Formation and early hydration characteristics of C_2.75B_1.25A₃$ in binary system of C_2.75B_1.25A₃$-C₂S

Shoude Wang*, Yongbo Huang, Chenchen Gong, Xinghua Fu, Lingchao Lu

Shandong Provincial Key Lab. of Preparation and Measurement of Building Materials, University of Jinan, (Jinan, China)

*personand98@163.com

1. INTRODUCTIONTOP

The Portland cement (PC) clinker manufacture consumes about 2900 to 3300 MJ per ton of clinker (1), which is considered to heat the raw meal to a temperature exceeding 1450 °C that allows alite phase to form (2). Moreover, around 830–970 kg CO₂ per ton (3, 4) are emitted of clinker in direct (5, 6, 7) and indirect ways (8, 9), which brings about enormous environmental footprint (6). In short, the cement industry is responsible for 5–7% of all anthropogenic emissions (10, 11) world wide. In order to attain sustainable development for manufacture of PC, there has been a revival of intensive interests in exploring high belite cement with low lime saturation factor (LSF), which leads to an increase in belite amount and a decrease in alite phase content in the clinker (12, 13, 14, 15, 16).

However, C₂S, even with high activity form (17, 18), is less active than C₃S, which has become the most significant limitation for the extensive application of belite cement (17, 19). Thus, many investigations were performed in an attempt to introduce another high early-strength mineral with low CaO such as C₄A₃$ (20, 21, 22, 23) or C_2.75B_1.25A₃$ (2.75CaO•1.25BaO• 3Al₂O₃•SO₃) (24, 25) into belite cement clinker system. Compared with C₄A₃$, C_2.75B_1.25A₃$, it possesses much higher early-strength (26, 27). Consequently, belite-C_2.75B_1.25A₃$ system, with the designed composition of 9.0% C_2.75B_1.25A₃$, 75% silicate mineral and 16% intermediate phase (by weight, as following) (28, 29), has the potential to receive attention nowadays.

In belite-C_2.75B_1.25A₃$ system, the formation of C_2.75B_1.25A₃$ governed directly the early-strength of cement clinker. Consequently, it is necessary to reveal the influence of silicates and intermediate phases on the formation of C_2.75B_1.25A₃$, which is not explored in previous work. On account of the belite as one of the main phases and the complication of phase composition for this new type of cement clinker system, the binary system of C₂S-C_2.75B_1.25A₃$ was designed to gain a deep insight into the effect of the high content of C₂S on the formation of C_2.75B_1.25A₃$. Furthermore, for that the early mechanical property of belite cement is connected with its early hydration characteristics, the hydration and evolution process of this binary system is monitored by ESEM at early stage. Moreover, this research also lays foundation for the basic study of sulphoaluminate mineral modified silicate cement.

2. EXPERIMENTALTOP

2.1. Specimens PreparationTOP

The used chemicals CaCO₃, Al₂O₃, SiO₂, BaCO₃ and BaSO₄ were reagent grade, which come from Sinopharm Chemical Reagent Co., Ltd, China. All regents were weighted accurately according to the anticipated proportion (Table 1) to obtain the mixtures. The mixtures were blended uniformly with water by planetary ball mill and placed in the drying oven at 105 °C for 4 h. After that, they were pressed into discs with the size of 40 mm×40 mm×3 mm at 10MPa. Then the discs were heated to different high temperatures (1000 °C, 1100 °C, 1150 °C, 1200 °C, 1250 °C, 1300 °C and 1350 °C) for 2 hours with the rise speed of 5 °C per minute in high temperature furnace and cooled with forced air rapidly. Finally, the specimens were ground to pass 200 mesh sieve for the next analysis.

Three kinds of specimens were prepared in this experiment. The binary system clinker of C₂S-C_2.75B_1.25A₃$ (37.5:9.0 in weight) was synthesized and named as specimen No.1. Moreover, the pure minerals of C_2.75B_1.25A₃$ and C₂S were also fabricated and named as the reference specimen No.2 and No.3 respectively. Due to the solid solution of BaO and SO₃ (30), excessive 6.44wt% (as carbonate) BaCO₃ and 4.41wt% (as sulfate) CaSO₄ (taking the extra calcium into consideration) were added into the raw material of specimen No.1. Table 1 displays the proportioning of the specimens.

2.2. Specimen testingTOP

Free lime displays the burnability of the specimen. F-CaO content of the specimens was determined by ethanol-glycerin method. X-ray diffraction analysis was performed by X-ray diffraction (D8 Advance, Germany) using Cu Kα radiation with an increment of 0.01° and maintaining time of 0.5s at a voltage of 40 kv and current of 40mA in 5-60 2θ range. The DSC-TGA analysis was carried out by a simultaneous thermal analyzer (TGA/DSC1/1600HT, Germany) with heating rate of 10 °C·min⁻¹ in flowing Ar at rate of 50 ml·min⁻¹. FT-IR spectroscopy analysis was conducted by infrared spectrometer (Nicolet 380, America) with a detector DGTS CsI and 32 scans were recorded to register each specimen. The scans were taken in the mid-infrared region at frequencies of 4000 cm⁻¹ to 400 cm⁻¹, with a spectral resolution of 4 cm⁻¹. Specimens for SEM-EDS analysis were prepared by cutting with a diamond saw and polishing with silicon carbide discs. The prepared samples were coated with 12 nm thick gold and examined by field emission scanning electron microscopy (SEM: QUANTAFEG, America; EDS: INCA, England) at a voltage of 20 kv and current of 20 mA. Specimen for ESEM analysis did not require special treatment. Early stage hydration characteristics of specimen No.1 prepared at 1350 °C were monitored by ESEM with Carl Zeiss EVO 15. The morphology of water drops and the ternary phase diagram of H₂O were shown in Figure 1. When the work condition of ESEM was controlled at the center of the shuriken in the phase diagram, specimen in the chamber was subjected to a relative humidity (RH) of 95% and 1 °C. Moreover, six specimens were used for the determination of the measure data.

2.3. Quantitative analysisTOP

Due to the fact that so far we do not have the crystal structure parameter of C_2.75B_1.25A₃$, reference intensity ratio (RIR) method is chosen to quantify the mineralogical data. This is one of the most simple and quickest ways to quantify X-ray diffraction data (31) and it has been proven effective in the quantification of mineralogical data (32). To confirm the weight fraction of C_2.75B_1.25A₃$ mineral in specimen No.1 and No.2, RIR method was conducted. Mixtures were prepared by mixing specimen No.1 and No.2 with CaF₂ in the weight ratio of 10:1. This mixture was dispersed in a 0.5% (w/v) aqueous solution of polyvinyl alcohol to form a suspension in which the solid-to-liquid ratio was 1:2. The suspension was spray dried at 105 °C and the collected particles were used for the quantitative analysis using the RIR method. The pure C_2.75B_1.25A₃$ mineral used in the quantitative analysis was fabricated by sintering the discs at 1350 °C for 4 h (33).

Figure 2 shows the XRD pattern of specimen No.1 and No.2 sintered at 1000 °C. From Figure 2, it is noticed that a certain amount of aluminate mineral (CA and C₁₂A₇) forms in specimen No.1 and No.2 at 1000 °C. Meanwhile, not any obvious f-CaO characteristic peaks are detected in specimen No.2, while three obvious f-CaO characteristic peaks appear in specimen No.1. So the majority of f-CaO used to form C_2.75B_1.25A₃$ was consumed at 1000 °C. In addition, the f-CaO content considered to form C_2.75B_1.25A₃$ is no more than 4.0% in specimen No.1 by weight. Therefore, it is reasonable to regard the f-CaO in specimen No.1 as the part which is applied to form C₂S. Therefore, it is reasonable to calculate the extent of formation of C₂S in specimen No.1 and No.3 by indirectly or directly using the following equation Eq.[1]:

Where α is the extent of formation of C₂S and f-CaO_C2S is the CaO content in 2CaO•SiO₂. For specimen No.3, f-CaO is the lime content of the specimen at any temperature. For specimen No.1, f-CaO is 5.17 times the lime content of specimen No. 2 at any temperature.

3. RESULTS AND DISCUSSIONTOP

3.1. Variation of phase compositionTOP

3.1.1. Extent of formationTOP

To inspect the effect of C₂S on the formation mechanism of C_2.75B_1.25A₃$ mineral, the extent of formation of C_2.75B_1.25A₃$ in the specimen No.1 and No.2 were measured by the RIR method. The methodology used to calculate the extent of formation of C₂S is based on the assumption that the free CaO in specimen No.1 belonged to the part applied to form C₂S. This methodology has been proven in the experimental part. It is highlighted that free CaO content is a key factor for the quantitative analysis of C₂S. Due to its easy hydration, it is mandatory to perform the free CaO content test immediately after the sintering of the specimens.

A comparison of the extent of formation of C_2.75B_1.25A₃$ and C₂S in specimen No.1, No.2 and No.3 is shown in Figure 3. It is observed that in the range of 1000 °C to 1200 °C, the extent of formation of C₂S in specimen No.1 is relatively higher than that in specimen No.3. In particular, when sintering temperature proceeds in the range of 1200 °C to 1350 °C, C₂S can form in a much faster rate in specimen No.1 than in specimen No.3. As for C_2.75B_1.25A₃$ mineral, in the range of 1000 °C to 1200 °C, its rate of formation in specimen No.1 is much slower than in specimen No.2. When the temperature changes into the range of 1200 °C to 1350 °C, the extent of formation of C_2.75B_1.25A₃$ in specimen No.1 increases much faster than in specimen No.2. Eventually, the extent of formation of C_2.75B_1.25A₃$ in specimen No.1 exceeds that of specimen No.2 at 1350 °C.

3.1.2. X ray diffraction analysisTOP

Figure 4 illustrated the XRD patterns of specimen No.1 sintered at different temperatures. As shown in Figure 4 (a), in the range of 1100–1200 °C, the main phases are BaSO₄ and free CaO. Also, slight amounts of intermediate phases, such as BA, CA and C₁₂A₇, are found. The characteristic peaks of C_2.75B_1.25A₃$ and C₂S are detectable but weak at 1200 °C. As seen from Figure 3 (a) and (b), in the range of 1250–1350 °C, the diminution of diffraction peaks of free CaO and the appearance of characteristic peaks of β-C₂S and γ-C₂S can be observed, proving that a large amount of C₂S is formed. From Figure 4 (b) and (c), it is seen that with the rise of sintering temperature, the intensity of characteristic peaks of β-C₂S increases, while the intensity of characteristic peaks of γ-C₂S displays a contrary tendency. Ultimately, the characteristic peaks of γ-C₂S are not detectable in specimen No.1 sintered at 1350 °C, due to its absence or small content. In specimen No.3, γ-C₂S is the main polymorph. This is different from that in specimen No.1, whose main polymorph is β-C₂S. The reason is the partial substitution of Ba²⁺ and [SO₄]⁴⁻ for Ca²⁺ and [SiO₄]⁴⁻ in C₂S, which results in a rise in both disorder state of lattice and entropy of the system (19) and supports the maintenance of high temperature form of β-C₂S at ambient temperature (3, 7, 34). As shown from Figure 4 (a) and (c), it is observed that the intensity of the characteristic peak for C_2.75B_1.25A₃$ (3.7888 Å, 2.2010 Å, 1.9494 Å) increases rapidly at 1350 °C. Meanwhile, the characteristic peaks of BA, CA and C₁₂A₇ disappear simultaneously. Combination with the discussion of previous works (35), this indicated that the formation of C_2.75B_1.25A₃$ mineral is sufficient.

3.1.3. FT-IR spectrum analysisTOP

The FT-IR spectrum shows the main absorption band and identification group of the functional group. The FT-IR results for specimens sintered at different temperatures is illustrated in Figure 5. As shown in Figure 5, at high wave-number stage the major absorption peaks concentrate at 3640 cm⁻¹, which is assigned to the vibration of [OH]. The reason is that partial f-CaO reacts with H₂O and generates Ca(OH)₂ during the time of the preservation of the specimens. The above mentioned absorption peak intensity presents a decreasing trend with the rise of temperature, indicating the reduction of CaO content in the specimen. The result is in accordance with the X-ray diffraction analysis. At 1000 °C, BaCO₃ decomposes thoroughly (35). Therefore, the vibration band at 1450 cm⁻¹ is mainly attributed to the portlandite carbonation. The vibration band at 1180 cm⁻¹ and 1080 cm⁻¹ is mainly due to the asymmetric stretching vibration of [SO₄].

As shown in Figure 5, the asymmetric stretching vibration of [SiO₄] tetrahedron of C₂S is displayed at 856 cm⁻¹–950 cm⁻¹. When the sintering temperature exceeds 1350 °C, the peak becomes more and more obvious, which indicates that a large amount of C₂S is formed. The bending vibration of [SO₄] tetrahedron sites at 600 cm⁻¹–700 cm⁻¹. Owing to the bending coupling vibration of [SO₄] tetrahedron and [AlO₄] tetrahedron, obvious peaks emerge at 683 cm⁻¹, 640 cm⁻¹ and 615 cm⁻¹ (36) when the sintering temperature is 1350 °C, indicating that C_2.75B_1.25A₃$ can form perfectly at this temperature. All of the above stated results are identical with those acquired from the XRD analysis.

3.1.4. Differential scanning calorimetric analysisTOP

Differential scanning calorimetric analysis is applied to monitor the evolution of solid phase reaction during the processing of the raw meals being sintered. The heat evolution during the process of sintering the specimen is illustrated in Figure 6. Decomposition of CaCO₃ is characterized by evident endothermic peak at 600–850 °C. The beginning temperature for the decomposition of CaCO₃ in specimen No.1 is almost identical with that of specimen No.3, but the terminated temperature lowers down sharply from 865 °C to 835 °C, proving that CaCO₃ decomposition rate is promoted in specimen No.1. As seen from Figure 6, an obvious exothermal peak attributed to the formation of β-C₂S shows at around 1360 °C in the curve of specimen No.3. Due to no mineralizer, its formation temperature is relatively higher than that in cement clinker. By contrast, the dispersed exothermal peak corresponding to the formation of β-C₂S in the curve of specimen No.1 shows at around 1270 °C, which is prior to the formation of β-C₂S in specimen No.3.

This is due to the existence of Ba and S element, which tends to lower down the eutectic point of the system. A strong and sharp endothermal peak due to the decomposition of C_2.75B_1.25A₃$ mineral is displayed at around 1370 °C in the curve of specimen No.2. While at the same temperature, no such peak is observed in the curve of specimen No.1. Consequently, we come to a conclusion that the decomposition of C_2.75B_1.25A₃$ in specimen No.1 is postponed or inhibited.

3.2. Phases growth and distribution in the binary systemTOP

Textual relationships, mineral grain morphology and elemental distribution were studied by SEM images and EDS spectra. Figure 7 shows two representative SEM images and four EDS spectra of specimen No.1 obtained at 1350 °C. The EDS result proves that the mass at point ‘1’position presenting red blood cell shape with a hole in the middle is C_2.75B_1.25A₃$ and the mass at point ‘2’position is belite solid solution (partial substitution of Ba and S for Ca and Si in C₂S). Therefore, it is concluded that C₂S and C_2.75B_1.25A₃$ can coexist in the binary system.

3.3. Formation mechanism discussionsTOP

In the range of 1100 °C–1200 °C, the primarily formed C₂S is not conductive to the combination of calcium, barium, aluminum and sulfur from hindering the formation of C_2.75B_1.25A₃$. It explains the reason why the extent of formation of C_2.75B_1.25A₃$ in specimen No.1 is relatively lower in the temperature range of 1100 °C-1200 °C. As is known to all, crystal formation undergoes the following two steps of nucleation and crystal embryos growth (37). Nucleation normally can be classified to two categories of heterogeneous nucleation and homogeneous nucleation. Heterogeneous nucleation tends to nucleate on the surface, interface and wall of container, which contributes to cutting down the potential barrier (ΔG_k^*) of the heterogeneous nucleation. X ray diffraction analysis and FT-IR spectrum analysis indicate that the large amount formation temperature of C₂S and C_2.75B_1.25A₃$ are 1250 °C and 1350 °C respectively. Therefore, in the range of 1200 °C-1350 °C, primarily formed C₂S can provide surface for the nucleation of C_2.75B_1.25A₃$, which cuts down the ΔG_k^* for the heterogeneous nucleation of C_2.75B_1.25A₃$. Besides, the condition of high temperature can boost the frequency of collisions between elements of calcium, barium, aluminum and sulfur which are considered to form C_2.75B_1.25A₃$. The above stated interprets the growth rhythm of C_2.75B_1.25A₃$ and the reason why the rate of extent of formation of C_2.75B_1.25A₃$ in specimen No.1 is relatively higher in the temperature range of 1200 °C-1350 °C. At 1350 °C, the extent of formation of C_2.75B_1.25A₃$ in specimen No.1 exceeds that in specimen No.2. It may attribute to that the large amount of previously formed C₂S prevent sulfur element from evaporation, which is benefit to the formation of C_2.75B_1.25A₃$ (38).

3.4. Characteristics of early hydrationTOP

Figure 8 displays the hydration process of specimen No.1 without CaSO₄·2H₂O monitored by ESEM under the condition of 95% RH and 1 °C. As shown from Figure 8, four locations of specimen are analyzed. From the micrographs of random location 1 at 0.5 h, 1.5 h, 3 h, it is easy to notice the interlocking of the hydration products, which is always dominating the setting of cement (21). At 6 h, it is observed that a continuous hydration film is formed on the surface of specimen, which are consist of hydrated calcium sulphoaluminate and alumina gel. From the micrographs of location 2 at which much C_2.75B_1.25A₃$ mineral exists, flake monosulfate (AFm, 3CaO·Al₂O₃·CaSO₄·12H₂O) is observed after 2 h. With the prolonging of curing time, no ore AFm forms is observed at 4 h, proving that AFm mainly forms within 2 h. With hydration time prolonging, the amount of AFm does not increase significantly. This means that AFm could form sufficiently. However, from the micrographs of typical location 3 and 4, minor AFt with needle shape is observed after 12 h. Previous investigation also found that the main hydrate of C_2.75B_1.25A₃$ in belite-barium calcium sulphoaluminate cement with 8% CaSO₄·2H₂O is ettringite (AFt, 3CaO·Al₂O₃·3CaSO₄·32H₂O) (26). Therefore, it is reasonable to speculate that the solid solution of sulphur for belite in the binary system lowers down, which inevitably leads to an excessive amount of sulphur in the specimen No.1 and promotes the formation of AFt. At hydration age of 12 h, no plate Ca(OH)₂ is found in Figure 8. This indicates that C₂S mineral is unhydrated at this stage.

4. CONCLUSIONSTOP

In the range of 1100 °C-1200 °C, the primarily formed C₂S is not conductive to the element combination of calcium, barium, aluminum and sulfur, hindering the formation of C_2.75B_1.25A₃$. It explains the reason why the extent of formation of C_2.75B_1.25A₃$ in the binary system is low in this temperature range. On the contrary, when the temperature is in the range of 1200 °C-1350 °C, the primarily formed C₂S contributed to the formation of C_2.75B_1.25A₃$, which could provide surface for the nucleation of C_2.75B_1.25A₃$ and cut down the potential barrier (ΔG_k^*) for the heterogeneous nucleation of C_2.75B_1.25A₃$. Besides, the condition of high temperature can boost the collision frequency of the elements of calcium, barium, aluminum and sulfur. The above stated phenomenon facilitates the formation of C_2.75B_1.25A₃$. At 1350 °C, the extent of formation of C_2.75B_1.25A₃$ in specimen No.1 exceeds that in specimen No.2. This may attribute to the fact that the large amount of previously formed C₂S prevent sulfur element from evaporation, which is a benefit to improve the extent of formation for C_2.75B_1.25A₃$. In early hydration age, AFm and AFt originating from C_2.75B_1.25A₃$ hydration are found within 2 h and 12 h under 95% RH at 1 °C, respectively, whereas C₂S is unhydrated at this moment.

ACKNOWLEDGEMENTSTOP

This work is supported by Natural Science Foundations of China (Grant 51272091) and Shandong Province Bold Talent program (ZR2015EM002). Meanwhile, this work was supported by Program for Scientific Research Innovation Team in Colleges and Universities of Shandong Province.

REFERENCESTOP