Corrosion Mechanism of Cemented Soil in MgCl2 Solution

8/10/2019 Corrosion Mechanism of Cemented Soil in MgCl2 Solution

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P.-J. HAN et al.: Corrosion Mechanism of Cemented Soil in MgCl2Solution, Kem. Ind. 63 (9-10) 311316 (2014) 311

Introduction

The cemented soil technique is dened as a method of im-proving soil properties by mixing cements with in-situsoils.Cemented soils can be used in structure retaining and seep-age control of small-scale irrigation in hydraulic engineeringby using a form of cushion material on large channels, roadsand compound pile foundation of dam slope and other pro-jects. They can be applied in unpolluted and polluted con-ditions, such as industrial land and seawater. MgCl2 is oneof the commonest causes resulting in pollution of cementedsoils. The corrosive environment may cause the change inmechanical and engineering properties of cemented soils.Much research attention has been focused on the inuenceof corrosive environment on cemented soil. Iorliam1 andMoses2recommended using CKD as a modier for stabiliz-ing BCS with cement or other additives. Koliasstudied theeffectiveness of high calcium y ash and cement in stabiliz -ing ne-grained clayey soils in the laboratory and the resultsobviously presented the technical benets of stabilizing clay-ey soils using y ash and cement.3Ning et al. investigatedthe behaviour of cemented soil under various environmentalconditions and concluded that the environmental corrosionhad little effect on the fracture process in contrast to the ev-ident inuence on mechanical strength.4Xing et al.showed

that Mg2+, Cl and SO42not only caused a change in themicrostructure of salt-rich soil-cements, but also reducedthe strength of the soil-cement composite.57To evaluate thisadverse effect on the behaviour of soil-cement composite,Yanganalysed the factors contributing to cement-based so-lidication/stabilization of the polluted soil and the optimumparameters.8Bai and Dong tested mechanical properties andelectrical resistivity of cement-soil polluted by H2SO4.910Yang et al.studied the strength of soil stabilized by cementin the corrosive environment of seawater and measured thestrength distribution by using micro cone-penetration tests.11

Liuand Qu studied the durability of cemented soil underbrine corrosion.12However, the studies of the inuence ofMgCl2 on properties of cemented soil are limited. To un-derstand MgCl2corrosion, a series of laboratory tests wereconducted in this study on the cemented-soil blocks curedin MgCl2solutions of different concentrations.

ExperimentalPreparations for cemented soil blocks

The air-dried soil with plasticity index (Ip) of 8.1 was takenfrom a building site in Taiyuan, China. The cement used wasordinary Portland cement (OPC), brand 32.5#, produced

in Taiyuan Shitou Cement Co. Ltd. Soil, cement, and wa-ter were mixed in a certain proportion as shown in Table 1by a HJW-30 blender at a rotating speed of 48 rounds perminute. After mixing well, the mixture was put into a steel

* Corresponding author: Assoc. professor Peng-Ju Hane-mail: [email protected]

Corrosion Mechanism of Cemented Soilin MgCl2Solution

Mechanical properties of the cemented soil will be reduced when cemented soils are applied in theMgCl2corrosive environment to conduct seepage control in hydraulic engineering. The corrosiveconditions will inevitably cause serious damage to the cement-soil composite, and further reducestablity of the whole structure. Therefore, this cementation technique could be commonly used tostabilize infrastructures. More engineering practices regarding the reduction in mechanical proper-ties of cemented soil in the MgCl2environment are required. To simulate and study the corrosionprocess, a series of tests including photographing, unconned compression tests, and measuringMg2+and Clconcentrations of solution were conducted on cured cemented soil blocks with dif-ferent concentrations of MgCl2 solutions. Results show that the surface corrosion of the sample

increases while the compression strength decreases with the increase in solution concentrationgiven the same curing time of the concrete block. Chemical analysis of the corrosive environmentindicates that the volumes of new products such as CaCl2 6H2O and Mg2(OH)3Cl4H2O amountto seven times that of Ca(OH)2after reaction. The corrosion of cemented soil is a sort of crystalliz -ing corrosion. The Mg(OH)2takes chemical reactions with 3 CaO2SiO2 3H2O and forms newproducts such as MgOSiO2 H2O, which is a sort of dissolving corrosion. In addition, the authorsanalysed the relationships between the unconned compressive strength of cemented soils curedfor 28 days and the initial concentrations of Mg2+and Cl. Finally, the regression equations of thestrength were established.

Key words: Cemented soil, compressive strength, modied coefcient, corrosion

P.-J. Han,a,b*W.-B. Zhang,aX.-H. Bai,aand T. Tonga

DOI: 10.15255/KUI.2013.025KUI-19/2014

Original scientic paperReceived August 17, 2013

Accepted December 27, 2013

a College of Architecture and Civil Engineering, Taiyuan University of Technology,030 024 Taiyuan, PR China

b College of Materials, Science and Engineering of Taiyuan, University of Technology,030 024 Taiyuan, PR China
mailto:13834569544%40163.com?subject=http://dx.doi.org/10.15255/KUI.2013.025http://dx.doi.org/10.15255/KUI.2013.025mailto:13834569544%40163.com?subject=


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P.-J. HAN et al.: Corrosion Mechanism of Cemented Soil in MgCl2Solution, Kem. Ind. 63 (9-10) 311316 (2014)312

mould to form a cemented soil block of 10 1010 cm3.After 24 hours, the blocks were taken out of the moulds,cured in a standard conservation box for seven days. Thecemented soil blocks were then soaked in MgCl2solutionsof various concentration prepared in advance. The mainchemical composition of the cemented soil is given in Table2.

T a b l e 1 General composition of cemented soilT a b l i c a 1 Opi sastav cementiranog tla

m(air-dried soil) gm(tlo osueno na zraku) g

m(cement) gm(cement) g

m(tap water) gm(vodovodna

voda) g

100 15 50

Preparation of MgCl2solution

The concentrations of the MgCl2 solution used in this experi-ment were selected as 1.5 gl1, 4.5 gl1, 9.0 gl1, 18.0 gl1,and 22.5 gl1respectively, based on the Code for Investi-

gation of Geotechnical Engineering (GB 500212001, 2009)and the Code for Anticorrosion Design of Industrial Construc-tions (GB 500462008, 2008).13,14For MgCl2 solution, theMg2+and Clconcentrations were measured and the corro-sion degrees evaluated (Table 3) according to the Code forInvestigation of Geotechnical Engineering. The MgCl2 solu-tions were graded as B.

Testing procedure

In order to simulate the corrosion process and study thecorrosion mechanism, the following test procedures wereperformed:

Step 1: Preparing the 72 blocks following the requirementsgiven above.

Step 2: Soaking the blocks in MgCl2solutions of concentra-tions 0, 1.5, 4.5, 9.0, 18.0 and 22.5 gl1, respec-tively.

Step 3: To observe the changes in appearance during cor-

rosion, photographing the blocks after soaking inMgCl2 solutions for 3, 7, 14 and 28 days.

Step 4: Carrying out unconned compression tests on theblocks soaked for 3, 7, 14, and 28 days, respectively.The unconned compression strength was measuredby universal testing machine (Model-5105A) withmaximum load capacity of 100 kN, and loading rate5 mm per minute. At each curing time, three blockswere tested in parallel with the one concentrationof the solution. The average value of the three testresults is used as the block strength.

Step 5: Measuring of Mg2+

and Cl

solution concentrationsimmediately after the blocks were taken out.

Results and discussionVisual observations

Figs. 1 and 2 shows the photos of the cemented soil blocksafter soaking for 28 days in MgCl2solutions of various con-centrations. It can be seen that the crystallization is found onthe surface of the cement-soil block in the MgCl2condition.The inuence of MgCl2on the blocks is more obvious withthe increase in the concentration of MgCl2and curing time.

T a b l e 3 Chemical ingredients of solutions and corrosion evaluationT a b l i c a 3 Kemijski sastav otopina i ocjena korozije

mg l1Mg2+ Cl

mg l1 Corrosion evaluationOcjena korozije mg l1 Corrosion evaluation

Ocjena korozije

tap watervodovodna voda 19.37

none

43.06none

1 500 93.90weakslabo 227.10

none

4 500 278.42weakslabo 584.21

none

9 000 317.81weakslabo 795.77

none

18 000 402.40 weakslabo 1100.84 none

22 500 471.60weakslabo 1881.69

none

T a b l e 2 Main chemical composition of cemented soilT a b l i c a 2 Osnovni kemijski sastav cementiranog tla

pHw(Ca2+)

mg kg1

w(Mg2+)mg kg1

w(SO42)mg kg1

w(NH4+)mg kg1

w(Cl)mg kg1

w(CO32)mg kg1

w(OH)mg kg1

9.80 427.84 354.17 2879.10 1.29 282.32 1090.8 496.64


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0 gl1 1.5 gl1

9.0 gl1 22.5 gl1

F i g. 2 Photos of the cemented soil blocks surfaces aftersoaking in MgCl2solution of different concentrationsfor 28 days

S l i k a 2 Fotograje povrina uzoraka cementiranog tla nakon28 dana moenja u otopinama MgCl2razliitih kon-centracija

Unconned compressive strength testThe unconned compressive strengths of the cemented soilblocks are summarized in Table 4. The compressive strengthof cemented soil decreases with the increase in corrosivesolution concentration during the same corrosion time, andthe corrosion degree increases with the corrosion time (Fig.3). The cemented soil modied coefcient, , is dened asfollows:

fcu'= fcu , (1)

where fcu' is compressive strength of the block cured inMgCl2solution; is a modied coefcient expressing degreeof blocks corrosion; fcuis compressive strength of the blockcured in pure water.

Table 4 presents the corresponding to different corrosiontimes and concentrations. All coefcients are greater than 1in MgCl2 solution with concentration of 1.5 gl1. This result

implies that the low concentration of MgCl2 may be bene-cial to improving strength of cemented soil. In addition, thestrength modied coefcients in other environments in thisstudy are less than 1, indicating that MgCl2 solutions can in-

duce obvious corrosion and greatly reduce the strength of ce-mented soils when its concentration is greater than 1.5 gl1.Therefore, to ensure safety, optimize design, cost-efciencyand suitable measurement, the environmental condition ofthe in-service cemented soils should be taken into accountin the design and calculation of the bearing capacity and thesettlement of a foundation.

F i g. 3 Relationship between the compressive strength ofsoils and time

S l i k a 4 Veza izmeu tlane vrstoe tla i vremena

T a b l e 4 Compressive strength modied coefcients of ce-

mented soil in MgCl2solutionT a b l i c a 4 Koecijent promjene tlane vrstoe cementira-

nog tla u otopini MgCl2

Corrosion timeTrajanje korozije

(MgCl2) gl13 days3 dana

7 days7 dana

14 days14 dana

28 days28 dana

0 1.000 1.000 1.000 1.000

1.5 1.029 1.060 1.089 1.089

4.5 1.039 1.000 0.991 0.981

9.0 1.010 0.962 0.920 0.88018.0 0.942 0.819 0.799 0.768

22.5 0.922 0.742 0.732 0.718

(a) 1.5 gl1 (b) 4.5 gl1 (c) 9.0 gl1 (d) 18.0 gl1 (e) 22.5 gl1

F i g. 1 Photos of the cemented soil blocks soaked in MgCl2solution of different concentrations for 28 daysS l i k a 1 Fotograje uzoraka cementiranog tla moenih 28 dana u otopinama MgCl2razliitih koncentracija


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Changes of Mg2+ and Cl concentration with corrosiontime and regression analysis

Figs. 4 and 5 shows the relationships between the Mg2+orClconcentration and the corrosion time with various MgCl2concentrations. The concentration of Mg2+in the solutionsincreases with curing time. The curve of Mg2+concentrationversus corrosion time can be divided into two parts: when

the corrosion time is less than seven days, the Mg2+

con-centration in the solutions rapidly increases and then tendsto stabilize (Fig. 4). The Mg2+concentration increases withthe increase in MgCl2 solution. Tables 1 and 2 show that theMg2+concentration was 354.17 mgl1in the cemented soil,and greater than that of the MgCl2solution (concentration:1.5 gl1, 4.5 gl1, and 9.0 gl1). To reach the chemical equi-librium state, Mg2+is likely to leach from cemented soils. Bycontrast, the Clconcentration of the solution almost keepssteady when the corrosion time increases with the givenconcentration of MgCl2 solution, and it increases with theconcentration of MgCl2solution at the same corrosion time.

F i g. 4 Plot of Mg2+concentration versus timeS l i k a 4 Koncentracija Mg2+u odnosu na vrijeme

F i g. 5 Plot of Clconcentration versus timeS l i k a 5 Koncentracija Clu odnosu na vrijeme

The corrosion mechanismThe relationships between unconned compressive strengthsof the cemented soil cured for 28 days and initial concen -trations of Mg2+and Clions are analysed by regression pro-

cedure. Regression equations are established in Table 5. Theresults of regression shows that the unconned compressivestrength of the cemented soil decreases with increase inconcentration of Mg2+or Cl for the same corrosion time.This means that the compressive strength of cemented soilsis closely related to the concentrations of Mg2+and Clionsin an MgCl2corrosive environment.

T a b l e 5 Strength formulae of cemented soil in depend-ance of Mg2+ concentration, (Mg2+), and Clconcentration, (Cl), at cured time of 28 days

T a b l i c a 5 vrstoa cementiranog tla kao funkcija koncen-tracije iona Mg2+, (Mg2+), i Cl, (Cl), nakonizloenosti otopini MgCl2tijekom 28 dana

Cemented soil strengthvrstoa cementiranog tla

Mg2+ Cl

fcu = 8.80 [(Mg2+) mgl1]0.241;

R2

=0.80

fcu= 9.06 [(Cl) mgl1]

C0.210

R2

= 0.93

In the MgCl2solution environment, the main chemical reac-tion of the soil and MgCl2can be described as:

MgCl2+ Ca(OH)2+ 6H2O CaCl2 6H2O + Mg(OH)2 (2)

3Mg(OH)2+ MgCl2+ 8H2O 2Mg2(OH)3Cl4H2O (3)

It is well known that the volumes of the resultant are seventimes that of Ca(OH)2. When the amount of the MgCl2 in

the solution is suitable, the increased volume produced byCaCl26H2Oand Mg2(OH)3Cl4H2O may ll the void inthe cemented soil, and result in the increase in compressivestrength of the cemented soil. However, when the amountof MgCl2in the solution is large enough, the increased vol-ume caused by formation of CaCl26H2O and Mg2(OH)3Cl4H2Owill not only ll the voids, but inate the cement-ed soil, crack and even break. This could explain why theunconned compressive strength of cemented soil blockssoaked in pure water is less than that of blocks soaked inlow concentration solutions and greater than that of blockssoaked in high concentration solutions.

As dened in Eq. 3, Mg(OH)2is the product of the reactionbetween cemented soil and MgCl2. In the cemented soilwith high level of Mg(OH)2, the following chemical reactionis presented:

2Mg(OH)2+ 3CaO2SiO23H2O 2(MgOSiO2H2O) + 3Ca(OH)2 (4)

In the soil, 3CaO2SiO23H2O is a sticking material whichcan increase the strength of cemented soils by sticking thesoil particles together. The product, MgOSiO2H2O (MSH)of chemical reaction (4), is not a sticking material, and pro-duces an unsteady structure that reduces the strength of thesoil.

In summary, the cemented soils undergo MgCl2 corrosionduring the crystallization and dissolution process.


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Conclusions

(1) The unconned compressive strength of the cementedsoil decreases with increase in concentration of the MgCl2solution and corrosion time.

(2) Based on the relationships between the unconned com-pressive strength of cemented soils cured for 28 days and

the initial concentration of Mg2+

and Cl

in laboratory tests,regression equations of strength have been established.

(3) The chemical mechanism of the MgCl2 corrosive solutionon cemented soils is discussed and analysed. The MgCl2cor-rosion is a combinative corrosion during the crystallizationand dissolution process.

ACKNOWLEDGEMENTThis work was supported by the National Natural ScienceFoundation of China (5107825351208333), The Ph.D.Programs Foundation of Ministry of Education of China(20111402120001) and Natural Science Foundation for

Young Scientists of Shanxi Province (2010021020-3).

List of symbols and abbreviationsPopis simbola i kratica

fcu compressive strength of the material cured in pure wa-ter, MPa

posmina vrstoa materijala obraenog istom vo-dom, MPa

fcu' compressive strength of the material cured in MgCl2

solution, MPa posmina vrstoa materijala obraenog otopinomMgCl2, MPa

Ip plasticity index indeks plastinosti

R2 coefcient of determination koecijent determinacije

m mass, g masa, g

w mass fraction, mgkg1 maseni udjel, mgkg1

soil modied coefcient

koecijent promjene svojstava tla mass concentration, mgl1, gl1 masena koncentracija, mgl1, gl1

BCS black cotton soil vertisol, tip humusno-akumulacijskog tla

CKD cement kiln dust praina klinkera

MSH MgOSiO2H2O

OPC ordinary Portland cement obini portland-cement

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SAETAKMehanizam korozije cementiranog tla u otopini MgCl2

P.-J. Han,a,b*W.-B. Zhang,aX.-H. Baiai T. Tonga

Mehanika svojstva cementiranog tla slabe kada je cementacija primjenjena u hidrotehnici zakontrolu propusnosti tla u korodirajuem okruenju s MgCl2. Korozija uzrokuje ozbiljnu tetukompozitu tla i cementa te dodatno smanjuje stabilnost cijele strukture. Stoga je potrebno po-drobnije istraiti mehanizme kojima opadaju mehanika svojstva cementiranog tla pod utjecajemMgCl2. Kako bi se simulirali i prouavali erozijski procesi, proveden je niz ispitivanja na blokovimacementiranog tla u otopinama MgCl2 razliitih koncentracija, ukljuujui fotograranje, ispitivanjeneograniene tlane vrstoe i mjerenja koncentracije Mg2+ i Cl. S poveanjem koncentracijeraste stupanj korozije na povrini uzoraka dok se posmina vrstoa smanjuje. Kemijska analiza

korozijskog okolia pokazuje da novonastali spojevi kao to su CaCl26H2O i Mg2(OH)3Cl4H2Oimaju sedam puta obujam vei od Ca(OH)2. Korozija cementiranog tla vrsta je kristalne korozije iMg(OH)2reagira s 3CaO2SiO23H2O te nastaje MgOSiO2H2O. Istodobno je analizirana vezaizmeu poetnih koncentracija iona Mg2+i Cli vrstoe te naene regresijske funkcije vrstoe.

Prispjelo 17. kolovoza 2013.Prihvaeno 27. prosinca 2013.

a College of Architecture and Civil EngineeringTaiyuan University of Technology030 024 TaiyuanKina

b College of Materials Science and Engineeringof TaiyuanUniversity of Technology030 024 TaiyuanKina

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Corrosion Mechanism of Cemented Soil in MgCl2 Solution