materials

Article Degradation Mechanism of Subjected to External Attack: Comparison of Different Curing Conditions

Gaowen Zhao 1,2,*, Mei Shi 3,*, Mengzhen Guo 1 and Henghui Fan 4 1 School of Highway, Chang’an University, Xi’an 710064, China; [email protected] 2 Department of Geotechnical Engineering, Tongji University, Shanghai 200092, China 3 College of Natural Resources and Environment, Northwest A&F University, Yangling 712100, China 4 College of Conservancy and Architectural Engineering, Northwest A&F University, Yangling 712100, China; [email protected] * Correspondence: [email protected] (G.Z.); [email protected] (M.S.)

 Received: 4 June 2020; Accepted: 14 July 2020; Published: 16 July 2020 

Abstract: Sulfate induced degradation of concrete brings great damage to concrete structures in saline or offshore areas. The degradation mechanism of cast-in-situ concrete still remains unclear. This paper investigates the degradation process and corresponding mechanism of cast-in-situ concrete when immersed in sulfate-rich corrosive environments. Concrete samples with different curing conditions were prepared and immersed in sulfate solutions for 12 months to simulate the of precast and cast-in-situ concrete structures, respectively. Tests regarding the changes of physical, chemical, and mechanical samples were conducted and recorded continuously during the immersion. Micro-structural and methods were performed to analyze the changes of concrete samples after immersion. Results indicate that the corrosion process of cast-in-situ concrete is much faster than the degradation of . Chemical attack is the main cause of degradation for both precast and cast-in-situ concrete. Concrete in the environment with higher sulfate concentration suffers more severe degradation. The water/ ratio has a significant influence on the durability of concrete. A lower water/cement ratio results in obviously better resistance against sulfate attack for both precast and cast-in-situ concrete.

Keywords: cast-in-situ concrete; degradation mechanism; sulfate attack; sulfate diffusion

1. Introduction , chlorides, and other corrosive sources are widely observed in soils, ground water, river water, and in salty saline and offshore areas [1–5]. Among the mostly observed ions in the environment, sulfates have been claimed to be one of the most aggressive ions to bring great damage to concrete structures [6–11]. Experimental and theoretical studies regarding the degradation of concrete caused by sulfate attack have been well reported in recent decades. The corrosion products such as ettringite, gypsum, or thaumasite were reported after sulfate corrosion [12,13]. In addition, researchers found that hydrates of sulfates, especially mirabilite, were responsible for the development of cracks in concrete in sulfate-rich environments [14,15]. Meanwhile, some researchers proposed that both chemical and physical attack induced by sulfate presence should be considered, which directly lead to the degradation of concrete, since both chemical corrosion products and crystallization hydrates have been detected in the damaged concrete [16]. Therefore, it is widely accepted that the degradation of concrete induced by sulfate attack can be divided into physical sulfate attack, chemical sulfate attack, and physical–chemical combined degradation [17,18].

Materials 2020, 13, 3179; doi:10.3390/ma13143179 www.mdpi.com/journal/materials Materials 2020, 13, 3179 2 of 17

Concrete subjected to sulfate attack always results in great expansion as well as the risk of failure [19–21]. The expansion is reported to be caused by the corrosion products or hydrates induced during sulfate attack [22,23]. It has been reported that the volume of chemical corrosion products, especially ettringite, far surpasses the volume of reactants during chemical corrosion [24–26]. In addition, the strength of corrosion products is relatively low, leading to a great decrease of concrete strength [27]. It is notable that the volume of hydrates, Na SO 10H O, is several times greater than the volume of 2 4· 2 Na2SO4 [28–30]. Therefore, chemical or physical attack by sulfates has a significant negative effect on the performance of concrete [31]. It is clear that previous studies regarding the degradation of concrete induced by sulfate attack are of great importance for further research. By paying close attention to the published research, most of the previous studies were conducted on precast concrete samples. However, it should be pointed out that most concrete structures in salty saline or offshore areas are cast-in-situ concrete structures, especially for underground concrete structures. It should be noted that the construction processes of precast and cast-in-situ concrete structures are quite different. For cast-in-situ concrete, the corrosion process starts immediately after the casting of fresh concrete in the sulfate-rich environments, while for precast concrete structures, the hardening process has finished before being installed in the environment. Moreover, the effects of the water/cement ratio on the sulfate induced degradation of cast-in-situ concrete are rarely reported, which plays an important role in affecting the properties and safety of cast-in-situ structures. Consequently, the degradation process and mechanism between the precast and cast-in-situ concrete are quite different. Moreover, the safety of cast-in-situ concrete cannot be ensured if the design of cast-in-situ concrete structures is based on the research conducted on precast concrete samples. Thus, it is necessary to perform a specific experiment to reveal the degradation process and mechanism differences between the precast and cast-in-situ concrete. To study the degradation process and mechanism of cast-in-situ concrete caused by sulfate attack, concrete samples in cylinder shapes were prepared and immersed in sulfate solutions. Different curing conditions were used in the present study. To highlight the differences of specimens with different curing conditions, specimens cured for 12 h were described as cast-in-situ concrete and specimens cured for 28 days were described as precast concrete in the present study. Tests were conducted to record the physical, chemical, and mechanical properties changes during the 12-month immersion. The micro-structural and mineral properties of concrete were analyzed after immersion by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), and scanning electron microscopy (SEM) tests.

2. Materials and Methods

2.1. Materials To prepare concrete specimens, P.C. 32.5 R (Conch, Shanghai, China) was used in the present experiment [32,33]. The chemical composition of cement is presented in Table1. River sand was used in this study as fine aggregate. The maximum particle size was 2.36 mm, and the fineness modulus was 2.6. Artificial crushed stone was used as coarse aggregate. Excessively large aggregate in concrete would result in great discreteness of the test results due to the size of specimens, thus the maximum size of crushed stone was 10 mm. Distilled water was used to prepare concrete samples and sulfate corrosive solutions.

Table 1. Chemical components of cement.

Chemical Components Al2O3 SO3 Fe2O3 MgO CaO Content (%) 6.82 1.96 2.13 1.92 60.05

To investigate the effects of water/cement ratio on the degradation of concrete caused by sulfate attack, three kinds of concrete samples were designed in the present study. Mixture proportions of concrete samples in the present study are listed in Table2. Materials 2020, 13, 3179 3 of 17

Table 2. Mixture proportions of concrete in the present study.

Concrete Components w/c Water (kg/m3) Cement (kg/m3) Sand (kg/m3) Gravel (kg/m3) 0.40 162 405 600 1275 0.48 175 365 638 1238 0.55 189 345 685 1210 Note: w/c is the water/cement ratio.

2.2. Sample Preparation and Testing Methods

2.2.1. Sample and Solution Preparation

Na2SO4 was selected to prepare sulfate corrosive solutions. The sulfate concentrations of different solutions were set at 0%, 3%, 5%, and 10% (named as CK, 3S, 5S, and 10S, as shown in Table3) in the present study. It should be noted that the sulfate concentration and liquid depth of each solution were checked every month to ensure that the sulfate concentration was constant and all the specimens were fully immersed in the solutions. The details of specimens and solutions are presented in Table3. It should be emphasized that to get a relatively accurate value of concrete properties, three parallel tested samples were prepared simultaneously in different tests in the present study.

Table 3. The details of specimens and solutions.

Specimens Immersion Solution Specimens Immersion Solution PCK Distilled water CCK Distilled water P3S 3% Na2SO4 solution C3S 3% Na2SO4 solution P5S 5% Na2SO4 solution C5S 5% Na2SO4 solution P10S 10% Na2SO4 solution C10S 10% Na2SO4 solution Note: P for precast concrete specimens and C for cast-in-situ concrete specimens.

Raw materials, such as river sand, crushed stone, and cement, were weighed and mixed in the mixer before the distilled water was added. Freshly mixed concrete was then put in the modules, and the size of concrete specimens was 100 mm 200 mm (diameter height). Samples with modules were × × then put in the curing chamber with a temperature of 22 ◦C and relative humidity of >95%. The top and bottom surfaces of each specimen were sealed with epoxy resin. For precast concrete, the samples were cured for 28 days before being put into the solutions, while for cast-in-situ concrete, the samples were put into the solutions after being cured for 12 h.

2.2.2. Physical Properties

It should be noted that the specimens were put into an oven at a temperature of 60 ◦C for 48 h after being immersed in the solutions for 1, 3, 6, 9, and 12 months. To determine the diameter and weight changes of concrete samples during the immersion, a Vernier caliper and an electronic scale were selected to measure the diameter and weight of specimens. The diameter of each specimen was measured 5 times near the mid-height of specimens to obtain an average diameter. The weight of each specimen was weighed twice, and the average value was recorded.

2.2.3. Compressive Strength An unconfined compressive strength test was performed to determine the compressive strength of each specimen during the immersion. A loading system (the equipment model was POPWIL (HSR)) with a maximum capacity of 1000 kN was used to conduct the compressive strength test, and the loading rate during the tests was set as 5 kN/s. Materials 2020, 13, x FOR PEER REVIEW 4 of 17

(HSR)) with a maximum capacity of 1000 kN was used to conduct the compressive strength test, and the loading rate during the tests was set as 5 kN/s.

2.2.4. Sulfate Concentration

MaterialsTo determine2020, 13, 3179 the sulfate concentration in the concrete specimens, a drill was used to collect4 of 17 the powder samples by drilling perpendicular to the lateral surface of each specimen. Five powder samples (from the surface to a depth of 25 mm beneath the exposed surface of specimens) were obtained2.2.4. Sulfate and sealed Concentration in the plastic zip bags. Each powder sample was mixed with 50 mL distilled water andTo determine put into a the bottle sulfate for concentration 48 h. Then the in thesolu concretetion was specimens, filtered through a drill was filter used paper to collect to remove the residues.powder The samples sulfate by drilling concentration perpendicular of each to the soluti lateralon surfacewas then of each determined specimen. by Five standard powder sampleschemical titration(from the[34]. surface to a depth of 25 mm beneath the exposed surface of specimens) were obtained and sealed in the plastic zip bags. Each powder sample was mixed with 50 mL distilled water and put into 2.2.5.a bottle Micro-Structural for 48 h. Then and the Mineral solution Analyses was filtered through filter paper to remove residues. The sulfate concentration of each solution was then determined by standard chemical titration [34]. SEM (by ZEISS-SIGMA 300 system, Carl Zeiss (Shanghai), Shanghai, China) and XRD (by model RINT2.2.5. 2000 Micro-Structural X-ray diffractometer, and Mineral Rigku Analyses (Beijing), Beijing, China) analyses were performed to observe the micro-structures and to determine the mineral composition of concrete samples after being SEM (by ZEISS-SIGMA 300 system, Carl Zeiss (Shanghai), Shanghai, China) and XRD (by model immersed in sulfate solutions for 12 months. The element analysis (by EDS (ZEISS-SIGMA 300, Carl RINT 2000 X-ray diffractometer, Rigku (Beijing), Beijing, China) analyses were performed to observe the Zeiss (Shanghai), Shanghai, China)) was also performed in the SEM test to determine the corrosion micro-structures and to determine the mineral composition of concrete samples after being immersed products. The samples used in the SEM analysis were concrete core samples drilled from the lateral in sulfate solutions for 12 months. The element analysis (by EDS (ZEISS-SIGMA 300, Carl Zeiss surface(Shanghai), of specimens Shanghai, with China)) a depth was of also 10 performed mm beneat inh the the SEM exposed test to surface. determine The the powder corrosion samples products. used in Thethe XRD samples analysis used inwere the drilled SEM analysis from the were exposed concrete surface core sampleswith a depth drilled of from 5 mm the and lateral passed surface through of a 75-µmspecimens sieve. with a depth of 10 mm beneath the exposed surface. The powder samples used in the XRD analysis were drilled from the exposed surface with a depth of 5 mm and passed through a 75-µm sieve. 3. Results and Discussion 3. Results and Discussion It is well accepted that the micro-structural and mineral properties play an important role in affectingIt isthe well basic accepted physical that and the mechanical micro-structural properties and of mineral concrete properties structures. play Therefore, an important the results role in of thea ffmicro-structuralecting the basic physicaland mineral and mechanical analyses were properties given ofand concrete discussed structures. firstly.Therefore, Additionally, the results sulfates penetrateof the micro-structural concrete and react and mineralwith concrete analyses compos were givenitions and to generate discussed corrosion firstly. Additionally, products and sulfates hence changepenetrate the properties concrete and of concrete. react with Thus, concrete the diffusion compositions process to generate of sulfates corrosion into concrete products is subsequently and hence introduced.change the Finally, properties results of concrete. of other Thus, properties the diff areusion given process and ofdiscussed. sulfates into concrete is subsequently introduced. Finally, results of other properties are given and discussed. 3.1. Micro-Structural and Mineral Analyses 3.1. Micro-Structural and Mineral Analyses ToTo show show the the micro-structures micro-structures ofof concrete concrete after afte immersionr immersion in sulfate in sulfate solutions solutions for 12 for months, 12 months, SEM SEMobservation observation was was conducted, conducted, and the and results the results are presented are presented in Figure 1in. SEMFigure images 1. SEM in Figure images1a,b in were Figure 1a,btaken were from taken precast from concrete precast and concrete cast-in-situ and concrete cast-in-situ specimens concrete after specimens immersion after in 10% immersion sulfate solution in 10% sulfatefor 12 solution months, for with 12 the months, same waterwith /thecement same ratio water/ of 0.55.cement To give ratio a relativelyof 0.55. To solid give determination a relatively solid of determinationthe corrosion of products the corrosion after immersion. products after XRD immersion. tests were performed XRD tests on were different performed concrete on samples different concreteafter immersion, samples after and immersion, the results areand shown the results in Figure are shown2. The EDSin Figure analysis 2. The results EDS areanalysis also listed results on are alsothe listed right on side the of right the SEM side imagesof the SEM to give images reliable to proof.give reliable The results proof. of The XRD results analysis of XRD in Figure analysis2a,b in Figurewere 2a,b obtained were from obtained the precast from and the cast-in-situprecast and concrete cast-in-situ specimens concrete immersed specimens in 10% immersed sulfate solution, in 10% sulfateand thesolution, water/ cementand the ratios water/cement were 0.55. ratios were 0.55.

(a)

Figure 1. Cont. MaterialsMaterials2020 2020, 13,, 13 3179, x FOR PEER REVIEW 5 of5 17of 17 Materials 2020, 13, x FOR PEER REVIEW 5 of 17

(b) (b) Figure 1. SEM images and corresponding EDS results in concrete exposed to 10% sulfate solutions for Figure 1. SEM images and corresponding EDS results in concrete exposed to 10% sulfate solutions for Figure(a) precast 1. SEM concrete, images and magnification corresponding = 5 K, EDS and results (b) cast-in-situ in concrete concrete, exposed magnification to 10% sulfate = solutions 2.5 K. for (a) precast concrete, magnificationmagnification == 5 K, and ( b) cast-in-situ concrete, magnificationmagnification = 2.5 K.

□ □ □ ◆ E □ SiO2 □ ◆ E◇★ G □ SiO Sodium2 Sulfate Hydrate ◇★ G Sodium Sulfate Hydrate

□ □ □ □ □ □ □ □ □ □ ◇ □ □ ◇ ◆ ◆ □ ★ ◇ □ ◆ ◆◆ ◆ ◇ ◇ ◆ ◇ ◆ ◇ ◇ ◆ ★ ◇ ◇ ◆ ◇ ◆ ★ ◆ ★ ◆ ◆ ◇ ◇ ◇ ◇ ★ ★ ◇ 10 20 30 40 50 60 70 10 20 30 40 50 60 70 2θ 2θ(a) (a) □ □ □ ◆ E □ SiO2 □ ◆ E◇★ G □ SiO Sodium2 Sulfate Hydrate ◇★ G Sodium Sulfate Hydrate

□ □

□ □ □ □ □ □ □ □ ◇◆ □ □ ◆ ★ ◆ ◇ □ ◇ ★ ◇◆◆ ◇ ◆ ◇□ ◆ ◆ ★ ◆ ◇ ◇ ◆ ◇ ◆ ◇ ◇ ★ ◆ ◇ ◇ ★ ◆ ◆ ◇ ★ 10 20 30 40 50 60 70 10 20 30 40 50 60 70 2θ 2θ (b) (b) FigureFigure 2. 2.XRD XRD analysis analysis results results ofof corrosioncorrosion products in in the the concrete concrete for for (a ()a )cast-in-situ cast-in-situ concrete, concrete, and Figureand(b ()b precast) 2. precast XRD concrete. analysis concrete. results of corrosion products in the concrete for (a) cast-in-situ concrete, and (b) precast concrete. It is clear in the SEM images and EDS analysis results that sulfate corrosion products such as ettringiteIt is clear and in gypsum the SEM were images observed and EDS in both analysis precast results and thatcast-in-situ sulfate concretecorrosion samples. products It suchshould as be ettringite and gypsum were observed in both precast and cast-in-situ concrete samples. It should be

Materials 2020, 13, 3179 6 of 17

It is clear in the SEM images and EDS analysis results that sulfate corrosion products such as ettringite and gypsum were observed in both precast and cast-in-situ concrete samples. It should be noted that the peaks of Au and S shared little overlap in the EDS results, and a and b in the SEM images in Figure1 were the sampling points of EDS analysis. The analysis results of XRD tests also indicated that the peaks of corrosion products of ettringite (marked as E in XRD patterns), gypsum (marked as G in XRD patterns), and sulfate hydrates were detected in the concrete after immersion in sulfate solutions. The involved chemical corrosion reactions of sulfate attack can be summarized as follows [35–37]: + 2 2Na + SO − + 10H O Na SO 10H O (1) 4 2 ⇔ 2 4 · 2 3CaO Al O + 3(CaSO 2H O) + 26H O 3CaO Al O 3CaSO 32H O (2) · 2 3 4 · 2 2 → · 2 3 · 4 · 2 3CaO Al O CaSO 12H O + 2(CaSO 2H O) + 16H O 3CaO Al O 3CaSO 32H O (3) · 2 3 · 4 · 2 4 · 2 2 → · 2 3 · 4 · 2 4CaO Al O 13H O + 3(CaSO 2H O) + 14H O 3CaO Al O 3CaSO 32H O + CaO H O (4) · 2 3 · 2 4 · 2 2 → · 2 3 · 4 · 2 · 2 Ca(OH) +Na SO +2H O CaSO 2H O + 2NaOH (5) 2 2 4 2 → 4 · 2 The results of micro-structural and mineral analyses illustrate that the main corrosion products in both precast and cast-in-situ concrete were the same. Thus, one can conclude that both precast and cast-in-situ concrete mainly suffer chemical sulfate attack, especially when the concrete is fully immersed in the sulfate-rich environments. The peaks of sulfate hydrates detected in concrete indicated that crystallization of sulfates also occurred in the cracks and pores of concrete. However, it should be emphasized that fully immersed concrete samples stayed beneath the liquid surface all the time, and the evaporation process was not involved during the immersion. Thus, it was inferred that the crystallization hydrates of sulfates were generated during the 48-hours drying process instead of the immersion period. The generation of sulfate corrosion products would generally affect the properties of concrete in the following three ways. Firstly, the corrosion products would be generated and embedded in concrete, affecting the subsequent development of other properties. The reason is that the properties of sulfate-induced corrosion products and concrete are quite not the same. Secondly, the generated corrosion products would be gradually accumulated in the initial cracks and pores in concrete, influencing the density and strength of the outer layer of concrete. At last, it is well accepted that the corrosion products would cause great volume expansion by loading great pressure on the walls of internal cracks and pores, putting the outer layer of concrete under great tensile .

3.2. Sulfate Concentration Concentration of sulfates in different concrete samples was determined, and the results are shown in Figure3. Overall, the results clearly indicated that the concentration of sulfates was relatively high near the surface, and then decreased fast with the growth of depth beneath the exposed surface of concrete specimens. For precast concrete specimens, it is clear that the penetration of sulfate ions was relatively slow, especially for concrete immersed in 3% sulfate solution. The concentration of sulfates in concrete immersed in 5% and 10% sulfate solutions was higher and the penetration depth was deeper. Moreover, it is clear that the sulfate concentration in concrete with lower water/cement ratio is lower than that in concrete with greater water/cement ratio. As for cast-in-situ concrete, it was noticed that the sulfate concentration was relatively higher than that in precast concrete specimens. In addition, sulfates reached deeper depth in cast-in-situ concrete. Thus, sulfates diffused faster in cast-in-situ concrete, especially for concrete specimens with high water/cement ratio. Materials 2020, 13, 3179 7 of 17 Materials 2020, 13, x FOR PEER REVIEW 7 of 17

6 C10S(1 Months) C10S(3 Months) 5 C10S(6 Months) C10S(9 Months) 4 C10S(12 Months) P10S(1 Months) P10S(3 Months) 3 P10S(6 Months) P10S(9 Months) 2 P10S(12 Months)

Sulfate Concentration (%) 1

0 0 4 8 12162024 Depth (mm)

(a) 4 CCK(12 Months) C3S(12 Months) C5S(12 Months) 3 C10S(12 Months) PCK(12 Months) P3S(12 Months) P5S(12 Months) 2 P10S(12 Months)

1 Sulfate Concentration (%)

0 0 4 8 12162024

Depth (mm) (b)

4 CCK(12 Months) C3S(12 Months) C5S(12 Months) 3 C10S(12 Months) PCK(12 Months) P3S(12 Months) P5S(12 Months) 2 P10S(12 Months)

1 Sulfate Concentration (%) Concentration Sulfate

0 0 4 8 12 16 20 24 Depth (mm) (c)

Figure 3. Cont.

Materials 2020, 13, 3179 8 of 17 Materials 2020, 13, x FOR PEER REVIEW 8 of 17

CCK(12 Months) 4 C3S(12 Months) C5S(12 Months) C10S(12 Months) 3 PCK(12 Months) P3S(12 Months) P5S(12 Months) P10S(12 Months) 2

1 Sulfate Concentration (%)

0 0 4 8 12 16 20 24 Depth (mm)

(d)

FigureFigure 3. 3.SulfateSulfate concentration concentration of of concrete concrete wi withth different different waterwater//cementcement ratios: ratios: ( a()a) 0.55, 0.55, in in 10% 10% sulfate sulfate solution,solution, (b ()b )0.40, 0.40, in didifferentfferent solutions, solutions, (c) 0.48,(c) 0.48, in di ffinerent differen solutions,t solutions, and (d) 0.55,and in(d di) ff0.55,erent in solutions. different solutions. The diffusion of sulfates into concrete can be divided into two steps. In the first step, the soluble sulfates such as Na SO dissolve in water to form a sulfate solution in which sulfate ions can move For precast concrete2 4specimens, it is clear that the penetration of sulfate ions was relatively slow, freely. In the second step, sulfate ions would move from positions with higher sulfate concentrations especially for concrete immersed in 3% sulfate solution. The concentration of sulfates in concrete to positions with lower sulfate concentrations. In this regard, the diffusion of sulfate ions is mainly immersed in 5% and 10% sulfate solutions was higher and the penetration depth was deeper. motived by the concentration gradient between the concrete and the outer environments. In addition, Moreover,it should it be is emphasizedclear that the that sulfate the porosityconcentration of concrete in concrete also plays with an lower important water/cement role in a ratioffecting is lower the thandiff thatusion inof concrete sulfates. with It is obviousgreater water/cement that concrete with ratio. higher porosity can physically provide more paths forAs the for penetration cast-in-situ of sulfates.concrete, Thus, it was the noticed sulfate concentrationthat the sulfate in concreteconcentration with a higherwas relatively water/cement higher thanratio that is relativelyin precast high, concrete while onspecimens. the contrary, In sulfatesaddition, only sulfates reach a reached shallower deeper depth beneathdepth in the cast-in-situ exposed concrete.surface Thus, of concrete sulfates with diffused a lower fa waterster in/cement cast-in-situ ratio. concrete, especially for concrete specimens with high water/cementIt should be notedratio. that the diffusion of sulfates in precast concrete and cast-in-situ concrete is diffTheerent. diffusion First of all,of sulfates sulfate ionsinto can concrete diffuse can directly be divi intoded concrete into two once steps. the fresh In the concrete first step, is put the into soluble the sulfatesenvironment such as during Na2SO the4 dissolve hardening in process.water to Then, form the a sulfate penetrated solution sulfate in ions which can reactsulfate with ions the can cement move freely.composition In the second to generate step, sulfate sulfate ions corrosion would products move from such positions as gypsum with and higher ettringite. sulfate The concentrations embedded to corrosionpositions productswith lower aff ectsulfate the integrity concentrations. of concrete In this and regard, hence increase the diffusion the initial of sulfate porosity ions of concrete. is mainly motivedAdditionally, by the concentration the strength of gradient sulfate induced between corrosion the concrete products and isthe relatively outer environments. low, making it In easier addition, for it shouldexpansive be productsemphasized to generate that the more porosity cracks of in concrete the upcoming also plays stage. an Thus, important the diff usionrole in of affecting sulfatesin the diffusioncast-in-situ of sulfates. concrete It is is easier obvious and fasterthat concrete than that with in precast higher concrete porosity structures. can physically provide more paths forIt is the clear penetration that the porosity of sulfates. of concrete Thus, with the diff suerentlfate water concentration/cement ratios in is concrete different. with The porositya higher water/cementis lower when ratio the is water relatively/cement high, ratio while is lower, on resultingthe contrary, in better sulfates waterproofness. only reach Thus,a shallower it would depth be beneathharder the for exposed an external surface sulfate of solution concrete to with permeate a lower into water/cement the internal concrete ratio. areas and hence decreases the diffusion speed of sulfate ions. Therefore, the sulfate concentration is low and the penetration It should be noted that the diffusion of sulfates in precast concrete and cast-in-situ concrete is depth is shallow in concrete with a lower water/cement ratio. Furthermore, concrete with a lower different. First of all, sulfate ions can diffuse directly into concrete once the fresh concrete is put into water/cement ratio has a higher tensile strength, making it difficult for sulfate corrosion products to the environment during the hardening process. Then, the penetrated sulfate ions can react with the generate new cracks, while for concrete with a high water/cement ratio, it would be easier for expansive cement composition to generate sulfate corrosion products such as gypsum and ettringite. The products to extend initial cracks or generate new cracks, leading to a faster degradation. embedded corrosion products affect the integrity of concrete and hence increase the initial porosity of concrete. Additionally, the strength of sulfate induced corrosion products is relatively low, making it easier for expansive products to generate more cracks in the upcoming stage. Thus, the diffusion of sulfates in cast-in-situ concrete is easier and faster than that in precast concrete structures. It is clear that the porosity of concrete with different water/cement ratios is different. The porosity is lower when the water/cement ratio is lower, resulting in better waterproofness. Thus, it would be harder for an external sulfate solution to permeate into the internal concrete areas and hence decreases the diffusion speed of sulfate ions. Therefore, the sulfate concentration is low and the penetration depth is shallow in concrete with a lower water/cement ratio. Furthermore, concrete with a lower water/cement ratio has a higher tensile strength, making it difficult for sulfate corrosion

Materials 2020, 13, x FOR PEER REVIEW 9 of 17 products to generate new cracks, while for concrete with a high water/cement ratio, it would be easier for expansive products to extend initial cracks or generate new cracks, leading to a faster degradation.

Materials 2020, 13, 3179 9 of 17 3.3. Physical Properties The diameter and weight of different specimens in different solutions were measured and 3.3. Physical Properties recorded during the immersion. To give a clear comparison of diameter for different concrete specimens,The diameter the change and ratios weight of of diameter different specimenswere calculated in different by solutions were measured and recorded during the immersion. To give a clear comparison of diameter for different concrete specimens, the change ratios of diameter were calculated by − ddt 0 a =×100% (6) dt d0d a = − 0 100% (6) d0 × in which a is the change ratio, d0 is the initial diameter, and dt is the diameter of the specimen after a in which a is the change ratio, d0 is the initial diameter, and dt is the diameter of the specimen after a specificspecific immersion immersion period period of of tt. .The The results results are shownshown inin FigureFigure4 .4.

CCK PCK 0.003 C3S P3S C5S P5S C10S P10S 0.002

0.001 Diameter change ratio Diameter change

0.000

024681012 Time (Month)

(a)

0.006 CCK PCK C3S P3S 0.004 C5S P5S C10S P10S

0.002

0.000 Diameter change ratio Diameter

-0.002 024681012 Time (Month)

(b) Figure 4. Cont.

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0.006 CCK PCK C3S P3S C5S P5S 0.004 C10S P10S

0.002

0.000 Diameter change ratio change Diameter

-0.002 024681012

Time (Month) (c)

FigureFigure 4. 4.DiameterDiameter change change ratios ratios for for concrete concrete with with different different water/cement water/cement ratios: ratios: (a) ( a0.40,) 0.40, (b ()b 0.48,) 0.48, and (c)and 0.55. (c ) 0.55.

The comparison clearly indicates that concrete in sulfate solutions suffered radical expansion. The comparison clearly indicates that concrete in sulfate solutions suffered radical expansion. For precast concrete specimens, the diameter showed a relatively stable increase tendency during the For precast concrete specimens, the diameter showed a relatively stable increase tendency during the immersion. Only the specimen immersed in 10% sulfate solution showed a relatively slight decrease immersion. Only the specimen immersed in 10% sulfate solution showed a relatively slight decrease after the ninth month (with a water/cement ratio of 0.55). Additionally, one could observe that concrete after the ninth month (with a water/cement ratio of 0.55). Additionally, one could observe that in solution with a higher concentration of sulfates suffered more radical expansion in the early stage, concrete in solution with a higher concentration of sulfates suffered more radical expansion in the indicating that concrete suffered more serious degradation when immersed in the environments with a earlyrelatively stage, higherindicating sulfate that concentration. concrete suffered more serious degradation when immersed in the environmentsCompared with to a precast relatively concrete higher specimens, sulfate concentration. it can be seen in Figure4 that cast-in-situ concrete obviouslyCompared encountered to precast more concrete volume specimens, changes during it can the be immersion. seen in Figure As for 4 concrete that cast-in-situ immersion concrete in 5% obviouslyand 10% encountered solutions, the radiusmore volume increased changes quickly induring the early the corrosionimmersion. stage As but for soon concrete began immersion to decrease in 5%even and after 10% the solutions, sixth month. the radius Moreover, increased one could quickl observey in the visible early cracks corrosion on the stage surface but of soon cast-in-situ began to decreaseconcrete even specimens. after the The sixth spalling month. and sheddingMoreover, of on concretee could powder observe and visible fragments cracks were on also the observed surface of cast-in-situon the surface concrete of concrete specimens. in a 10% The sulfate spalling solution. and shedding of concrete powder and fragments were also observedFor precast on the concrete surface samples, of concrete the in weight a 10% increased sulfate solution. quickly in the early corrosion stage and becameFor precast relatively concrete stable aftersamples, nine months.the weight Furthermore, increased therequickly was in a slightthe early diff erencecorrosion between stage the and becameconcrete relatively in distilled stable water after and nine sulfate months. solutions, Furthermore, which was there caused was bya slight the accumulation difference between of sulfate the concretecorrosion in distilled products. water In addition, and sulfate it can solutions, be seen thatwhich concrete was caused immersed by the in accumulation solutions with of higher sulfate corrosionsulfate concentration products. In addition, obviously it experienced can be seen more that concrete fluctuation immersed during the in solutions immersion, with and higher the weight sulfate concentrationincrement was obviously relatively greater,experienced especially more at the fluctu earlyation stage. during However, the it canimmersion, be seen that and the the weight weight of incrementcast-in-situ was concrete relatively experienced greater, moreespecially significant at the changes early stage. during However, the 12-month it can immersion. be seen that The the weight weight of ofcast-in-situ specimens concrete increased experienced quickly at the more early signif stage,icant then changes became stableduring or the even 12-month began to immersion. decrease after The weightthe sixth of month,specimens especially increased for specimens quickly at immersed the early in 10%stage, sulfate then solutionbecame (shown stable inor Figure even 5beganc). to decrease after the sixth month, especially for specimens immersed in 10% sulfate solution (shown in Figure 5c).

Materials 2020, 13, 3179 11 of 17 Materials 2020, 13, x FOR PEER REVIEW 11 of 17

0.05 CCK PCK C3S P3S 0.04 C5S P5S C10S P10S 0.03

0.02

Weight change ratio Weight change 0.01

0.00 024681012 Time (Month)

(a) 0.05 CCK PCK C3S P3S 0.04 C5S P5S C10S P10S

0.03

0.02 Weight change ratio change Weight 0.01

0.00 024681012 Time (Month)

(b) 0.05 CCK PCK C3S P3S 0.04 C5S P5S C10S P10S

0.03

0.02

Weight change ratio 0.01

0.00 024681012 Time (Month)

(c)

FigureFigure 5. WeightWeight change change ratios ratios fo forr concrete concrete with with different different water/cement water/cement ratios: ratios: (a) 0.40, (a) 0.40, (b) 0.48, (b) 0.48, and and(c) 0.55. (c) 0.55.

To give a clear comparison of degradation between the precast and cast-in-situ concrete, the photos of the exposed surface of specimens after immersion are shown together. The image in Figure

Materials 2020, 13, 3179 12 of 17

Materials 2020, 13, x FOR PEER REVIEW 12 of 17 To give a clear comparison of degradation between the precast and cast-in-situ concrete, the photos 6aof theshows exposed the exposed surface ofsurface specimens of the after precast immersion concrete are with shown a water/cement together. The ratio image ofin 0.48 Figure immersed6a shows in athe 10% exposed sulfate surface solution. of the The precast images concrete in Figure with a6b,c water show/cement the ratioexposed of 0.48 surfaces immersed of the in a 10%cast-in-situ sulfate concretesolution. Theimmersed images in in Figure10% sulfate6b,c show solutions, the exposed whil surfacese the water/cement of the cast-in-situ ratios concrete are 0.48 immersed and 0.55, in 10%respectively. sulfate solutions, The comparison while the distinctly water/cement shows ratios that aremore 0.48 potential and 0.55, cracks respectively. were generated The comparison on the exposeddistinctly surface shows of that cast-in-situ more potential concrete. cracks Moreover were generated, there was on clearly the exposed more surfaceshedding of of cast-in-situ concrete powderconcrete. and Moreover, fragments there on the was cast-in-situ clearly more concrete shedding surface. of concrete The appearance powder comparison and fragments illustrates on the thatcast-in-situ precast concrete concrete surface. has a better The appearanceresistance perfor comparisonmance illustratesagainst sulfate that precast attack concretethan that has of acast-in- better situresistance concrete. performance Additionally, against the sulfate results attack illustrate than th thatat concrete of cast-in-situ with concrete.a low wa Additionally,ter/cement ratio the clearly results suffersillustrate less that degradation concrete with and a shows low water a better/cement perfor ratiomance clearly against suffers sulfate less degradation attack compared and shows to the a samplebetter performance with a high againstwater/cement sulfate ratio. attack compared to the sample with a high water/cement ratio.

(a) (b) (c)

Figure 6. Surface photosphotos ofof specimens specimens with with di ffdifferenterent water water/cement/cement ratios: ratios: (a) 0.40,(a) 0.40, (b) 0.48,(b) 0.48, and (andc) 0.55. (c) 0.55. It is clear that the diameter and weight of concrete samples are significantly affected by sulfate attack.It is In clear the that early the corrosion diameter stage, and weight sulfates of penetrates concrete samples internal are concrete significantly and reacts affected with by concrete sulfate attack.composition In the to early generate corrosion ettringite stage, and sulfates gypsum. penetr The corrosionates internal products concrete are and accumulated reacts with in theconcrete initial compositioncracks and pores to generate with immersion ettringite and time. gypsum. Soon after, The thecorrosion initial products cracks are are fully accumulated filled by the in corrosionthe initial products,cracks and and pores excessive with immersion pressure is time. loaded Soon on theafter, walls the of initial cracks cracks and pores are fully to cause filled volume by the expansion.corrosion products,During this and stage, excessive the diameter pressure and is weight loaded of specimenson the walls increase of cracks with and immersion pores time,to cause while volume in the expansion.later corrosion During stage, this the stage, excessive the diameter filling of corrosionand weight products of specimens loads great increase pressure with on immersion the walls oftime, the whilecrack, in causing the later the corrosion extending stage, of initial the cracks excessive and generationfilling of corrosion of new cracks. products Moreover, loads great the development pressure on theof a walls crack of system the crack, on the causing concrete the surface extending causes of initial serious cracks spalling and and generation shedding of ofnew concrete cracks. powder Moreover, and thefragments, development which of cause a crack a decrease system of on both the theconcrete diameter surface and weightcauses serious of concrete spalling specimens. and shedding Therefore, of concretethe diameter powder of concrete and fragments, specimens which is acause comprehensive a decrease of result both of the the diameter increase and tendency weight inducedof concrete by specimens.the proceeding Therefore, of expansion the diameter and the of decrease concrete tendency specimens induced is a comprehensive by the falling result off of concreteof the increase on the tendencyexposed surface. induced The by weightthe proceeding of concrete of expansion specimens and is a comprehensivethe decrease tendency result of induced the increase by the tendency falling offinduced of concrete by the on accumulation the exposed ofsurface. sulfate The corrosion weight productsof concrete and specimens the decrease is a comprehensive tendency induced result by ofthe the shedding increase of tendency concrete induced fragments. by the Furthermore, accumulati theon of diameter sulfate corrosion and weight products of specimens and the are decrease mainly tendencycontrolled induced by the increase by the shedding tendency of in concrete the early frag corrosionments. stage,Furthermore, while in the diameter later stage, and the weight decrease of specimenstendency plays are mainly the leading controlled role. Hence by the both increase the diameter tendency and in weightthe early of concretecorrosion specimens stage, while encounter in the latera rapid stage, decrease the decrease at the end tendency of the immersion, plays the leadin especiallyg role. for Hence concrete both with the a diameter high water and/cement weight ratio of concreteimmersed specimens in the solution encounter with a arapid high decrease sulfate concentration at the end of (suchthe immersion, as the specimen especially C10S for shown concrete in with a high water/cement ratio immersed in the solution with a high sulfate concentration (such as the specimen C10S shown in Figures 4c and 5c). From the results, it can be inferred that cast-in-situ concrete suffered more severe degradation in the later corrosion stage than precast concrete. In

Materials 2020, 13, 3179 13 of 17

Figures4c and5c). From the results, it can be inferred that cast-in-situ concrete su ffered more severe degradation in the later corrosion stage than precast concrete. In addition, the spalling and shedding of concrete fragments became more serious at the later stage, which damaged the integrity of the outer layer of concrete and then caused potential loss of concrete strength.

3.4. Mechanical Properties The results of compressive strength tests are presented in Figure7. It is clear that the strength of different specimens in different solutions is quite different. MaterialsFor 2020 precast, 13, x FOR concrete PEER REVIEW specimens, the strength showed a relatively stable increase tendency during13 of 17 the immersion, especially for concrete specimens immersed in 3% and 5% sulfate solutions. For concrete addition, the spalling and shedding of concrete fragments became more serious at the later stage, immersed in 10% sulfate solution (with water/cement ratio of 0.55), the strength began to decrease after which damaged the integrity of the outer layer of concrete and then caused potential loss of concrete the ninth month, while for concrete specimens with water/cement ratios of 0.40 and 0.48, the strength strength. remained increasing or stable even at the end of the immersion. It is noteworthy that the strength of cast-in-situ concrete increased quickly, and the value was 3.4. Mechanical Properties relatively low in the early stage, while after that, the increase tendency of strength became stable or evenThe began results to decrease of compressive after the strength sixth month. tests are In presente addition,d onein Figure could 7. notice It is clear that that concrete the strength lost more of differentstrength whenspecimens immersed in differen in thet solutions withis quite higher different. sulfate concentration.

65 CCK PCK C3S P3S 60 C5S P5S C10S P10S 55

50

45

40 Compressive Strength (MPa) Strength Compressive

35 024681012 Time (Months) (a) 55 CCK PCK C3S P3S 50 C5S P5S C10S P10S 45

40

35

30 Compressive Strength (MPa) Strength Compressive

25 024681012 Time (Months)

(b)

Figure 7. Cont.

Materials 2020, 13, 3179 14 of 17 Materials 2020, 13, x FOR PEER REVIEW 14 of 17

55 CCK PCK 50 C3S P3S C5S 45 P5S C10S P10S 40

35

30

25

Compressive Strength (MPa) Strength Compressive 20

15 024681012 Time (Months)

(c)

Figure 7. CompressiveCompressive strength strength of of concrete concrete wi withth different different water/cement water/cement ratios: ratios: (a) 0.40, (a) 0.40, (b) 0.48, (b) 0.48,and and(c) 0.55. (c) 0.55.

ItFor is precast clearly illustratedconcrete specimens, in Figure 7the that strength the water showed/cement a relatively ratio had stable a significant increase tendency influence during on the strengththe immersion, of concrete especially specimens. for concrete Generally, specimens the strength immersed of concrete in 3% with and a greater5% sulfate water solutions./cement ratio For wasconcrete clearly immersed lower than in the10% strength sulfate solution of concrete (with with wa ater/cement lower water ratio/cement of 0.55), ratio. the Moreover, strength the began results to distinctlydecrease after indicated the ninth that themonth, strength while of for concrete concrete with specimens a higher waterwith water/cement/cement ratio beganratios toof decrease0.40 and earlier,0.48, the and strength the strength remained was increasing relatively or low stable at the even end at of the the end immersion. of the immersion. In addition, the strength of concreteIt is innoteworthy the solution that with thea strength higher sulfate of cast-in-situ concentration concrete decreased increased faster quickly, and lostand morethe value strength was afterrelatively immersion. low in the early stage, while after that, the increase tendency of strength became stable or evenFor began precast to decrease concrete after specimens, the sixth the month. hardening In ad processdition, andone strengthcould notice development that concrete were lost finished more beforestrength being when put immersed into the sulfatein the solutions solutions. with Thus, higher sulfate sulfate ions hadconcentration. to penetrate internal concrete from the outside-inIt is clearly with illustrated immersion in Figure time. Sulfate7 that the ions wa penetratedter/cement the ratio outer had layer a significant of concrete influence together on with the thestrength permeating of concrete of pore specimens. water during Generally, the immersion. the strength Then, of concrete the penetrated with a greater sulfate water/cement ions reacted withratio thewas concrete clearly lower composition than the to strength generate of corrosion concrete with products, a lower which water/cement accumulated ratio. in Moreover, the initial cracksthe results and poresdistinctly to cause indicated expansion that the and strength degradation. of concrete Therefore, with a the higher degradation water/cement of precast ratio concrete began to proceeded decrease fromearlier, the and surface the strength into the was internal relatively areas oflow the at concretethe end of gradually. the immersion. However, In addition, for cast-in-situ the strength concrete, of theconcrete strength in the development solution with of concretea higher wassulfate negatively concentration affected decreased in the early faster corrosion and lost stage, more sincestrength the hardeningafter immersion. process of concrete was not complete during this stage. Hence, the strength of cast-in-situ concreteFor wasprecast lower concrete than that specimens, of precast the concrete, hardening even process when theand specimens strength development were immersed were in thefinished same sulfatebefore being solutions, put into especially the sulfate when solutions. the sulfate Thus, concentration sulfate ions was had high. to penetrate internal concrete from the outside-inIt has to be with emphasized immersion that time. the Sulfate water/ ionscement pene ratiotrated has the asignificant outer layer e offfect concrete on the together strength with and resistancethe permeating performance of pore water of concrete during against the immersion. sulfate attack. Then, Itthe is penetrated well accepted sulfate that ions concrete reacted with with a lowerthe concrete water/ compositioncement ratio to always generate results corrosion in a relatively products, higher which density accumulated and lower in the porosity. initial cracks Therefore, and thepores strength to cause of expansion specimens and with degradation. lower water Therefor/cemente, ratio the isdegradation relatively high. of precast In addition, concrete the proceeded porosity offrom concrete the surface also plays into the a significant internal areas role inof atheffecting concrete the gradually. diffusion speed However, of sulfate for cast-in-situ ions, subsequently concrete, influencingthe strength the development mechanical of property concrete of was concrete. negatively affected in the early corrosion stage, since the hardeningThe generation process of and concrete accumulation was not ofcomplete sulfate corrosionduring this products stage. Hence, in the cracksthe strength and pores of cast-in-situ influence theconcrete properties was lower of concrete. than that In theof precast very early concrete, corrosion even stage, when the the cracks specimens and pores were are immersed not fully in filled; the hence,same sulfate the accumulation solutions, especially of corrosion when products the sulfate do not concentration affect the strength was ofhigh. concrete. Then, the excessive fillingIt ofhas corrosion to be emphasized products generate that the pressurewater/cement on the ra wallstio has of a internal significant cracks effect and on pores, the strength making and the concreteresistance denser performance and causing of concrete a slight against increase sulfate in strength. attack. It Then,is well initial accepted cracks that are concrete extended with and a lower new crackswater/cement are generated, ratio always decreasing results the in strength a relatively of concrete higher todensity a large and extent. lower In porosity. this regard, Therefore, for concrete the withstrength a higher of specimens water/cement with ratio,lower itwater/cement takes less time ratio for sulfatesis relatively to penetrate high. In internaladdition, concrete the porosity regions of andconcrete brings also a great plays deal a significant of damage role to concrete in affectin structures.g the diffusion Moreover, speed the tensileof sulfate strength ions, ofsubsequently the internal influencing the mechanical property of concrete.

Materials 2020, 13, 3179 15 of 17 concrete area with a higher water/cement ratio is low, making it easier for sulfate corrosion products to generate new cracks and bring great damage.

4. Conclusions Experiments were performed to investigate the degradation of precast and cast-in-situ concrete caused by sulfate attack. Parameters regarding physical, chemical, and mechanical properties were determined and compared in detail. Methods of SEM, EDS, and XRD were used to reveal the micro-structural and mineral changes after immersion. Based on the experiments in the present study, the main conclusions are as follows: Degradation of both precast and cast-in-situ concrete is caused by chemical sulfate attack when the concrete is fully immersed in the sulfate-rich environments. The main corrosion products induced by sulfate attack are ettringite and gypsum. Sulfate attack results in great expansion and weight loss in the later corrosion stage, putting concrete structures in great danger, especially for cast-in-situ concrete structures. The diffusion of sulfates is faster in cast-in-situ concrete than that in the precast concrete due to the faster development of cracks in the cast-in-situ concrete. The faster strength increase of cast-in-situ concrete in the early corrosion stage is induced by the faster accumulation of corrosion products. Cast-in-situ concrete obviously suffers more strength loss during corrosion. Precast concrete has better resistance performance against sulfate attack than cast-in-situ concrete structures, indicating that precast concrete structures are more suitable in sulfate-rich environments.

Author Contributions: Conceptualization, G.Z. and M.S.; methodology, G.Z. and H.F.; software, M.G.; validation, G.Z.; formal analysis, M.S.; investigation, M.G. and H.F.; resources, G.Z.; data curation, M.G.; writing—original draft preparation, G.Z.; writing—review and editing, M.S.; visualization, G.Z.; supervision, G.Z.; project administration, G.Z.; funding acquisition, G.Z. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China, grant number 51908466; the Post-Doctoral Innovative Talent Support Program, grant number BX20200287; the Fundamental Research Funds for the Central Universities, CHD, grant number 300102210305; and the Natural Science Foundation of Shaanxi Province, grant numbers 2020JQ-361 and 2020JQ-271. Acknowledgments: The authors would like to thank Longfei Xu (in Chang’an University) for giving us technical support during the experiments. Conflicts of Interest: The authors declare no conflict of interest.

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