The water components and microstructure evolution in cellulose ether modified sulphoaluminate cement (CSA) slurry were studied by low-field nuclear magnetic resonance and thermal analyzer. The results showed that after the addition of cellulose ether, it adsorbed water between the flocculation structures, which was characterized as the third relaxation peak in the transverse relaxation time (T2) spectrum, and the amount of adsorbed water was positively correlated with the dosage. In addition, cellulose ether significantly facilitated the water exchange between the interior and inter-floc structures of CSA flocs. Although the addition of cellulose ether has no effect on the types of hydration products of sulphoaluminate cement, it will affect the amount of hydration products of a specific age.
Key words: cellulose ether; sulfoaluminate cement; water; hydration products
0、Preface
Cellulose ether, which is processed from natural cellulose through a series of processes, is a renewable and green chemical admixture. Common cellulose ethers such as methylcellulose (MC), ethylcellulose (HEC), and hydroxyethylmethylcellulose (HEMC) are widely used in medicine, construction and other industries. Taking HEMC as an example, it can significantly improve the water retention and consistency of Portland cement, but delay the setting of cement. At the microscopic level, HEMC also has a significant effect on the microstructure and pore structure of cement paste. For example, the hydration product ettringite (AFt) is more likely to be short rod-shaped, and its aspect ratio is lower; at the same time, a large number of closed pores are introduced into the cement paste, reducing the number of communicating pores.
Most of the existing studies on the influence of cellulose ethers on cement-based materials focus on Portland cement. Sulphoaluminate cement (CSA) is a low-carbon cement independently developed in my country in the 20th century, with anhydrous calcium sulphoaluminate as the main mineral. Because a large amount of AFt can be generated after hydration, CSA has the advantages of early strength, high impermeability, and corrosion resistance, and is widely used in the fields of concrete 3D printing, marine engineering construction, and rapid repair in low temperature environments. In recent years, Li Jian et al. analyzed the influence of HEMC on CSA mortar from the perspectives of compressive strength and wet density; Wu Kai et al. studied the effect of HEMC on the early hydration process of CSA cement, but the water in the modified CSA cement The law of evolution of components and slurry composition is unknown. Based on this, this work focuses on the distribution of transverse relaxation time (T2) in the CSA cement slurry before and after adding HEMC by using a low-field nuclear magnetic resonance instrument, and further analyzes the migration and change law of water in the slurry. The composition change of cement paste was studied.
1. Experiment
1.1 Raw materials
Two commercially available sulphoaluminate cements were used, denoted as CSA1 and CSA2, with a loss on ignition (LOI) of less than 0.5% (mass fraction).
Three different hydroxyethyl methylcelluloses are used, which are denoted as MC1, MC2 and MC3 respectively. MC3 is obtained by mixing 5% (mass fraction) polyacrylamide (PAM) in MC2.
1.2 Mixing ratio
Three kinds of cellulose ethers were mixed into the sulphoaluminate cement respectively, the dosages were 0.1%, 0.2% and 0.3% (mass fraction, the same below). The fixed water-cement ratio is 0.6, and the water-cement ratio of the water-cement ratio has good workability and no bleeding through the water consumption test of the standard consistency.
1.3 Method
The low-field NMR equipment used in the experiment is the PQ⁃001 NMR analyzer from Shanghai Numei Analytical Instrument Co., Ltd. The magnetic field strength of the permanent magnet is 0.49T, the proton resonance frequency is 21MHz, and the temperature of the magnet is kept constant at 32.0°C. During the test, the small glass bottle containing the cylindrical sample was put into the probe coil of the instrument, and the CPMG sequence was used to collect the relaxation signal of the cement paste. After inversion by the correlation analysis software, the T2 inversion curve was obtained by using the Sirt inversion algorithm. Water with different degrees of freedom in the slurry will be characterized by different relaxation peaks in the transverse relaxation spectrum, and the area of the relaxation peak is positively correlated with the amount of water, based on which the type and content of water in the slurry can be analyzed. In order to generate nuclear magnetic resonance, it is necessary to ensure that the center frequency O1 (unit: kHz) of the radio frequency is consistent with the frequency of the magnet, and O1 is calibrated every day during the test.
The samples were analyzed by TG?DSC with STA 449C combined thermal analyzer from NETZSCH, Germany. N2 was used as the protective atmosphere, the heating rate was 10 °C/min, and the scanning temperature range was 30-800 °C.
2. Results and discussion
2.1 Evolution of water components
2.1.1 Undoped cellulose ether
Two relaxation peaks (defined as the first and second relaxation peaks) can be clearly observed in the transverse relaxation time (T2) spectra of the two sulphoaluminate cement slurries. The first relaxation peak originates from the inside of the flocculation structure, which has a low degree of freedom and a short transverse relaxation time; the second relaxation peak originates from between the flocculation structures, which has a large degree of freedom and a long transverse relaxation time. In contrast, the T2 corresponding to the first relaxation peak of the two cements is comparable, while the second relaxation peak of CSA1 appears later. Different from sulphoaluminate cement clinker and self-made cement, the two relaxation peaks of CSA1 and CSA2 partially overlap from the initial state. With the progress of hydration, the first relaxation peak gradually tends to be independent, the area gradually decreases, and it disappears completely at about 90 minutes. This shows that there is a certain degree of water exchange between the flocculation structure and the flocculation structure of the two cement pastes.
The change of the peak area of the second relaxation peak and the change of the T2 value corresponding to the apex of the peak respectively characterize the change of free water and physically bound water content and the change of the degree of freedom of water in the slurry. The combination of the two can more comprehensively reflect the The hydration process of the slurry. With the progress of hydration, the peak area gradually decreases, and the shift of T2 value to the left gradually increases, and there is a certain corresponding relationship between them.
2.1.2 Added cellulose ether
Taking CSA2 mixed with 0.3% MC2 as an example, the T2 relaxation spectrum of sulphoaluminate cement after adding cellulose ether can be seen. After adding cellulose ether, the third relaxation peak representing the adsorption of water by cellulose ether appeared at the position where the transverse relaxation time was greater than 100ms, and the peak area gradually increased with the increase of cellulose ether content.
The amount of water between the flocculation structures is affected by the migration of water inside the flocculation structure and the water adsorption of cellulose ether. Therefore, the amount of water between the flocculation structures is related to the internal pore structure of the slurry and the water adsorption capacity of cellulose ether. The area of the second relaxation peak varies with The content of cellulose ether varies with different types of cement. The area of the second relaxation peak of CSA1 slurry decreased continuously with the increase of cellulose ether content, and was the smallest at 0.3% content. In contrast, the second relaxation peak area of CSA2 slurry increases continuously with the increase of cellulose ether content.
List the change of the area of the third relaxation peak with the increase of the content of cellulose ether. Since the peak area is affected by the quality of the sample, it is difficult to ensure that the quality of the added sample is the same when loading the sample. Therefore, the area ratio is used to characterize the signal amount of the third relaxation peak in different samples. From the change of the area of the third relaxation peak with the increase of the content of cellulose ether, it can be seen that with the increase of the content of cellulose ether, the area of the third relaxation peak basically showed an increasing trend (in CSA1, when the content of MC1 was 0.3%, it was more The area of the third relaxation peak decreases slightly at 0.2%), indicating that with the increase of the content of cellulose ether, the adsorbed water also gradually increases. Among CSA1 slurries, MC1 had better water absorption than MC2 and MC3; while among CSA2 slurries, MC2 had the best water absorption.
It can be seen from the change of the area of the third relaxation peak per unit mass of the CSA2 slurry with time at the content of 0.3% cellulose ether that the area of the third relaxation peak per unit mass decreases continuously with the hydration, indicating that Since the hydration rate of CSA2 is faster than that of clinker and self-made cement, cellulose ether has no time for further water adsorption, and releases the adsorbed water due to the rapid increase of the liquid phase concentration in the slurry. In addition, the water adsorption of MC2 is stronger than that of MC1 and MC3, which is consistent with the previous conclusions. It can be seen from the change of the peak area per unit mass of the third relaxation peak of CSA1 with time at different 0.3% dosages of cellulose ethers that the change rule of the third relaxation peak of CSA1 is different from that of CSA2, and the area of CSA1 increases briefly in the early stage of hydration. After increasing rapidly, it decreased to disappear, which may be due to the longer clotting time of CSA1. In addition, CSA2 contains more gypsum, hydration is easy to form more AFt (3CaO Al2O3 3CaSO4 32H2O), consumes a lot of free water, and the rate of water consumption exceeds the rate of water adsorption by cellulose ether, which may lead to The area of the third relaxation peak of CSA2 slurry continued to decrease.
After incorporation of cellulose ether, the first and second relaxation peaks also changed to some extent. It can be seen from the peak width of the second relaxation peak of the two kinds of cement slurry and the fresh slurry after adding cellulose ether that the peak width of the second relaxation peak of the fresh slurry is different after adding cellulose ether. increase, the peak shape tends to be diffuse. This shows that the incorporation of cellulose ether prevents the agglomeration of cement particles to a certain extent, makes the flocculation structure relatively loose, weakens the binding degree of water, and increases the degree of freedom of water between the flocculation structures. However, with the increase of the dosage, the increase of the peak width is not obvious, and the peak width of some samples even decreases. It may be that the increase of the dosage increases the viscosity of the liquid phase of the slurry, and at the same time, the adsorption of cellulose ether to the cement particles is enhanced to cause flocculation. The degree of freedom of moisture between the structures is reduced.
Resolution can be used to describe the degree of separation between the first and second relaxation peaks. The degree of separation can be calculated according to the degree of resolution = (Afirst component-Asaddle)/Afirst component, where Afirst component and Asaddle represent the maximum amplitude of the first relaxation peak and the amplitude of the lowest point between the two peaks, respectively. The degree of separation can be used to characterize the degree of water exchange between the slurry flocculation structure and the flocculation structure, and the value is generally 0-1. A higher value for Separation indicates that the two parts of water are more difficult to exchange, and a value equal to 1 indicates that the two parts of water cannot exchange at all.
It can be seen from the calculation results of the separation degree that the separation degree of the two cements without adding cellulose ether is equivalent, both are about 0.64, and the separation degree is significantly reduced after adding cellulose ether. On the one hand, the resolution decreases further with the increase of the dosage, and the resolution of the two peaks even drops to 0 in the CSA2 mixed with 0.3% MC3, indicating that cellulose ether significantly promotes the exchange of water inside and between the flocculation structures . Based on the fact that the incorporation of cellulose ether has basically no effect on the position and area of the first relaxation peak, it can be speculated that the decrease in resolution is partly due to the increase in the width of the second relaxation peak, and the loose flocculation structure makes the water exchange between the inside and the outside easier. In addition, the overlapping of cellulose ether in the slurry structure further improves the degree of water exchange between the inside and outside of the flocculation structure. On the other hand, the resolution reduction effect of cellulose ether on CSA2 is stronger than that of CSA1, which may be due to the smaller specific surface area and larger particle size of CSA2, which is more sensitive to the dispersion effect of cellulose ether after incorporation.
2.2 Changes in slurry composition
From the TG-DTG spectra of CSA1 and CSA2 slurries hydrated for 90 min, 150 min and 1 day, it can be seen that the types of hydration products did not change before and after adding cellulose ether, and AFt, AFm and AH3 were all formed. The literature points out that the decomposition range of AFt is 50-120 °C; the decomposition range of AFm is 160-220 °C; the decomposition range of AH3 is 220-300 °C. With the progress of hydration, the weight loss of the sample gradually increased, and the characteristic DTG peaks of AFt, AFm and AH3 gradually became obvious, indicating that the formation of the three hydration products gradually increased.
From the mass fraction of each hydration product in the sample at different hydration ages, it can be seen that the AFt generation of the blank sample at 1d age exceeds that of the sample mixed with cellulose ether, indicating that cellulose ether has a great influence on the hydration of the slurry after coagulation. There is a certain delay effect. At 90 minutes, the AFm production of the three samples remained the same; at 90-150 minutes, the production of AFm in the blank sample was significantly slower than that of the other two groups of samples; after 1 day, the content of AFm in the blank sample was the same as that of the sample mixed with MC1, and the AFm content of the MC2 sample was significantly lower in other samples. As for the hydration product AH3, the generation rate of the CSA1 blank sample after hydration for 90 minutes was significantly slower than that of the cellulose ether, but the generation rate was significantly faster after 90 minutes, and the AH3 production amount of the three samples was equivalent at 1 day.
After the CSA2 slurry was hydrated for 90min and 150min, the amount of AFT produced in the sample mixed with cellulose ether was significantly less than that of the blank sample, indicating that cellulose ether also had a certain retarding effect on the CSA2 slurry. In the samples at 1d age, it was found that the AFt content of the blank sample was still higher than that of the sample mixed with cellulose ether, indicating that cellulose ether still had a certain retardation effect on the hydration of CSA2 after final setting, and the degree of retardation on MC2 was greater than that of the sample added with cellulose ether. MC1. At 90 minutes, the amount of AH3 produced by the blank sample was slightly less than that of the sample mixed with cellulose ether; at 150 minutes, the AH3 produced by the blank sample exceeded that of the sample mixed with cellulose ether; at 1 day, the AH3 produced by the three samples was equivalent.
3. Conclusion
(1) Cellulose ether can significantly promote the water exchange between the flocculation structure and the flocculation structure. After incorporation of cellulose ether, the cellulose ether adsorbs the water in the slurry, which is characterized as the third relaxation peak in the transverse relaxation time (T2) spectrum. With the increase of the content of cellulose ether, the water absorption of cellulose ether increases, and the area of the third relaxation peak increases. The water absorbed by cellulose ether is gradually released into the flocculation structure with the hydration of the slurry.
(2) The incorporation of cellulose ether prevents the agglomeration of cement particles to a certain extent, making the flocculation structure relatively loose; and with the increase of the content, the liquid phase viscosity of the slurry increases, and the cellulose ether has a greater effect on the cement particles. The enhanced adsorption effect reduces the degree of freedom of water between the flocculated structures.
(3) Before and after the addition of cellulose ether, the types of hydration products in the sulphoaluminate cement slurry did not change, and AFt, AFm and aluminum glue were formed; but cellulose ether slightly delayed the formation of hydration products effect.