Influence of Lithium Carbonate on C3A Hydration

Key Laboratory of Roadway Bridge and Structure Engineering, Wuhan University of Technology, Wuhan, Hubei 430070, China School of Civil Engineering and Architecture, Wuhan University of Technology, Wuhan, Hubei 430070, China State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan, Hubei 430070, China School of Materials Science and Engineering, Wuhan University of Technology, Wuhan, Hubei 430070, China


Introduction
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 CaCl 2 .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 [1], expansion caused by the ASR [2], long-term strength [3], and the amount of hydration products [1].However, most researches focus on the inhibition of alkali-aggregate reaction (AAR), and there are few reports on the cement hydration.ere are ongoing debates on the mechanism of lithium salts, especially Li + , on the hydration and properties of cement paste and concrete.Ong [4] 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 [5].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][7][8][9][10][11] and even the content of lithium salts in cement paste [12].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 [13] points out that the content of C-S-H produced by cement hydration is reduced for lithium ion intervening AAR.erefore, it is necessary to investigate the mechanism of effects of lithium ions on cement hydration.
Tricalcium aluminate (C 3 A) 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).C 3 A reacts with water to form calcium hydroaluminates (i.e., AFt-type phase and C 3 AH 6 ), which induces a stiffening to the hardened paste.
In this study, the influence of lithium carbonate on the hydration of C 3 A with or without CaSO 4 •2H 2 O is investigated.A range of analytical techniques, such as conductimetry, isothermal calorimetry, X-ray di raction (XRD), and Fourier transformation infrared spectrometer (FTIR), are performed to characterize the hydration process and reveal the mechanism behind.

Materials and Methods
Tricalcium aluminate used in this study is synthesized by heating highly pure limestone (99.0%CaCO 3 ) and technically pure Al 2 O 3 (99.0%)with molar ratio 3 : 1 at 1380 °C.e mixture of limestone and Al 2 O 3 is rst 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.e 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.en the powders are remoulded and heated at 1380 °C for 4 hours.e process (Figure 1) is repeated for three times, and nally the specimens are crushed, nely ground, and passed 80 μm sieve.e synthesized powder is analysed by XRD, and almost pure cubic Ca 3 Al 2 O 6 (C 3 A) phase is identi ed (Figure 2), with its speci c surface area of 350 m 2 /kg (Blaine's method).e contents of its free lime and insoluble residue are 0.27% and 0.10%, respectively.C 3 A 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.
e C 3 A hydration with di erent additives, Li 2 CO 3 (analytical reagent, 1.5% by weight related to the C 3 A, CCL paste) or/and CaSO 4 •2H 2 O (analytical reagent, 20% by weight related to the C 3 A, CCLS paste) and the control paste (20% by weight related to the C 3 A) (Figure 3), is also studied based on CC paste proportion.Conductimetry and isothermal microcalorimetry of hydration system are carried out based on the literature [14].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, ermo Electron Scienti c Instruments, USA) with a frequency range of 4000-399 cm −1 .

Conductivity and pH Value.
e pH value and conductivity of pastes are shown in Figures 4 and 5, respectively.When Li 2 CO 3 is added to the CC hydration system, the following reaction takes place [15]: ( With a higher activity compared to calcium ion, the addition of Li 2 CO 3 improves the alkalinity of C 3 A hydration environment.e dissolution of aluminum ion or its group is promoted as a result of the water molecules around Al 3+ being replaced by OH − , which favors the C 3 A 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 CaSO 4 •2H 2 O is added to the CCL paste, the hydration of CCL paste is retarded or delayed by sulfate ion [18], and the ion concentration is signi cantly reduced due to the lower solubility of CaCO 3 , formed as follows: ( Consequently, CCLS hydration system resulted in lower pH value and conductivity than those of CCL paste.

Heat Evolution during Hydration.
According to the results from calorimetry (Figure 6), the addition of lithium carbonate and CaSO 4 •2H 2 O signi cantly a ects the heat evolution rate (heat ow) of pastes during the rst 48 hours of hydration.e 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 C 3 A is added into saturated Ca(OH) 2 solution, the rst peak of heat evolution rate is generated from the dissolution of C 3 A grains and precipitation of 3CaO•Al  Advances in Materials Science and Engineering and the hydration progress is gradually controlled by ion di usion. 3CaO ( As for the hydration of CCL paste, C 3 A grains are quickly dissolved, and a large amount of heat is released by the positive e ect of Li 2 CO 3 .However, the rst maximum heat evolution rate is reduced because a large amount of microcrystalline hydration products are formed and precipitated on the surfaces of C 3 A grains [19], 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 C 3 A hydration products (4) [19,20].After 0.85 hour, the heat ow of CCL paste is higher than that of CC paste at the same time due to Li 2 CO 3 and carbonation.
When CaSO 4 •2H 2 O is added to CCL paste, the rst maximum heat evolution rate is reduced further due to the retardation of CaSO 4 •2H 2 O on the C 3 A hydration despite the positive e ect from Li 2 CO 3 .en, a broad and lower peak of heat evolution rate is observed, attributing to the formation of 3CaO•Al 2 O 3 •3CaSO 4 •32H 2 O (ettringite) (5), which results in a higher heat ow 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. e order of total heat evolved changes with the time of hydration but eventually reaches an order (by 48 hours): CCL paste is greater than Advances in Materials Science and Engineering CCLS paste, CCLS paste is greater than CC paste, and CC paste is greater than control paste.
ese indicate that Li 2 CO 3 improves the degree of C 3 A hydration.

Phase Analysis.
In order to obtain better understanding on the di erences of heat ow 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).e di raction patterns of CC paste, CCL paste, and CCLS paste at various ages are shown in Figure 8.According to the methods reported in literature [21], the content of hydration products in these specimens is calculated and shown in Table 3. e content of C 3 A in CCL paste is lower than that of in CC paste at the same age, indicating that Li 2 CO 3 has a positive e ect on C 3 A hydration.e 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 Li 2 CO 3 improves the C 3 A 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 C 3 AH 6 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 C 3 AH 6 at 4.5 hours is smaller than that of C 3 AH 6 at 2.1 hours, which may be related to the carbonation of C 3 AH 6 (4).
As for CCLS paste (Figure 8(c)), the main crystalline phases are anhydrous C 3 A, ettringite, Mc, Ms, and Time (h)   [14].In Table 3, as for CCL paste, the ratio of Mc to C 3 A of CCLS paste is higher, indicating that Li 2 CO 3 can partly promote the C 3 A hydration in the presence of CaSO 4 •2H 2 O, which is consistent with the result of heat evolution (Figure 7).e contents of Mc and CaSO 4 •2H 2 O show the conventional change with the increase of age, but those of Ms and E have a di erent trend, which is slightly di erent from previous research on C 3 A-CaSO 4 •2H 2 O hydration system [14].is is mainly due to that Li 2 CO 3 a ects the amount of ettringite (E) and Ms (( 6) and ( 7)) [20].For C 3 A-CaSO 4 •2H 2 O-Ca(OH) 2 -H 2 O hydration system, with the addition of Li 2 CO 3 , the hydration of C 3 A is promoted and the amount of ettringite (E) and Ms is a ected.
FTIR is performed to charaterize the phases of three pastes at 0.85 hour, 2.1 hours, and 4.5 hours (Figure 9).e bands around 3662 cm −1 (OH free ) and at 530 cm −1 (ν-AlO 6 ) are observed in the spectra of CC (Figure 9   FTIR on CC, CCL, and CCLS pastes, the addition of Li 2 CO 3 significantly affects the types of hydration products of C 3 A hydration system, which coincides with the XRD results.

Figure 6 :
Figure 6: Heat ow over time during cement hydration.

Table 1 :
Maximum heat evolution rate and peak position.

Table 3 :
e content of hydration products in the pastes at various ages (wt.%).
[23]trum of CCL paste presents an absorption band around 3540 cm −1 , probably due to the carbonation of hydration products[23], which is di erent 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 Li 2 CO 3 changes the hydration products of C 3 A hydration system.As for CCLS paste (Figure9(c)), the bands at 3605 cm −1 , 3641 cm −1 , and 1683 cm −1 are observed, attributing to AFt and CaSO 4 •2H 2 O.e 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