WANG Xiuhai ,TIAN Zhuangcai ,ZHANG Yanan ,SU Xiuting ,LIU Hongjun ,and LIU Tao,

1) College of Environmental Science and Engineering, Ocean University of China, Qingdao 266100,China

2) Shandong Provincial Key Laboratory of Marine Environment and Geological Engineering, Ocean University of China, Qingdao 266100,China

3) State Key Laboratory for Geomechanics and Deep Underground Engineering, China University of Mining and Technology, Xuzhou 221116, China

Abstract Saline soil is widely distributed in the marine sediments along the coast of the world and the arid-semi-arid areas of the Middle East and Iraq,and calcium sulfate erosion has become one of the important factors affecting the durability of concrete in this area.In order to clarify the mechanism of sulfate ion damage to concrete,this paper mainly takes saline soil with high sulfate content in coastal area as well as arid-semi-arid area as the research object,and uses indoor geotechnical test,field test and numerical simulation to study the influence of different dry-wet cycle times on the unconfined compressive strength of concrete test blocks,and puts forward the relationship between the erosion arrival depth and time of sulfate ion in concrete,so as to predict the long-term erosion depth by using the erosion depth of sulfate ion in concrete in short time.The results show that the shorter the erosion time when the erosion reaches a certain depth,and the larger the erosion reaches when the erosion time is the same,the faster the erosion reaches the depth with the increase of erosion time.Compared with rectangular section concrete,circular section concrete penetrates faster.The results of this study can provide a reference for the durability design of concrete in saline soil sites containing sulfate.

Key words wetting-drying cycle;sulfate ion;concrete;corrosion

1 Introduction

The saline soils are widely distributed in the marine sediments along the coast of the world and the aridsemi-arid regions of the Middle East and Tigris and Euphrates of Iraq,and the calcium sulfate corrosion has become one of the important factors impairing the durability of concrete,which seriously affect the safety,stability and normal use of buildings.

The processes of sulfate corrosion on the concrete,including its diffusion in the concrete,chemical reaction between sulfate ions and concrete components,and formation of some insoluble salt crystals involving the ettringite,gypsum,etc.,and change in internal stress of the concrete arising out of expansive deformation of reaction products,are extremely complex (Huanget al.,2008;Liet al.,2009).The sulphate solution will enter the concrete along the crack and continue to undergo a series of physical and chemical reactions like the crystallization,dissolution,recrystallization,etc.,further aggravating the sulfate corrosion process of the concrete,the microstructure of and concrete will be seriously damaged with extremely short time,causing crisiscrossing cracks inside and outside the concrete structure and eventually resulting in the collapse or instability of the structure (Valenza and Scherer,2007;Leng,2012).Presently,the Fick’s second law and its mathematical model are utilized in most cases to conduct one-dimensional analysis on the sulfate corrosion,concentrating on the diffusion law of sulfate ions,formation mechanism of ettringite and changes in mechanical properties of the concrete materials (Jinet al.,2006).

With respect to experiments,Zhanget al.(2014) established the corrosion strength model of concrete under sulfate environment based on the concepts including the thickness of concrete corrosion layer and loss rate of compressive strength of concrete in the corrosion area,and derived the calculation formula for resistance coefficient of compressive strength in the concrete.According to the corrosion experiments of three groups of fully immersed concrete,the calculation formula for compressive strength and corrosion resistance coefficient of concrete were subject to the fitting and parameters analysis.Cao(1991) studied the diffusion and reaction of corrosive materials in the concrete,and established the functional relations between the degradation of concrete properties and diffusion ability and reaction coefficient of corrosive materials in the concrete.

Theoretically,the cement-based composite material is considered as a type of uniform and continuous composite in the ‘Concrete Centriole Hypothesis’,in which the dispersions are thought to be evenly distributed inside the composite (Wu and Lian,1999).The theory,based on the elastic theory,is used to construct the theoretical model for microstructure during cement hardening and to thereby explain mechanical properties of cement composites.The growth of eroded crystals in the concrete inside pores will cause the expansion stress on the pore wall and damage the concrete,and the prevention on the nucleation and crystallization of erosive material as acts as an important method to avoid material damage (Tixier and Mobasher,2003).The crystal stress in the concrete is featured with upper bound under supersaturated and frozen state,and the crystal stress in the single pore is insufficient to cause damages on the materials.No tensile stress generates when the contact angle between the crystal and pore wall is less than 90? (Young,1998).Other scholars proposed chemically and mechanically coupling damage model to predict the expansion of the concrete under sulfate corrosion (Doehne,2002).Basista and Weglewski (2009) summarized the generation of the crystallization pathway and pressure in the soil under laboratory conditions and on-site subsurface conditions:The crystallization of sodium sulphate ore in supersaturated solutions caused porous materials to become main cause of corrosion damage.

The effects of sulphate on unconfined compressive strength of concrete test blocks are studied by laboratory tests in this paper.The relation between the depth of corrosion and time is proposed by numerical simulation.The research results may reference for research on concrete durability in sulfate saline soil site.

2 Test Overview

In this paper,the soil with high sulfate content in aridsemi-arid countries was tested in laboratory.The concrete specimens sized in 100 mm × 100 mm × 100 mm were used for experiment.No.42.5 ordinary Portland cement and anti-sulfate cement were used as concrete materials.The mixture ratio is shown in Table 1.

Table 1 Table for concrete mixing proportions

Dismantle concrete specimens from forms after one day of pouring and place them in a standard curing room (temperature at 20℃ ± 3℃) for the first 2 d of the 28-day period.

The steps of the wetting-drying cycle are shown as follows:

1) Remove the concrete specimens to be subject to wettingdrying cycles from the concrete curing box,dry the moisture left on the surface at the same time,and then place the specimens in an the oven and keep baking for 48 h at 80℃.

2) Place the specimens in dry environment and cool to room temperature after completing drying,and then place such specimens in the specimen box.

3) Pour 5% Na2SO4prepared solution into the specimen box,simulate the corrosive environment to soak the specimens,and make sure that the solution exceeds the upper surface of specimens by 2 cm with the steeping duration in 15 h.

4) Take out the specimens and have them undergo airdrying for 30 min.Immediately put the specimens into the drying oven after the air-drying process is over,place the specimens into the drying oven and bake for 6 h at 80℃.After the drying is completed,and place the specimens in dry environment and cool for 30 min.The above process is a complete wetting-drying cycle.Replace the specimens in the solution for soaking,and then repeat the above steps (see Fig.1).

Fig.1 Photos of concrete specimens.

5) Check pH value of the solution regularly during the experiment and always maintain pH value of the solution between 6 and 8.

To study the sulfate ion diffusion process,other surfaces of some specimens are coated with the epoxy resin except that the poured surfaces are retained as corrosion surfaces,and the specimens for compressive strength comparison are made with the same method in the same batch in addition to the specimens for sulfate corrosion resistance.

Have the specimens standardized and cured for 28 d divided into 4 experimental group,namely 15 N group,30 N group,45 N group and 60 N group,or 15 wettingdrying cycles group,30 wetting-drying cycles group,45 wetting-drying cycles group and 60 wetting-drying cycles group respectively,and have the specimens subjected to the compressive strength when the soaking age becomes matured.The pressure testing machine is shown below in Fig.2.

Fig.2 Photo of concrete pressure testing machine.

3 Model Calculation

Sulfate ions enter the concrete and react with calcium hydroxide in the pore solution of concrete to form secondary gypsum:

The secondary gypsum,as an intermediate product,continues to react with calcium aluminate to form ettringite:

These reactions can be simplified as follows:

where the CAare calcium aluminate substances (tricalcium aluminate,calcium aluminate hydrate,calcium monosulfur aluminate),andqis the weighted average coefficient of the CSH2 after equation simplification,which is about 8/3.

3.1 Transmission Equation

According to the second law of Fick and the kinetic theory of chemical reaction,the diffusion reaction equation of rectangular section concrete subjected to sulfate erosion is established:

Among them,Uis sulfate ion concentration;tis erosion time;xis to the erosion surface distance;Duis sulfate ion diffusion coefficient;kis chemical reaction rate constant;CCAis the equivalent total concentration of calcium aluminate;qis the equivalent reaction coefficient of calcium aluminate to produce ettringite,q=8/3;CCA0is theinitial calcium aluminate concentration;φ0is the initial porosity;U0is sulfate solution concentration.

In the actual environment,the surface of concrete components is not all plane.For cylindrical components such as concrete sewage pipes,the circumference of the section of the component decreases gradually during the diffusion of sulfate ions from the surface.The second law of Fick considering the influence of circular section should be deduced again.

Diffusion in two-dimensional space can be described as the Fick second law:

Cis ion concentration,tis diffusion time,xis transverse diffusion depth,yis vertical diffusion depth,andDis ion diffusion coefficient.

For circular section components,polar coordinate systems can be used to transform:

The two items on the right of the upper equal sign can be transformed into:

Cis ion concentration;tis diffusion time,xis diffusion depth;Dis ion diffusion coefficient;andris erosion depth.

For cylindrical concrete components,the diffusion reaction equation considering the effect of circular section:

3.2 Diffusion Coefficient

In sulfate erosion,the porosity of concrete changes,and the diffusion coefficient is not quantitative,but a function related to porosity:

Among them,φis the porosity of concrete during sulfate erosion;Dsis the initial diffusion coefficient of sulfate ions in the pore solution of concrete.

The porosity of concrete is related to the degree of cement hydration,and the damage of concrete is very large in the process of sulfate erosion of concrete.The porosity function related to cement hydration and concrete damage is established:

whereφwis the porosity of concrete affected by cement hydration andD(C,t) is the damage function of concrete material.

Thefcis the volume fraction of cement,and thehαis the degree of hydration of cement,

where theτis cement hydration time.

whereφ0is initial porosity;dimensionless parameterC0=0.08;aD,bDandcDare parameters to be fitted.Whenw/c=0.45,aD=3.189,bD=1.74,cD=6.75;t0=720 d.

A cube with a length of 100 mm was simulated and immersed in 5% sodium sulfate solution.Five of the surfaces were sealed with epoxy resin,leaving only one surface subjected to sulfate erosion.The numerical calculation parameters are shown in Table 2.

Table 2 Calculating relevant parameters

4 Corrosive Results and Analysis of Sulfate

Example 1:The cube with a length of 100 mm is simulated and immersed in 5% sodium sulfate solution.Five surfaces are sealed with epoxy resin;only one surface is eroded by sulfate.The numerical calculation parameters are shown in the above table.

Fig.3 shows the relationship between sulfate ion concentration and erosion depth at different erosion time.It can be seen from the diagram that the content of sulfate ion decreases with the increase of erosion depth at the same erosion time.At the same erosion depth,the content of sulfate ion increases with the increase of erosion time.With the increase of erosion timeΔT=500 d,curve becomes smaller and smaller,indicating that the rate of sulfate ion erosion becomes slower and slower with the increase of erosion time.When the erosion depth is 20 mm and at the first 2500 d,which the concentration of sulfate ion is 0,the concentration increases gradually with the increase of erosion time,and the concentration is about 5 mol m-3at 4000 d.

Fig.3 Curve of ion concentration of sulfate with erosion depth.

Fig.4 shows the relationship of sulfate ion concentration and erosion time at erosion depth of 10 mm.As can be seen in the first 400 d,the concentration of sulfate ion is 0,and then the concentration increases gradually;the curve of 400 d to 1500 d,becomes steep and the concentration increases rapidly;after 1500 d,the curve slowed down and the concentration of sulfate ion slowed down.This is due to the reaction of sulfate ions to form ettringite after reaching this depth.With the consumption of calcium aluminate,the reaction rate slows down,the consumption of sulfate ions slows down,and the concentration of sulfate ions increases faster.After a period of time,the reaction reaches equilibrium.

Fig.4 10 mm change curve of sulfate ion concentration with erosion time.

Example 2:A cylinder with a diameter of 100 mm and a cube with a section of 100 mm × 100 mm are simulated.The top and bottom surfaces of the cylinder and cube are sealed with epoxy resin,immersed in 5% sodium sulfate solution,and only eroded by sulfate on the side.The numerical calculation parameters are the same as example 1.

Fig.5 shows the relationship between sulfate ion concentration and erosion depth under different erosion time in circular section concrete.It can be seen that the variation of sulfate ion concentration is similar to that of onedimensional condition,but at the same erosion time,the concentration of sulfate ion is much higher than that of one-dimensional condition.When the erosion time is 4000 d and at the erosion depth of 20 mm,the concentration of sulfate ion is about 9.8 mol m-3;where the erosion depth is 25 mm,the concentration is about 3.8 mol m-3.

Fig.5 Curve of ion concentration of sulfate with erosion depth.

Fig.6 shows the concentration distribution of sulfate ions in 4000 d of rectangular section and circular section concrete.At the erosion time of 4000 d,the concentration distribution of sulfate ions in rectangular section,the closer to the boundary and corner,the greater the concentration of sulfate ions.In the circular section,the concentration of sulfate ion is concentric.

Fig.6 Distribution of sulfate ions at 4000 d at different interfaces.

Fig.7 shows the variation of sulfate ion concentration with erosion depth at different erosion times along the central axis and diagonal direction of rectangular section concrete,in which the dashed line indicates the direction along the central axis and the solid line indicates the direction along the diagonal line.As we can see,at 4000 d,along the central axis,the concentration of sulfate ion is about 5 mol m-3at 35 mm erosion depth;on the diagonal direction,at the erosion depth of 20 mm,the concentration of sulfate ion is as high as 21 mol m-3,until the erosion depth of 35 mm,the concentration of sulfate ion is reduced to 5.5 mol m-3.At the same erosion time and the same erosion depth,the concentration of sulfate ion is much higher than that along the central axis along the diagonal direction.Therefore,the closer to the inflection point of the rectangular section,the greater the concentration of sulfate ions,the easier it is to form ettringite,which leads to the expansion and the destruction of concrete.

Fig.7 Variation curve of sulfate ion concentration with erosion depth along different directions.

Fig.8 shows the variation of sulfate ion concentration of concrete with time at 10 mm of erosion depth,circular section and rectangular section.It can be seen that the concentration of sulfate ions in the circular section is between the central axis and diagonal line of the rectangular section at the same erosion depth.It shows that replacing rectangular section concrete components with circular section concrete components can improve the sulfate resistance of concrete structures.

Fig.8 Changes of ion concentration of sulfate with erosion time at 10 mm erosion depth.

Fig.9 represents the compressive strength of concrete changes over time.It can be seen that the corrosion of sulfate-based saline soil mainly destroys cement hydration products through physical and chemical interactions,causing the concrete to differentiate,shed and lose strength (Huang,2013).And this process is a progressive process rather than straight line relationship.As the sodium sulphate crystallizes inside the concrete,the strength of the concrete at the age of 30 N approximately intensifies,and the interior of the concrete will be destroyed with repeated crystallizations,thereby lowering the strength of the concrete.The variation tendency of strength of the sulfate-resistant cement is basically consistent with that of ordinary Portland cement.The variation is slightly slower than that of ordinary Portland cement,while the compressive strength is significantly higher than that of ordinary Portland cement.

Fig.9 Curve for compressive strength versus duration.

5 Discussion and Conclusions

1) As the sodium sulfate crystallizes inside the concrete,the strength of the concrete strengthens at 30 N of age approximately,while the interior of the concrete will be destroyed as the process of crystal dissolution is repeated,thereby impairing the strength of the concrete.The variation tendency of strength of sulfate-resistant cement is basically the same as that of ordinary Portland cement.The variation is slightly slower than that of ordinary Portland cement,but the compressive strength is obviously higher than that of ordinary Portland cement.

2) The results of sulfate corrosion study show that the closer to the section inflection point,the greater the concentration of sulfate ion,the easier it is to form ettringite,which leads to the expansion and destruction of concrete.Therefore,replacing rectangular section concrete components with circular section concrete components can improve the sulfate resistance of concrete structures.

3) The shorter corrosion duration of the specimen is when the corrosion hits a fixed depth,greater hitting depth of corrosion of the specimen will when the corrosion duration remains unchanged,the faster hitting depth of corrosion will be with the increase in corrosion duration,and greater hitting depth of long-term corrosion will be.

Acknowledgements

The study is supported by the Fundamental Research Funds for the Central Universities (No.201962011),the Laboratory for Marine Geology,Qingdao National Laboratory for Marine Science and Technology (No.MGQN LM-KF201804),the National Natural Science Foundation of China (No.41672272).