine iron tailing powder show a large amount of hydration heat, high non-evaporable water content, fine pore structure of hardened paste and high compressive strength of mortar. The iron tailing powder significantly promotes the hydration of binder at low water-to-binder ratio. The iron tailing powder particles are well graded with cement particles in the paste, but the large iron tailing powder particles bond poorly with the surrounding hydrates. The decreasing ratio of compressive strength of mortar is much lower than the replacement level of iron tailing powder, especially for the mortar containing fine iron tailing powder at low water-to-binder ratio. © 2017 Elsevier B.V. All rights reserved.
1. Introduction With the rapid development of the national economy, the construction of infrastructure and buildings has greatly increased in China. The increasing demand for natural energy and resources has produced considerable industrial wastes. To reduce the environmental impact and achieve sustainability, industrial wastes should be used for secondary purposes. Most industrial waste is used in the concrete industry [1], where these wastes can be used to partially replace clinker in blended cement or to partially replace cement or aggregate in concrete [2–5]. Using industrial waste saves energy and resources, reduces the cost of concrete and decreases carbon dioxide emissions. The most commonly used industrial wastes in modern concrete are ground granulated blast furnace slag, fly ash and silica fume. The physical and chemical properties of these materials contribute to the strength development and durability of concrete [6]. However, at present, the quantity of these industrial wastes is limited in China and their prices are increasing rapidly. To ensure the sustainable development of the concrete industry, other industrial wastes must be used in concrete. Iron tailings are mining waste obtained during the beneficiation process to concentrate the iron ore [7]. More than six hundred million tons of iron tailings are generated in China each year. The accumulated ⁎ Corresponding author. E-mail address: [email protected] (F. Han).
http://dx.doi.org/10.1016/j.powtec.2017.04.022 0032-5910/© 2017 Elsevier B.V. All rights reserved.
quantity of iron tailings has reached five billion tons, which accounts for more than 80% of industrial solid waste [8,9]. However, the utilization rate of iron tailings in China is only 7% [10]. Thus, large quantities of iron tailings are piling up, occupying land, and polluting air and water. Therefore, disposing iron tailings using proper methods is crucial. Because iron tailings mainly contain silica, alumina, iron, magnesium, and calcium, iron tailings can be used in the construction industry [11]. Shettima et al. [12] found that iron tailings can be used as fine aggregate replacement in concrete. Adding iron tailings decreased the workability of concrete while increasing the strength and elasticity modulus compared to conventional concrete. Zhao et al. [13] found that the mechanical properties of ultra-high performance concrete were comparable to control concrete when no more than 40% of the fine aggregate was replaced with iron tailings. Huang et al. [14] reported that engineered cementitious composites using iron tailings as the aggregate obtained good tensile and compressive strength and that fine iron tailings resulted in better fiber dispersion. Li et al. [15] found that the mechanical properties of cementitious material prepared by blending 30% clinker, 34% blast furnace slag, 30% iron tailings and 6% gypsum were comparable with those of 42.5 ordinary Portland cement. Yunhong et al. [16] reported that mechanochemically activated iron tailings have pozzolanic characteristics and can be used as supplementary cementitious material in concrete; in ordinary concrete, the maximum replacement level is 30%, but could be up to 40% for concrete at low waterto-binder ratio. Ma et al. [17] noted that substituting 40%–60% of silica
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Table 1 Chemical compositions of cement and iron tailing powder wt%. Composition
SiO2
Cement Iron tailing powder
20.55 4.59 67.29 8.49
Al2O3 Fe2O3 CaO 3.27 8.95
MgO SO3
62.50 2.61 3.63 4.80
Na2Oeq f-CaO LOI
2.93 0.53 0.45 2.90
0.83 –
2.08 2.39
Na2Oeq = Na2O + 0.658K2O.
sand with iron tailings in autoclaved aerated concrete could sufficiently improve the density and compressive strength of the concrete to satisfy the requirements of B05, A2.5 grade in the Chinese National Standard. The C-S-H gel and tobermorite contribute to the compressive strength. Iron tailings were also used for the production of fired brick [18] and in combination with fly ash to produce geopolymers [19,20]. According to the current literatures, iron tailings are usually used as fine aggregate in concrete. There is little information about using iron tailings as supplementary cementitious material in concrete, mainly due to its low activity; the content of iron tailings in clinker is only 5% [21]. In addition, the early-age hydration characteristics of composite binder are very important to understand the hydration mechanism, but there is little deep study on the early-age hydration characteristics. In this paper, iron tailings are ground to decrease their particle size. Then, the iron tailing powder is used partially replace cement in composite binder. The early-age hydration characteristics of the composite binder containing iron tailing powder are investigated. The fineness of the iron tailing powder, the water-to-binder ratio, and the replacement level are all considered. The purpose of this research is to investigate the effect of iron tailing powder on the hydration mechanism of the binder and the possibility of using iron tailing powder as mineral admixture in concrete. 2. Experimental
Fig. 2. Morphology of iron tailing powder.
quartz. Fig. 2 shows the morphology of the iron tailing powder; many fine particles exist after mechanical grinding along with some coarse particles with irregular shapes. The coarse iron tailing powder and fine iron tailing powder are used in this study; these powders were ground in a ball mill for 20 min and 60 min, respectively. The particle size distributions of the raw materials are given in Fig. 3, which clearly shows that the iron tailing powder is finer than Portland cement. The medium particle size (d50) for the cement, coarse iron tailing powder and fine iron tailing powder are 17.17 μm, 7.31 μm and 3.56 μm, respectively. The water requirement ratio of fine iron tailing powder is 104%, which indicates that the water requirement of iron tailing powder is slightly higher than that of Portland cement.
2.1. Raw materials 2.2. Mix proportions P.I 42.5 Portland cement that conformed to Chinese National Standard GB 175-2007 and iron tailings from a mining enterprise in Miyun County of Beijing were used in this study. The chemical compositions of the cement and iron tailing powder determined using X-ray fluorescence (XRF) are shown in Table 1. The main chemical composition of the iron tailing powder is SiO2, followed by Fe2O3 and Al2O3. The iron tailings fall into the category of high-silicon iron tailings. The X-ray diffraction (XRD) pattern of the iron tailing powder is presented in Fig. 1, which shows that the main mineral phase of the iron tailing powder is
Fig. 1. XRD patterns of iron tailing powder.
The mix proportions of pastes and mortars containing iron tailing powder are shown in Tables 2 and 3, respectively. The replacement levels of iron tailing powder were 0, 20% and 50% by mass, and waterto-binder (w/b) ratios of 0.3 and 0.4 were used in this study. The pastes were prepared by mixing binder with water according to Table 2. The mortars were prepared by mixing ISO standard sand, binder and
Fig. 3. Particle size distributions of raw materials.
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Table 2 Mix proportions of pastes. Sample
Cem CTP20 CTP50 FTP20 FTP50
w/b
0.4/0.3
Mass fraction (%) Cement
Coarse iron tailing powder
Fine iron tailing powder
100 80 50 80 50
0 20 50 0 0
0 0 0 20 50
water according to Table 3. The ISO standard sand-to-binder ratio (s/b) is 3:1. Polycarboxylic superplasticizer was used to adjust the fluidity of the pastes and mortars. To determine the hydration heat characteristics of the iron tailing powder, iron tailing powder paste was prepared by mixing fine iron tailing powder with NaOH solution (with a pH value of 13.3) at a ratio of NaOH solution to fine iron tailing powder of 0.4.
Fig. 4. Hydration heat of fine iron tailing powder in NaOH solution of pH 13.3.
2.3. Test methods
3. Results and discussion
The hydration heat evolution rate and cumulative hydration heat of the iron tailing powder and the composite binder with iron tailing powder were measured using a TAM Air isothermal calorimeter at 20 °C over 72 h. After evenly stirring the pastes in the vial, the vial was immediately sealed and placed in the chamber of isothermal calorimeter that was set to a constant temperature of 20 °C. Then, the hydration heat evolution rate and cumulative hydration heat of the pastes were continuously measured as a function of hydration time. The composite binder pastes containing iron tailing powder were placed in plastic tubes with a diameter of 15 mm and a length of 80 mm. Then, the sealed plastic tubes were placed in a standard curing room at 20 ± 1 °C and 95% RH. At the age of 3 days, the pastes were cut into slices and then soaked in ethanol. The ethanol was absorbed into the pores of the pastes, thus replacing the water in the pastes and ceasing further hydration. The differential pore volume and cumulative pore volume of the pastes were measured using mercury intrusion porosimeter (MIP). After the pastes cured for 3 days, the non-evaporable water contents were determined as the mass difference between pastes heated at 80 °C and 1000 °C, and normalized by the mass after heating at 80 °C. The loss on ignition of raw materials was also considered to correct the results [22]. The morphologies of the pastes cured for 3 days were investigated using an FEI Quanta 200FEG scanning electron microscope. Freshly cut sections of the pastes were coated with gold to prevent a charging effect. The mortars were cast into moulds with dimensions of 40 × 40 × 160 mm. Then, all the mortars were placed in a standard curing room (20 ± 1 °C, 95% RH). At the age of 1 day, the mortars were stripped from the moulds. After curing for 3 days, the compressive strengths of the mortars were measured as per Chinese National Standard GB/ T17671-1999.
3.1. Hydration heat of the composite binder containing iron tailing powder The hydration heat of the fine iron tailing powder in NaOH solution are shown in Fig. 4. The pH value of the cement pore solution was
Table 3 Mix proportions of mortars. Sample
MCem MC20 MC50 MF20 MF50
w/b
0.4/0.3
s/b
3
Mass fraction (%) Cement
Coarse iron tailing powder
Fine iron tailing powder
100 80 50 80 50
0 20 50 0 0
0 0 0 20 50
Fig. 5. Hydration heat of composite binder containing coarse iron tailing powder at w/b ratio of 0.4. (a) Hydration heat evolution rate; (b) Cumulative hydration heat.
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The addition of coarse iron tailing powder reduces the cement content in the composite binder. Furthermore, as discussed above, the hydration rate of coarse iron tailing powder is very low at early age. Thus, the overall hydration rate of the composite binder is low, and less amount of hydration heat is generated. The ratios of the second exothermic peak values of samples CTP20 and CTP50 to that of sample Cem are 84.52% and 63.20%, respectively. It is clear that the decreasing ratios of the exothermic rates are 15.48% and 36.80%, which are lower than the replacement ratio of the coarse iron tailing powder, especially for the composite binder containing a large amount of coarse iron tailing powder. Similar results are observed for the cumulative hydration heat (Fig. 5(b)). The dilution effect of the iron tailing powder increases the effective water-to-cement ratio, further promoting the hydration of cement in the composite binder. Moreover, the surface of the iron tailing powder acts as nucleation site for hydrates of cement, further increases the hydration degree of the cement [25]. As shown in Fig. 6, the hydration heat evolution rate of sample FTP20 is approximately identical to that of sample Cem during the acceleration period. Although the hydration heat evolution rate of sample FTP50 is still lower than that of sample Cem, the difference between the two clearly smaller compared to sample CTP50 (Fig. 5(a)). This result is due to the high specific surface area of the fine iron tailing powder, which provides more additional nucleation sites. For sample FTP20, the filler effect of the fine iron tailing powder can compensate for the decreased cement content at early stage of hydration. Adding 20% fine iron tailing powder has little effect on the cumulative hydration heat
Fig. 6. Hydration heat of composite binder containing fine iron tailing powder at w/b ratio of 0.4. (a) Hydration heat evolution rate; (b) Cumulative hydration heat.
greater than 13.0 after a short hydration time [23]. The NaOH solution with a pH value of 13.3 simulates the hydration environment of iron tailing powder in the composite binder paste. Fig. 4 shows that there is only one exothermic peak on the hydration heat evolution rate curve. It is due to the heat of immersion of the fine iron tailing powder. The value of exothermal peak is approximately 0.8 J∙g−1 h−1. After this point, the hydration heat evolution rate rapidly decreases. The cumulative hydration heat within 72 h is only 2.69 J∙g−1. The hydration of the fine iron tailing powder in the NaOH solution releases a small amount of heat, indicating that the reaction degree of fine iron tailing powder in the composite binder paste is extremely low at early age. So the hydration of iron tailing powder makes less contribution to the total hydration heat of composite binder. The activity of supplementary cementitious material can be significantly improved after mechanical activation due to the increased specific surface area and altered internal structure [24]. Fig. 4 shows that the reaction degree of the fine iron tailing powder is very small, which indicates that the coarse iron tailing powder also presents extremely low reaction degree in the NaOH solution. Therefore, at early age, the chemical effect of the iron tailing powder is small, while the physical effect plays an important role in the hydration process of the composite binder. Figs. 5 and 6 present the hydration heat of the composite binder containing coarse iron tailing power and that containing fine iron tailing powder at w/b ratio of 0.4, respectively. As shown in Fig. 5, the hydration heat evolution rate and cumulative hydration heat of the composite binder decrease with increasing the coarse iron tailing powder content.
Fig. 7. Hydration heat of composite binder containing coarse iron tailing powder at w/b ratio of 0.3. (a) Hydration heat evolution rate; (b) Cumulative hydration heat.
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within 20 h (Fig. 6(b)). However, for sample FTP50, the cement content is significantly decreased due to the high replacement level. Therefore, low cumulative hydration heat is shown for sample FTP50 (Fig. 6(b)). Figs. 7 and 8 show the hydration heat of the composite binder containing coarse iron tailing powder and that containing fine iron tailing powder at w/b ratio of 0.3, respectively. Fig. 7 shows that the hydration heat evolution rate and cumulative hydration heat of the composite binder containing coarse iron tailing powder at w/b ratio of 0.3 present trends similar to those at w/b ratio of 0.4 (Fig. 5). However, decreasing the w/b ratio from 0.4 to 0.3 greatly promotes the hydration of composite binder containing fine iron tailing powder (Figs. 6 and 8). The hydration heat evolution rate of sample FTP20 is clearly higher than that of sample Cem during the acceleration period. The hydration heat evolution rate of sample FTP50 is approximately the same as that of sample Cem during the initial acceleration period (Fig. 8(a)). The cumulative hydration heat of samples FTP20 and FTP50 clearly increase at early stage of hydration (Fig. 8(b)). The fine iron tailing powder greatly promotes the hydration of composite binder at low w/b ratio. Other researchers also found that supplementary cementitious materials have a greater effect on cement hydration at lower w/b ratio [26–28]. Although the water requirement of fine iron tailing powder is greater than that of Portland cement, the negative effect of water requirement of fine iron tailing powder on hydration is smaller than the promoting effect of iron tailing powder on hydration, especially at low water-tobinder ratio.
At high w/b ratio, the amount of water provided for hydration is sufficient, so the dilution effect of the iron tailing powder is insignificant. In addition, the nucleating effect of coarse iron tailing powder is smaller than that of fine iron tailing powder. Thus, the hydration heat evolution rate and cumulative hydration heat of samples CTP20 and CTP50 are much lower than those of sample Cem (Fig. 5). The increased waterto-cement ratio at low w/b ratio significantly promotes the hydration of binder due to the marginal effect, especially for samples FTP20 and FTP50 (Fig. 8). At low w/b ratio, the concentrations of Ca2 + and OH– reach supersaturation in a short time [29]. Then, the hydration products are rapidly generated and more heat is released. For samples CTP20 and CTP50 hydrated at low w/b ratio, the hydration heat evolution rate and cumulative hydration heat are still lower than those of cement due to the small nucleating effect of coarse iron tailing powder (Fig. 7). At later stage of hydration, the hydration heat of the composite binder containing iron tailing powder is lower than that of cement owing to the cement content and the low activity of the iron tailing powder.
Fig. 8. Hydration heat of composite binder containing fine iron tailing powder at w/b ratio of 0.3. (a) Hydration heat evolution rate; (b) Cumulative hydration heat.
Fig. 9. Non-evaporable water content of hardened paste containing iron tailing powder. (a) at w/b ratio of 0.4; (b) at w/b ratio of 0.3.
3.2. Non-evaporable water content of paste containing iron tailing powder Fig. 9 presents the non-evaporable water content of hardened paste containing iron tailing powder. The non-evaporable water content represents not only the amount of hydration products but also the degree
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Table 3 shows the increasing ratio of the non-evaporable water content of hardened paste, η, which is determined using Eq. (1).
Table 4 Increasing ratio of non-evaporable water content of hardened paste. w/b
0.4
Sample
CTP20 CTP50 FTP20 FTP50 CTP20 CTP50 FTP20 FTP50
0.3
Increasing ratio of η/%
7.62
49.34
18
69.6
9.02
53.68
327
20.73
η ¼ wntp −ð1−f Þ∙ wncem =½ð1−f Þ∙ wncem 100%
ð1Þ
71.17
of hydration. Fig. 9(a) clearly shows that the non-evaporable water contents of the pastes containing iron tailing powder are lower than that of cement paste and that increasing the replacement level of the iron tailing powder decreases the non-evaporable water content. As explained above, the reduction of cement content is significant in samples CTP20 and CTP50 at high w/b ratio, which produces small amount of hydrates. The non-evaporable water contents of samples FTP20 and FTP50 are higher than those of samples CTP20 and CTP50, respectively. The fine iron tailing powder promotes the early-age hydration of the composite binder (Fig. 6), obtaining a greater quantity of hydrates. The hydration heat evolution rate of sample FTP20 clearly decreases after 20 h and its 72 h hydration heat is lower than that of cement (Fig. 6). The heat is generated by the hydration of cement due to the almost no reaction of fine iron tailing powder at early age (Fig. 4). Thus, the quantity of hydrates in sample FTP20 is lower than that in sample Cem. Similar trends are shown for the non-evaporable water contents of hardened pastes at w/b ratio of 0.3 (Fig. 9(b)). Owing to the significant promoting effect of the fine iron tailing powder on hydration at low w/b ratio, the nonevaporable water content of sample FTP20 is close to that of sample Cem.
Fig. 10. Pore structure of hardened paste containing coarse iron tailing powder at w/b ratio of 0.4. (a) Differential pore volume; (b) Cumulative pore volume.
where f is the replacement level of the iron tailing powder, wntp is the non-evaporable water content of paste containing iron tailing powder when the replacement level is f, and wncem is the non-evaporable water content of the cement paste. As shown in Table 4, positive increasing ratios of the non-evaporable water contents are obtained for all composite binder pastes. The pastes containing large amounts of iron tailing powder have higher increasing ratios, which prove that adding iron tailing powder enhances the hydration of cement. This effect increases with increasing the content of iron tailing powder, which is attributed to the enhanced filler effect of the iron tailing powder. The increasing ratio of the non-evaporable water content of paste containing fine iron tailing powder is higher than that of paste containing coarse iron tailing powder at the same replacement level. This result further confirms that fine iron tailing powder promotes the early-age hydration more than coarse iron tailing powder. For the same sample, the increasing ratio of the non-evaporable water content at w/b ratio of 0.3 is higher than that at w/b ratio of 0.4. Therefore, decreasing w/b ratio increases the promoting effect of the iron tailing powder on the early-age hydration. These results are in agreement with the above findings of the hydration heat (Figs. 5–8). Because the total amount of cement is low in the composite binder paste and the
Fig. 11. Pore structure of hardened paste containing fine iron tailing powder at w/b ratio of 0.4. (a) Differential pore volume; (b) Cumulative pore volume.
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(Fig. 11(a)). Therefore, the negative impact of the coarse iron tailing powder on the pore structure can be reduced by decreasing the particle size of the iron tailing powder. Although fine iron tailing powder shows very low hydration activity at early age (Fig. 4), but its filler effect significantly promotes the hydration of the composite binder (Fig. 6). Thus, a greater number of hydration products are generated, increasing the density of the paste. Moreover, the microaggregate effect of the fine iron tailing powder cannot be neglected due to the small particle size (Fig. 3). These effects of the fine iron tailing powder increase the content of pores that are smaller than 100 nm in the hardened paste. Adding fine iron tailing powder also induces large pores, which indicates that at high w/b ratio, decreasing the particle size of iron tailing powder cannot completely compensate for the negative impact of iron tailing powder on the pore structure of hardened paste. The pore structures of hardened paste containing coarse iron tailing powder and that containing fine iron tailing powder at w/b ratio of 0.3 are given in Figs. 12 and 13, respectively. Fig. 12(a) shows that the pore diameters related to the maximum peaks of samples CTP20 and CTP50 are smaller than that of sample Cem. It indicates that decreasing the w/b ratio results in the pore structure of hardened paste with coarse iron tailing powder finer than that of cement paste. At low w/b ratio, as explained above, the dilution effect of the iron tailing powder is significant. The increased ion concentration of the pore solution at low w/b ratio also promotes the nucleation of hydration products. In addition, the small amount of hydration products can increase the density of the pore structure due to the low initial porosity of the hardened paste at low w/b ratio [31]. Moreover, the content of pores that are larger than 100 nm clearly decreases in samples CTP20 and CTP50. The
Fig. 12. Pore structure of hardened paste containing coarse iron tailing powder at w/b ratio of 0.3. (a) Differential pore volume; (b) Cumulative pore volume.
reaction degree of iron tailing powder is extremely low, the nonevaporable water content at 3 days is low (Fig. 9). 3.3. Pore structure of hardened paste containing iron tailing powder The pore structures of hardened paste containing coarse iron tailing powder and that containing fine iron tailing powder at w/b ratio of 0.4 are shown in Figs. 10 and 11, respectively. The pore diameter related to the maximum peak of the differential pore volume curve for the hardened paste is the most probable pore diameter or critical diameter [30]. Fig. 10(a) shows that increasing the coarse iron tailing powder content increases the pore diameter related to the maximum peak. It indicates that coarse iron tailing powder has a negative impact on the pore structure of hardened paste. Incorporating coarse iron tailing powder decreases the content of pores that are smaller than 100 nm, but obviously increases the content of large pores, especially for sample CTP50 (Fig. 10(b)). Because the coarse iron tailing powder has smaller particle size than cement (Fig. 3), it can fill the pores of the hardened paste, making the pore structure finer. However, at w/b ratio of 0.4, the filler effect and the microaggregate effect of the coarse iron tailing powder cannot compensate for the decreased cement content. A small quantity of hydration products is generated in the hardened paste (Fig. 9(a)). Thus, coarser pore structures are observed in samples CTP20 and CTP50. Compared to the pore structures of samples CTP20 and CTP50, the pore structures of samples FTP20 and FTP50 are much finer (Fig. 11). The pore diameter related to the maximum peak of sample FTP20 is smaller than that of sample Cem. The pore diameter related to the maximum peak of sample FTP50 is almost identical to that of sample Cem
Fig. 13. Pore structure of hardened paste containing fine iron tailing powder at w/b ratio of 0.3. (a) Differential pore volume; (b) Cumulative pore volume.
F. Han et al. / Powder Technology 315 (2017) 322–331
cumulative pore volume of sample CTP20 is much smaller than that of sample Cem. Sample CTP50 has an increased content of pores that are smaller than 50 nm (Fig. 12(b)), which has little influence on the strength and permeability of the concrete [32]. As shown in Fig. 13, at low w/b ratio, the pore structure of hardened paste containing fine iron tailing powder becomes finer. For samples FTP20 and FTP50, the pore diameters related to the maximum peaks are much smaller than that of sample Cem, and the cumulative pore volumes decrease further, especially for the content of pores that are larger than 50 nm. The early-age hydration of the binder at low w/b ratio is significantly improved by fine iron tailing powder (Fig. 8 and Table 4). The ratios of non-evaporable water contents of samples FTP20 and FTP50 at w/b ratio of 0.3 to those at w/b ratio of 0.4 are 98% and 97%, respectively. These results indicate that decreasing the w/b ratio almost does not decrease the non-evaporable water content. Moreover, the initial porosity of the paste at low w/b ratio is lower than that at high w/b ratio.
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Therefore, samples FTP20 and FTP50 have denser structures at low w/ b ratio. Decreasing the w/b ratio has a greater effect on the pore structure than on the hydration heat and non-evaporable water content of hardened paste with iron tailing powder (Figs. 7–9), which is mainly attributed to the microaggregate effect of iron tailing powder. 3.4. Morphology of hardened paste containing iron tailing powder The morphologies of hardened pastes containing coarse iron tailing powder at w/b ratio of 0.3 are shown in Fig. 14. Fig. 14(a) shows plenty of hydration products in the paste. The C-S-H gel and ettringite are shown around the unhydrated particles and tend to fill the pores. After 3 days of hydration, the microstructure of the paste is not sufficiently dense, and the bond between the hydrates is not tight. Some pores can be clearly seen. The coarse iron tailing powder is finer than cement (Fig. 3), and the finer particles are distributed uniformly in the
Fig. 14. Morphologies of hardened pastes containing coarse iron tailing powder at w/b ratio of 0.3. (a)–(c) CTP20, and (d)–(e) CTP50.
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which is attributed to the large amount of iron tailing powder in the paste filling the pores and increasing the density of the structure. Some large iron tailing powder particles appear in the microstructure. Fig. 14(e) shows the structure of hardened paste at a high magnification; this figure confirms that the centre particle is iron tailing powder based on the energy spectrum (Fig. 14(f)). A large iron tailing powder particle with smooth surface and clear edge is observed in the paste (Fig. 14(e)). Moreover, the large iron tailing powder particle bonds poorly with its surroundings. The large iron tailing powder particles could be weak load-bearing points in the paste. 3.5. Compressive strength of mortar containing iron tailing powder
Fig. 15. Compressive strength of mortar containing iron tailing powder. (a) at w/b ratio of 0.4; (b) at w/b ratio of 0.3.
paste and can be well graded with cement particles. Sample CTP20 contains only 20% coarse iron tailing powder, resulting in a very small amount of iron tailing powder in the hardened paste. Fig. 14(b) shows the structure of the paste at a high magnification. The energy spectrum of the particle in the centre position of Fig. 14(b) is given in Fig. 14(c). Fig. 14(c) confirms that this particle is iron tailing powder due to the high Si content and low Ca content. The iron tailing powder particle is surrounded by hydration products, but its partial surface is very smooth; this finding further confirms that the reaction degree of iron tailing powder is extremely low at early age. These results are in agreement with the hydration heat of the iron tailing powder (Fig. 4). As shown in Fig. 14(d), the sheet Ca(OH)2, needle ettringite and C-SH gel are observed in the sample CTP50. Compared to sample CTP20, the amount of hydration products in sample CTP50 is small. However, the microstructure of sample CTP50 is as dense as that of sample CTP20, Table 5 Relative compressive strength of mortar containing iron tailing powder. w/b
0.4
0.3
Sample
MC20 MC50 MF20 MF50 MC20 MC50 MF20
Relative compressive strength/%
85.63
54.60
90.80 60.06 90.55
63.74
MF50
106.15 73.19
The compressive strengths of mortars containing iron tailing powder are shown in Fig. 15. At w/b ratio of 0.4, increasing the replacement level of the iron tailing powder decreases the compressive strength of the mortar, while increasing the fineness of the iron tailing powder increases the compressive strength of the mortar at the same replacement level. The trend of the compressive strength is similar to that of the nonevaporable water content (Fig. 9(a)). The amount of hydration products in the mortar containing coarse iron tailing powder is much smaller (Fig. 9(a)). In addition, coarse pore structures are observed in pastes CTP20 and CTP50 (Fig. 10). Thus, the compressive strengths of mortars MC20 and MC50 are clearly lower than that of mortar MCem (Fig. 15(a)). Owing to the small promoting effect of fine iron tailing powder at high w/b ratio, some large pores remain in the paste (Fig. 11), decreasing the strength. As shown in Fig. 15(b), at w/b ratio of 0.3, the compressive strengths of mortars MC20 and MC50 are still lower than that of mortar MCem. As mentioned previously, the pore structures of pastes CTP20 and CTP50 become finer than that of cement paste at low w/b ratio (Fig. 12). However, the pore structure is not the only factor that affects the strength; the cohesion of compositions in the mix is an important factor that affects the strength [33]. At early age, the chemical effect of iron tailing powder is very low (Fig. 4). Iron tailing powder with smooth surface almost acts as inert material in the paste, and its cohesion with the surrounding hydrates is poor (Fig. 14(b) and (e)). This phenomenon is more serious in the paste containing coarse iron tailing powder. Thus, low strengths are obtained for mortars MC20 and MC50. It is noted that the compressive strength of mortar MF20 is higher than that of mortar MCem at w/b ratio of 0.3. The pore structure of paste FTP20 is much finer than that of paste Cem at low w/b ratio (Fig. 13). The fine iron tailing powder decreases the adverse effect of poor cohesion between the iron tailing powder and hydrates on the strength. Moreover, the fine iron tailing powder can fill the interfacial transition zone between sand and hardened paste, and the dense structure of the interfacial transition zone is beneficial to the strength development. Due to its low cement content, mortar MF50 shows low compressive strength at w/b ratio of 0.3. Table 5 gives the relative compressive strength of mortar containing iron tailing powder, which is defined as the ratio of the compressive strength of mortar containing iron tailing powder to that of the Portland cement mortar. It is clear that the relative compressive strength of mortar containing fine iron tailing powder is higher than that of mortar containing coarse iron tailing at the same replacement level. For one sample, the relative compressive strength at w/b ratio of 0.3 is higher than that at w/b ratio of 0.4. These results confirm that fine iron tailing powder or hydration at low w/b ratio can improve the strength development. The relative compressive strengths of all samples are smaller than 100% except that of mortar MF20. However, compared to the strength of the Portland cement mortar, the decreasing ratios of strengths, which are obtained by subtracting the relative compressive strengths from 100%, of mortars MC20, MC50, MF20 and MF50 at w/b ratio of 0.4 are 14.37%, 45.40%, 9.20% and 39.94%, respectively, and at w/b ratio of 0.3, the decreasing ratios become 9.45%, 36.26%, − 6.15% and 26.81%, respectively. The decreasing ratio of strength is much lower than the replacement level of iron tailing powder, especially for
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mortars containing fine iron tailing powder. The best early-age properties of the composite binder are obtained with fine tailing powder and low w/b ratio. The long-term properties of composite binder containing iron tailing powder will be further investigated in future studies. 4. Conclusions (1) The reaction degree of iron tailing powder is extremely low at early age. The hydration heat evolution rate and cumulative hydration heat decrease with increasing the content of coarse iron tailing powder at the two studied w/b ratios. Incorporating fine iron tailing powder increases the early-stage hydration heat evolution rate and cumulative hydration heat. Iron tailing powder significantly promotes the early hydration at low w/b ratio. (2) Increasing the iron tailing powder content decreases the nonevaporable water content of the paste. The non-evaporable water content of paste containing coarse iron tailing powder is lower than that of paste containing fine iron tailing powder at the same replacement level. The increasing ratio of nonevaporable water content of paste is higher at low w/b ratio. (3) The addition of coarse iron tailing powder results in the coarse pore structure of hardened paste at high w/b ratio. Increasing the fineness of the iron tailing powder results in finer pore structure at high w/b ratio, but some large pores remain in the paste. At low w/b ratio, the pore structure of paste containing iron tailing powder is finer than that of cement paste, especially for the paste containing fine iron tailing powder. (4) The iron tailing powder is well graded with cement particles in the paste. Large iron tailing powder particle with smooth surface and clear edge is observed. The large iron tailing powder particle bonds poorly with the surrounding hydration products. (5) The compressive strength of mortar containing iron tailing powder is lower than that of cement mortar at high w/b ratio. At low w/b ratio, the compressive strength of mortar containing 20% of fine iron tailing powder is higher than that of Portland cement mortar. Increasing the fineness of the iron tailing powder or decreasing the w/b ratio leads to higher relative compressive strength of mortar. The decreasing ratio of compressive strength is much lower than the replacement level of the iron tailing powder. Acknowledgement Authors would like to acknowledge the National Natural Science Foundation of China (No. 51578039), China Postdoctoral Science Foundation (Nos. 2015 M580992 and 2016T90036), and Open Fund of State Key Laboratory of High Performance Civil Engineering Materials (No. 2015CEM010). References [1] P.K. Mehta, P.J. Monteiro, Concrete: Microstructure, Properties, and Materials, 3rd Ed, McGraw Hill, New York, 2006. [2] B. Lothenbach, K. Scrivener, R.D. Hooton, Supplementary cementitious materials, Cem. Concr. Res. 41 (2011) 1244–1256. [3] L.H. Buruberri, M.P. Seabra, J.A. Labrincha, Preparation of clinker from paper pulp industry wastes, J. Hazard. Mater. 286 (2015) 252–260.
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[4] J. Mohammadi, W. South, Effect of up to 12% substitution of clinker with limestone on commercial grade concrete containing supplementary cementitious materials, Constr. Build. Mater. 115 (2016) 555–564. [5] N. Palankar, A.U. Ravi Shankar, B.M. Mithun, Durability studies on eco-friendly concrete mixes incorporating steel slag as coarse aggregates, J. Clean. Prod. 129 (2016) 437–448. [6] M.C.G. Juenger, R. Siddique, Recent advances in understanding the role of supplementary cementitious materials in concrete, Cem. Concr. Res. 78 (2015) 71–80. [7] K.J. Osinubi, P. Yohanna, A.O. Eberemu, Cement modification of tropical black clay using iron ore tailings as admixture, Transp. Geotech. 5 (2015) 35–49. [8] Y.F. Li, J.L. Bao, J.R. Zhang, Status analysis of iron tailings comprehensive utilization, China Min. Mag. 24 (2015) 77–81 (in Chinese). [9] S. Zhang, X. Xue, X. Liu, et al., Current situation and comprehensive utilization of iron ore tailing resources, J. Min. Sci. 42 (2006) 403–408. [10] W. Deng, D. Jiang, B. Yang, et al., Comprehensive utilization status and existing problems of iron tailings in China, Mod. Min. 521 (2012) 1–3 (in Chinese). [11] M. Yellishetty, V. Karpe, E.H. Reddy, et al., Reuse of iron ore mineral wastes in civil engineering constructions: a case study, Resour. Conserv. Recycl. 52 (2008) 1283–1289. [12] A.U. Shettima, M.W. Hussin, Y. Ahmad, et al., Evaluation of iron ore tailing as replacement for fine aggregate in concrete, Constr. Build. Mater. 120 (2016) 72–79. [13] S. Zhao, J. Fan, W. Sun, Utilization of iron ore tailings as fine aggregate in ultra-high performance concrete, Constr. Build. Mater. 50 (2014) 540–548. [14] X. Huang, R. Ranade, W. Ni, et al., Development of green engineered cementitious composites using iron ore tailings as aggregates, Constr. Build. Mater. 44 (2013) 757–764. [15] C. Li, H. Sun, Z. Yi, et al., Innovative methodology for comprehensive utilization of iron ore tailings part 2: the residues after iron recovery from iron ore tailings to prepare cementitious material, J. Hazard. Mater. 174 (2010) 78–83. [16] C. Yunhong, H. Fei, L. Wenchuan, et al., Test research on the effects of mechanochemically activated iron tailings on the compressive strength of concrete, Constr. Build. Mater. 118 (2016) 164–170. [17] B. Ma, L. Cai, X. Li, et al., Utilization of iron tailings as substitute in autoclaved aerated concrete: physico-mechanical and microstructure of hydration products, J. Clean. Prod. 127 (2016) 162–171. [18] C. Yang, C. Cui, J. Qin, et al., Characteristics of the fired bricks with low-silicon iron tailings, Constr. Build. Mater. 70 (2014) 36–42. [19] P. Duan, C. Yan, W. Zhou, et al., Fresh properties, compressive strength and microstructure of fly ash geopolymer paste blended with iron ore tailing under thermal cycle, Constr. Build. Mater. 118 (2016) 76–88. [20] P. Duan, C. Yan, W. Zhou, et al., Development of fly ash and iron ore tailing based porous geopolymer for removal of cu(II) from wastewater, Ceram. Int. 42 (2016) 13507–13518. [21] J. Wang, Y. Zhao, P. Xu, Experiment studies and mechanism analyses of Portland cement with iron-tailing as raw materials, Bull. Chin. Ceram. Soc. 6 (1999) 32–37 (in Chinese). [22] F.H. Han, P.Y. Yan, Hydration characteristics of slag-blended cement at different temperatures, J. Sust. Cement-Based Mater. 4 (2015) 34–43. [23] H.F.M. Taylor, Cement Chemistry, second ed. Thomas Telford, London, 1997. [24] Z. Zhang, Q. Wang, H. Chen, Properties of high-volume limestone powder concrete under standard curing and steam-curing conditions, Powder Technol. 301 (2016) 16–25. [25] K.L. Scrivener, P. Juilland, P.J.M. Monteiron, Advances in understanding hydration of Portland cement, Cem. Concr. Res. 78 (2015) 38–56. [26] D.P. Bentz, Influence of water-to-cement ratio on hydration kinetics: simple models based on spatial considerations, Cem. Concr. Res. 36 (2006) 238–244. [27] P.C. Aïtcin, The importance of the water-cement and water-binder ratios, Science and Technology of Concrete Admixtures, first ed. 2016, pp. 3–13. [28] V. Tydlitát, T. Matas, R. Černý, Effect of w/c and temperature on the early-stage hydration heat development in Portland-limestone cement, Constr. Build. Mater. 50 (2014) 140–147. [29] J. Hu, Z. Ge, K. Wang, Influence of cement fineness and water-to-cement ratio on mortar early-age heat of hydration and se times, Constr. Build. Mater. 50 (2014) 657–663. [30] E. Berodier, K. Scrivener, Evolution of pore structure in blended systems, Cem. Concr. Res. 73 (2015) 25–35. [31] W. Qiang, Y. Peiyu, Y. Jianwei, et al., Influence of steel slag onmechanical properties and durability of concrete, Constr. Build. Mater. 47 (2013) 1414–1420. [32] P.K. Mehta, P. Monteiro, Concrete: Structure, Properties and Materials, McGraw-Hill Professional, 2014. [33] P. Yan, G. Mi, Q. Wang, A comparison of early hydration properties of cement-steel slag binder and cement-limestone powder binder, J. Therm. Anal. Calorim. 115 (2014) 193–200002E.
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Powder Technology journal homepage: www.elsevier.com/locate/powtec
Early-age hydration characteristics of composite binder containing iron tailing powder Fanghui Han a,⁎, Li Li b, Shaomin Song b, Juanhong Liu a a b
School of Civil and Resource Engineering, University of Science and Technology Beijing, Beijing, China School of Civil and Transportation Engineering, Beijing University of Civil Engineering and Architecture, Beijing, China
a r t i c l e
i n f o
Article history: Received 10 October 2016 Received in revised form 9 February 2017 Accepted 3 April 2017 Available online 7 April 2017 Keywords: Iron tailing powder Hydration Pore structure Morphology Strength
a b s t r a c t The early-age hydration characteristics of composite binder containing iron tailing powder were investigated by determining the hydration heat, non-evaporable water content, pore structure, and morphology of hardened paste as well as the compressive strength of mortar. The results show that the reaction degree of iron tailing powder is extremely low at early age. Addition of coarse iron tailing powder negatively affects the properties of composite binder paste and mortar. Compared to samples containing coarse iron tailing powder, the samples containing fine iron tailing powder show a large amount of hydration heat, high non-evaporable water content, fine pore structure of hardened paste and high compressive strength of mortar. The iron tailing powder significantly promotes the hydration of binder at low water-to-binder ratio. The iron tailing powder particles are well graded with cement particles in the paste, but the large iron tailing powder particles bond poorly with the surrounding hydrates. The decreasing ratio of compressive strength of mortar is much lower than the replacement level of iron tailing powder, especially for the mortar containing fine iron tailing powder at low water-to-binder ratio. © 2017 Elsevier B.V. All rights reserved.
1. Introduction With the rapid development of the national economy, the construction of infrastructure and buildings has greatly increased in China. The increasing demand for natural energy and resources has produced considerable industrial wastes. To reduce the environmental impact and achieve sustainability, industrial wastes should be used for secondary purposes. Most industrial waste is used in the concrete industry [1], where these wastes can be used to partially replace clinker in blended cement or to partially replace cement or aggregate in concrete [2–5]. Using industrial waste saves energy and resources, reduces the cost of concrete and decreases carbon dioxide emissions. The most commonly used industrial wastes in modern concrete are ground granulated blast furnace slag, fly ash and silica fume. The physical and chemical properties of these materials contribute to the strength development and durability of concrete [6]. However, at present, the quantity of these industrial wastes is limited in China and their prices are increasing rapidly. To ensure the sustainable development of the concrete industry, other industrial wastes must be used in concrete. Iron tailings are mining waste obtained during the beneficiation process to concentrate the iron ore [7]. More than six hundred million tons of iron tailings are generated in China each year. The accumulated ⁎ Corresponding author. E-mail address: [email protected] (F. Han).
http://dx.doi.org/10.1016/j.powtec.2017.04.022 0032-5910/© 2017 Elsevier B.V. All rights reserved.
quantity of iron tailings has reached five billion tons, which accounts for more than 80% of industrial solid waste [8,9]. However, the utilization rate of iron tailings in China is only 7% [10]. Thus, large quantities of iron tailings are piling up, occupying land, and polluting air and water. Therefore, disposing iron tailings using proper methods is crucial. Because iron tailings mainly contain silica, alumina, iron, magnesium, and calcium, iron tailings can be used in the construction industry [11]. Shettima et al. [12] found that iron tailings can be used as fine aggregate replacement in concrete. Adding iron tailings decreased the workability of concrete while increasing the strength and elasticity modulus compared to conventional concrete. Zhao et al. [13] found that the mechanical properties of ultra-high performance concrete were comparable to control concrete when no more than 40% of the fine aggregate was replaced with iron tailings. Huang et al. [14] reported that engineered cementitious composites using iron tailings as the aggregate obtained good tensile and compressive strength and that fine iron tailings resulted in better fiber dispersion. Li et al. [15] found that the mechanical properties of cementitious material prepared by blending 30% clinker, 34% blast furnace slag, 30% iron tailings and 6% gypsum were comparable with those of 42.5 ordinary Portland cement. Yunhong et al. [16] reported that mechanochemically activated iron tailings have pozzolanic characteristics and can be used as supplementary cementitious material in concrete; in ordinary concrete, the maximum replacement level is 30%, but could be up to 40% for concrete at low waterto-binder ratio. Ma et al. [17] noted that substituting 40%–60% of silica
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Table 1 Chemical compositions of cement and iron tailing powder wt%. Composition
SiO2
Cement Iron tailing powder
20.55 4.59 67.29 8.49
Al2O3 Fe2O3 CaO 3.27 8.95
MgO SO3
62.50 2.61 3.63 4.80
Na2Oeq f-CaO LOI
2.93 0.53 0.45 2.90
0.83 –
2.08 2.39
Na2Oeq = Na2O + 0.658K2O.
sand with iron tailings in autoclaved aerated concrete could sufficiently improve the density and compressive strength of the concrete to satisfy the requirements of B05, A2.5 grade in the Chinese National Standard. The C-S-H gel and tobermorite contribute to the compressive strength. Iron tailings were also used for the production of fired brick [18] and in combination with fly ash to produce geopolymers [19,20]. According to the current literatures, iron tailings are usually used as fine aggregate in concrete. There is little information about using iron tailings as supplementary cementitious material in concrete, mainly due to its low activity; the content of iron tailings in clinker is only 5% [21]. In addition, the early-age hydration characteristics of composite binder are very important to understand the hydration mechanism, but there is little deep study on the early-age hydration characteristics. In this paper, iron tailings are ground to decrease their particle size. Then, the iron tailing powder is used partially replace cement in composite binder. The early-age hydration characteristics of the composite binder containing iron tailing powder are investigated. The fineness of the iron tailing powder, the water-to-binder ratio, and the replacement level are all considered. The purpose of this research is to investigate the effect of iron tailing powder on the hydration mechanism of the binder and the possibility of using iron tailing powder as mineral admixture in concrete. 2. Experimental
Fig. 2. Morphology of iron tailing powder.
quartz. Fig. 2 shows the morphology of the iron tailing powder; many fine particles exist after mechanical grinding along with some coarse particles with irregular shapes. The coarse iron tailing powder and fine iron tailing powder are used in this study; these powders were ground in a ball mill for 20 min and 60 min, respectively. The particle size distributions of the raw materials are given in Fig. 3, which clearly shows that the iron tailing powder is finer than Portland cement. The medium particle size (d50) for the cement, coarse iron tailing powder and fine iron tailing powder are 17.17 μm, 7.31 μm and 3.56 μm, respectively. The water requirement ratio of fine iron tailing powder is 104%, which indicates that the water requirement of iron tailing powder is slightly higher than that of Portland cement.
2.1. Raw materials 2.2. Mix proportions P.I 42.5 Portland cement that conformed to Chinese National Standard GB 175-2007 and iron tailings from a mining enterprise in Miyun County of Beijing were used in this study. The chemical compositions of the cement and iron tailing powder determined using X-ray fluorescence (XRF) are shown in Table 1. The main chemical composition of the iron tailing powder is SiO2, followed by Fe2O3 and Al2O3. The iron tailings fall into the category of high-silicon iron tailings. The X-ray diffraction (XRD) pattern of the iron tailing powder is presented in Fig. 1, which shows that the main mineral phase of the iron tailing powder is
Fig. 1. XRD patterns of iron tailing powder.
The mix proportions of pastes and mortars containing iron tailing powder are shown in Tables 2 and 3, respectively. The replacement levels of iron tailing powder were 0, 20% and 50% by mass, and waterto-binder (w/b) ratios of 0.3 and 0.4 were used in this study. The pastes were prepared by mixing binder with water according to Table 2. The mortars were prepared by mixing ISO standard sand, binder and
Fig. 3. Particle size distributions of raw materials.
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Table 2 Mix proportions of pastes. Sample
Cem CTP20 CTP50 FTP20 FTP50
w/b
0.4/0.3
Mass fraction (%) Cement
Coarse iron tailing powder
Fine iron tailing powder
100 80 50 80 50
0 20 50 0 0
0 0 0 20 50
water according to Table 3. The ISO standard sand-to-binder ratio (s/b) is 3:1. Polycarboxylic superplasticizer was used to adjust the fluidity of the pastes and mortars. To determine the hydration heat characteristics of the iron tailing powder, iron tailing powder paste was prepared by mixing fine iron tailing powder with NaOH solution (with a pH value of 13.3) at a ratio of NaOH solution to fine iron tailing powder of 0.4.
Fig. 4. Hydration heat of fine iron tailing powder in NaOH solution of pH 13.3.
2.3. Test methods
3. Results and discussion
The hydration heat evolution rate and cumulative hydration heat of the iron tailing powder and the composite binder with iron tailing powder were measured using a TAM Air isothermal calorimeter at 20 °C over 72 h. After evenly stirring the pastes in the vial, the vial was immediately sealed and placed in the chamber of isothermal calorimeter that was set to a constant temperature of 20 °C. Then, the hydration heat evolution rate and cumulative hydration heat of the pastes were continuously measured as a function of hydration time. The composite binder pastes containing iron tailing powder were placed in plastic tubes with a diameter of 15 mm and a length of 80 mm. Then, the sealed plastic tubes were placed in a standard curing room at 20 ± 1 °C and 95% RH. At the age of 3 days, the pastes were cut into slices and then soaked in ethanol. The ethanol was absorbed into the pores of the pastes, thus replacing the water in the pastes and ceasing further hydration. The differential pore volume and cumulative pore volume of the pastes were measured using mercury intrusion porosimeter (MIP). After the pastes cured for 3 days, the non-evaporable water contents were determined as the mass difference between pastes heated at 80 °C and 1000 °C, and normalized by the mass after heating at 80 °C. The loss on ignition of raw materials was also considered to correct the results [22]. The morphologies of the pastes cured for 3 days were investigated using an FEI Quanta 200FEG scanning electron microscope. Freshly cut sections of the pastes were coated with gold to prevent a charging effect. The mortars were cast into moulds with dimensions of 40 × 40 × 160 mm. Then, all the mortars were placed in a standard curing room (20 ± 1 °C, 95% RH). At the age of 1 day, the mortars were stripped from the moulds. After curing for 3 days, the compressive strengths of the mortars were measured as per Chinese National Standard GB/ T17671-1999.
3.1. Hydration heat of the composite binder containing iron tailing powder The hydration heat of the fine iron tailing powder in NaOH solution are shown in Fig. 4. The pH value of the cement pore solution was
Table 3 Mix proportions of mortars. Sample
MCem MC20 MC50 MF20 MF50
w/b
0.4/0.3
s/b
3
Mass fraction (%) Cement
Coarse iron tailing powder
Fine iron tailing powder
100 80 50 80 50
0 20 50 0 0
0 0 0 20 50
Fig. 5. Hydration heat of composite binder containing coarse iron tailing powder at w/b ratio of 0.4. (a) Hydration heat evolution rate; (b) Cumulative hydration heat.
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The addition of coarse iron tailing powder reduces the cement content in the composite binder. Furthermore, as discussed above, the hydration rate of coarse iron tailing powder is very low at early age. Thus, the overall hydration rate of the composite binder is low, and less amount of hydration heat is generated. The ratios of the second exothermic peak values of samples CTP20 and CTP50 to that of sample Cem are 84.52% and 63.20%, respectively. It is clear that the decreasing ratios of the exothermic rates are 15.48% and 36.80%, which are lower than the replacement ratio of the coarse iron tailing powder, especially for the composite binder containing a large amount of coarse iron tailing powder. Similar results are observed for the cumulative hydration heat (Fig. 5(b)). The dilution effect of the iron tailing powder increases the effective water-to-cement ratio, further promoting the hydration of cement in the composite binder. Moreover, the surface of the iron tailing powder acts as nucleation site for hydrates of cement, further increases the hydration degree of the cement [25]. As shown in Fig. 6, the hydration heat evolution rate of sample FTP20 is approximately identical to that of sample Cem during the acceleration period. Although the hydration heat evolution rate of sample FTP50 is still lower than that of sample Cem, the difference between the two clearly smaller compared to sample CTP50 (Fig. 5(a)). This result is due to the high specific surface area of the fine iron tailing powder, which provides more additional nucleation sites. For sample FTP20, the filler effect of the fine iron tailing powder can compensate for the decreased cement content at early stage of hydration. Adding 20% fine iron tailing powder has little effect on the cumulative hydration heat
Fig. 6. Hydration heat of composite binder containing fine iron tailing powder at w/b ratio of 0.4. (a) Hydration heat evolution rate; (b) Cumulative hydration heat.
greater than 13.0 after a short hydration time [23]. The NaOH solution with a pH value of 13.3 simulates the hydration environment of iron tailing powder in the composite binder paste. Fig. 4 shows that there is only one exothermic peak on the hydration heat evolution rate curve. It is due to the heat of immersion of the fine iron tailing powder. The value of exothermal peak is approximately 0.8 J∙g−1 h−1. After this point, the hydration heat evolution rate rapidly decreases. The cumulative hydration heat within 72 h is only 2.69 J∙g−1. The hydration of the fine iron tailing powder in the NaOH solution releases a small amount of heat, indicating that the reaction degree of fine iron tailing powder in the composite binder paste is extremely low at early age. So the hydration of iron tailing powder makes less contribution to the total hydration heat of composite binder. The activity of supplementary cementitious material can be significantly improved after mechanical activation due to the increased specific surface area and altered internal structure [24]. Fig. 4 shows that the reaction degree of the fine iron tailing powder is very small, which indicates that the coarse iron tailing powder also presents extremely low reaction degree in the NaOH solution. Therefore, at early age, the chemical effect of the iron tailing powder is small, while the physical effect plays an important role in the hydration process of the composite binder. Figs. 5 and 6 present the hydration heat of the composite binder containing coarse iron tailing power and that containing fine iron tailing powder at w/b ratio of 0.4, respectively. As shown in Fig. 5, the hydration heat evolution rate and cumulative hydration heat of the composite binder decrease with increasing the coarse iron tailing powder content.
Fig. 7. Hydration heat of composite binder containing coarse iron tailing powder at w/b ratio of 0.3. (a) Hydration heat evolution rate; (b) Cumulative hydration heat.
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within 20 h (Fig. 6(b)). However, for sample FTP50, the cement content is significantly decreased due to the high replacement level. Therefore, low cumulative hydration heat is shown for sample FTP50 (Fig. 6(b)). Figs. 7 and 8 show the hydration heat of the composite binder containing coarse iron tailing powder and that containing fine iron tailing powder at w/b ratio of 0.3, respectively. Fig. 7 shows that the hydration heat evolution rate and cumulative hydration heat of the composite binder containing coarse iron tailing powder at w/b ratio of 0.3 present trends similar to those at w/b ratio of 0.4 (Fig. 5). However, decreasing the w/b ratio from 0.4 to 0.3 greatly promotes the hydration of composite binder containing fine iron tailing powder (Figs. 6 and 8). The hydration heat evolution rate of sample FTP20 is clearly higher than that of sample Cem during the acceleration period. The hydration heat evolution rate of sample FTP50 is approximately the same as that of sample Cem during the initial acceleration period (Fig. 8(a)). The cumulative hydration heat of samples FTP20 and FTP50 clearly increase at early stage of hydration (Fig. 8(b)). The fine iron tailing powder greatly promotes the hydration of composite binder at low w/b ratio. Other researchers also found that supplementary cementitious materials have a greater effect on cement hydration at lower w/b ratio [26–28]. Although the water requirement of fine iron tailing powder is greater than that of Portland cement, the negative effect of water requirement of fine iron tailing powder on hydration is smaller than the promoting effect of iron tailing powder on hydration, especially at low water-tobinder ratio.
At high w/b ratio, the amount of water provided for hydration is sufficient, so the dilution effect of the iron tailing powder is insignificant. In addition, the nucleating effect of coarse iron tailing powder is smaller than that of fine iron tailing powder. Thus, the hydration heat evolution rate and cumulative hydration heat of samples CTP20 and CTP50 are much lower than those of sample Cem (Fig. 5). The increased waterto-cement ratio at low w/b ratio significantly promotes the hydration of binder due to the marginal effect, especially for samples FTP20 and FTP50 (Fig. 8). At low w/b ratio, the concentrations of Ca2 + and OH– reach supersaturation in a short time [29]. Then, the hydration products are rapidly generated and more heat is released. For samples CTP20 and CTP50 hydrated at low w/b ratio, the hydration heat evolution rate and cumulative hydration heat are still lower than those of cement due to the small nucleating effect of coarse iron tailing powder (Fig. 7). At later stage of hydration, the hydration heat of the composite binder containing iron tailing powder is lower than that of cement owing to the cement content and the low activity of the iron tailing powder.
Fig. 8. Hydration heat of composite binder containing fine iron tailing powder at w/b ratio of 0.3. (a) Hydration heat evolution rate; (b) Cumulative hydration heat.
Fig. 9. Non-evaporable water content of hardened paste containing iron tailing powder. (a) at w/b ratio of 0.4; (b) at w/b ratio of 0.3.
3.2. Non-evaporable water content of paste containing iron tailing powder Fig. 9 presents the non-evaporable water content of hardened paste containing iron tailing powder. The non-evaporable water content represents not only the amount of hydration products but also the degree
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Table 3 shows the increasing ratio of the non-evaporable water content of hardened paste, η, which is determined using Eq. (1).
Table 4 Increasing ratio of non-evaporable water content of hardened paste. w/b
0.4
Sample
CTP20 CTP50 FTP20 FTP50 CTP20 CTP50 FTP20 FTP50
0.3
Increasing ratio of η/%
7.62
49.34
18
69.6
9.02
53.68
327
20.73
η ¼ wntp −ð1−f Þ∙ wncem =½ð1−f Þ∙ wncem 100%
ð1Þ
71.17
of hydration. Fig. 9(a) clearly shows that the non-evaporable water contents of the pastes containing iron tailing powder are lower than that of cement paste and that increasing the replacement level of the iron tailing powder decreases the non-evaporable water content. As explained above, the reduction of cement content is significant in samples CTP20 and CTP50 at high w/b ratio, which produces small amount of hydrates. The non-evaporable water contents of samples FTP20 and FTP50 are higher than those of samples CTP20 and CTP50, respectively. The fine iron tailing powder promotes the early-age hydration of the composite binder (Fig. 6), obtaining a greater quantity of hydrates. The hydration heat evolution rate of sample FTP20 clearly decreases after 20 h and its 72 h hydration heat is lower than that of cement (Fig. 6). The heat is generated by the hydration of cement due to the almost no reaction of fine iron tailing powder at early age (Fig. 4). Thus, the quantity of hydrates in sample FTP20 is lower than that in sample Cem. Similar trends are shown for the non-evaporable water contents of hardened pastes at w/b ratio of 0.3 (Fig. 9(b)). Owing to the significant promoting effect of the fine iron tailing powder on hydration at low w/b ratio, the nonevaporable water content of sample FTP20 is close to that of sample Cem.
Fig. 10. Pore structure of hardened paste containing coarse iron tailing powder at w/b ratio of 0.4. (a) Differential pore volume; (b) Cumulative pore volume.
where f is the replacement level of the iron tailing powder, wntp is the non-evaporable water content of paste containing iron tailing powder when the replacement level is f, and wncem is the non-evaporable water content of the cement paste. As shown in Table 4, positive increasing ratios of the non-evaporable water contents are obtained for all composite binder pastes. The pastes containing large amounts of iron tailing powder have higher increasing ratios, which prove that adding iron tailing powder enhances the hydration of cement. This effect increases with increasing the content of iron tailing powder, which is attributed to the enhanced filler effect of the iron tailing powder. The increasing ratio of the non-evaporable water content of paste containing fine iron tailing powder is higher than that of paste containing coarse iron tailing powder at the same replacement level. This result further confirms that fine iron tailing powder promotes the early-age hydration more than coarse iron tailing powder. For the same sample, the increasing ratio of the non-evaporable water content at w/b ratio of 0.3 is higher than that at w/b ratio of 0.4. Therefore, decreasing w/b ratio increases the promoting effect of the iron tailing powder on the early-age hydration. These results are in agreement with the above findings of the hydration heat (Figs. 5–8). Because the total amount of cement is low in the composite binder paste and the
Fig. 11. Pore structure of hardened paste containing fine iron tailing powder at w/b ratio of 0.4. (a) Differential pore volume; (b) Cumulative pore volume.
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(Fig. 11(a)). Therefore, the negative impact of the coarse iron tailing powder on the pore structure can be reduced by decreasing the particle size of the iron tailing powder. Although fine iron tailing powder shows very low hydration activity at early age (Fig. 4), but its filler effect significantly promotes the hydration of the composite binder (Fig. 6). Thus, a greater number of hydration products are generated, increasing the density of the paste. Moreover, the microaggregate effect of the fine iron tailing powder cannot be neglected due to the small particle size (Fig. 3). These effects of the fine iron tailing powder increase the content of pores that are smaller than 100 nm in the hardened paste. Adding fine iron tailing powder also induces large pores, which indicates that at high w/b ratio, decreasing the particle size of iron tailing powder cannot completely compensate for the negative impact of iron tailing powder on the pore structure of hardened paste. The pore structures of hardened paste containing coarse iron tailing powder and that containing fine iron tailing powder at w/b ratio of 0.3 are given in Figs. 12 and 13, respectively. Fig. 12(a) shows that the pore diameters related to the maximum peaks of samples CTP20 and CTP50 are smaller than that of sample Cem. It indicates that decreasing the w/b ratio results in the pore structure of hardened paste with coarse iron tailing powder finer than that of cement paste. At low w/b ratio, as explained above, the dilution effect of the iron tailing powder is significant. The increased ion concentration of the pore solution at low w/b ratio also promotes the nucleation of hydration products. In addition, the small amount of hydration products can increase the density of the pore structure due to the low initial porosity of the hardened paste at low w/b ratio [31]. Moreover, the content of pores that are larger than 100 nm clearly decreases in samples CTP20 and CTP50. The
Fig. 12. Pore structure of hardened paste containing coarse iron tailing powder at w/b ratio of 0.3. (a) Differential pore volume; (b) Cumulative pore volume.
reaction degree of iron tailing powder is extremely low, the nonevaporable water content at 3 days is low (Fig. 9). 3.3. Pore structure of hardened paste containing iron tailing powder The pore structures of hardened paste containing coarse iron tailing powder and that containing fine iron tailing powder at w/b ratio of 0.4 are shown in Figs. 10 and 11, respectively. The pore diameter related to the maximum peak of the differential pore volume curve for the hardened paste is the most probable pore diameter or critical diameter [30]. Fig. 10(a) shows that increasing the coarse iron tailing powder content increases the pore diameter related to the maximum peak. It indicates that coarse iron tailing powder has a negative impact on the pore structure of hardened paste. Incorporating coarse iron tailing powder decreases the content of pores that are smaller than 100 nm, but obviously increases the content of large pores, especially for sample CTP50 (Fig. 10(b)). Because the coarse iron tailing powder has smaller particle size than cement (Fig. 3), it can fill the pores of the hardened paste, making the pore structure finer. However, at w/b ratio of 0.4, the filler effect and the microaggregate effect of the coarse iron tailing powder cannot compensate for the decreased cement content. A small quantity of hydration products is generated in the hardened paste (Fig. 9(a)). Thus, coarser pore structures are observed in samples CTP20 and CTP50. Compared to the pore structures of samples CTP20 and CTP50, the pore structures of samples FTP20 and FTP50 are much finer (Fig. 11). The pore diameter related to the maximum peak of sample FTP20 is smaller than that of sample Cem. The pore diameter related to the maximum peak of sample FTP50 is almost identical to that of sample Cem
Fig. 13. Pore structure of hardened paste containing fine iron tailing powder at w/b ratio of 0.3. (a) Differential pore volume; (b) Cumulative pore volume.
F. Han et al. / Powder Technology 315 (2017) 322–331
cumulative pore volume of sample CTP20 is much smaller than that of sample Cem. Sample CTP50 has an increased content of pores that are smaller than 50 nm (Fig. 12(b)), which has little influence on the strength and permeability of the concrete [32]. As shown in Fig. 13, at low w/b ratio, the pore structure of hardened paste containing fine iron tailing powder becomes finer. For samples FTP20 and FTP50, the pore diameters related to the maximum peaks are much smaller than that of sample Cem, and the cumulative pore volumes decrease further, especially for the content of pores that are larger than 50 nm. The early-age hydration of the binder at low w/b ratio is significantly improved by fine iron tailing powder (Fig. 8 and Table 4). The ratios of non-evaporable water contents of samples FTP20 and FTP50 at w/b ratio of 0.3 to those at w/b ratio of 0.4 are 98% and 97%, respectively. These results indicate that decreasing the w/b ratio almost does not decrease the non-evaporable water content. Moreover, the initial porosity of the paste at low w/b ratio is lower than that at high w/b ratio.
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Therefore, samples FTP20 and FTP50 have denser structures at low w/ b ratio. Decreasing the w/b ratio has a greater effect on the pore structure than on the hydration heat and non-evaporable water content of hardened paste with iron tailing powder (Figs. 7–9), which is mainly attributed to the microaggregate effect of iron tailing powder. 3.4. Morphology of hardened paste containing iron tailing powder The morphologies of hardened pastes containing coarse iron tailing powder at w/b ratio of 0.3 are shown in Fig. 14. Fig. 14(a) shows plenty of hydration products in the paste. The C-S-H gel and ettringite are shown around the unhydrated particles and tend to fill the pores. After 3 days of hydration, the microstructure of the paste is not sufficiently dense, and the bond between the hydrates is not tight. Some pores can be clearly seen. The coarse iron tailing powder is finer than cement (Fig. 3), and the finer particles are distributed uniformly in the
Fig. 14. Morphologies of hardened pastes containing coarse iron tailing powder at w/b ratio of 0.3. (a)–(c) CTP20, and (d)–(e) CTP50.
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which is attributed to the large amount of iron tailing powder in the paste filling the pores and increasing the density of the structure. Some large iron tailing powder particles appear in the microstructure. Fig. 14(e) shows the structure of hardened paste at a high magnification; this figure confirms that the centre particle is iron tailing powder based on the energy spectrum (Fig. 14(f)). A large iron tailing powder particle with smooth surface and clear edge is observed in the paste (Fig. 14(e)). Moreover, the large iron tailing powder particle bonds poorly with its surroundings. The large iron tailing powder particles could be weak load-bearing points in the paste. 3.5. Compressive strength of mortar containing iron tailing powder
Fig. 15. Compressive strength of mortar containing iron tailing powder. (a) at w/b ratio of 0.4; (b) at w/b ratio of 0.3.
paste and can be well graded with cement particles. Sample CTP20 contains only 20% coarse iron tailing powder, resulting in a very small amount of iron tailing powder in the hardened paste. Fig. 14(b) shows the structure of the paste at a high magnification. The energy spectrum of the particle in the centre position of Fig. 14(b) is given in Fig. 14(c). Fig. 14(c) confirms that this particle is iron tailing powder due to the high Si content and low Ca content. The iron tailing powder particle is surrounded by hydration products, but its partial surface is very smooth; this finding further confirms that the reaction degree of iron tailing powder is extremely low at early age. These results are in agreement with the hydration heat of the iron tailing powder (Fig. 4). As shown in Fig. 14(d), the sheet Ca(OH)2, needle ettringite and C-SH gel are observed in the sample CTP50. Compared to sample CTP20, the amount of hydration products in sample CTP50 is small. However, the microstructure of sample CTP50 is as dense as that of sample CTP20, Table 5 Relative compressive strength of mortar containing iron tailing powder. w/b
0.4
0.3
Sample
MC20 MC50 MF20 MF50 MC20 MC50 MF20
Relative compressive strength/%
85.63
54.60
90.80 60.06 90.55
63.74
MF50
106.15 73.19
The compressive strengths of mortars containing iron tailing powder are shown in Fig. 15. At w/b ratio of 0.4, increasing the replacement level of the iron tailing powder decreases the compressive strength of the mortar, while increasing the fineness of the iron tailing powder increases the compressive strength of the mortar at the same replacement level. The trend of the compressive strength is similar to that of the nonevaporable water content (Fig. 9(a)). The amount of hydration products in the mortar containing coarse iron tailing powder is much smaller (Fig. 9(a)). In addition, coarse pore structures are observed in pastes CTP20 and CTP50 (Fig. 10). Thus, the compressive strengths of mortars MC20 and MC50 are clearly lower than that of mortar MCem (Fig. 15(a)). Owing to the small promoting effect of fine iron tailing powder at high w/b ratio, some large pores remain in the paste (Fig. 11), decreasing the strength. As shown in Fig. 15(b), at w/b ratio of 0.3, the compressive strengths of mortars MC20 and MC50 are still lower than that of mortar MCem. As mentioned previously, the pore structures of pastes CTP20 and CTP50 become finer than that of cement paste at low w/b ratio (Fig. 12). However, the pore structure is not the only factor that affects the strength; the cohesion of compositions in the mix is an important factor that affects the strength [33]. At early age, the chemical effect of iron tailing powder is very low (Fig. 4). Iron tailing powder with smooth surface almost acts as inert material in the paste, and its cohesion with the surrounding hydrates is poor (Fig. 14(b) and (e)). This phenomenon is more serious in the paste containing coarse iron tailing powder. Thus, low strengths are obtained for mortars MC20 and MC50. It is noted that the compressive strength of mortar MF20 is higher than that of mortar MCem at w/b ratio of 0.3. The pore structure of paste FTP20 is much finer than that of paste Cem at low w/b ratio (Fig. 13). The fine iron tailing powder decreases the adverse effect of poor cohesion between the iron tailing powder and hydrates on the strength. Moreover, the fine iron tailing powder can fill the interfacial transition zone between sand and hardened paste, and the dense structure of the interfacial transition zone is beneficial to the strength development. Due to its low cement content, mortar MF50 shows low compressive strength at w/b ratio of 0.3. Table 5 gives the relative compressive strength of mortar containing iron tailing powder, which is defined as the ratio of the compressive strength of mortar containing iron tailing powder to that of the Portland cement mortar. It is clear that the relative compressive strength of mortar containing fine iron tailing powder is higher than that of mortar containing coarse iron tailing at the same replacement level. For one sample, the relative compressive strength at w/b ratio of 0.3 is higher than that at w/b ratio of 0.4. These results confirm that fine iron tailing powder or hydration at low w/b ratio can improve the strength development. The relative compressive strengths of all samples are smaller than 100% except that of mortar MF20. However, compared to the strength of the Portland cement mortar, the decreasing ratios of strengths, which are obtained by subtracting the relative compressive strengths from 100%, of mortars MC20, MC50, MF20 and MF50 at w/b ratio of 0.4 are 14.37%, 45.40%, 9.20% and 39.94%, respectively, and at w/b ratio of 0.3, the decreasing ratios become 9.45%, 36.26%, − 6.15% and 26.81%, respectively. The decreasing ratio of strength is much lower than the replacement level of iron tailing powder, especially for
F. Han et al. / Powder Technology 315 (2017) 322–331
mortars containing fine iron tailing powder. The best early-age properties of the composite binder are obtained with fine tailing powder and low w/b ratio. The long-term properties of composite binder containing iron tailing powder will be further investigated in future studies. 4. Conclusions (1) The reaction degree of iron tailing powder is extremely low at early age. The hydration heat evolution rate and cumulative hydration heat decrease with increasing the content of coarse iron tailing powder at the two studied w/b ratios. Incorporating fine iron tailing powder increases the early-stage hydration heat evolution rate and cumulative hydration heat. Iron tailing powder significantly promotes the early hydration at low w/b ratio. (2) Increasing the iron tailing powder content decreases the nonevaporable water content of the paste. The non-evaporable water content of paste containing coarse iron tailing powder is lower than that of paste containing fine iron tailing powder at the same replacement level. The increasing ratio of nonevaporable water content of paste is higher at low w/b ratio. (3) The addition of coarse iron tailing powder results in the coarse pore structure of hardened paste at high w/b ratio. Increasing the fineness of the iron tailing powder results in finer pore structure at high w/b ratio, but some large pores remain in the paste. At low w/b ratio, the pore structure of paste containing iron tailing powder is finer than that of cement paste, especially for the paste containing fine iron tailing powder. (4) The iron tailing powder is well graded with cement particles in the paste. Large iron tailing powder particle with smooth surface and clear edge is observed. The large iron tailing powder particle bonds poorly with the surrounding hydration products. (5) The compressive strength of mortar containing iron tailing powder is lower than that of cement mortar at high w/b ratio. At low w/b ratio, the compressive strength of mortar containing 20% of fine iron tailing powder is higher than that of Portland cement mortar. Increasing the fineness of the iron tailing powder or decreasing the w/b ratio leads to higher relative compressive strength of mortar. The decreasing ratio of compressive strength is much lower than the replacement level of the iron tailing powder. Acknowledgement Authors would like to acknowledge the National Natural Science Foundation of China (No. 51578039), China Postdoctoral Science Foundation (Nos. 2015 M580992 and 2016T90036), and Open Fund of State Key Laboratory of High Performance Civil Engineering Materials (No. 2015CEM010). References [1] P.K. Mehta, P.J. Monteiro, Concrete: Microstructure, Properties, and Materials, 3rd Ed, McGraw Hill, New York, 2006. [2] B. Lothenbach, K. Scrivener, R.D. Hooton, Supplementary cementitious materials, Cem. Concr. Res. 41 (2011) 1244–1256. [3] L.H. Buruberri, M.P. Seabra, J.A. Labrincha, Preparation of clinker from paper pulp industry wastes, J. Hazard. Mater. 286 (2015) 252–260.
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