Study on Solidification Mechanism of Chloride Salt in Base Course. Material of Cement-Fly-ash-Flushed-by-Seawater

Transcription

1 Advanced Materials Research Online: ISSN: , Vols , pp doi: / Trans Tech Publications, Switzerland Study on Solidification Mechanism of Chloride Salt in Base Course Material of Cement-Fly-ash-Flushed-by-Seawater Longsheng Bao a, Xiaofang Zhang b *, Ling Yu c, Guangshan Zhu d School of Civil Engineering, Shenyang Jianzhu University, Shenyang, Liaoning, , China a baolongsheng710605@163.com, b xiaofangzhang111@163.com, c yulingsy@163.com, d cegszhu@sjzu.edu.cn * Longsheng Bao(1971-), male, associate professor, School of Civil Engineering,Shenyang Jianzhu University; mainly interested in teaching and research in bridge engineering. baolongsheng710605@163.com. Key words: fly-ash-flushed-by-seawater; chloride; Friedel salt; Cement gel Abstract: Through analyzing the influence of different slat content on the microstructure of cement and fly-ash-flushed-by-seawater binder, the solidification mechanism of salt added cement and fly-ash-flushed-by-seawater binder is investigated. The Scanning Electron Microscope test, X-Ray diffraction and theoretical analysis method are adopted to study the performance and the microstructure of cement and fly-ash-flushed-by-seawater, and to analyze the solidification mechanism of chloride in the mixture. When content of the chloride ions is added to the cement and fly-ash-flushed-by-seawater binder, a new kind of crystal-friedel can be generated in the age of 7d and 28d. According to the unconfined strength test on the specimens which contain 0.5% chloride ions, the strength is high in 7d, highest in 28d. The chloride ions of the fly-ash-flushed-by-seawater can be solidified in the cement and fly-ash-flushed-by-seawater binder, which can increase the strength of the binder. Introduction At present, there is a peak period of building seaside roads in some coastal cities, and the fly-ash as one of base materials reduces day by day. Through the investigation, a lot of low activity fly-ash soaking by seawater is hoarded in some coastal industry clusters (like Liaoning Yingkou Bayuquan Power Plant). In this case, how to solve the shortage of fly-ash by replacing fly-ash with fly-ash-flushed-by-seawater in road construction with a scientific and reasonable method becomes the key problem. There is a large amount of chlorine ions in fly-ash-flushed-by-seawater. The earlier study [2-5] shows that free chlorine ions in the mixture of cement with fly-ash-flushed-by-seawater greatly influence the road performance of base material; the chlorine ions solidified is relative stable, which nearly have no influence on road performance. Study on application of fly-ash-flushed-by-seawater to the construction of base course is to solidify the free chlorine ions to form the stable crystal so that the road performance of base material can satisfy the request of Specifications for Design of Highway Asphalt Pavement JTG D There are rare research reports on the chlorine ion solidification mechanism to the mixture of the cement with fly-ash-flushed-by-seawater. The chlorine ion solidification mechanism is revealed and chlorine ion solidification process is confirmed through the microcosmic-test such as Scanning Electron Microscope (SEM) and X-ray diffraction (XRD), which provide theoretical foundation for applying fly-ash-flushed-by-seawater to the road basic construction successfully. 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, (ID: , Pennsylvania State University, University Park, USA-09/05/16,21:08:12)

2 756 Application of Chemical Engineering Microcosmic tests The component analysis of raw materials The cement adopted in the test is slag portland cement (P.S32.5R) produced by Liaoning Hengwei Cement Corporation (Group). The chemical composition is shown in Table 1 Table 1 The chemical composition of cement [%] W(MgO) W(Al 2 O 3 ) W(SiO 2 ) W(SO 3 ) W(K 2 O) W(Na 2 O) W(CaO) W(Fe 2 O 3 ) W(LOSS) Fly-ash-flushed-by-seawater from the gray pool in Yingkou Bayuquan Power Plant, with a loss on ignition of 3.16%, satisfies the regulation of Specification for Construction and Acceptance of Fly-ash-Lime-Stabilized Materials for Road Base CJJ4-97. Content of chlorine ion in sample fly-ash-flushed-by-seawater is 0.8%. Its chemical composition is shown in Table 2. Table 2 The chemical composition of fly-ash-flushed-by-seawater [%] W(MgO) W(Al 2 O 3 ) W(SiO 2 ) W(CaO) W(Fe 2 O 3 ) W(LOSS) Test method The optimum mass ratio of cement and fly-ash-flushed-by-seawater (hereinafter referred to lime-fly-ash) is 1 2 according to the mix proportion design of them. Under the optimum water content and the maximum dry density, the lime-fly-ash specimen is made as cylinders of 10cm 10cm. The specimen is cured in the standard maintainer for 28 days as temperature is 20±1 and relative humidity is above 90%. Then the hydration is terminated by using anhydrous ethanol. When start the test, the specimen pulverized is analyzed in its microstructure through the microcosmic tests such as Scanning Electron Microscope (SEM) and X-ray diffraction (XRD). Analysis of XRD The analysis on lime-fly-ash specimen with chlorine salt in 28th day by XRD (see fig.1) shows that there is a new product F-salt discovered in lime-fly-ash which is not found in the mixture with no chlorine ion. Fig.1: The XRD spectrum of cement with fly-ash-flushed-by-seawater mixture

3 Advanced Materials Research Vols Analysis of SEM From SEM image of the same specimen, F-salt, a hexagon tabular crystal, is also discovered. See in Fig.2. Fig.2: The SEM spectrum of cement with fly-ash-flushed-by-seawater mixture Results analysis The results of these two microcosmic tests fully prove that partial chloride ions are solidified to form a new indissoluble product during the intensity forming process of cement with fly-ash-flushed-by-seawater mixture. The analysis of solidification Solidification theory After entering into the cement-base material, chlorine ion in cement mixture has two existing forms: the solidified chlorine ion (b-cl - ) which is combined through chemical bonds to form stable salts; the free chlorine ions (f-cl - ) which are dissolved in the pore solution. The solidification of chlorine ions is mainly divided into chemical solidification and physical adsorption. Chemical solidification: As to the mixture of cement with fly-ash-flushed-by-seawater, the main component to solidify chlorine ion is Friedel salt (hereinafter referred to F-salt) that is produced by the reaction of C 3 A with chlorine ion. There are two explanations [2-3] to the formation mechanism of F-salt at present: one explanation is that chlorine ions directly react with C 3 A to produce F-salt. The basic reaction process: first, chlorine ions react with Ca(OH) 2 hydrated by C 3 S to form CaCl 2, then CaCl 2 reacts with unhydrated C 3 A to form F-salt. If the C 3 A content is high in the cement, calcium chloroaluminate with three chlorine ions will be formed at first, then the calcium chloroaluminate with one chlorine ion is formed with the decrease of C 3 A content. The chemical reaction equations are as following:

4 758 Application of Chemical Engineering 2(C 3 S)+6H 2 O C3S2H+3Ca(OH)2. Ca(OH) 2 +2Cl - CaCl 2 +2OH -. C 3 A+3CaCl 2 +10H 2 O 3CaO Al 2 O 3 CaCl 2 10H 2 O (calcium chloroaluminate with one chlorine ion). C 3 A+3CaCl 2 +32H 2 O 3CaO Al 2 O 3 3CaCl 2 32H 2 O (calcium chloroaluminate with three chlorine ions). The other explanation is that anions such as SO 4 - in AFm or Aft will be replaced by chlorine ions to form F-salt. The theory of replacement: in the early stage of hydration when CaSO 4 2H 2 O is relatively abundant, C 3 A in cement will react with CaSO 4 2H 2 O first to form calcium sulphoaluminate with three sulfurs that is also called as ettringite (AFt). In the later stage, the C 6 AS 3 H 32 formed in the early stage will react with residual C 3 A to form calcium sulphoaluminate with one sulfur (AFm) with the decrease of CaSO 4 2H 2 O content. Cl - is combined with SO 4 2- Afm to form Cl-AFm (F-salt). Physical adsorption: Compared with the chemical solidification, the amount of chlorine ions solidified through physical adsorption is relatively small. The author thinks that the mechanism of physical adsorption is based on the theory of diffuse electric double layer and there is an electric double layer close to the solid surface: one is called fixed-layer which is an immobile layer close to the solid surface; the other is called overflowing scattered layer (or spread layer) which is flowable and the surplus ion of different electrical charges are reduced to zero gradually in it. The free chlorine ions are stabilized by the interaction of electric charges through immobile layer or overflowing scattered layer to form a new relatively stable electric double layer so as to physically absorb chlorine ions and reduce the free chlorine ion content in pores. Influence for solidification of fly-ash to chlorine ion The hydration rate of fly-ash is slower than cement, but the structural characteristic determines that the fly ash not only plays a good filling role and the micro-aggregate role in the hydration of cement, but also has different surface absorption and pozzolanic activity. Active SiO 2 of fly ash and Ca(0H) 2 of cement hydrated product conduct secondary reaction of hydration, and there are so many C-S-H gels produced in later stage of hydration which strengthen physical solidification to chlorine ion. The solidification mechanism Through the microcosmic-test of solidified chlorine ion, the hydration process of cement-fly-ash with chlorine ion is as following: (1) In initial stage of cement hydration, Ca(OH) 2 are released, the chlorine ion in fly-ash-flushed-by-seawater is dissolved in the solution containing the cement particle, and the spherical surface of fly-ash-flushed-by-seawater is surrounded by water film (shown in Fig. 3).

5 Advanced Materials Research Vols (2) Chlorine ion and unhydrated C 3 A react and form one stable crystal - F salt (shown in Fig.4). (3) The surface of fly-ash-flushed-by-seawater begins to be corroded when CH crystal enters and pozzolanic reaction begins (shown in Fig. 5). (4) Surface of fly-ash-flushed-by-seawater are corroded further, some Cl - replace SO 4 2- in Aft and produce F-salt, structures of fly-ash-flushed-by-seawater start to be destroyed, and produce large amount of aluminic acid and silicic acid. The later strength of macroscopic display increases (shown in Fig.6). The verification of chlorine ion solidification mechanism by analyzing the influence of lime-fly ash base material road performance The optimum mixture ratio of lime-fly ash macadam is determined through test, the optimum mass ratio of cement, fly-ash-flushed-by-seawater and macadam is In test, using three kinds of fly-ash-flushed-by-seawater with different salt content: one is no extra chlorine ion with the content of chlorine ion is 0.8%;the second one includes 0.5% chlorine ion with the content of chlorine ion is 1.3%; the last one includes 1.0% chlorine ion with the content of chlorine ion is 1.8%. Make 15cm 15cm test specimens by using the three kinds of fly-ash-flushed-by-seawater, and put them in the standard curing box to maintain under the condition of 20±1 and relative humidity more than 90% for 7d and 28d. The curve of unconfined strength is shown in Fig.7.

6 760 Application of Chemical Engineering Fig.7 The variation curve of unconfined strength of lime-fly ash macadam combined by fly-ash-flushed-by-seawater with different salt content It can be seen from the macroscopic results: the strength of fly-ash-flushed-by-seawater added chloride ion is lower than that without chloride ion in 7th day; in 28th day, the strength of that added 0.5% chloride ion grows quickly, but the strength with 1% chloride ion is lower than the other two groups of specimens. Through the explanation mentioned above, the solidification mechanism of chloride ion can be explained as: in 7 days, partial free chloride ion in binder react directly with the unhydrated C3A and produce the F salt at the initial of cement hydration, then F salt solidify the free chloride ion through its structure ( shown in Fig. 4 ). At this moment, it needs so many unhydrated C3A to produce F salt that the initial hydration rate of cement in the mixture with adding chloride ion is lower than that without chlorine ion. The reduction of cement hydration rate directly influences strength form of specimens. The more the chloride ion mixed in fly-ash-flushed-by-seawater, the more the chloride ion not solidified in specimens and the more the chloride ion dissolve, the looser the specimens and the lower the strength. The above reasons result in the more the chloride ion mixed in water, the lower the strength is. It can be seen in Fig. 7. With the growth of age (e.g. Fig. 5, Fig. 6), CH-crystal will enter into fly-ash-flushed-by-seawater and corrode it. Partial Cl - replaces SO 4 2- in Aft to produce some F salt which is the stable crystal. F salt increases gradually and the free chloride ion reduces unceasingly with chloride ion solidified, it is helpful for the intensity increasing in later period. The effective component SiO 2,Al 2 O 3 in fly-ash-flushed-by-seawater and Ca(OH) 2 produced by the cement hydration take place pozzolanic reaction, which is also a reason for the intensity increasing in later period. But the ability of mixture for solidifying chloride ion is limited, after the responds, there are so many odd chloride ions in specimens becoming loose and also the intensity becomes lower in the time of soaking in the water. Therefore, from macroscopic intensity curve it can be seen that the strength of specimens added 0.5% chloride ion is highest in 28th day. Conclusion (1) Test shows: comparing with the lime-fly ash without salt and with 1% chloride, the strength of those added 0.5% chloride is highest in 7th day and 28th day. (2) The mixture and free chloride ion react and produce plate-type and six side crystal Friedel-salt. There are two ways to form F-salt: one is Cl - and C 3 A react and product directly; another is Cl - replaces SO 4 2- in AFm or AFt to form the Friedel-salt.

7 Advanced Materials Research Vols (3) The microscopic analysis shows: author explains reaction process of chloride ion in mixture with the solidification theory, the chloride ion can be solidified completely and the later intensity grows quickly when the content of chloride ion in fly-ash-flushed-by-seawater is 0.5%. Therefore, fly-ash-flushed-by-seawater with a suitable salt content can be used as the path material. References [1] Kayyali O A.Free and water soluble chloride in concrete[j].cement and Concrete Research, 1995,25(4): [2] Bao Longsheng.The study of the fly-ash-flushed-by-seawater used in the road subbase course [D].Xi an:the Highway Faculty of Chang an University, [3] Bao Longsheng. Research on the application of cement fly-ash-drained-by-seawater stabilized aggregate for roadway basic layer[j]. Journal of Shenyang Jianzhu University: Natural Science,2008,24(4): [4] Bao Longsheng.Study on key techniques index for construction of base course with cement fly-ash-flushed-by-seawater stabilized crushed-stones[j]. Journal of Shenyang Jianzhu University:Natural Science,2008,24(5): [5] Bao Longsheng.Study on the mixture ratio design of construction of subbase with cement fly-ash-flushed-by-seawater stabilized crushed-stone [J]. Journal of Shenyang Jianzhu University:Natural Science,2008,24(6): [6] Bakkar W T.Production of properties of fly ash[c]//utilization of Ash Workgroup. North Dakota:University of North Dakota Grand forks,1987: [7] Brown P W.The distributions of bound sulfates and chlorides in concrete subjected to mixed NaCl,MgSO4,Na2SO4 attack[j]. Cement and Concrete Research, 2000(30) : [8] Joshi R C.Relationship between pozzolanic activity and chemical and physical of selected canadian fly ashes [J].Materials Research Society Symposia Proceedings,1997(86): [9] Hilmi A.Microstructure development of stabilized fly ash as pavement base material [J]. Journal of Materials in Civil Engineering,2000 (2): [10] Xu Y.The influence of sulphates on chloride binding and pore solution chemisrty[j]. Cement and Concrete Research,1997,27(12):

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