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Effects of cations in sulfate on the thaumasite form of sulfate attack of cementitious materials
Construction and Building Materials 229 (2019) 116865
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Effects of cations in sulfate on the thaumasite form of sulfate attack of cementitious materials Yaoling Luo a,b, Shuai Zhou a,⇑, Chong Wang a,⇑, Zheng Fang a a b
College of Materials Science and Engineering, Chongqing University, Chongqing 400045, China China West Construction Group Co., Ltd., Chengdu 610015, China
h i g h l i g h t s The degradation process of the TSA is studied in the sulfate solution. The effect of cations in the sulfate on the TSA has been investigated. The influence of mixed cations has been considered. 2+
The relative corrosivity of cations was: Mg
a r t i c l e
i n f o
Article history: Received 15 April 2019 Received in revised form 8 August 2019 Accepted 1 September 2019 Available online 9 September 2019 Keywords: Full immersion Mixed full immersion Types of cations TSA Cement-based materials
> Ca2+ > Na+.
a b s t r a c t The thaumasite form of sulfate attack (TSA) of cement-based materials is a severe threat. There is little research on the effect of cations in the sulfate on the TSA, especially the mixed cations. The effects of six conditions, namely, Na2SO4 full immersion, MgSO4 full immersion, CaSO4 full immersion, Na2SO4 & MgSO4 full immersion, Na2SO4 & CaSO4 full immersion and MgSO4 & CaSO4 full immersion, on the TSA were studied by the observed degradation grades, mass loss, Raman analysis and X-ray diffraction (XRD). Results show that the degradation grade, degradation rate and degradation products are influenced by cations greatly. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Sulfate attack is widely recognized to cause damage and degradation in cementitious materials. Vicat [1] previously researched the chemical corrosion of cement and concrete in briny environments. Candlot and Michaelis firstly reported sulfate attack at the end of the nineteenth century, and Michaelis called the degradation product ‘‘cement bacillus”, which is also known as ettringite [2]. Bied [3] invented calcium aluminate cement in order to prevent sulfate attack. Sulfate attack is caused by the chemical reaction between the hydration products and sulfates either inside the cement or from the external environment. In addition, sulfate attack also causes physical damage. The main chemical degradation products are gypsum, ettringite and thaumasite, among others. The type of sulfate attack is related to the type of cations
⇑ Corresponding authors. E-mail addresses: [email protected] (S. Zhou), [email protected] (C. Wang). https://doi.org/10.1016/j.conbuildmat.2019.116865 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
in the sulfate, and the most types of sulfates cause different levels of sulfate attack except BaSO4 and the PbSO4. The main corrosive sulfates include Na2SO4, MgSO4 and CaSO4. CaSO4 attack is the main cause of ettringite attack. Na2SO4 attack can cause ettringite attack and gypsum attack, and some studies have indicated that Na2SO4 attack also causes decalcification of C-S-H gels [4]. MgSO4 attack is the most serious form of degradation, as it allows both sulfate attack and magnesium salt attack. MgSO4 attack can also result in ettringite attack and gypsum attack. Meanwhile, C-S-H gels are transferred to M-S-H, which has no cementation [5–8]. The thaumasite form of sulfate attack (TSA) can occur when there is a carbonate near a corrosive sulfate at low temperatures (<15 °C). Limestone powder is extensively applied around the world [9–11]. TSA is a serious type of attack that can destroy cement completely [12–18]; hence, extensively applied limestone powder is an extremely severe threat to cement and concrete. The main test for sulfate attack is sole sulfate full immersion in the laboratory, but there are various sulfates in natural environments. To date, there have been no reports about compound
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sulfate attack based on the kinds of cations in the sulfate, and research about the effect of cations in sulfate on TSA lacks systematic experiments. Hence, this paper studied the effect of cations in the sulfate on the TSA, as well as the mixed sulfate. 2. Materials and methods 2.1. Preparation of basic materials The cement is composed of 97 wt% portland cement clinker and 3 wt% gypsum. The gypsum is CaSO42H2O. The limestone powder is from Bao Xing Sichuan China and the content of CaCO3 exceeds 98%. The specific surface area of the limestone powder is 266 kg/m2. The chemical components of the above materials are shown in Table 1. The XRD patterns of the clinker and limestone powder are exhibited in Fig. 1 and Fig. 2, respectively. The corrosive solutions are made up of MgSO4, Na2SO4 and CaSO42H2O. The concentrations of SO24 in the MgSO4 solution and Na2SO4 solution are both 33.8 g/l, and the CaSO4 solution is a saturated solution. 2.2. Experimental methods Fig. 1. XRD of cement clinker.
The cement paste samples were prepared according to Table 2. The sample size was 40 mm 40 mm 40 mm. These samples were placed in the curing chamber for 24 h after synthesis, and then were cured for 30 days. The low-temperature environment of 5 ± 2 °C was applied then. There were six test groups: Na2SO4 full immersion, MgSO4 full immersion, CaSO4 full immersion, Na2SO4 & MgSO4 full immersion (i.e., mixed Na2SO4 and MgSO4 solutions), Na2SO4 & CaSO4 full immersion, and MgSO4 & CaSO4 full immersion (illustrated in Table 3). The mass mix ratio of mixed solutions both was 1:1, and these solutions were replaced every 60 days. The appearance of the samples was observed and recorded every 60 days, and the damage grade of samples was evaluated according to Table 4. Micro-samples were created for XRD and Raman analysis. The broken pieces were soaked and rinsed using anhydrous alcohol, and dried in vacuum oven at 50 °C. The X-ray diffractometer was from the Ricoh Company, with D/MAX-IIIC and Co Ka radiation (0.2 Å), and the sample was scanned from 5°-75° within 5 min. Raman spectroscopic measurements were carried out using a HORIBA Jobin Yvon S. A. S spectrometer (product model: Lab RAM HR Evolution; spectral resolution: visible spectral 0.65 cm1; spectral region from 200 nm to 1050 nm). 3. Results and discussion
Fig. 2. XRD of limestone powder.
Table 2 Mixture proportions of cement paste.
3.1. Macro-properties
Cementitious materials (wt%)
The corrosive solutions were replaced every 60 days. At that time, the degradation grade of the samples was recorded. The degradation grade and the appearance of samples are shown in Figs. 3 and 4, respectively. The degradation grade of the six groups after 720 days is displayed in Fig. 3. Grade 10 means that the surface of the samples had been destroyed and produced some pulp. The degradation rate and degradation degrees with MgSO4 full
Cement
CaCO3 powder
70
30
Water/cementitious materials ratio 0.40
immersion, Na2SO4 & MgSO4 full immersion, and MgSO4 & CaSO4 full immersion were the fastest and most severe, as the surface of the above three samples had converted to pulp at 360 days.
Table 1 Chemical components of clinker, gypsum, and limestone powder (wt%). Materials
SiO2
Fe2O3
Al2O3
CaO
MgO
SO3
K2O
Na2O
LOI
Clinker Gypsum Limestone powder
19.99 4.47 0.23
2.98 0.36 0.21
4.80 0.99 –
61.22 34.05 55.46
3.27 1.84 –
0.23 40.61 –
0.88 0.23 –
0.18 0.08 –
3.52 16.87 41.75
Y. Luo et al. / Construction and Building Materials 229 (2019) 116865 Table 3 Experimental groups. Experimental group
Corrosive solution
Na Mg Ca Na & Mg Na & Ca Mg & Ca
Na2SO4- full immersion MgSO4- full immersion CaSO4- full immersion Na2SO4 & MgSO4- full immersion Na2SO4 & CaSO4- full immersion MgSO4 & CaSO4- full immersion
Table 4 Degradation grade from TSA. Degradation grades
Degradation features
1 2 3 4
No visible deterioration Some deterioration at corners Deterioration at corners and edges Deterioration at corners and some cracking along the edges Cracking and expansion along the edges Serious cracking and expansion Spalling and expansion of the surfaces Grey pulp at corners, edges and surface Amount of grey pulp at samples’ surface Complete pulp at samples’ surface
5 6 7 8 9 10
Fig. 3. The degradation classification of six samples within 720 days.
The degradation rate with Na2SO4 full immersion and Na2SO4 & CaSO4 full immersion was slower, and the sample converted to pulp at 480 days. The degradation rate of samples under CaSO4 full immersion was the slowest, and this sample converted to pulp at 660 days. Fig. 4 shows the appearance of these six samples at 360 days and 720 days, respectively. The surface of samples under the MgSO4 full immersion, Na2SO4 & MgSO4 full immersion, and MgSO4 & CaSO4 full immersion had completely converted to pulp. The surface of the samples under Na2SO4 & CaSO4 full immersion had partially converted to pulp. The samples under Na2SO4 full immersion and CaSO4 full immersion had no obvious degradation at 360 days. Fig. 4(b) shows a comparative diagram of the control group and test groups. The samples under MgSO4 full immersion, Na2SO4 & MgSO4 full immersion, and MgSO4 & CaSO4 full immersion had been dissolved by half. The degrees of degradation for the other three groups were lower. The degradation with sulfate full immersion is a spalling and softening destruction, as the sample converted to pulp gradually and the degradation occurred from the surface to the core of the sample as the degradation continued.
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The degradation degree of samples can be studied by statistical methods based on Fig. 3 [19–20]. The correlation coefficient between the time and the degradation degree is 0.96, 0.88, 0.97, 0.86, 0.95 and 0.84 under the Na2SO4 full immersion, MgSO4 full immersion, CaSO4 full immersion, Na2SO4 & MgSO4 full immersion, Na2SO4 & CaSO4 full immersion, and MgSO4 & CaSO4 full immersion, respectively. The degradation degree and the time are highly interrelated, and the Na2SO4 has the highest correlation. A more detailed statistical analysis by the machine learning method [21] will be provided when more data are obtained in our following research. The mass losses of the six groups were tested and are displayed in Fig. 5. The mass losses of the samples under MgSO4 full immersion, Na2SO4 & MgSO4 full immersion, and MgSO4 & CaSO4 full immersion were the most serious. In particular, the mass loss of the samples under MgSO4 full immersion had exceeded 50%. The mass loss of the samples under Na2SO4 full immersion, and Na2SO4 & CaSO4 full immersion were lower than the above three groups. The mass loss of the sample under CaSO4 full immersion was the lowest. Combined with the data above, the type of cations in the sulfate has a significant impact on the degradation. The calcium hydroxide is the weakest part of cementitious materials in a long-term period. Among the three kinds of ions, magnesium ion has the highest reaction activity with Ca(OH)2. The formation of Mg(OH)2 precipitation will consume lots of Ca(OH)2, reduce the alkalinity of the pore fluid, and thus decrease the stability of CASAH gel. Besides, magnesium ions also react directly with CASAH gel to form M-S-H and destroy the original CASAH gel. In Na2SO4 solution, Na2SO4 will react with Ca(OH)2 to form CaSO4. Calcium sulfate is a slightly soluble substance and will react with calcium aluminate hydrate to form ettringite, which also causes the degradation of cementitious materials. In CaSO4 solution, calcium ions in the solution can diffuse into the specimen and increase the calcium ion concentration in the specimen. Therefore, the high concentration of calcium ion can maintain the concentration of calcium hydroxide to some extent, which reduces the degradation of cement. Hence, the relative corrosivity of the cations is Mg2+ > Na+ > Ca2+. To avoid the influence of the concentration of sulfate ion, the same concentration of sulfate ion was used to prepare 6 groups of corrosive solution. The concentration of magnesium ion in MgSO4 solution is the highest, and the concentration of magnesium ion in MgSO4 & CaSO4 and MgSO4 & Na2SO4 is only half of that in MgSO4 solution. Therefore, we can see that the mass loss under MgSO4 soaking is the greatest and its degradation is the most serious. The concentration of magnesium ion in MgSO4 & CaSO4 and MgSO4 & Na2SO4 was the same, and the mass loss of the two groups was almost the same. Similarly, for sodium ions, the sodium ion concentration of Na2SO4 solution is the highest, and the sodium ion concentration of Na2SO4 & CaSO4 is only half of that. It can be seen that the mass loss of Na2SO4 solution is greater than that of Na2SO4 & CaSO4. Of the three cations, calcium ions have the lowest corrosivity. Hence, the samples in CaSO4 solution have the lowest mass loss. According to the corrosivity and concentration of cations, it can be concluded that the higher concentration of magnesium ion results in a more serious erosion. Further, the higher concentration of sodium ion induces a more serious erosion when there was no magnesium ion. The corrosivity of the mixed solutions depended on the most corrosive cation and its concentration. The chemical reaction and analysis are detailed in Section 3.2. 3.2. Micro-properties The thaumasite and ettringite were difficult to be distinguished by XRD due to their similar molecular structures. Hence, these degradation products were firstly identified by Raman
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Fig. 4. The appearance of the six samples at 360 days and 720 days.
spectroscopy. The samples with Mg2+ in the solution were taken every 60 days for 360 days to identify the degradation products by Raman spectroscopy. Then, after 360 days, these samples were identified every 180 days. The samples with Na+ in the solution were taken every 60 days within 480 days to identify the type of degradation products by Raman spectroscopy. Further, the samples were identified at 540 days and 720 days. The samples with Ca2+ in the solution were taken every 120 days within 720 days to identify the type of degradation product by Raman spectroscopy. After that, XRD was used to analyse the degradation process and mechanism.
3.2.1. Raman analysis The Raman spectra of the six groups within 720 days are indicated from Figs. 6–11. Fig. 6 shows the Raman spectra of the sample under Na2SO4 full immersion within 720 days. The characteristic thaumasite peak was found at 360 days, but there was only one characteristic peak, which illustrated that the degradation product was a compound of thaumasite and ettringite. The three characteristic peaks of thaumasite were found at 540 days, indicating that the degradation product was mainly thaumasite. These data indicated that the degradation product was mainly
Y. Luo et al. / Construction and Building Materials 229 (2019) 116865
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Fig. 5. The mass loss of the six samples at 720 days.
Fig. 8. Raman spectra of samples under CaSO4 full immersion within 720 days.
Fig. 6. Raman spectra of samples under Na2SO4 full immersion within 720 days.
Fig. 9. Raman spectra of samples under Na2SO4 & MgSO4 full immersion within 720 days.
Fig. 7. Raman spectra of samples under MgSO4 full immersion within 720 days.
ettringite within 360 days, a compound of ettringite, gypsum and thaumasite from 420 days to 540 days, and a compound of gypsum and thaumasite after 540 days. The sample was attacked by TSA in the Na2SO4 full immersion after 540 days, and thaumasite formation (TF) occurred between 420 days and 540 days.
Fig. 10. Raman spectra of samples under Na2SO4 & CaSO4 full immersion within 720 days.
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Fig. 11. Raman spectra of samples under MgSO4 & CaSO4 full immersion within 720 days.
Fig. 7 displays the Raman spectra of the samples under MgSO4 full immersion within 720 days. A characteristic peak for thaumasite was found at 240 days, and the three characteristic peaks of thaumasite were found at 300 days. These data indicated that the degradation product was mainly ettringite within 180 days. TF occurred between 180 days and 240 days, and the sample was attacked by TSA under the MgSO4 full immersion after 300 days. Fig. 8 exhibits the Raman spectra of the samples under CaSO4 full immersion within 720 days. The degradation rate of the samples under CaSO4 full immersion was the slowest; hence, Raman spectra were taken every 120 days. The thaumasite was found at 600 days, and the sample was attacked by TSA under the CaSO4 full immersion after 720 days. TF occurred during the period between 600 days and 720 days. Fig. 9 shows the Raman spectra of samples under Na2SO4 & MgSO4 full immersion within 720 days. The degradation process was the similar as that under MgSO4 full immersion, but the degradation rate is lower than the MgSO4 full immersion. The degradation products were the compound of ettringite and gypsum within 300 days, and TSA began after 360 days. Fig. 10 shows the Raman spectra of the samples under Na2SO4 & CaSO4 full immersion within 720 days. Thaumasite was found at 360 days, and the obvious characteristic thaumasite peaks were observed after 540 days. Combined with the above data, TSA was found at 480 days. Hence, the TF process occurred between 300 days and 420 days, and the sample was attacked by TSA after 420 days. The degradation products were ettringite and thaumasite from 360 days to 420 days, and they became gypsum and thaumasite after 480 days. Fig. 11 shows the Raman spectra of the samples under MgSO4 & CaSO4 full immersion within 720 days. Thaumasite was found at 300 days, and obvious TSA was observed after 360 days. The TF process occurred between 240 days and 300 days. The samples under MgSO4 full immersion, Na2SO4 & MgSO4 full immersion, and MgSO4 & CaSO4 full immersion were attacked by TSA after 360 days, and the TF process occurred from 240 days to 360 days. The samples under Na2SO4 full immersion, and Na2SO4 & CaSO4 full immersion were both attacked by TSA after 420 days, and the TF process occurred from 360 days to 420 days. The TF process began at 600 days for the samples under CaSO4 full immersion, and the TSA started after 720 days. The concentration of magnesium ions in the MgSO4 solution was greater than that in Na2SO4 & MgSO4 solution, and magnesium
ions have the greatest corrosivity among the 3 cations. Some characteristic peaks of thaumasite appeared after 180 days in MgSO4 solution, and the characteristic peaks of thaumasite were very obvious after 300 days of degradation. In the early degradation process, ettringite and gypsum are the main degradation products. The characteristic peaks of thaumasite were found only after 300 days of degradation, while the characteristic peaks of thaumasite were obvious after 360 days. Therefore, ettringite and gypsum mainly existed in the Na2SO4 & MgSO4 solution within 300 days. The degradation process of the two groups was similar, but the degradation rate of the group in Na2SO4 & MgSO4 solution was significantly slower than that of the group in MgSO4 solution. The above results of the mass loss of six groups of specimens also indicate that the corrosivity of sodium ions is greater than that of calcium ions. The sodium ion concentration in the Na2SO4 solution was significantly greater than that in the Na2SO4 & CaSO4 solution, so the degradation of the Na2SO4 solution group was more serious. Therefore, it can be observed from Fig. 11 and Fig. 8 that the production of TF and TSA of Na2SO4 solution group was significantly faster than that in the Na2SO4 & CaSO4 solution group. The Raman results agree well with the mass loss results. 3.2.2. XRD analysis In the XRD patterns, the three main ettringite diffraction peaks were 9.73, 5.61 and 3.88 Å, and the three main diffraction peaks of thaumasite were 9.56, 5.51 and 3.41 Å. The thaumasite and ettringite were difficult to be differentiated by XRD. Hence, Raman spectroscopy was used to differentiate the thaumasite and ettringite, and the following phase analysis of the surface region of samples by XRD supported the Raman data. Fig. 12 displays the XRD of the samples under Na2SO4 full immersion within 720 days. The main ettringite diffraction peak was at 9.73 Å, and was firstly enhanced and reached its maximum at 180 days. After that, the ettringite diffraction peak began to weaken until 360 days. Meanwhile, thaumasite was found at 360 days (see Raman data above), and then, thaumasite began to form quickly. In addition, gypsum was found after 300 days, and it began to produce significantly after 420 days; however, the gypsum disappeared after 540 days. The Na2SO4 diffused into the samples and reacted with Ca(OH)2 to generate gypsum (see Eq. (1)), and the Na2SO4 also participated to generate the ettringite (see Eq. (2)).
Fig. 12. XRD patterns of samples under Na2SO4 full immersion within 720 days.
Y. Luo et al. / Construction and Building Materials 229 (2019) 116865
Na2 SO4 þ CaðOHÞ2 þ 2H2 O ! CaSO4 2H2 O þ 2NaOH
7
ð1Þ
3CaO Al2 O3 þ 3CaSO4 2H2 O þ 26H2 O ! 3CaO Al2 O3 3CaSO4 32H2 O
ð2Þ
Fig. 12 indicates that the main chemical reaction was Eq. (2), which generated ettringite. The generation of gypsum followed in the early degradation stage. In the common Na2SO4 full immersion attack, gypsum is generated at first, and then more Na2SO4 diffuses into the sample, which leads to an excess concentration of sulfate and Ca2+, reducing the solubility of the gypsum. After that, the ettringite forms. In this test, the ettringite was generated at first, and then gypsum began to form due to the limestone powder. The limestone powder can promote the generation of ettringite. The ettringite diffraction intensity was weakened after 180 days. Meanwhile, the diffraction intensity of gypsum was enhanced due to the combined actions of ettringite decomposition to gypsum (see Eq. (3)) and its’ formation from Ca(OH)2 (see Eq. (1)). At later degradation times, the ettringite started to generate thaumasite and gypsum (see Eq. (4)). In addition, the gypsum disappeared after 540 days, and the diffraction intensity of thaumasite was stronger, indicating that the gypsum was consumed to generate thaumasite (see Eq. (5)). þ 3CaO Al2 O3 3CaSO4 32H2 O þ 4SO2 4 þ 8H
! 4CaSO4 2H2 O þ 2AlðOHÞ3 þ 12H2 O
ð3Þ
Ca6 AlðOHÞ6 2 ðSO4 Þ3 26H2 O þ CaCO3 þ CO2 þ Ca3 Si2 O7 3H2 O þ xH2 O ! Ca6 SiðOHÞ6 2 ðCO3 Þ2 ðSO4 Þ2 24H2 O þ Al2 O3 xH2 O þ CaSO4 2H2 O þ CaðOHÞ2 Ca3 Si2 O7 3H2 O þ 2CaSO4 2H2 O þ 2CaCO3 þ 24H2 O ! Ca6 SiðOHÞ6 2 ðCO3 Þ2 ðSO4 Þ2 24H2 O þ CaðOHÞ2
ð4Þ
ð5Þ
The TSA process in Na2SO4 full immersion can be concluded as follows: (1) the sulfate diffused into the sample and firstly reacted with C3A to generate ettringite. Then, the gypsum and thaumasite formed. (2) The formation of thaumasite involved both an indirect reaction mechanism and a direct reaction mechanism. The indirect reaction mechanism happened at first. After that, the direct reaction mechanism occurred. In addition, the diffraction intensity of Ca(OH)2 enhanced gradually from 480 days to 720 days. Meanwhile, the diffraction intensity of gypsum weakened gradually. This process corresponded with Eq. (5), which suggests that the gypsum was consumed to generate thaumasite and Ca(OH)2. Fig. 13 illustrates the XRD patterns of the samples under MgSO4 full immersion within 720 days. The ettringite was also generated at early degradation times, and the highest diffraction intensity of ettringite was also achieved at 180 days. After that, the diffraction intensity of ettringite began to weaken. With MgSO4 full immersion, gypsum was generated faster than that with Na2SO4 full immersion because Eq. (6) could happen more easily.
MgSO4 þ CaðOHÞ2 þ 2H2 O ! CaSO4 2H2 O þ MgðOHÞ2
ð6Þ
The degradation rate of samples under MgSO4 full immersion was faster due to Mg2+. Mg2+ can react with OH to generate Mg (OH)2, which leads to a reduced pH value. The lower pH can reduce the stability of the CASAH gel. Meanwhile, Mg2+ can react with the CASAH gel to generate M-S-H (see Eq. (7)).
2xMg2þ þ 2xSO2 4 þ 2½2 xCaO 2SiO2 aq þ xH2 O ! 3MgO 2SiO2 2H2 O þ 2x½2 CaSO4 2H2 O þ ð2x 3ÞMgðOHÞ2
Fig. 13. XRD patterns of samples under MgSO4 full immersion within 720 days.
ð7Þ
The generation of thaumasite and gypsum was faster after 240 days, which indicated that some ettringite was consumed to generate thaumasite, and some Ca(OH)2 was consumed to generate gypsum. The main area of the gypsum diffraction peak was calculated from the XRD pattern of jade, and the results were 6834 counts at 540 days and 6140 counts at 720 days. The data indicated that gypsum was also consumed to generate thaumasite at later degradation times, and the direct reaction mechanism also existed in the MgSO4 full immersion. The TSA process with MgSO4 full immersion can be concluded as follows: (1) the degradation process was similar to that of samples under Na2SO4 full immersion, where the ettringite formed at first, and then the thaumasite and gypsum formed. (2) The amount of generated gypsum was much more than that under Na2SO4 full immersion, and the generation of thaumasite was faster. (3) The degradation products were a mixture of thaumasite, gypsum and brucite in MgSO4 full immersion, while the degradation product of samples under Na2SO4 full immersion was thaumasite. The XRD patterns of samples under CaSO4 full immersion are shown in Fig. 14. The CaSO4 solution was a saturated solution. The sulfate content was lower than those of the Na2SO4 solution and the MgSO4 solution. There were hardly any degradation products within 480 days except a little ettringite. Thaumasite and gypsum were found at 600 days, and the sample was attacked by TSA after 720 days. The TSA process in the CaSO4 full immersion can be concluded as follows: (1) The degradation process was similar to the above groups, where the ettringite formed at first, and then the thaumasite and gypsum produced quickly. (2) Some thaumasite was generated by the indirect mechanism, and the degradation products of samples under CaSO4 full immersion were a mixture of thaumasite and gypsum. The XRD patterns of the samples under Na2SO4 & MgSO4 full immersion are shown in Fig. 15. The degradation process was the same as that under MgSO4 full immersion, such that the ettringite was found at first and the diffraction intensity of ettringite reached its maximum at 180 days. After that, the gypsum and thaumasite began to form significantly. The corrosivity of Mg2+ plays the leading role in the mixed Na2SO4 & MgSO4 solution. Fig. 16 shows the XRD patterns of samples under Na2SO4 & CaSO4 full immersion within 720 days. The degradation rate of
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Fig. 14. XRD patterns of samples under CaSO4 full immersion within 720 days. Fig. 16. XRD patterns of samples under Na2SO4 & CaSO4 full immersion within 720 days.
Fig. 15. XRD patterns of samples under Na2SO4 & MgSO4 full immersion within 720 days.
the sample was slow within 300 days. The diffraction intensity of ettringite reached its maximum at 180 days. Then, the ettringite was consumed and thaumasite and gypsum were found in the sample. The corrosivity of Na+ played a leading role in the mixed Na2SO4 & CaSO4 solution. The degradation products were a mixture of thaumasite and gypsum. Fig. 17 exhibits the XRD patterns of samples under MgSO4 & CaSO4 full immersion within 720 days. The degradation process of this sample was the same as that under MgSO4 full immersion, and the corrosivity of Mg2+ played a leading role in the mixed MgSO4 & CaSO4 solution. The degradation products were a mixture of thaumasite, gypsum and brucite. The XRD patterns of the six groups at 720 days are exhibited in Fig. 18. The degradation products of samples under Na2SO4 full immersion were mainly thaumasite, and gypsum was not found at 720 days. The degradation products of the other five groups both were a mixture of thaumasite and gypsum. The corrosivity of Mg2+ was the most serious, followed by Na+. The corrosivity of Ca2+ was the lowest. The corrosivity of the mixed solutions depended on the corrosivity of the cations. Meanwhile, the degree of degradation and type of degradation products both counted on the corrosivity of cations. In addition, the degradation processes for the above six groups were the same: ettringite formed at first, and then
Fig. 17. XRD patterns of samples under MgSO4 & CaSO4 full immersion within 720 days.
Fig. 18. XRD patterns of the six groups at 720 days.
Y. Luo et al. / Construction and Building Materials 229 (2019) 116865
9
gypsum and thaumasite began to form. The main degradation mechanism was the indirect mechanism, and the direct mechanism was observed at later degradation times.
Declaration of Competing Interest
4. Conclusion
References
To study the effect of cations in sulfate on the TSA of cementbased materials, Na2SO4 full immersion, MgSO4 full immersion, CaSO4 full immersion, Na2SO4 & MgSO4 full immersion, Na2SO4 & CaSO4 full immersion, and MgSO4 & CaSO4 full immersion were adopted. The degree of degradation of the six groups was quantified by their appearance and mass loss within 720 days. The degradation products was identified by Raman spectroscopy, and the degradation processes and mechanisms were studied by XRD. The conclusions were as follows:
[1] W. Kurdowski, Cement and Concrete Chemistry, first ed., Springer, Dordrecht, 2014. [2] P.K. Mehta, P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials, third ed., McGraw-Hill, New York, 2013. [3] J. Bied, Recherches industrielles sur la chaux, ciments et mortiers, Dunod, Paris, 1926. [4] R.S. Gollop, H.F.W. Taylor, Microstructural and microanalytical studies of sulfate attack. II. sulfate-resisting Portland cement: Ferrite composition and hydration chemistry, Cem. Concr. Res. 24 (1994) 1347–1358. [5] Z. Liu, D. Deng, G. De Schutter, et al., The effect of MgSO4 on thaumasite formation, Cem. Concr. Compos. 35 (2013) 102–108. [6] X. Jia, T. Wu, Q. Guo, et al., Effects of MgSO4 on thaumasites formation, Concrete 11 (2014) 70–74. [7] M. Santhanam, M.D. Cohen, J. Olek, Sulfate attack research-whither now, Cem. Concr. Res. 31 (2001) 845–851. [8] S.A. Hartshorn, J.H. Sharp, R.N. Swamy, Thaumasite formation in Portlandlimestone cement pastes, Cem. Concr. Res. 29 (1999) 1331–1340. [9] M. Schmidt, Cement with interground additives-capabilities and environmental relief, part 1, Zement-Kalk-Gips 45 (1992) 64–69. [10] ASTM Standard C595. Standard specification for blended hydraulic cements. West Conshohocken, PA: ASTM International, 2012. [11] CEMBUREAU, Cement Standards of the World 1991 (CEMBUREAU, Brussels, 1991) Candlot, E.: Sur les propriétés des produits hydrauliques. Bull. Soc. Encour. Ind. Nat. 89, 1890, 682-689. [12] M.A. Halliwell, N.J. Crammond, A.P. Barker, The thaumasite form of sulfate attack in limestone-filled cement mortars, IHS BRE Press, Bracknell, 1996. [13] R.D. Hooton, in Effects of Carbonate Additions on Heat of Hydration and Sulfate Resistance of Portland Cement, ed. by P. Klieger, R.D. Hooton. Carbonate Additions to Cement, ASTM STP 1064 (American Society for Testing and Materials, Philadelphia, 1990), 73-81. [14] D. Stark, Performance of concrete in sulfate environments, PCA R&D Bull. RD129 (2002) 28. [15] P.C. Taylor, Sulfate resistance tests on type V cements containing limestone, PCA R&D Serial No. 2182a (1998) 1–8. [16] A.P. Barker, D.W. Hobbs, Performance of Portland limestone cements in mortar prisms immersed in sulfate solutions at 5 , Cement Concr. Compos. 21 (1999) 129–137. [17] G. Kakali, S. Tsivilis, A. Skaropoulou, J.H. Sharp, R.N. Swamy, Parameters affecting thaumasite formation in limestone cement mortar, Cement Concr. Compos. 25 (2003) 977–981. [18] S.A. Hartshorn, J.H. Sharp, R.N. Swamy, Engineering properties and structural implications of Portland limestone cement mortar exposed to magnesium sulfate attack, Adv. Cement Res. 13 (2001) 31–46. [19] K.M. Hamdia, M. Silani, X.Y. Zhuang, P.F. He, T. Rabczuk, Stochastic analysis of the fracture toughness of polymeric nanoparticle composites using polynomial chaos expansions, Int. J. Fracture 206 (2017) 215–227. [20] N. Vu-Bac, T. Lahmer, X. Zhuang, T. Nguyen-Thoi, T. Rabczuk, A software framework for probabilistic sensitivity analysis for computationally expensive models, Adv. Eng. Softw. 100 (2016) 19–31. [21] C. Anitescu, E. Atroshchenko, N. Alajlan, T. Rabczuk, Artificial neural network methods for the solution of second order boundary value problems, Comput. Mater. Con. 59 (2019) 345–359.
(1) TSA degradation can be accelerated by Mg2+, wherein TF can be found at 240 days and obvious TSA is observed for MgSO4 full immersion, Na2SO4 & MgSO4 full immersion and MgSO4 & CaSO4 full immersion at 360 days. TF was found at 360 days and obvious TSA was observed in the Na2SO4 full immersion and Na2SO4 & CaSO4 full immersion at 480 days. The TF and TSA of the CaSO4 full immersion were the slowest and observed at 600 days and 720 days, respectively. (2) The same degradation processes and mechanisms were observed in the six groups. The early degradation was caused by ettringite attack, and the ettringite diffraction intensity reached its maximum at 180 days. After that, gypsum and thaumasite began to form. The main degradation mechanism was the indirect mechanism, and the direct mechanism was found during the last stage of degradation. The degradation products of the six groups were not exactly the same, as the degradation products under Na2SO4 full immersion were mainly thaumasite, and the other five groups showed a mixture of thaumasite and gypsum. The relative corrosivity of cations was: Mg2+ > Ca2+ > Na+, and the overall corrosivity of the mixed solutions depended on the most corrosive cation.
Acknowledgements The authors wish to acknowledge the financial support from the National Natural Science Foundation of China (No. 51572038) and the Fundamental Research Funds for the Central Universities (No. 2019CDXYCL0031).
None.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Effects of cations in sulfate on the thaumasite form of sulfate attack of cementitious materials Yaoling Luo a,b, Shuai Zhou a,⇑, Chong Wang a,⇑, Zheng Fang a a b
College of Materials Science and Engineering, Chongqing University, Chongqing 400045, China China West Construction Group Co., Ltd., Chengdu 610015, China
h i g h l i g h t s The degradation process of the TSA is studied in the sulfate solution. The effect of cations in the sulfate on the TSA has been investigated. The influence of mixed cations has been considered. 2+
The relative corrosivity of cations was: Mg
a r t i c l e
i n f o
Article history: Received 15 April 2019 Received in revised form 8 August 2019 Accepted 1 September 2019 Available online 9 September 2019 Keywords: Full immersion Mixed full immersion Types of cations TSA Cement-based materials
> Ca2+ > Na+.
a b s t r a c t The thaumasite form of sulfate attack (TSA) of cement-based materials is a severe threat. There is little research on the effect of cations in the sulfate on the TSA, especially the mixed cations. The effects of six conditions, namely, Na2SO4 full immersion, MgSO4 full immersion, CaSO4 full immersion, Na2SO4 & MgSO4 full immersion, Na2SO4 & CaSO4 full immersion and MgSO4 & CaSO4 full immersion, on the TSA were studied by the observed degradation grades, mass loss, Raman analysis and X-ray diffraction (XRD). Results show that the degradation grade, degradation rate and degradation products are influenced by cations greatly. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Sulfate attack is widely recognized to cause damage and degradation in cementitious materials. Vicat [1] previously researched the chemical corrosion of cement and concrete in briny environments. Candlot and Michaelis firstly reported sulfate attack at the end of the nineteenth century, and Michaelis called the degradation product ‘‘cement bacillus”, which is also known as ettringite [2]. Bied [3] invented calcium aluminate cement in order to prevent sulfate attack. Sulfate attack is caused by the chemical reaction between the hydration products and sulfates either inside the cement or from the external environment. In addition, sulfate attack also causes physical damage. The main chemical degradation products are gypsum, ettringite and thaumasite, among others. The type of sulfate attack is related to the type of cations
⇑ Corresponding authors. E-mail addresses: [email protected] (S. Zhou), [email protected] (C. Wang). https://doi.org/10.1016/j.conbuildmat.2019.116865 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
in the sulfate, and the most types of sulfates cause different levels of sulfate attack except BaSO4 and the PbSO4. The main corrosive sulfates include Na2SO4, MgSO4 and CaSO4. CaSO4 attack is the main cause of ettringite attack. Na2SO4 attack can cause ettringite attack and gypsum attack, and some studies have indicated that Na2SO4 attack also causes decalcification of C-S-H gels [4]. MgSO4 attack is the most serious form of degradation, as it allows both sulfate attack and magnesium salt attack. MgSO4 attack can also result in ettringite attack and gypsum attack. Meanwhile, C-S-H gels are transferred to M-S-H, which has no cementation [5–8]. The thaumasite form of sulfate attack (TSA) can occur when there is a carbonate near a corrosive sulfate at low temperatures (<15 °C). Limestone powder is extensively applied around the world [9–11]. TSA is a serious type of attack that can destroy cement completely [12–18]; hence, extensively applied limestone powder is an extremely severe threat to cement and concrete. The main test for sulfate attack is sole sulfate full immersion in the laboratory, but there are various sulfates in natural environments. To date, there have been no reports about compound
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sulfate attack based on the kinds of cations in the sulfate, and research about the effect of cations in sulfate on TSA lacks systematic experiments. Hence, this paper studied the effect of cations in the sulfate on the TSA, as well as the mixed sulfate. 2. Materials and methods 2.1. Preparation of basic materials The cement is composed of 97 wt% portland cement clinker and 3 wt% gypsum. The gypsum is CaSO42H2O. The limestone powder is from Bao Xing Sichuan China and the content of CaCO3 exceeds 98%. The specific surface area of the limestone powder is 266 kg/m2. The chemical components of the above materials are shown in Table 1. The XRD patterns of the clinker and limestone powder are exhibited in Fig. 1 and Fig. 2, respectively. The corrosive solutions are made up of MgSO4, Na2SO4 and CaSO42H2O. The concentrations of SO24 in the MgSO4 solution and Na2SO4 solution are both 33.8 g/l, and the CaSO4 solution is a saturated solution. 2.2. Experimental methods Fig. 1. XRD of cement clinker.
The cement paste samples were prepared according to Table 2. The sample size was 40 mm 40 mm 40 mm. These samples were placed in the curing chamber for 24 h after synthesis, and then were cured for 30 days. The low-temperature environment of 5 ± 2 °C was applied then. There were six test groups: Na2SO4 full immersion, MgSO4 full immersion, CaSO4 full immersion, Na2SO4 & MgSO4 full immersion (i.e., mixed Na2SO4 and MgSO4 solutions), Na2SO4 & CaSO4 full immersion, and MgSO4 & CaSO4 full immersion (illustrated in Table 3). The mass mix ratio of mixed solutions both was 1:1, and these solutions were replaced every 60 days. The appearance of the samples was observed and recorded every 60 days, and the damage grade of samples was evaluated according to Table 4. Micro-samples were created for XRD and Raman analysis. The broken pieces were soaked and rinsed using anhydrous alcohol, and dried in vacuum oven at 50 °C. The X-ray diffractometer was from the Ricoh Company, with D/MAX-IIIC and Co Ka radiation (0.2 Å), and the sample was scanned from 5°-75° within 5 min. Raman spectroscopic measurements were carried out using a HORIBA Jobin Yvon S. A. S spectrometer (product model: Lab RAM HR Evolution; spectral resolution: visible spectral 0.65 cm1; spectral region from 200 nm to 1050 nm). 3. Results and discussion
Fig. 2. XRD of limestone powder.
Table 2 Mixture proportions of cement paste.
3.1. Macro-properties
Cementitious materials (wt%)
The corrosive solutions were replaced every 60 days. At that time, the degradation grade of the samples was recorded. The degradation grade and the appearance of samples are shown in Figs. 3 and 4, respectively. The degradation grade of the six groups after 720 days is displayed in Fig. 3. Grade 10 means that the surface of the samples had been destroyed and produced some pulp. The degradation rate and degradation degrees with MgSO4 full
Cement
CaCO3 powder
70
30
Water/cementitious materials ratio 0.40
immersion, Na2SO4 & MgSO4 full immersion, and MgSO4 & CaSO4 full immersion were the fastest and most severe, as the surface of the above three samples had converted to pulp at 360 days.
Table 1 Chemical components of clinker, gypsum, and limestone powder (wt%). Materials
SiO2
Fe2O3
Al2O3
CaO
MgO
SO3
K2O
Na2O
LOI
Clinker Gypsum Limestone powder
19.99 4.47 0.23
2.98 0.36 0.21
4.80 0.99 –
61.22 34.05 55.46
3.27 1.84 –
0.23 40.61 –
0.88 0.23 –
0.18 0.08 –
3.52 16.87 41.75
Y. Luo et al. / Construction and Building Materials 229 (2019) 116865 Table 3 Experimental groups. Experimental group
Corrosive solution
Na Mg Ca Na & Mg Na & Ca Mg & Ca
Na2SO4- full immersion MgSO4- full immersion CaSO4- full immersion Na2SO4 & MgSO4- full immersion Na2SO4 & CaSO4- full immersion MgSO4 & CaSO4- full immersion
Table 4 Degradation grade from TSA. Degradation grades
Degradation features
1 2 3 4
No visible deterioration Some deterioration at corners Deterioration at corners and edges Deterioration at corners and some cracking along the edges Cracking and expansion along the edges Serious cracking and expansion Spalling and expansion of the surfaces Grey pulp at corners, edges and surface Amount of grey pulp at samples’ surface Complete pulp at samples’ surface
5 6 7 8 9 10
Fig. 3. The degradation classification of six samples within 720 days.
The degradation rate with Na2SO4 full immersion and Na2SO4 & CaSO4 full immersion was slower, and the sample converted to pulp at 480 days. The degradation rate of samples under CaSO4 full immersion was the slowest, and this sample converted to pulp at 660 days. Fig. 4 shows the appearance of these six samples at 360 days and 720 days, respectively. The surface of samples under the MgSO4 full immersion, Na2SO4 & MgSO4 full immersion, and MgSO4 & CaSO4 full immersion had completely converted to pulp. The surface of the samples under Na2SO4 & CaSO4 full immersion had partially converted to pulp. The samples under Na2SO4 full immersion and CaSO4 full immersion had no obvious degradation at 360 days. Fig. 4(b) shows a comparative diagram of the control group and test groups. The samples under MgSO4 full immersion, Na2SO4 & MgSO4 full immersion, and MgSO4 & CaSO4 full immersion had been dissolved by half. The degrees of degradation for the other three groups were lower. The degradation with sulfate full immersion is a spalling and softening destruction, as the sample converted to pulp gradually and the degradation occurred from the surface to the core of the sample as the degradation continued.
3
The degradation degree of samples can be studied by statistical methods based on Fig. 3 [19–20]. The correlation coefficient between the time and the degradation degree is 0.96, 0.88, 0.97, 0.86, 0.95 and 0.84 under the Na2SO4 full immersion, MgSO4 full immersion, CaSO4 full immersion, Na2SO4 & MgSO4 full immersion, Na2SO4 & CaSO4 full immersion, and MgSO4 & CaSO4 full immersion, respectively. The degradation degree and the time are highly interrelated, and the Na2SO4 has the highest correlation. A more detailed statistical analysis by the machine learning method [21] will be provided when more data are obtained in our following research. The mass losses of the six groups were tested and are displayed in Fig. 5. The mass losses of the samples under MgSO4 full immersion, Na2SO4 & MgSO4 full immersion, and MgSO4 & CaSO4 full immersion were the most serious. In particular, the mass loss of the samples under MgSO4 full immersion had exceeded 50%. The mass loss of the samples under Na2SO4 full immersion, and Na2SO4 & CaSO4 full immersion were lower than the above three groups. The mass loss of the sample under CaSO4 full immersion was the lowest. Combined with the data above, the type of cations in the sulfate has a significant impact on the degradation. The calcium hydroxide is the weakest part of cementitious materials in a long-term period. Among the three kinds of ions, magnesium ion has the highest reaction activity with Ca(OH)2. The formation of Mg(OH)2 precipitation will consume lots of Ca(OH)2, reduce the alkalinity of the pore fluid, and thus decrease the stability of CASAH gel. Besides, magnesium ions also react directly with CASAH gel to form M-S-H and destroy the original CASAH gel. In Na2SO4 solution, Na2SO4 will react with Ca(OH)2 to form CaSO4. Calcium sulfate is a slightly soluble substance and will react with calcium aluminate hydrate to form ettringite, which also causes the degradation of cementitious materials. In CaSO4 solution, calcium ions in the solution can diffuse into the specimen and increase the calcium ion concentration in the specimen. Therefore, the high concentration of calcium ion can maintain the concentration of calcium hydroxide to some extent, which reduces the degradation of cement. Hence, the relative corrosivity of the cations is Mg2+ > Na+ > Ca2+. To avoid the influence of the concentration of sulfate ion, the same concentration of sulfate ion was used to prepare 6 groups of corrosive solution. The concentration of magnesium ion in MgSO4 solution is the highest, and the concentration of magnesium ion in MgSO4 & CaSO4 and MgSO4 & Na2SO4 is only half of that in MgSO4 solution. Therefore, we can see that the mass loss under MgSO4 soaking is the greatest and its degradation is the most serious. The concentration of magnesium ion in MgSO4 & CaSO4 and MgSO4 & Na2SO4 was the same, and the mass loss of the two groups was almost the same. Similarly, for sodium ions, the sodium ion concentration of Na2SO4 solution is the highest, and the sodium ion concentration of Na2SO4 & CaSO4 is only half of that. It can be seen that the mass loss of Na2SO4 solution is greater than that of Na2SO4 & CaSO4. Of the three cations, calcium ions have the lowest corrosivity. Hence, the samples in CaSO4 solution have the lowest mass loss. According to the corrosivity and concentration of cations, it can be concluded that the higher concentration of magnesium ion results in a more serious erosion. Further, the higher concentration of sodium ion induces a more serious erosion when there was no magnesium ion. The corrosivity of the mixed solutions depended on the most corrosive cation and its concentration. The chemical reaction and analysis are detailed in Section 3.2. 3.2. Micro-properties The thaumasite and ettringite were difficult to be distinguished by XRD due to their similar molecular structures. Hence, these degradation products were firstly identified by Raman
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Fig. 4. The appearance of the six samples at 360 days and 720 days.
spectroscopy. The samples with Mg2+ in the solution were taken every 60 days for 360 days to identify the degradation products by Raman spectroscopy. Then, after 360 days, these samples were identified every 180 days. The samples with Na+ in the solution were taken every 60 days within 480 days to identify the type of degradation products by Raman spectroscopy. Further, the samples were identified at 540 days and 720 days. The samples with Ca2+ in the solution were taken every 120 days within 720 days to identify the type of degradation product by Raman spectroscopy. After that, XRD was used to analyse the degradation process and mechanism.
3.2.1. Raman analysis The Raman spectra of the six groups within 720 days are indicated from Figs. 6–11. Fig. 6 shows the Raman spectra of the sample under Na2SO4 full immersion within 720 days. The characteristic thaumasite peak was found at 360 days, but there was only one characteristic peak, which illustrated that the degradation product was a compound of thaumasite and ettringite. The three characteristic peaks of thaumasite were found at 540 days, indicating that the degradation product was mainly thaumasite. These data indicated that the degradation product was mainly
Y. Luo et al. / Construction and Building Materials 229 (2019) 116865
5
Fig. 5. The mass loss of the six samples at 720 days.
Fig. 8. Raman spectra of samples under CaSO4 full immersion within 720 days.
Fig. 6. Raman spectra of samples under Na2SO4 full immersion within 720 days.
Fig. 9. Raman spectra of samples under Na2SO4 & MgSO4 full immersion within 720 days.
Fig. 7. Raman spectra of samples under MgSO4 full immersion within 720 days.
ettringite within 360 days, a compound of ettringite, gypsum and thaumasite from 420 days to 540 days, and a compound of gypsum and thaumasite after 540 days. The sample was attacked by TSA in the Na2SO4 full immersion after 540 days, and thaumasite formation (TF) occurred between 420 days and 540 days.
Fig. 10. Raman spectra of samples under Na2SO4 & CaSO4 full immersion within 720 days.
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Fig. 11. Raman spectra of samples under MgSO4 & CaSO4 full immersion within 720 days.
Fig. 7 displays the Raman spectra of the samples under MgSO4 full immersion within 720 days. A characteristic peak for thaumasite was found at 240 days, and the three characteristic peaks of thaumasite were found at 300 days. These data indicated that the degradation product was mainly ettringite within 180 days. TF occurred between 180 days and 240 days, and the sample was attacked by TSA under the MgSO4 full immersion after 300 days. Fig. 8 exhibits the Raman spectra of the samples under CaSO4 full immersion within 720 days. The degradation rate of the samples under CaSO4 full immersion was the slowest; hence, Raman spectra were taken every 120 days. The thaumasite was found at 600 days, and the sample was attacked by TSA under the CaSO4 full immersion after 720 days. TF occurred during the period between 600 days and 720 days. Fig. 9 shows the Raman spectra of samples under Na2SO4 & MgSO4 full immersion within 720 days. The degradation process was the similar as that under MgSO4 full immersion, but the degradation rate is lower than the MgSO4 full immersion. The degradation products were the compound of ettringite and gypsum within 300 days, and TSA began after 360 days. Fig. 10 shows the Raman spectra of the samples under Na2SO4 & CaSO4 full immersion within 720 days. Thaumasite was found at 360 days, and the obvious characteristic thaumasite peaks were observed after 540 days. Combined with the above data, TSA was found at 480 days. Hence, the TF process occurred between 300 days and 420 days, and the sample was attacked by TSA after 420 days. The degradation products were ettringite and thaumasite from 360 days to 420 days, and they became gypsum and thaumasite after 480 days. Fig. 11 shows the Raman spectra of the samples under MgSO4 & CaSO4 full immersion within 720 days. Thaumasite was found at 300 days, and obvious TSA was observed after 360 days. The TF process occurred between 240 days and 300 days. The samples under MgSO4 full immersion, Na2SO4 & MgSO4 full immersion, and MgSO4 & CaSO4 full immersion were attacked by TSA after 360 days, and the TF process occurred from 240 days to 360 days. The samples under Na2SO4 full immersion, and Na2SO4 & CaSO4 full immersion were both attacked by TSA after 420 days, and the TF process occurred from 360 days to 420 days. The TF process began at 600 days for the samples under CaSO4 full immersion, and the TSA started after 720 days. The concentration of magnesium ions in the MgSO4 solution was greater than that in Na2SO4 & MgSO4 solution, and magnesium
ions have the greatest corrosivity among the 3 cations. Some characteristic peaks of thaumasite appeared after 180 days in MgSO4 solution, and the characteristic peaks of thaumasite were very obvious after 300 days of degradation. In the early degradation process, ettringite and gypsum are the main degradation products. The characteristic peaks of thaumasite were found only after 300 days of degradation, while the characteristic peaks of thaumasite were obvious after 360 days. Therefore, ettringite and gypsum mainly existed in the Na2SO4 & MgSO4 solution within 300 days. The degradation process of the two groups was similar, but the degradation rate of the group in Na2SO4 & MgSO4 solution was significantly slower than that of the group in MgSO4 solution. The above results of the mass loss of six groups of specimens also indicate that the corrosivity of sodium ions is greater than that of calcium ions. The sodium ion concentration in the Na2SO4 solution was significantly greater than that in the Na2SO4 & CaSO4 solution, so the degradation of the Na2SO4 solution group was more serious. Therefore, it can be observed from Fig. 11 and Fig. 8 that the production of TF and TSA of Na2SO4 solution group was significantly faster than that in the Na2SO4 & CaSO4 solution group. The Raman results agree well with the mass loss results. 3.2.2. XRD analysis In the XRD patterns, the three main ettringite diffraction peaks were 9.73, 5.61 and 3.88 Å, and the three main diffraction peaks of thaumasite were 9.56, 5.51 and 3.41 Å. The thaumasite and ettringite were difficult to be differentiated by XRD. Hence, Raman spectroscopy was used to differentiate the thaumasite and ettringite, and the following phase analysis of the surface region of samples by XRD supported the Raman data. Fig. 12 displays the XRD of the samples under Na2SO4 full immersion within 720 days. The main ettringite diffraction peak was at 9.73 Å, and was firstly enhanced and reached its maximum at 180 days. After that, the ettringite diffraction peak began to weaken until 360 days. Meanwhile, thaumasite was found at 360 days (see Raman data above), and then, thaumasite began to form quickly. In addition, gypsum was found after 300 days, and it began to produce significantly after 420 days; however, the gypsum disappeared after 540 days. The Na2SO4 diffused into the samples and reacted with Ca(OH)2 to generate gypsum (see Eq. (1)), and the Na2SO4 also participated to generate the ettringite (see Eq. (2)).
Fig. 12. XRD patterns of samples under Na2SO4 full immersion within 720 days.
Y. Luo et al. / Construction and Building Materials 229 (2019) 116865
Na2 SO4 þ CaðOHÞ2 þ 2H2 O ! CaSO4 2H2 O þ 2NaOH
7
ð1Þ
3CaO Al2 O3 þ 3CaSO4 2H2 O þ 26H2 O ! 3CaO Al2 O3 3CaSO4 32H2 O
ð2Þ
Fig. 12 indicates that the main chemical reaction was Eq. (2), which generated ettringite. The generation of gypsum followed in the early degradation stage. In the common Na2SO4 full immersion attack, gypsum is generated at first, and then more Na2SO4 diffuses into the sample, which leads to an excess concentration of sulfate and Ca2+, reducing the solubility of the gypsum. After that, the ettringite forms. In this test, the ettringite was generated at first, and then gypsum began to form due to the limestone powder. The limestone powder can promote the generation of ettringite. The ettringite diffraction intensity was weakened after 180 days. Meanwhile, the diffraction intensity of gypsum was enhanced due to the combined actions of ettringite decomposition to gypsum (see Eq. (3)) and its’ formation from Ca(OH)2 (see Eq. (1)). At later degradation times, the ettringite started to generate thaumasite and gypsum (see Eq. (4)). In addition, the gypsum disappeared after 540 days, and the diffraction intensity of thaumasite was stronger, indicating that the gypsum was consumed to generate thaumasite (see Eq. (5)). þ 3CaO Al2 O3 3CaSO4 32H2 O þ 4SO2 4 þ 8H
! 4CaSO4 2H2 O þ 2AlðOHÞ3 þ 12H2 O
ð3Þ
Ca6 AlðOHÞ6 2 ðSO4 Þ3 26H2 O þ CaCO3 þ CO2 þ Ca3 Si2 O7 3H2 O þ xH2 O ! Ca6 SiðOHÞ6 2 ðCO3 Þ2 ðSO4 Þ2 24H2 O þ Al2 O3 xH2 O þ CaSO4 2H2 O þ CaðOHÞ2 Ca3 Si2 O7 3H2 O þ 2CaSO4 2H2 O þ 2CaCO3 þ 24H2 O ! Ca6 SiðOHÞ6 2 ðCO3 Þ2 ðSO4 Þ2 24H2 O þ CaðOHÞ2
ð4Þ
ð5Þ
The TSA process in Na2SO4 full immersion can be concluded as follows: (1) the sulfate diffused into the sample and firstly reacted with C3A to generate ettringite. Then, the gypsum and thaumasite formed. (2) The formation of thaumasite involved both an indirect reaction mechanism and a direct reaction mechanism. The indirect reaction mechanism happened at first. After that, the direct reaction mechanism occurred. In addition, the diffraction intensity of Ca(OH)2 enhanced gradually from 480 days to 720 days. Meanwhile, the diffraction intensity of gypsum weakened gradually. This process corresponded with Eq. (5), which suggests that the gypsum was consumed to generate thaumasite and Ca(OH)2. Fig. 13 illustrates the XRD patterns of the samples under MgSO4 full immersion within 720 days. The ettringite was also generated at early degradation times, and the highest diffraction intensity of ettringite was also achieved at 180 days. After that, the diffraction intensity of ettringite began to weaken. With MgSO4 full immersion, gypsum was generated faster than that with Na2SO4 full immersion because Eq. (6) could happen more easily.
MgSO4 þ CaðOHÞ2 þ 2H2 O ! CaSO4 2H2 O þ MgðOHÞ2
ð6Þ
The degradation rate of samples under MgSO4 full immersion was faster due to Mg2+. Mg2+ can react with OH to generate Mg (OH)2, which leads to a reduced pH value. The lower pH can reduce the stability of the CASAH gel. Meanwhile, Mg2+ can react with the CASAH gel to generate M-S-H (see Eq. (7)).
2xMg2þ þ 2xSO2 4 þ 2½2 xCaO 2SiO2 aq þ xH2 O ! 3MgO 2SiO2 2H2 O þ 2x½2 CaSO4 2H2 O þ ð2x 3ÞMgðOHÞ2
Fig. 13. XRD patterns of samples under MgSO4 full immersion within 720 days.
ð7Þ
The generation of thaumasite and gypsum was faster after 240 days, which indicated that some ettringite was consumed to generate thaumasite, and some Ca(OH)2 was consumed to generate gypsum. The main area of the gypsum diffraction peak was calculated from the XRD pattern of jade, and the results were 6834 counts at 540 days and 6140 counts at 720 days. The data indicated that gypsum was also consumed to generate thaumasite at later degradation times, and the direct reaction mechanism also existed in the MgSO4 full immersion. The TSA process with MgSO4 full immersion can be concluded as follows: (1) the degradation process was similar to that of samples under Na2SO4 full immersion, where the ettringite formed at first, and then the thaumasite and gypsum formed. (2) The amount of generated gypsum was much more than that under Na2SO4 full immersion, and the generation of thaumasite was faster. (3) The degradation products were a mixture of thaumasite, gypsum and brucite in MgSO4 full immersion, while the degradation product of samples under Na2SO4 full immersion was thaumasite. The XRD patterns of samples under CaSO4 full immersion are shown in Fig. 14. The CaSO4 solution was a saturated solution. The sulfate content was lower than those of the Na2SO4 solution and the MgSO4 solution. There were hardly any degradation products within 480 days except a little ettringite. Thaumasite and gypsum were found at 600 days, and the sample was attacked by TSA after 720 days. The TSA process in the CaSO4 full immersion can be concluded as follows: (1) The degradation process was similar to the above groups, where the ettringite formed at first, and then the thaumasite and gypsum produced quickly. (2) Some thaumasite was generated by the indirect mechanism, and the degradation products of samples under CaSO4 full immersion were a mixture of thaumasite and gypsum. The XRD patterns of the samples under Na2SO4 & MgSO4 full immersion are shown in Fig. 15. The degradation process was the same as that under MgSO4 full immersion, such that the ettringite was found at first and the diffraction intensity of ettringite reached its maximum at 180 days. After that, the gypsum and thaumasite began to form significantly. The corrosivity of Mg2+ plays the leading role in the mixed Na2SO4 & MgSO4 solution. Fig. 16 shows the XRD patterns of samples under Na2SO4 & CaSO4 full immersion within 720 days. The degradation rate of
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Y. Luo et al. / Construction and Building Materials 229 (2019) 116865
Fig. 14. XRD patterns of samples under CaSO4 full immersion within 720 days. Fig. 16. XRD patterns of samples under Na2SO4 & CaSO4 full immersion within 720 days.
Fig. 15. XRD patterns of samples under Na2SO4 & MgSO4 full immersion within 720 days.
the sample was slow within 300 days. The diffraction intensity of ettringite reached its maximum at 180 days. Then, the ettringite was consumed and thaumasite and gypsum were found in the sample. The corrosivity of Na+ played a leading role in the mixed Na2SO4 & CaSO4 solution. The degradation products were a mixture of thaumasite and gypsum. Fig. 17 exhibits the XRD patterns of samples under MgSO4 & CaSO4 full immersion within 720 days. The degradation process of this sample was the same as that under MgSO4 full immersion, and the corrosivity of Mg2+ played a leading role in the mixed MgSO4 & CaSO4 solution. The degradation products were a mixture of thaumasite, gypsum and brucite. The XRD patterns of the six groups at 720 days are exhibited in Fig. 18. The degradation products of samples under Na2SO4 full immersion were mainly thaumasite, and gypsum was not found at 720 days. The degradation products of the other five groups both were a mixture of thaumasite and gypsum. The corrosivity of Mg2+ was the most serious, followed by Na+. The corrosivity of Ca2+ was the lowest. The corrosivity of the mixed solutions depended on the corrosivity of the cations. Meanwhile, the degree of degradation and type of degradation products both counted on the corrosivity of cations. In addition, the degradation processes for the above six groups were the same: ettringite formed at first, and then
Fig. 17. XRD patterns of samples under MgSO4 & CaSO4 full immersion within 720 days.
Fig. 18. XRD patterns of the six groups at 720 days.
Y. Luo et al. / Construction and Building Materials 229 (2019) 116865
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gypsum and thaumasite began to form. The main degradation mechanism was the indirect mechanism, and the direct mechanism was observed at later degradation times.
Declaration of Competing Interest
4. Conclusion
References
To study the effect of cations in sulfate on the TSA of cementbased materials, Na2SO4 full immersion, MgSO4 full immersion, CaSO4 full immersion, Na2SO4 & MgSO4 full immersion, Na2SO4 & CaSO4 full immersion, and MgSO4 & CaSO4 full immersion were adopted. The degree of degradation of the six groups was quantified by their appearance and mass loss within 720 days. The degradation products was identified by Raman spectroscopy, and the degradation processes and mechanisms were studied by XRD. The conclusions were as follows:
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(1) TSA degradation can be accelerated by Mg2+, wherein TF can be found at 240 days and obvious TSA is observed for MgSO4 full immersion, Na2SO4 & MgSO4 full immersion and MgSO4 & CaSO4 full immersion at 360 days. TF was found at 360 days and obvious TSA was observed in the Na2SO4 full immersion and Na2SO4 & CaSO4 full immersion at 480 days. The TF and TSA of the CaSO4 full immersion were the slowest and observed at 600 days and 720 days, respectively. (2) The same degradation processes and mechanisms were observed in the six groups. The early degradation was caused by ettringite attack, and the ettringite diffraction intensity reached its maximum at 180 days. After that, gypsum and thaumasite began to form. The main degradation mechanism was the indirect mechanism, and the direct mechanism was found during the last stage of degradation. The degradation products of the six groups were not exactly the same, as the degradation products under Na2SO4 full immersion were mainly thaumasite, and the other five groups showed a mixture of thaumasite and gypsum. The relative corrosivity of cations was: Mg2+ > Ca2+ > Na+, and the overall corrosivity of the mixed solutions depended on the most corrosive cation.
Acknowledgements The authors wish to acknowledge the financial support from the National Natural Science Foundation of China (No. 51572038) and the Fundamental Research Funds for the Central Universities (No. 2019CDXYCL0031).
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