Research on the Hydration Mechanism of Portland Cement with Magnesium Slag

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1 Applied Mechanics and Materials Submitted: ISSN: , Vol. 576, pp Accepted: doi:1.428/ Online: Trans Tech Publications, Switzerland Research on the Hydration Mechanism of Portland Cement with Magnesium Slag Junping Deng 1,a, Xiaoliang Wang 1,b,and Yiping Guo 1,c 1 Dept. of Materials Science and engineering, Xi an University of Science and Technology, Xi an, 71, China a jpdeng26@163.com, b kinzer824@sina.com, c ypguo21@163.com Keywords: magnesium slag cement, pozzolanic activity, hydration mechanism. Abstract. Magnesium slag is a kind of industrial waste during the silicothermic process for magnesium reduction which has potential hydration activity. By adding 2% to 5% of magnesium slag into Portland cement, the influence principle of the additive amount of magnesium slag on the pozzolanic activity is investigated through performance testing and hydration products analysis. During the early period of hydration process, the strength of pozzolanic effectiveness ratio weakens with the increasing additive amount of magnesium slag, while during the later period of hydration process with the additive amount of magnesium slag less than 4%, the strength of pozzolanic effectiveness ratio increases with increasing additive amount of magnesium slag. The activity index of magnesium slag increases with the increase of period and additive amount, and the strength brought by hydration of magnesium slag is a key source of the strength improvement of Portland cement with magnesium slag. 1. Introduction Magnesium slag is a kind of industrial waste during the silicothermic process for magnesium reduction. In this process, the dolomite and ferrosilicon of certain proportion are burnt and then grinded. After the grinding produces are pressed into lumps, they will be reduced to magnesium vapor under the condition of 12 and 13Pa, and then become rough magnesium through cooling and crystal. The output of magnesium in China tops the world successively since 1998 with 698, 3t in 212, occupying over 82% of the total output in the world [1]. All of them are produced through silicothermic process. At present, for the advanced enterprises in China, there are 5 to 6t magnesium slag output whenever they produce 1t raw magnesium [2]. It is estimated that there are around 4 million tons of magnesium slag produced all over the country every year, and the cumulated output has exceeded ten million tons. As the magnesium slag is usually stacked in the open air and occupies many lands, bringing about a lot of dust and seriously impacting the environment. It is an urgent need to make research on how to develop and utilize it. As the content and storage status of magnesium slag is similar to cement, great attention has been paid in domestic studies regarding applying magnesium slag in cement. Magnesium slag can replace the limestone as the raw material for cement[3,4], and those with low MgO content can be used as an active admixture[5,6] or other new-type wall material like building brick[7-9]. As the formation of magnesium slag is quite complicated and the content of residual MgO fluctuates greatly, it is indeed necessary to focus on the fundamental studies of the hydration activity of magnesium slag and their efficient utilization. This paper studies the hydration mechanism of Portland cement with magnesium slag. 2. Experimental Raw Material and Treatment 2.1 Magnesium slag. The magnesium slag in the experiment comes from some magnesium metal factory in Shanxi Province. It is gray yellow powder and the main chemical composition includes CaO, SiO 2, Fe 2 O 3, MgO and so on, as shown in Table. 1. After drying and ball-milling in laboratory, the crystalline structure of the samples was investigated by X-ray diffraction (XRD, X` Pert MPD All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (# , Pennsylvania State University, University Park, USA-18/9/16,7:54:43)

2 58 Materials Engineering PRO, Holland) using CuKa radiation (λ=.1546nm). It can be seen from Fig. 1, the main mineral composition is β- C 2 S, γ- C 2 S and small amount of dolomite. β- C 2 S has a certain hydration activity in cement. Intensity Intensity 2θ / o Fig. 1 XRD patterns of magnesium slag 2.2 Cement clinker and dehydrate gypsum. The Portland cement clinker and the dehydrate gypsum of Qinling Cement company are choosed as reference cement and their chemical composition is listed in Table 1. The materials have been ground in the lab as preparation. Table 1 Chemical composition of raw materials/wt% Raw materials SiO 2 Al 2 O 3 CaO Fe 2 O 3 MgO SO 3 CaF 2 K 2 O Na 2 O Loss Magnesium slag Cement clinker / Gypsum / / / 22.5 MgO is the harmful ingredient in magnesium slag. If the content of MgO is too much, it can lead to volume expansion and strength reduction. According to national standard GB/T23933, the magnesium slag is qualified when the content is less than 8%. In this work, the magnesium slag is not harmful to the products because of its content is 6.8%(see Table 1.) Besides, other studies have shown that magnesium slag is not radioactive and can be used as mixed material in cement. 3. Experimental Method 3.1 Experimental formulation and the preparation of magnesium slag cement. The magnesium slag cement samples were prepared by adding 2% to 5% magnesium slag respectively to the Portland reference cement. According to GB/T12957Test Method for Activity of Industrial Waste Slag Used as Addition to Cement, the hydration activities of magnesium slag were tested. The experimental formulations are shown in Table 2. Table 2 Mix proportion of magnesium slag cement /wt% Sample Magnesium slag Cement clinker Gypsum P 95 5 Z Z Z Z

3 Applied Mechanics and Materials Vol Physical performance test of magnesium slag cement. The fineness of magnesium slag cement is conducted based on GB/T Test Method for Fineness of Cement-Sieve. The normal consistency and setting time of magnesium slag cement were tested following GB/T Standard Test Method for Water Requirement of Normal Consistency, Setting Time and Soundness of the Portland cement. The strength of hydraulic cement mortar was tested according to GB/T Test Method for Strength of Hydraulic Cement Mortar. 3.3 Analytical method for the hydration products of magnesium slag cement. The cement pastes were prepared according to normal consistency, and were maintained in defined period in standard curing room, and then the samples were broken. After the hydration process stopped with absolute alcohol, the samples were dried for three hours in oven. At last, the microstructure observations of the samples were performed by scanning electron microscopy (SEM, FEI Quanta2). 4. Conclusion and Discussion 4.1 The impact of magnesium slag on the normal consistency and setting time of cement. As the magnesium slag strong water-absorbing and high fineness of powder, the more the additive amount, the more water is needed for cement normal consistency, but this will not impact the formation of cement mortar. As the additive amount increases, both of the initial and final setting time are prolonged gradually. The more additive amount, the longer the time needed for setting time. The variation trend is illustrated in Table 2. It can be seen that the initial setting time is longer than 45 minutes and the final setting time is less than 1 hours, which confines to the national standard. 4.2 The impact of magnesium slag on the strength of hydraulic cement mortar. The impact of the additive amount of magnesium slag on the compressive strength of cement is illustrated in Fig. 3. With the increase of the additive amount of magnesium slag, the breaking strength and compressive strength of cement decrease gradually. When the amount of magnesium slag is over 4%, the strength of cement decreased rapidly. The impact of the additive amount of magnesium slag is more evident in the early period than later period. When the amount of magnesium slag is 3%, the 28d breaking strength of cement reaches 8.MPa, and the compressive strength reaches 46.8MPa, fulfilling the requirement of national standard (GB/T23933), that is 42.5R[1]. setting time/minute initial final 2% 3% 4% 5% MS content strength/mpa d 28d 2% 3% 4% 5% MS content Fig. 2 Effect of MS content on setting time Fig. 3 Effect of MS content on compressive strength 4.3 Analysis of the pozzolanic activity of magnesium slag. For composite cement mixed with mineral admixtures, the cement is of independent hydraulicity, while the mineral admixtures don t have independent hydraulicity themselves. Its contribution to the strength of cement lies in Ca(OH) 2 brought by the hydration reaction it has with the cement, as well as the pozzolanic reaction it have with the extreme alkaline hydrated silicate. As a result, the strength of cement mixed with mineral admixtures comes from two parts, the contribution of cement hydration and the pozzolanic effect of mineral admixtures. In order to analyze these two parts, the paper references the concept of cement

4 6 Materials Engineering consumption specific strength put forward by Pu Xincheng, that is, the contribution of unit cement consumption to the strength of composite cement [11]. The definition is shown as below: For the composite cement mixed with mineral admixtures, its expression is: f cb=f c/q. (1) For expression (1): f cb is the specific strength of the composite cement mixed with admixtures (MPa); f cb is the absolute intensity of the composite cement mixed with admixtures (MPa); q is the percentage of pure cement among the composite cement. For the reference cement without admixtures, its expression is: f ob=fo/1. (2) For expression (2): f ob is the specific strength of the reference cement (MPa); f o is the absolute intensity of the reference cement (MPa). According to indexes such as the strength of pozzolanic effectiveness ratio f p, pozzolanic effectiveness strength contribution ratio P p, hydration strength contribution ratio P c, activity index A of mineral admixtures, it is easy to analyze the impact of the pozzolanic effectiveness of activity admixtures on cement. Its definition is: fp=fcb-fob. (3) Pp=fp/fcb=(fcb-fob)/fcb. (4) Pc=fob/fcb. (5) A=Pp/qc. (6) For expression (6) q c is the percentage of mineral admixtures among composite cement. According to the strength of pozzolanic effectiveness ratio, the analysis of the strength of pozzolanic effectiveness ratio of Portland cement with magnesium slag can be seen in Table 3 and Table 4: Table 3 Analysis of pozzolanic effective of magnesium slag portland cement for 3d Mixture ratio Index of pozzolanic effect Constitution of strength Sample cement/ % MS/% f cb /MPa f p /MPa P p /% P c /% A Cement/MPa MS/MPa P Z Z Z Z From Table 3, it is easy to see that the 3d compressive strength of Portland cement with magnesium slag weakens with the increase of magnesium slag admixture, and the contribution ratio of pozzolanic effectiveness is negative, illustrating that the magnesium slag is negative to the strength of cement in the early period as it will reduce the strength. When the additive amount of magnesium slag reaches 5%, the contribution ratio comes to the lowest, -26.3%.

5 Applied Mechanics and Materials Vol Table 4 Analysis of pozzolanic effect of magnesium slag portland cement for 28d Sample P Z1 Z2 Z3 Z4 Mixture ratio Cement/ MS/% % Index of pozzolanic effect Constitution of strength fcb/mpa fp/mpa Pp /% Pc /% A Cement/MPa MS/MPa Table 4 shows that the 28d compressive strength of magnesium slag cement will improve with the increase of the additive amount of magnesium slag. Meanwhile, the Pp changes from negative to positive. As the Pp increases with the additive amount of magnesium slag, the pozzolanic effectiveness improve too. Therefore, the pozzolanic effectiveness of magnesium emerges along with the hydration process, and it is estimated the effectiveness will continue to improve during later period. When the additive amount of magnesium slag reaches 5%, the cement strength will jump dramatically. Therefore, it is suggested that the additive amount of magnesium slag should be no more than 4%. 4.4 Analysis of the hydration products of Portland cement with magnesium slag. The main components of magnesium slag is CaO, SiO2, Al2O3 and so on, they are the effective constituent in cement. First of all, Ca(OH)2 and C-S-H gel will appear with the hydration of cement clinker. Meanwhile the β-c2s in magnesium slag will gradually produce Ca(OH)2 during hydration. Under the condition of alkaline medium, Ca(OH)2 will react with the active SiO2 in magnesium slag and generate C-S-H gel. With the progress of hydration, Ca(OH)2 crystal continues to dissolve and C-S-H gel goes on depositing to set and harden the paste, so the macro strength increasing gradually. During the later period of hydration, the needle-shaped C3A 3CaSO4 32H2O crystal staggered together with the stimulation of sulfates. This is the main reason that the cement has quite intensive strength during the later period. The hydration products of magnesium slag cement can be found in Fig. 4 and Fig. 5. According to Fig. 4, after 3d hydration, clumps-shaped C-S-H gel appear. Fig.5 shows that large amounts of C3A 3CaSO4 32H2O structure form after 28d hydration, connecting the hydration products of cement together to make it with strong strength. Fig. 4 SEM photography of hydration for 3d Fig. 5 SEM photography of hydration for 28d 5. Conclusion (1) The magnesium slag exhibits certain hydration activity as the main mineral composition is β-c2s and γ- C2S. When the additive amount of magnesium slag is 3%, the 28d ratio of compressive strength of magnesium slag cement reaches 84% with comparatively high activity. (2) When the additive amount of magnesium slag is less than 4% of cement clinker, 42.5 magnesium slag Portland cement can be prepared by using 52.5 Portland cement. When the additive amount of magnesium slag is 3%, the 28d breaking strength of magnesium slag cement reaches

6 62 Materials Engineering 8.MPa, and the compressive strength reaches 46.8MPa, fulfilling the requirement of national standard (GB/T23933), that is 42.5R. (3) During the early hydration process, the strength of pozzolanic effectiveness ratio weakens with the increasing additive amount of magnesium slag. While during the later hydration process, the strength of pozzolanic effectiveness ratio intensifies with the increasing additive amount of magnesium slag when the additive amount of magnesium slag is less than 4%. (4) The activity index of magnesium slag grows with the increases of magnesium slag cement age and additive amount. When the additive amount is less than 4%, the 28d activity index A>.6, the strength brought by hydration is a main source of the strength improvement of Portland cement with magnesium slag. Acknowledgement This work was financially supported by the Shanxi Natural Science Foundation (211JY7). References [1] D.B. Liu, Q. Sun: China Metal Bulletin. Vol.7 (211), p [2] R.Y. Xu, H.Z. Liu: World Nonferrous Metals. Vol. 1 (26), p [3] C.Y. Huang, J.S. Ke: Cement Guide for New Epoch. Vol. 5 (25), p [4] H.P. Zan, H.J. Zhao, J.R. Gao: Cement. Vol. 7 (27), p [5] C.Y. Huang, J.S. Ke: Cement Technology. Vol. 5 (25), p [6] Q.J. Ding, Y. Li, S.G. Hu: Cement Engineering. Vol.3 (1998), p [7] A.Q. Zhao: ShanXi Construction.Vol. 29 (23), p [8] E.Q. Chen: New Building Materials. Vol. 5 (27), p [9] W.H. Zhao, J. Hu, Y. Zhao: Block-Brick-Tile. Vol.9 (29), p [1] GB/T , Magnesium slag Portland cement national standards [S] [11] X.C. Pu: Concrete. Vol.5 (1998), p