Influence of Lithium Carbonate on C3A Hydration
Lithium salts, known to ameliorate the effects of alkali-silica reaction, can make significant effects on cement setting. However, the mechanism of effects on cement hydration, especially the hydration of C3A which is critical for initial setting time of cement, is rarely reported. In this study, the development of pH value of pore solution, conductivity, thermodynamics, and mineralogical composition during hydration of C3A with or without Li2CO3 are investigated. The results demonstrate that Li2CO3 promotes C3A hydration through high alkalinity, due to higher activity of lithium ion than that of calcium ion in the solution and carbonation of C3A hydration products resulted from Li2CO3. Li2CO3 favors the C3A hydration in C3A-CaSO4·2H2O-Ca(OH)2-H2O hydration system and affects the mineralogical variation of the ettringite phase(s).
Lithium, one of the most active alkali elements, has high charge density and stable electric double layer. Li+ has small radius, strong polarization effect, and a bigger hydration radius, which provide a higher chemical activity for lithium salts compared to other salts usually used in the cement paste, such as NaCl and CaCl2. Lithium salts have been known to ameliorate the effects of alkali-silica reaction (ASR) for many years. When lithium salts are added to the cement paste or concrete, they significantly affects the properties, especially the setting time , expansion caused by the ASR , long-term strength , and the amount of hydration products . However, most researches focus on the inhibition of alkali-aggregate reaction (AAR), and there are few reports on the cement hydration. There are ongoing debates on the mechanism of lithium salts, especially Li+, on the hydration and properties of cement paste and concrete. Ong  points out that lithium salts with lower solubility can restrain cement hydration, which is contrary to the fact that lithium ion has the function to accelerate cement hydration, according to the research from Millard and Kurtis . Some researches suggest that the effects of lithium salts on the properties of cement paste or concrete vary with the categories of cement and lithium salts [6–11] and even the content of lithium salts in cement paste . Some other research shows that lithium salts can react with cement hydration products and form lithium aluminate hydrate, which acts as seeds for more stable hydration products, resulting in the improvement of properties of cement or concrete. Nevertheless, Mo  points out that the content of C-S-H produced by cement hydration is reduced for lithium ion intervening AAR. Therefore, it is necessary to investigate the mechanism of effects of lithium ions on cement hydration.
Tricalcium aluminate (C3A) is the most important and reactive major component contributing to the early properties of cement or cementitious products and plays a critical role in the early stages of hydration process of Ordinary Portland Cement (OPC) and Calcium Aluminate Cements (CAC). C3A reacts with water to form calcium hydroaluminates (i.e., AFt-type phase and C3AH6), which induces a stiffening to the hardened paste.
In this study, the influence of lithium carbonate on the hydration of C3A with or without CaSO4·2H2O is investigated. A range of analytical techniques, such as conductimetry, isothermal calorimetry, X-ray diffraction (XRD), and Fourier transformation infrared spectrometer (FTIR), are performed to characterize the hydration process and reveal the mechanism behind.
2. Materials and Methods
Tricalcium aluminate used in this study is synthesized by heating highly pure limestone (99.0% CaCO3) and technically pure Al2O3 (99.0%) with molar ratio 3 : 1 at 1380°C. The mixture of limestone and Al2O3 is first mixed for 2 hours and pressed as a pellet with a diameter of about 40 mm and maximum press of 100 kN under a rate of 4 kN/s. The pellets are then calcined at 1000°C for 2 hours before cooling to room temperature and being crushed and ground in absolute ethanol to achieve homogeneity. Then the powders are remoulded and heated at 1380°C for 4 hours. The process (Figure 1) is repeated for three times, and finally the specimens are crushed, finely ground, and passed 80 μm sieve. The synthesized powder is analysed by XRD, and almost pure cubic Ca3Al2O6 (C3A) phase is identified (Figure 2), with its specific surface area of 350 m2/kg (Blaine’s method). The contents of its free lime and insoluble residue are 0.27% and 0.10%, respectively.
C3A hydration is conducted in a saturated calcium hydroxide solution (namely CC paste) in order to mimic the pore solution and avoid carbonation during early cement hydration. The C3A hydration with different additives, Li2CO3 (analytical reagent, 1.5% by weight related to the C3A, CCL paste) or/and CaSO4·2H2O (analytical reagent, 20% by weight related to the C3A, CCLS paste) and the control paste (20% by weight related to the C3A) (Figure 3), is also studied based on CC paste proportion. Conductimetry and isothermal microcalorimetry of hydration system are carried out based on the literature . Meanwhile, the paste is dried at 55°C and then ground to a particle size smaller than 45 μm, and the mineralogical compositions are performed by XRD (D8, Bruker AXS Corporation, Germany) with a scan rate of 1.000° per min and FTIR (Nicolet 6700, Thermo Electron Scientific Instruments, USA) with a frequency range of 4000–399 cm−1.
3. Results and Discussions
3.1. Conductivity and pH Value
With a higher activity compared to calcium ion, the addition of Li2CO3 improves the alkalinity of C3A hydration environment. The dissolution of aluminum ion or its group is promoted as a result of the water molecules around Al3+ being replaced by OH−, which favors the C3A hydration [12, 16, 17]. Consequently, the pH value and conductivity are increased due to a higher alkalinity and ion concentration in the hydration system.
When CaSO4·2H2O is added to the CCL paste, the hydration of CCL paste is retarded or delayed by sulfate ion , and the ion concentration is significantly reduced due to the lower solubility of CaCO3, formed as follows:
Consequently, CCLS hydration system resulted in lower pH value and conductivity than those of CCL paste.
3.2. Heat Evolution during Hydration
According to the results from calorimetry (Figure 6), the addition of lithium carbonate and CaSO4·2H2O significantly affects the heat evolution rate (heat flow) of pastes during the first 48 hours of hydration. The peak positions and heat evolution rates of pastes are shown in Table 1. And the heats evolved during hydration in three time ranges are shown in Table 2.
When C3A is added into saturated Ca(OH)2 solution, the first peak of heat evolution rate is generated from the dissolution of C3A grains and precipitation of 3CaO·Al2O3·6H2O (C3AH6) at 0.18 hour (3). As the hydration continues, the heat evolution rate decreases, and the hydration progress is gradually controlled by ion diffusion.
As for the hydration of CCL paste, C3A grains are quickly dissolved, and a large amount of heat is released by the positive effect of Li2CO3. However, the first maximum heat evolution rate is reduced because a large amount of microcrystalline hydration products are formed and precipitated on the surfaces of C3A grains , resulting in an earlier but smaller peak than that of CC paste. With the increase of hydration time, the second peak is generated which may be related to the carbonation of C3A hydration products (4) [19, 20]. After 0.85 hour, the heat flow of CCL paste is higher than that of CC paste at the same time due to Li2CO3 and carbonation.
When CaSO4·2H2O is added to CCL paste, the first maximum heat evolution rate is reduced further due to the retardation of CaSO4·2H2O on the C3A hydration despite the positive effect from Li2CO3. Then, a broad and lower peak of heat evolution rate is observed, attributing to the formation of 3CaO·Al2O3·3CaSO4·32H2O (ettringite) (5), which results in a higher heat flow than that of CCL paste after 4.5 hours at the same time.
As the hydration continues, the total heat evolved of three pastes is shown in Figure 7. The order of total heat evolved changes with the time of hydration but eventually reaches an order (by 48 hours): CCL paste is greater than CCLS paste, CCLS paste is greater than CC paste, and CC paste is greater than control paste. These indicate that Li2CO3 improves the degree of C3A hydration.
3.3. Phase Analysis
In order to obtain better understanding on the differences of heat flow and heat evolution rate of three pastes, the mineralogical compositions of pastes at 0.85 hour, 2.1 hours, and 4.5 hours are characterized by XRD (Figure 8).
The diffraction patterns of CC paste, CCL paste, and CCLS paste at various ages are shown in Figure 8. According to the methods reported in literature , the content of hydration products in these specimens is calculated and shown in Table 3. The content of C3A in CCL paste is lower than that of in CC paste at the same age, indicating that Li2CO3 has a positive effect on C3A hydration. The content of calcium hydroxide (CH) in CCL paste is lower than that of CH in CC paste at the same age, which agrees with the explication for the pH value and conductivity, so as to suggest that Li2CO3 improves the C3A hydration. Combined with the full width at half maximum (FWHM) during the calculation process as reported in literature [21, 22], the results suggest that the content of C3AH6 in CCL paste is lower than that of in CC paste at 0.85 hour, which is consistent with the result from calorimetry. For CC paste and CCL paste, the content of C3AH6 at 4.5 hours is smaller than that of C3AH6 at 2.1 hours, which may be related to the carbonation of C3AH6 (4).
As for CCLS paste (Figure 8(c)), the main crystalline phases are anhydrous C3A, ettringite, Mc, Ms, and CaSO4·2H2O, which are different from CC and CCL pastes but similar to C3A-CaSO4·2H2O hydration system . In Table 3, as for CCL paste, the ratio of Mc to C3A of CCLS paste is higher, indicating that Li2CO3 can partly promote the C3A hydration in the presence of CaSO4·2H2O, which is consistent with the result of heat evolution (Figure 7). The contents of Mc and CaSO4·2H2O show the conventional change with the increase of age, but those of Ms and E have a different trend, which is slightly different from previous research on C3A-CaSO4·2H2O hydration system . This is mainly due to that Li2CO3 affects the amount of ettringite (E) and Ms ((6) and (7)) . For C3A-CaSO4·2H2O–Ca(OH)2–H2O hydration system, with the addition of Li2CO3, the hydration of C3A is promoted and the amount of ettringite (E) and Ms is affected.
FTIR is performed to charaterize the phases of three pastes at 0.85 hour, 2.1 hours, and 4.5 hours (Figure 9).
The bands around 3662 cm−1 (OHfree) and at 530 cm−1 (ν-AlO6) are observed in the spectra of CC (Figure 9(a)) and CCL (Figure 9(b)) specimens, indicating the presence of C3AH6 in CC and CCL pastes [23–27]. The spectra of CC and CCL pastes present absorption bands near 789 cm−1 and 804 cm−1 indicating the presence of C3A. However, the spectrum of CCL paste presents an absorption band around 3540 cm−1, probably due to the carbonation of hydration products , which is different from that of CC paste. Combined with the XRD results of the CCL paste, it is proposed that the products from carbonation could be Mc, suggesting that addition of Li2CO3 changes the hydration products of C3A hydration system. As for CCLS paste (Figure 9(c)), the bands at 3605 cm−1, 3641 cm−1, and 1683 cm−1 are observed, attributing to AFt and CaSO4·2H2O. The absorption band around 3410 cm−1, indicating the presence of carbonation, suggests that Mc is also the hydration product of CCLS paste. According to the results of FTIR on CC, CCL, and CCLS pastes, the addition of Li2CO3 significantly affects the types of hydration products of C3A hydration system, which coincides with the XRD results.
In this study, the effect of Li2CO3 and CaSO4·2H2O on the hydration of C3A is investigated. A range of analytical techniques, such as conductimetry, isothermal calorimetry, XRD, and FTIR, are performed to characterize the hydration process. According to the results obtained, the following conclusions can be drawn:(1)Li2CO3 promotes the hydration of C3A, and the retarding of C3A hydration by CaSO4·2H2O can be reduced by Li2CO3 addition.(2)The positive effects of Li2CO3 on C3A hydration are mainly due to a high pH value resulted from lithium ion and carbonate released from Li2CO3, which promotes the formation of hydration products.(3)Li2CO3 can promote the C3A hydration in C3A-CaSO4·2H2O-Ca(OH)2-H2O hydration system and affect types and amount of hydration products.
Conflicts of Interest
The authors declare that they have no conflicts of interest regarding the publication of this study.
This study was financially supported by the National “Twelfth Five-Year” Plan for Science and Technology Support Development Program of China (no. 2014BAB15B01), National Natural Science Foundation of China (no. 51204128), the YangFan Innovative and Entrepreneurial Research Team Project (no. 201312C12), and the Fundamental Research Funds for the Central Universities (WUT: 2017II51GX).
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