Deterioration Mechanism of Concrete Subjected to Acid Rain Attack—An Experimental and Modeling Approach

LI Beixing, YUAN Xiaolu, ZHANG Yaming, ZHU Wenkai

(1. State Key Laboratory of Silicate Materials for Architecture, Wuhan University of Technology, Wuhan 430070, China; 2. Key Laboratory of Geological Hazards on Three Gorges Reservoir Area, Ministry of Education, China Three Gorges University, Yichang 443002 , China)

Abstract: The experimental and modeling approaches were taken to study the deterioration mechanism of concrete under acid rain attack. Concrete specimens were prepared and exposed to the simulated acid rain environment. The neutralization depth of concrete was measured, and the mineralogical composition and microstructure of concrete were analyzed using X-ray diffraction (XRD) and scanning electron microscope(SEM). The experimental results show that the degradation of concrete increases with the corrosion time and the decrease in pH value of acidic solution from 3.5 to 1.5. Concrete was corroded by H+ and SO42- in acid rain, producing gypsum and leading to the neutralization of concrete. The acid rain exposure also resulted in the decomposition of hydration products of cement, such as C-S-H and ettringite, forming the main corrosion products of gypsum and SiO2·nH2O. Based on the second Fick diffusion law, a model was developed to describe the deterioration mechanism of concrete exposed to acid rain mathematically, coupling the diffusion of reactive ions and the corrosion reaction. The simulation results and the experimental results were compared and discussed.

Key words: acid rain; deterioration mechanism; concrete; modeling

1 Introduction

Industrial and urban developments have brought about severe acid rain pollution. Acid rain is a strongly corrosive medium, containing not only H+but also SO42-, NO3-,etc.When concrete structure is exposed to acid rain, physical and chemical reactions occur between acid rain and concrete material[1]. This accelerates the degradation of concrete and the rusting of reinforced bars, bringing serious durability problem of concrete structure.

Experimental studies have been conducted on durability of concrete under acid rain environment.Okochiet al[2]carried out the investigation by laboratory and field exposure of cement mortar to acid deposition. Laboratory results have shown that the dissolved Ca(OH)2increases with the increase in the acidity of simulated acid rain solution and the decrease in the flow rate, and that the conditions of heating and cooling promote the neutralization in material. The field experiment for 2 years has indicated that the corrosion is limited to the surface of mortar. The mechanical properties of concrete suffering cyclic acid rain exposure were studied by Zhouet al[3]. The results have suggested that the pure mode II fracture toughness and the elastic modulus of concrete decrease with the increase of wetting-drying cycles, and increase with the rising of the pH value of simulated acid rain. There is a linear relationship between pure mode II fracture toughness and compressive strength of concrete under cyclic acid rain attack. Tripathiet al[4]and Sahooet al[5]have pointed out that the proper additions of mineral admixtures such as silica fume and fly ash can improve the durability of concrete exposed to acidic environment.

Several researchers have studied the deterioration mechanism of concrete under acid rain. However,owing to its complexity, the deterioration mechanism has not yet been well understood. For example, Xieet al[6]suggested that the corrosion of concrete under acid rain were caused by both H+and SO42-, producing the expansive CaSO4·2H2O, CaAl2Si2O8, Ca3Al6O12·CaSO4.By means of microscopic analysis methods, Chenet al[7]have concluded that the acid rain leads to the decomposition of cement hydration products including Ca(OH)2, calcium silicate hydrates, calcium aluminate hydrates and ettringite, forming nothing expansive but CaSO4.2H2O. Okochiet al[2]have found that the corrosion products include carbonate, chloride, nitrate and sulfate.

The objective of this paper is to explore the deterioration mechanism of concrete subjected to acid rain through experiments and modeling. Experiments were performed to analyze the composition and microstructure of concrete under acid rain attack, figuring out the corrosion reactions and the corrosion products. A model is to be developed to describe the deterioration mechanism of concrete mathematically, considering both the diffusion process of reactive ions and the corrosion reaction taking place in concrete. The proposed model is to be verified by comparing the simulation results with the experimental results.

2 Experimental

2.1 Materials and specimens preparation

Ordinary Portland cement (OPC) with strength grade 42.5 and ClassⅠfly ash (FA) with the specific surface area of 622 m2/kg were used in making concrete. Chemical compositions of cement and fly ash are shown in Table 1. Polycarboxylate superplasticizer with the water reducing rate of 28% was used to achieve the slump of 200 mm for fresh concrete. River sand with the fineness modulus of 2.93 and the coarse aggregate of crushed limestone with the size of 5-20 mm were used. The analytically pure concentrated sulfuric acid(H2SO4) and concentrated nitric acid (HNO3) were used to prepare the simulated acid rain solution.

Table 1 Chemical compositions of cement and fly ash /%

Table 2 Mix proportion of concrete /(kg/m3)

Mixture proportion of concrete is listed in Table 2.Concrete specimens (100 mm×100 mm×400 mm) were made and cured until 28 d in a standard curing condition (20±2 ℃, RH ＞95%).

Before exposure to the acid rain attack, the hardened cement paste was taken from concrete surface and ground into powder for the measurement of X-ray diffraction (XRD). XRD analysis was conducted using the D/Max-IIIA diffractometer made in Japanese Rigaku Company.

2.2 Experimental methods

(1) Simulated acid rain experiment

The acid rain in most areas of China has pH value＜4.5, and it is mainly the acid rain of sulfuric acid type containing increasing nitric acid[8]. Thus, the simulated acid rain solution was prepared by mixing the analytically pure H2SO4and HNO3. The molar concentration ratio of NO3-and SO42-in the solution was 5:1. The pH value of the solution was 1.5, 2.5 and 3.5, respectively.The ratio of the acidic solution volume to the whole surface area of concrete specimens was approximately 80:1. The surface of the simulated acid rain solution was 10 cm higher than the top surface of the specimens.

A periodic soaking-drying method was adopted.One erosion cycle was composed of exposure to the simulated acid rain solution for 6 d, subsequently air drying at room temperature for 12 h, oven drying at 60℃ for 8 h, and finally cooling at ambient temperature for 4 h. The pH value of the acidic solution was measured by a digital pH-meter and adjusted to original value using the mixed solution of sulfuric acid and nitric every 24 h, and the mixed acid solution was periodically refreshed every one erosion cycle.

Neutralization depth of concrete was measured every 30 days of exposure. After 270 days of exposure,cement paste samples were taken from the surface of concrete and examined for the composition through the XRD analysis. Scanning electron microscope (SEM)was performed on the concrete sample obtained from the surface of concrete by JSM-5610LV SEM.

(2) Neutralization depth test

The neutralization depth of concrete was measured as follows:

Every 30 days of exposure to the simulated acid rain solution, concrete specimen was taken out and cut in half to obtain a piece of segment with a cross section of 100 mm×100 mm. Then parallel measuring lines at 10 mm spacing distance were drawn on the cross section, on which a 0.1% phenolphthalein solution in alcohol was sprayed, shown in Fig.1. The length of red part for measuring lineiwas measured by vernier caliper,denoted asW(i),i= 1, 2, …, 8, 9. The neutralization depth for measuring linei(ND(i)) was calculated according to Formula (1):

The neutralization depth of the concrete specimen was obtained by averaging nine values ofND(i).

Fig.1 Cross section of concrete and measuring lines for neutralization depth

2.3 Modeling methodology

The modeling approach used the experimental results and the second Fick diffusion law. A suitable index for quantitatively representing the degradation of concrete structure is identified by experimentally studying the deterioration process of concrete under acid rain. Then, based on the second Fick diffusion law,a diffusion-reaction equation is established, expressing both the diffusion process of reactive ions and the corrosion reaction. According to the initial conditions and the boundary conditions, the diffusion-reaction equation is solved to obtain the concentration distributions of reactive ions in concrete. Finally, the deterioration model relating to the suitable index and the diffusion-reaction equation is proposed to describe the deterioration mechanism of concrete structure exposed to acid rain.

3 Results and discussion

3.1 Experimental results

3.1.1 Visual inspection

Under the simulated acid rain attack, concrete specimens presented the degradation of different degrees, shown in Fig.2. It can be observed that the visual appearance of concrete has to do with the corrosion time and pH value of the acidic solution. The surface roughness, swelling and scaling of concrete generally increase with corrosion time. Concrete over 90 d exposure showed evident surface scaling for pH = 1.5 and slight surface scaling for pH = 2.5; it kept intact for pH= 3.5. Over 180 d exposure, concrete specimens exhibit evident surface scaling for pH = 1.5 and 2.5; and for pH = 3.5, there is a slight scaling on concrete surface.Over 270 d exposure, concrete specimens present swelling and fine cracks for pH = 1.5 and 2.5, and the swelling and fine cracks of concrete increase with the decrease of pH value from 2.5 to 1.5; and for pH = 3.5,there is no swelling and fine cracks but a slight scaling on the surface of concrete.

Fig.2 Visual appearance of concrete specimens exposed to the acidic solution with the pH value of 1.5, 2.5 and 3.5

3.1.2 Neutralization depth

Fig.3 Experimental and simulation results for neutralization depth of concrete

Experimental results for the neutralization depth of concrete specimens exposed to the acid rain environment are presented in Fig.3. It can be seen that the acid rain exposure gives rise to the neutralization of concrete. The neutralization depth of all concrete specimens increase with corrosion time. The pH value of simulated acid rain solution has effect on the neutralization depth of concrete. As the pH value of the acidic solution is raised from 1.5 to 3.5, the neutralization depth of concrete decreases.

3.1.3 XRD analysis

XRD analysis was conducted to investigate the mineralogical composition of concrete before and after acid rain attack, and the results are shown in Fig.4.Portlandite and ettringite present relatively large intensity peaks before exposure; while their intensity peaks almost disappear after exposure. There are mainly two products detected after exposure, which were gypsum and SiO2. When the pH value of acidic solution dropped from 3.5 to 1.5, the intensity peaks of gypsum became higher and the intensity peaks of SiO2became lower. Especially when the pH value of acidic solution was 1.5, SiO2was hardly detected.

Fig.4 XRD analysis of cement paste before and after exposure

Fig.5 SEM image of concrete after exposure (pH=2.5)

The results indicate that the hydration product of portlandite is corroded by acid rain.

Formula (2) shows the formation of gypsum.This is the main reason that huge amounts of gypsum are detected in the XRD pattern after exposure. When pH value of acidic solution drops from 3.5 to 1.5, the addition of concentrated sulfuric acid increases in the simulated acid rain solution, leading to the increase of gypsum formed. Gypsum is a kind of expansive substance, and it accumulates largely, causing the swelling and cracking of concrete, seen in Table 3 (pH = 1.5 and 2.5, over 270 d corrosion).

Formula (3) is a neutralization reaction. As the acidity of acid rain solution and the corrosion time increase, the more Ca(OH)2is consumed by H+, resulting in the increase of the neutralization depth of concrete(Fig.3). The neutralization of concrete destroys the protection of concrete cover for reinforcing bars, leading to the rusting of them. Moreover, the neutralization of concrete induced the decomposition of some main hydration products of cement, such as C-S-H and ettringite[9,10].

Formula (4) produces SiO2·nH2O which dehydrates into SiO2. This explains why there exist diffraction peaks for SiO2after exposure in Fig.4. However,with the decrease of pH value of acidic solution from 3.5 to 1.5, SiO2might gradually become amorphous and undetectable.

Formula (5) produces gypsum and Al2O3·nH2O.Al2O3·nH2O is possibly corroded by H+in acidic solution.

Formula (6) may be the reason why there are not diffraction peaks for Al2O3in the XRD pattern after exposure.

3.1.4 SEM analysis

Fig.5 illustrates the SEM image of concrete subjected to simulated acid rain solution (pH = 2.5), showing the microstructure and morphology of concrete.Large amounts of white blocky crystals were observed in the image. They are supposed to be gypsum crystals according to the XRD results.

3.2 Model development and results

3.2.1 Choosing an index to express the deterioration of

concrete structure quantitatively

According to the preceding experimental results,the neutralization of concrete plays a key role in the degradation of concrete structure under acid rain exposure. It brings about not only the decomposition of hydration products of cement but also the rusting of reinforcing bars. The rusting of reinforcing bars is particularly important to influence the durability of concrete structure[11]. Therefore, the neutralization depth of concrete is chosen as the index to express concrete deterioration quantitatively.

When concrete is subjected to acid rain, H+in the acid rain diffuses in concrete because of concentration gradient. Ca(OH)2in concrete dissolves, forming Ca2+and OH-. Owing to concentration gradient, Ca2+and OH-diffuse outwards, reacting with H+.

According to Formula (7), it becomes neutral when the mole ratio of H+and OH-is 1:1. Thus, the neutralization depth of concrete is supposed to be the depth of concrete where the concentration of H+is equal to or higher than the concentration of OH-.

whereCHandCOHare the concentration distribution of H+and OH-in concrete respectively, which are to be figured out.

3.2.2 Establishment of the concentration distribution formulas of H+and OH-in concrete

Fig.6 Scheme of concrete under acid rain attack

Suppose that a concrete block is invaded by acid rain from one side, shown in Fig.6. The diffusion of H+and OH-is due to the concentration gradient. Therefore,the second Fick diffusion law could be used to describe the diffusion process of H+and OH-in concrete.

whereDHandDOHare the diffusion coefficients of H+and OH-in concrete respectively, which are assumed to be constant.xis the distance from certain point in concrete to the surface of concrete from which acid rain enters,tis the corrosion time.

Because the chemical reaction happens between H+and OH-during their diffusion process, the transport of H+and OH-is influenced by not only the concentration gradient but also the neutralization reaction. Then,the reaction factor is introduced into the second Fick diffusion equation[12], and Formulas (9) and (10) are changed as:wherekis a constant to express the rate of neutralization reaction taking place in concrete. Initially, there is no H+in concrete and the concentration of OHis regarded as the concentration of OH-in saturated Ca(OH)2solution. Then the initial conditions for Formulas (11) and (12) are respectively described as:

whereCHrainis the concentration of H+in acid rain.

According to the initial conditions and the boundary conditions, Equations (11) and (12) are solved, and the concentration distributions of H+and OH-in concrete are obtained as:

3.2.3 Simulation results

To verify the model proposed, the simulation results for the neutralization depth of concrete are compared with the experimental results, shown in Fig.3.It is seen that the simulation values are not always consistent with the experimental ones. When pH value of acidic solution is 1.5 and 2.5, the simulation results have good consistency with experimental ones at early age of about 60 d; while afterwards they become inconsistent, and the difference between them increases significantly especially after 180 d. When pH value of acidic solution is 3.5, the simulation results have good consistency with experimental ones at early age of about 150 d and afterwards their difference increases.

It is indicated that the developed model has some limitations. This may be because that concrete exhibits cracking and scaling on surface at later period of acid rain exposure (in Fig.2), leading to the change of the diffusion coefficientsDHandDOH. Therefore, the next work is to study a formula relating to the diffusion coefficient of concreteD, the corrosion timetand the positionx, and improve this model by introducing the formula into it.

4 Conclusions

A methodology based on experiments and modeling has been applied to study the deterioration mechanism of concrete under acid rain attack.

a) Experimental results suggest the corrosion reactions and corrosion products. Ca(OH)2in hardened cement paste is corroded by acid rain, forming gypsum and leading to the neutralization of concrete. As pH value of acidic solution drops from 3.5 to 1.5, gypsum formed increases, causing the swelling and cracking in concrete. The neutralization of concrete increases with the corrosion time and the decrease in pH value of acidic solution from 3.5 to 1.5, bringing about not only the rusting of reinforcing bars but also the decomposition of hydration products of cement, such as C-S-H and ettringite. The corrosion products are a mixture mainly consisting of gypsum and SiO2·nH2O.

b) A model has been proposed to describe the deterioration mechanism of concrete exposed to acid rain mathematically. The model is based on the second Fick diffusion law, and coupled the diffusion equation and the corrosion reaction. The model can be used to simulate the neutralization depth of concrete under acid rain.The simulation results were compared with the experimental results, and some difference between them was discussed. To improve the modeling, a formula relating to the diffusion coefficient of concreteD, the corrosion timetand the positionxshould be established and introduced into the model.