CORROSION MECHANISMS FOR CEMENTED SOILS IN THREE DIFFERENT SULFATE SOLUTIONS Abstract In order to simulate and study the corrosion effects on the compressive strength of cemented soils that could be exposed in a polluted environment, a series of tests were conducted on cemented soil blocks cured with different concentrations of H2SO4, MgSO4, and Na2SO4 solutions. The test results show that the corrosion degree generally increases with the corrosion time and the solution concentration, while the compressive strength decreases with the increasing corrosion degree. The corrosion degree is highest for the Na2SO4 solution, followed by the MgSO4 and H2SO4 solutions. Namely, when the SO42- ion exists in a solution, the corrosion degree for the positive ions follows this descending order: Na+, Mg2+, and H+. X-ray diffraction (XRD) phase analyses were performed for the cemented soil samples after corrosion and ionic concentrations. The results show that the compressive strength decreases with an increase of the Mg2+ concentration in the MgSO4 solution and the Na+ concentration in the Na2SO4 solution. At the same time, the strength increases with an increase of the pH value of the H2SO4 solution. Based on the chemical analysis results, the corrosion of H2SO4 or MgSO4 solutions on cemented soils is deemed as a composite action involving the combined resolving and crystallizing corrosion processes. Furthermore, the corrosion of the Na2SO4 solution of cemented soil is a composite action consisting of dissolving and crystallizing. Pengju Han Taiyuan University of Technology, College of Architecture and Civil Engineering Taiyuan, Shanxi 030024 P. R. China Chao Ren Taiyuan University of Technology, College of Architecture and Civil Engineering Taiyuan, Shanxi 030024 P. R. China Xiaohong Bai Taiyuan University of Technology, College of Architecture and Civil Engineering Taiyuan, Shanxi 030024 P. R. China Y. Frank Chen (corresponding author) The Pennsylvania State University, Department of Civil Engineering Middletown, PA 17057, USA E-mail: yxc2@psu.edu Keywords cemented soil, compressive strength, corrosion mechanism, sulfate, pollution, solution 1 INTRODUCTION The cemented soil technique is a method of mixing the cement with in-situ soils in order to improve the soil's properties. Cement-stabilized soils may or may not work in corrosive conditions. Cemented soils would be influenced under the environment of acid rain, seawater invasion, or industrial pollution. These adverse effects could result in a structural deterioration. At a corrosive site, the strength of the soil stabilized by the cement is increased at the beginning of the stabilization, but it will be decreased due to the deterioration over time. Cemented soils are utilized in a SO42- corrosive environment when the groundwater is polluted or used under the seawater. Such a corrosive environment will inevitably corrode the cemented soil and thus change its mechanical properties. This serious issue must be considered in any practical project. Several researchers have looked at the influence of a corrosive environment on the properties of cemented soils for various aspects. Venkatarama and Jagadish [1] performed an experimental study on the influence of soil composition on the strength and durability characteristics of soil-cement blocks. Walker [2] summarized Acta Geotechnica Slovenica, 2015/2 25. P. Han et al.: Corrosion mechanisms for cemented soils in three different sulfate solutions the results from a comprehensive investigation undertaken to assess the influence of soil characteristics and cement content on the physical properties of stabilized soil blocks. Pei and Yang [3] stated that the influence of Na2SO4 on cemented soils contributed to the diffusion of Na+ and SO42- and that their chemical reaction with Ca(OH)2 weakened the strength of the cemented soils due to decomposition and breaking. Shihata and Bagh-dadi [4] investigated the durability characteristics and compressive strength of cemented soils after a prolonged exposure to saline ground water. Jiao and Liu [5] analyzed the influence of the cement-soil strength in an acid environment. Kolias et al. [6] studied the effectiveness of high-calcium fly ash and cement in stabilizing fine-grained clayey soils in the laboratory. Ning et al. [7, 8] investigated the behaviors of cemented soils under various environmental conditions and concluded that, in contrast to the mechanical strength, the environmental corrosion had little effect on the fracturing process. Dong et al. [9, 10] tested the mechanical properties and electrical resistance of cement-soil polluted by H2SO4. Iyengar and Al-Tabbaa [11] presented two effective magnesia-based cements for the stabilization/ solidification (S/S) of contaminated soils. Xing et al. [12, 13] showed that Mg2+, Cl-, and SO42- caused not only a change in the microstructures of salt-rich soil-cements, but also reduced the strength of the soil-cement composite. Zandieh and Yasrobi [14] proved that two polymer materials could be used to stabilize the polluted soils on road shoulders, slopes, and military and airport pads. Heineck et al. [15] analyzed the microstructural behavior of composite mixtures of residual soils and sodic bentonites that were used as contaminant barriers. They carried out a series of microstructural analyses, including X-ray diffraction (XRD), scanning electronic microscopy (SEM), and energy-dispersive spectrometry (EDS). Voglar and Lestan [16] set up a strength model that the formulations of ordinary Portland cement (OPC), calcium aluminate cement (CAC) and pozzolanic cement (PC) and additives were used for the solidification/stabilization (S/S) of soils from a contaminated industrial brownfield. Liu et al. [17] deduced the relationship between the ion concentration and the corrosion time based on the theory of chemical dynamics, damage mechanics, and the chemical reaction formula between MgCl2 and the cement soil. Yang et al. [18, 19] considered the factors of cement content, curing age concentrations of magnesium sulfate, and pH value and concluded that the strength of the cemented soil increased with the cement content and curing age. They also analyzed the microscopic mechanism of failure. Nevertheless, studies on the influence of different sulfate solutions on the properties of cemented soils are still rather limited. To further investigate the corrosion effect and process in a sulfate corrosion environment, a series of tests, including unconfined compressive tests, were conducted on the cemented soil blocks that were cured by H2SO4, MgSO4, and Na2SO4 solutions with different concentrations. Photos of the blocks' appearances, XRD phase of the corrosion power of the cemented soils, and the concentrations of SO42-, Mg2+, H+ or Na+ in the corrosive solutions after curing the blocks were also measured. Corrosion mechanisms for different sulfate solutions on cemented soils were investigated. 2 EXPERIMENTAL PROCEDURES 2.1 Materials The main chemical composition of the cemented soils for testing is listed in Table 1. A summary of the materials and the mixing machine is provided as follows. (1) Soil: Air-dried silt soil with plasticity index (Ip) = 8.1, uniformity coefficient (Cu) = 26.67, and curative coefficient (Cc) = 1.35. (2) Cement: Ordinary Portland cement (OPC) with compressive strength = 32.5MPa after 28 days of curing, produced by a local cement company in Taiyuan, a northern city in China. (3) Proportion of cemented soil contents: soil in mass: cement: water = 100:20:50. Tap water is used for the cemented soil, and water content value in cemented soil = 29.4%. The cement content value is 11.8%. (4) Mixing machine: HJW-30 blender having a body volume = 30L and a rotational speed = 48 rpm. (5) Typical size of scaled cemented soil samples: 70.7 x 70.7 x 70.7 mm3. (6) Standard curing time for the cemented soil prior to submersion with a sulfate solution: 7 days. Table 1. Main chemical composition of cemented soils. pH Ca2+ Mg2+ SO42- Cl- CO32- OH- (-) (mg/kg) (mg/kg) (mg/kg) (mg/kg) (mg/kg) (mg/kg) 9.80 427.84 354.17 2879.10 282.32 1090.8 496.64 2.2 Preparations of H2SO4, MgSO4, and Na2SO4 solutions Based on the Chinese national standards GB 50021 (2009) [20] and GB 50046 (2008) [21], the following concentrations of H2SO4, MgSO4, and Na2SO4 sulfate solutions were considered in this study: 1.5g/L, 4.5g/L, 9.0g/L, and 18.0g/L. The corrosion results for the three solutions are summarized in Tables 2-4, in which the corrosion ratings were based on GB 50021 (2009) [20]. 78. Acta Geotechnica Slovenica, 2015/2 P. Han et al.: Corrosion mechanisms for cemented soils in three different sulfate solutions According to GB 50021 (2009) [20], the circumstance would be graded as B for samples fully cured by pure water. For either solution with lower concentration, the corrosion rating is consistent as weak (for 1.5g/L concentration) or medium (for 4.5g/L concentration), Tables 2-4. For higher concentration (9.0 or 18.0g/L), no consistency for the corrosion rating is noted. Table 2. Chemical ingredients of H2SO4 solution and corrosion-evaluation results. Concen- pH SO 24 Corrosion Corrosion Corrosion Corrosion (g/L) degree degree (g/L) rating (g/L) rating 1.5 5.4 weak 1.302 weak 4.5 4.3 medium 2.401 medium 9.0 2.0 strong 3.358 strong 18.0 1.6 strong 3.889 strong Table 3. Chemical ingredients of MgSO4 solution and corrosion-evaluation results. Concen- Mg2+ SO 24 Corrosion Corrosion Corrosion Corrosion (g/L) degree degree (g/L) rating (g/L) rating 1.5 2.196 weak 0.960 weak 4.5 2.293 weak 2.000 medium 9.0 2.368 weak 2.168 medium 18.0 2.538 weak 2.500 medium Table 4. Chemical ingredients of Na2SO4 solution and corrosion-evaluation results. Concentration Na+ SO 24 Corrosion Corrosion Corrosion Corrosion (g/L) degree degree (g/L) rating (g/L) rating 1.5 0.521 no 1.001 weak 4.5 1.198 no 2.300 medium 9.0 1.461 no 2.805 medium 18.0 1.670 no 3.207 strong 2.3 Testing Procedure The testing procedure consisted of the following steps. (1) Prepare the cemented soil blocks, as described in the above. (2) Cure the blocks with MgSO4, H2SO4, and Na2SO4 solutions in five different concentrations, i.e., 0, 1.5, 4.5, 9.0 and 18.0g/L. (3) Photograph each soil block at the curing times of 3, 7, 14, and 28 days to observe the change in appearance during the corrosion process. (4) Conduct the unconfined compression tests for the blocks at the same curing time as described in Step 3. For each curing time, three blocks were tested simultaneously for each prescribed concentration in a solution. The average value from the three tests is used as the block strength. (5) Measure the main ionic concentrations of the solutions immediately after the blocks are removed. (6) Perform phase analyses for the cemented soil samples in powder form after corrosion using the TD 3500 X-ray diffraction (XRD) machine, made in Denmark. The cemented soil powder samples were taken from MgSO4, H2SO4, and Na2SO4 solutions with a concentration of 18.0g/L. The XRD test was a continuous scan with a scanning angle of 200-700 and a scanning speed of 0.020/s. After the test, the corrosive powders were analyzed using JADE5.0 software to determine their chemical components [22]. 3 RESULTS AND DISCUSSION_ 3.1 Change of block appearance Figures 1-3 show the photographed appearances of the samples after 28 days of curing in the solutions. From these photos, it is clear that the sulfate solution changes the look of a cement-soil block by ways of peeling, size reduction, and cracking. Detailed observations are described as follows. (1) As shown in Figure 1, for the samples cured in the H2SO4 solution the main phenomena are peeling, size reduction, and increasing corrosion degree with the greater concentration. (2) Referring to Figure 2, for the MgSO4 solution with a lower concentration (i.e., 1.5-4.5 g/L), only crystal-like material was seen on the surface. The change in the surface size is not obvious. However, for a higher concentration (i.e., 9.0-18.0g/L), the corrosion degree becomes more serious so that both crystal-like material and peeling were observed. The corrosion degree is less than that in the H2SO4 solution. At the onset of the surface peeling, the soil sample started to crack at a concentration of 18.0g/L. (3) For samples in the more dilute Na2SO4 solution with the lower concentration (1.5-4.5g/L), similar to the MgSO4 solution only crystal-like material was observed on the surface (Figure 3). For a higher concentration (9.0-18.0g/L), the corrosion degree is more significant and the sample begins to break. For samples cured in the solution with the concentration of 18.0g/L, the damage was too severe to perform a strength test. 78. Acta Geotechnica Slovenica, 2015/2 P. Han et al.: Corrosion mechanisms for cemented soils in three different sulfate solutions (a) 1.5 g/L (b) 4.5 g/L (c) 9.0 g/L (d) 18.0 g/L Figure 1. Photos for the sample blocks in the H2SO4 solution after 28 days of curing. (a) 1.5 g/L (b) 4.5 g/L (c) 9.0 g/L (d) 18.0 g/L Figure 2. Photos for the sample blocks in the MgSO4 solution after 28 days of curing. (a) 1.5 g/L (b) 4.5 g/L (c) 9.0 g/L (d) 18.0 g/L Figure 3. Photos for the sample blocks in the Na2SO4 solution after 28 days of curing. Based on the above observations, the influence of the sulfate solution on the soil blocks increases with the increase of the solution concentration and the curing time. For inorganic compounds with a high solution concentration (9.0-18.0g/L), the influence degree for the three solutions follows such a descending order: Na2SO4 > MgSO4 > H2SO4. Namely, when the SO42- ion exists in a solution, the positive Na+ ion has the highest corrosion degree, followed by Mg2+ and H+. 3.2 Unconfined compressive-strength test results The compressive strength of the soil block cured in a sulfate solution (f'cu) is determined by Equation 1. f = a f J cu J a (1) where a is the modified coefficient (Table 5) reflecting the corrosion degree and fcu is the compressive strength of the block cured in pure water (i.e., when a = 1). The calculated unconfined compression strengths for the cemented soil blocks are shown in Figure 4. As demonstrated, the compressive strength decreases with the increase of the solution concentration. Generally, it increases with the corrosion time, except for the Na2SO4 solution with the high concentration (18.0g/L), where the strength is reducing to zero as the sample becomes broken. Coefficients a greater than 1 are indicated in boldface in Table 5. For the a coefficients corresponding to 28 days of curing, we observed the following findings from Table 5. 78. Acta Geotechnica Slovenica, 2015/2 P. Han et al.: Corrosion mechanisms for cemented soils in three different sulfate solutions Table 5. Modified coefficients (a) of cemented soils. Concentration H2SO 4 MgSO4 Na2SO4 (g/L) 3 d1 7 d 14 d 28 d 3 d 7 d 14 d 28 d 3 d 7 d 14 d 28 d 1.5 1.00 0.87 0.89 0.78 1.25 1.24 1.14 1.13 1.30 1.21 1.19 1.17 4.5 0.99 0.82 0.88 0.70 1.12 1.03 0.95 0.87 1.15 1.12 1.07 0.97 9.0 0.99 0.79 0.84 0.64 1.00 0.98 0.83 0.63 0.92 0.9 0.83 0.53 18.0 0.98 0.70 0.55 0.49 0.97 0.92 0.78 0.56 0.90 0.85 0.44 0 1 d = days (1) For the MgSO4 and Na2SO4 solutions with a 1.5g/L concentration, the coefficient a is greater than 1. This is a favorable environment as the compressive strength of the cemented soils will be increased. (2) For all the other concentrations of either solution the a coefficient is less than 1, indicating that the degree of corrosion is higher and the strength is reducing. The worst case occurs when the sample is cured in the Na2SO4 solution with a 18g/L concentration, where the strength becomes zero (i.e., a = 0) due to broken samples (Table 5). For safer, optimal, and more economical designs, it is suggested that the existing environmental condition of the cemented soils be taken into account when determining the foundation bearing capacity and settlement. 3.3 Relationship between the compressive strength (y and the ion concentration of solution (C) (1) fcu versus C of SO42- The relationship between fcu and C of the SO42- ion is shown in Figure 5. As indicated, the test data seem to be encompassed by a triangle with the slant described by fcu = -0.0008C + 4. This signifies the close relationship between the compressive strength and the SO42- concentration. 3.5 2.8 n 2.1 :> 1.4 0.7 0.0 "Og/L ■ 1.5g/L ■4.5g/L "9.0g/L ■18.Og/L 7 14 21 28 Time (days) (a) H2SO4 solution 7 14 21 Time (days) (b) MgSO4 solution ■Og/L ■1.5g/L ■4.5g/L ■9.Og/L " 18.Og/L 14 21 : Time (days) (c) Na2SO4 solution Figure 4. Relationship between the compressive strength (fcu) of the cemented soils and the corrosion time. 5,0 4,0 3,0 TO 2,0 1,0 0,0 fcu = -0.0008C +4 s ^ ------ 1000 2000 C (mg/L) 3000 4000 Figure 5. Relationship between fcu and C of SO42-. 78. Acta Geotechnica Slovenica, 2015/2 P. Han et al.: Corrosion mechanisms for cemented soils in three different sulfate solutions (2) fcu versus the concentration of a positive ion (Mg2+, H+, or Na+) The relation between fcu and the positive ion concentration or the pH values is shown in Figure 6. It is clear that the compressive strength fcu) decreases with the increase of the Mg2+ concentration (C) of the MgSO4 solution, nonlinearly expressed by fcu = -0.136ln(C) + 2.1322, where fcu is in MPa and C is in mg/L (Figure 6a). fcu decreases with the increase of the Na+ concentration of the Na2SO4 solution linearly described by fcu = -0.0006C + 2 (Figure 6c). Again, a close relationship between the compressive strength and the ion concentration is noted. fcu increases with an increase of the pH value of the H2SO4 solution, also linearly represented by fcu = 0.0897V + 0.7372, where V is the pH value (Figure 6b). The above-derived mathematical expressions for fcu could be useful for practical designs. 3.4 Analysis of the cemented soil phases and discussion of the corrosive mechanisms After 28 days of curing, XRD tests were performed for the corrosive powder samples that were taken from the MgSO4, H2SO4, and Na2SO4 solutions with a concentration of 18.0g/L. At the conclusion of the testing, the powder chemical products (phases) were analyzed using JADE5.0 software [22]. The analysis results are presented in Figures 7-9. As seen from the figures, the resulting chemical products are very different among the different sulfate solutions and pure water. Therefore, the chemical mechanism can be determined based on the X-ray diffraction (XRD) results and the chemical formulas. (1) Corrosion mechanism for the cemented soil in the H2SO4 solution The main chemical reaction for the cemented soil in the H2SO4 solution is expressed by Equations (2)-(4). (2) Ca(OH)2 + H2SO4 = CaSO4-2H2O 3CaO-2SiO2-3H2O + 3H2SO4 = 3[CaSO4-2H2O] + 2SiO2+ 6H2O (3) 3CaO-Al2O3-3H2O + 3H2SO4 = 3[CaSO4-2H2O] + Al2O3 + 6H2O (4) As indicated in the chemical formulas (2)-(4), H2SO4 reacts with Ca(OH)2 3CaO-2SiO2-3H2O (C-S-H) and 3CaO-Al2O3'3H2O(C-A-H), where H+ participates actively and causes the cemented soil to form an unsteady structure. Therefore, the corrosion of the cemented soils in the H2SO4 solution is deemed as being a "resolving corrosion". 2,5 2,0 Q_ 1,5 2 1,0 0,5 0,0 fa,= -0.1361n(C) +2.1322 O 0 600 2400 1200 1800 C (mg/L) (a) Compressive strength (fcu) versus Mg2+ concentration (C). fcu = 0.0897V + 0.7372 2,5 - 2,0 - 1,5 - 2 J 1,0 - 0,5 ■ 0,0 ■ 2 4 6 8 10 12 pH Value (b) Compressive strength (fcu) versus pH value (V). 2,5 -I 2,0 - 1,5 - 1,0 ■ 0,5 ■ 0,0 ■ o o o fcu = -0.0006C +2.13 200 600 1000 1400 1800 2200 C (mg/L) (c) Compressive strength (fcu) versus Na+ concentration (C). Figure 6. Relationship between the compressive strength (fcu) and the positive ion concentration (C) or pH Value (V). Figure 7 gives the corrosive products for the cemented soil in the H2SO4 solution with a 18.0g/L concentration. As shown, the peaks for the C-S-H (3CaO-2SiO2-3H2O) phase diffraction are small and there is virtually no peak for either Ca(OH)2 (C-H) or 3CaO-Al2O3-3H2O (C-A-H) phase diffraction. For the H2SO4 solution with a high concentration of 18.0g/L, the cementing agent virtually does not function as intended. Based on the above results, the H2SO4 solution is acting in the acid state. These chemical reactions resolve the main cementing agents contained in the cemented soil, i.e., Ca(OH)2, 3CaO-2SiO2-3H2O, and 3CaO-Al2O3-3H2O, and cause the surface peeling and strength reduction. 78. Acta Geotechnica Slovenica, 2015/2 P. Han et al.: Corrosion mechanisms for cemented soils in three different sulfate solutions Figure 7. Chemical products in the H2SO4 solution (HS4) with a 18.0g/L concentration and in pure water (W). l:Si02 2:A1;Ö3 3:CaSÖ4 4:CaC03 SCaSiOjj 6:Na;Si03 7:C-S-H S:C-A-S-H 7s itJn «I^jil Jm 11 Uli. I H S4 4wtwJlliJ ..I. .M'i ni—i'1 L■ Jim A .....- ■ Jt 1 Aj w 20 30 40 _ 50 60 Diffraction angle 26/ ° 70 Figure 8. Chemical products in MgSO4 solution (MS4) with 18.0g/L concentration and in pure water (W). Figure 7 also shows the apparently intense peaks for the C-A-S-H and CaSO4 phase diffractions, where C-A-S-H means 3CaO-Al2O3-3CaSO4-32H2O and 3CaO-Al2O3-CaSO4-18H2O. Besides the resolving action, the dissociative SO42- in the solution may take the following chemical reactions. 3CaSO4 + 4CaO-Al2O3-19H2O + 14H2O = (5 3CaO-Al2O3-3CaSO4-32H2O + Ca(OH)2 ( ) 2CaSO4 + 3CaO-Al2O3-CaSO4-18H2O + 14H2O = 3CaO-Al2O3-3CaSO4-32H2O (6) CaCO3 + Ca(OH)2 + SiO2 + CaSO4-2H2O + 12H2O = CaCO3-CaSO4-CaSiO3-15H2O (7) The effects of these reactions on the compressive strength are less significant for the cemented soil in the H2SO4 solution with a lower concentration as the ion H+ plays the main role. Those effects become more significant when the concentration is higher. The crystallizing resultants of 3CaO-Al2O3-3CaSO4-32H2O (C-A-S-H) and CaCO3-CaSO4-CaSiO3-15H2O (C-S-C-H) are apparently larger than the reactant. As such, their inflating force is greater than the sticking force in the cemented soil, and thus causes the cracking in the cemented soil block and the strength reduction. In conclusion, the H2SO4 solution results in a crystallizing corrosion in addition to the resolving corrosion. (2) Corrosion mechanism for the cemented soil in the MgSO4 solution Figure 8 gives the corrosive products for the cemented soil in the MgSO4 solution with a 18.0g/L concentration. From the figure, the peaks for the products C-A-S-H, M-A-H (MgO-Al2O3-H2O), M-S-H (MgO-SiO2-H2O), and CaSO4 appear more intense. The main chemical reactions for the cemented soil in the MgSO4 solution are expressed using Equations (8)-(13). 3CaO-2SiO2'3H2O (C-S-H) + 3MgSO4 + 6H2O= 3[CaSO4-2H2O] + 3Mg(OH)2 + 2SiO2 (8) 3CaO-Al2O3-3H2O(C-A-H) + 3MgSO4 + 6H2O= 3[CaSO4-2H2O] + 3Mg(OH)2 + Al2O3 MgCl2 + Ca(OH)2 + 6H2O = CaCl2-6H2O + Mg(OH)2 3Mg(OH)2 + MgCl2 + 8H2O = 2Mg2(OH)3Cl-4H2O 2Mg(OH)2 + 3CaO-2SiO2-3H2O(C-S-H) = 2[MgO-SiO2-H2O] (M-S-H) + 3Ca(OH)2 Mg(OH)2 + 3CaO-Al2O3-3H2O(C-A-H)= MgO-Al2O3-H2O (M-A-H) + 3Ca(OH)2 (9) (10) (11) (12) (13) Equations (8) and (9) indicate a chemical reaction will take place to dissolve the main cementing agent (i.e., 3CaO-2SiO2-3H2O and 3CaO-2Al2O3-3H2O), causing surface peeling for the cemented soil and strength reduction. It has been well documented that the volumes for the resultants CaCl2-6H2O and Mg2(OH)3Cl-4H2O [i.e., Equations (10) and (11)] are seven times those of Ca(OH)2. So, the new products, CaCl2-6H2O and Mg2(OH)3Cl-4H2O, can fill in the voids of the cemented soil. This is beneficial for the unconfined compressive strength of cemented soils when the mass of MgCl2 is suitable and the new reactants (CaCl2-6H2O and Mg2(OH)3Cl-4H2O) are small in each MgSO4 solution. At a 1.5g/L concentration, the volume of CaSO4-2H2O resultants [from Equations (8) and (9)] is two times that of the Ca(OH)2. CaSO4-2H2O is desirable for the 78. Acta Geotechnica Slovenica, 2015/2 P. Han et al.: Corrosion mechanisms for cemented soils in three different sulfate solutions compressive strength of cemented soil as the mass of CaSO4'2H2O is deemed suitable. This explains why the strength of the cemented soil cured in MgSO4 solutions with a 1.5g/L concentration is greater. However, the product of Mg(OH)2 may react with 3CaO-2SiO2-3H2O (C-S-H) and 3CaO-Al2O3-3H2O (C-A-H) if there is enough Mg(OH)2 in the MgSO4 solution. The new reactants such as MgO-SiO2-H2O (M-S-H) and MgO-Al2O3-H2O (M-A-H) have poor coagulation, resulting in an unsteady structure and a reduced soil strength. Consequently, the corrosion of the cemented soil in the MgSO4 solution is a resolving corrosion. Besides the resolving action, the dissociative product CaSO4-2H2O [produced by Equations (8) and (9)] may take the chemical reactions as expressed by Equations (5)-(7). The products 3CaO-Al2O3-3CaSO4-32H2O and 3CaO-Al2O3-CaSO4-18H2O (C-A-S-H) and CaSO4-2H2O may also have crystallizing corrosion. So, the corrosion for the cemented soil cured in the MgSO4 solution is a combined resolving and crystallizing corrosion. (3) Corrosion mechanism for the cemented soil in the Na2SO4 solution Figure 9 gives the corrosive products for the cemented soil in the Na2SO4 solution with a 18.0g/L concentration. From the figure, the peaks for products C-A-S-H and CaSO4 are more apparent. In addition to Equations (5)-(7), other chemical reactions for the cemented soil in the Na2S04 solution include Equations (14)-(17). Ll uL.....11 L nf^trntm w tylfc l:SiCb 3:Ca(OH>2 4:CaS04 5:CaCOj 6:CaSiOj 7:Na2SiÔ3 8C-S-H 9:C-A-S-H • NS4 «jL w 20 30 _ 40 50 Diffraction angle 29/ ' 60 70 Figure 9. Chemical products in Na2SO4 solution (NS4) with 18.0g/L concentration and in pure Water (W). Na2SO4 + 10H2O = Na2SO4-1QH2O Ca(OH)2 + Na2SO4-10H2O = CaSO4-2H2O + 2NaOH + 8H2O 3CaO-2SiO2-3H2O (C-S-H) + 4NaOH = 3Ca(OH)2 + 2Na2SiO3 + 2H2O (14) (15) (16) 3CaO-Al2O3-6H2O(C-A-H)+2NaOH = 3Ca(OH)2 + Na2O-Al2O3 + 4H2O (17) For the Na2SO4 solution with a 1.5g/L concentration, with suitable mass of Na2SO4, the new reactants (i.e., C-A-S-H and CaSO4-2H2O) can fill the voids of the cemented soil and thus increase the unconfined compressive strength. This also explains why the strength of the cemented soil cured in the Na2SO4 solution with a lower concentration at 1.5g/L is larger. For higher concentrations of the Na2SO4 solution, due to the excessive mass of Na2SO4, the Na2SO4 will be combined with H2O to produce the new crystallizing product of Na2SO4-10H2O [Equation (14)]. Na2SO4-10H2O tends to inflate with the cemented soil with C-A-S-H and CaSO4-2H2O, the so-called "crystallizing inflation", and reduces the strength. One the other hand, Na2SO4 combined with Ca(OH)2 produces a new product, NaOH [Equatiion (15)]. NaOH may react with 3CaO-2SiO2-3H2O and 3CaO-Al2O3-6H2O, when the amount of NaOH is enough in the cemented soil [Equations (16) and (17)]. Therefore, the sticking materials in the cemented soil will be decomposed and the new poorly coagulating reactants such as Na2SiO3 and Na2O-Al2O3 will be formed, which dissolve the cemented soil. Hence, the corrosion of the cemented soil cured in the Na2SO4 solution is a combined dissolving and crystallizing corrosion. In summary, the corrosion mechanism for the cemented soil in a H2SO4 or MgSO4 solution is a composite type involving resolving and crystallizing. In addition, it is a composite type consisting of dissolving and crystallizing for the materials in a Na2SO4 solution. 4 CONCLUSIONS_ Based on the results, the following conclusions can be drawn. (1) The sulfate solution changes the cement-soil block including peeling, size reduction, and cracking. The effect of the solution on the block increases with the increase of the concentration of the solution and the curing time. (2) The unconfined compressive strength of the cemen-ted-soil block decreases with the increase of sulfate solution concentration and the curing time. The derived a coefficients can be used to predict the modified compressive strength of cemented soil in various concentrations of corrosive solution. 84. Acta Geotechnica Slovenica, 2015/2 P. Han et al.: Corrosion mechanisms for cemented soils in three different sulfate solutions (3) The degree of the corrosion effect from the various solutions is in the descending order: Na2SO4 > MgSO4 > H2SO4. When the SO42- ion exists in a solution, the corrosion degree for the positive ions follows this descending order Na+ > Mg2+ > H+. (4) In terms of the corrosion mechanism for the cemented soil, the corrosion type is found to be a combined resolving and crystallizing for the H2SO4 and MgSO4 solutions and a combined dissolving and crystallizing for the Na2SO4 solution. Acknowledgement This study was financially supported by the National Natural Science Foundation of China (Grant No. 51078253 & 51208333) and Natural Science Foundation of Shanxi Province (2014011036-1, 2014131019, TYUT2014YQ017, 0IT2015). The opinions expressed in this paper are solely of the authors, however. REFERENCES [1] Venkatarama-Reddy, B.V. and Jagadish, K.S. 1995. 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