Cellulose ether modified cement slurry

The effect of different molecular structure of non-ionic cellulose ether on the pore structure of cement slurry was studied by performance density test and macroscopic and microscopic pore structure observation. The results show that nonionic cellulose ether can increase the porosity of cement slurry. When the viscosity of non-ionic cellulose ether modified slurry is similar, the porosity of hydroxyethyl cellulose ether (HEC) modified slurry is smaller than that of hydroxypropyl methyl cellulose ether (HPMC) and methyl cellulose ether (MC) modified slurry. The lower the viscosity/relative molecular weight of HPMC cellulose ether with similar group content, the smaller the porosity of its modified cement slurry. Non-ionic cellulose ether can reduce the surface tension of liquid phase and make the cement slurry easy to form bubbles. Non-ionic cellulose ether molecules are directionally adsorbed at the gas-liquid interface of the bubbles, which also increases the viscosity of the cement slurry phase and enhances the ability of the cement slurry to stabilize the bubbles.

Key words: nonionic cellulose ether; Cement slurry; Pore structure; Molecular structure; Surface tension; viscosity

Nonionic cellulose ether (hereinafter referred to as cellulose ether) has excellent thickening and water retention, and is widely used in dry mixed mortar, self-compacting concrete and other new cement-based materials. Cellulose ethers used in cement-based materials usually include methyl cellulose ether (MC), hydroxypropyl methyl cellulose ether (HPMC), hydroxyethyl methyl cellulose ether (HEMC) and hydroxyethyl cellulose ether (HEC), among which HPMC and HEMC are the most common applications.

Cellulose ether can significantly affect the pore structure of cement slurry. Pourchez et al., through apparent density test, pore size test (mercury injection method) and sEM image analysis, concluded that cellulose ether can increase the number of pores with a diameter of about 500nm and pores with a diameter of about 50-250μm in cement slurry. Moreover, for hardened cement slurry, The pore size distribution of low molecular weight HEC modified cement slurry is similar to that of pure cement slurry. The total pore volume of high molecular weight HEC modified cement slurry is higher than that of pure cement slurry, but lower than that of HPMC modified cement slurry with roughly the same consistency. Through SEM observation, Zhang et al. found that HEMC could significantly increase the number of pores with a diameter of about 0.1mm in cement mortar. They also found through mercury injection test that HEMC could significantly increase the total pore volume and average pore diameter of cement slurry, resulting in a significant increase in the number of large pores with a diameter of 50nm ~ 1μm and large pores with a diameter of more than 1μm. However, the number of pores with diameter less than 50nm was significantly reduced. Saric-Coric et al. believed that cellulose ether would make cement slurry more porous and lead to the increase of macropores. Jenni et al. tested the performance density and determined that the pore volume fraction of HEMC modified cement mortar was approximately 20%, while pure cement mortar contained only a small amount of air. Silva et al. found that in addition to the two peaks at 3.9 nm and 40 ~ 75nm as pure cement slurry, there were also two peaks at 100 ~ 500nm and greater than 100μm through mercury injection test. Ma Baoguo et al. found that cellulose ether increased the number of fine pores with diameters less than 1μm and large pores with diameters greater than 2μm in cement mortar through mercury injection test. As for the reason that cellulose ether increases the porosity of cement slurry, it is usually believed that cellulose ether has surface activity, will enrich in the air and water interface, forming a film, so as to stabilize the bubbles in cement slurry.

Through the above literature analysis, it can be seen that the effect of cellulose ether on the pore structure of cement-based materials has received great attention. However, there are many kinds of cellulose ether, the same kind of cellulose ether, its relative molecular weight, group content and other molecular structure parameters are also very different, and domestic and foreign researchers on cellulose ether selection is only limited to their respective application field, lack of representation, the conclusion is inevitable “overgeneralization”, so that the explanation of cellulose ether mechanism is not deep enough. In this paper, the effect of cellulose ether with different molecular structure on the pore structure of cement slurry was studied by apparent density test and macroscopic and microscopic pore structure observation.

1. Test

1.1 Raw Materials

The cement was a P·O 42.5 ordinary Portland cement manufactured by Huaxin Cement Co., LTD., in which the chemical composition was measured by AXIOS Ad-Vanced wavelength dispersion-type X-ray fluorescence spectrometer (PANa — lytical, Netherlands), and the phase composition was estimated by Bogue method.

Cellulose ether selected four kinds of commercial cellulose ether, respectively methyl cellulose ether (MC), hydroxypropyl methyl cellulose ether (HPMC1, HPMC2) and hydroxyethyl cellulose ether (HEC), HPMC1 molecular structure and HPMC2 similar, but the viscosity is much less than HPMC2, That is, the relative molecular mass of HPMC1 is much smaller than that of HPMC2. Because of the similar properties of hydroxyethyl methyl cellulose ether (HEMc) and HPMC, HEMCs were not selected in this study. To avoid influence of moisture content on test results, all cellulose ethers were baked at 98℃ for 2h before use.

The viscosity of cellulose ether was tested by NDJ-1B rotary viscosimeter (Shanghai Changji Company). The test solution concentration (mass ratio of cellulose ether to water) was 2.0%, the temperature was 20℃, and the rotation rate was 12r/min. The surface tension of cellulose ether was tested by the ring method. The test instrument was JK99A automatic tensiometer (Shanghai Zhongchen Company). The concentration of the test solution was 0.01% and the temperature was 20℃. Cellulose ether group content is provided by the manufacturer.

According to the viscosity, surface tension and group content of cellulose ether, when the solution concentration is 2.0%, the viscosity ratio of HEC and HPMC2 solution is 1:1.6, and the viscosity ratio of HEC and MC solution is 1: 0.4, but in this test, the water-cement ratio is 0.35, the maximum cement ratio is 0.6%, the mass ratio of cellulose ether to water is about 1.7%, less than 2.0%, and the synergistic effect of cement slurry on viscosity, so the viscosity difference of HEC, HPMC2 or MC modified cement slurry is small.

According to the viscosity, surface tension and group content of cellulose ether, the surface tension of each cellulose ether is different. Cellulose ether has both hydrophilic groups (hydroxyl and ether groups) and hydrophobic groups (methyl and glucose carbon ring), is a surfactant. Cellulose ether is different, the type and content of hydrophilic and hydrophobic groups are different, resulting in different surface tension.

1.2 Test methods

Six kinds of cement slurry were prepared, including pure cement slurry, four cellulose ether (MC, HPMCl, HPMC2 and HEC) modified cement slurry with 0.60% cement ratio and HPMC2 modified cement slurry with 0.05% cement ratio. Ref, MC — 0.60, HPMCl — 0.60, Hpmc2-0.60. HEC 1-0.60 and hpMC2-0.05 indicate that the water-cement ratio is both 0.35.

Cement slurry first in accordance with GB/T 17671 1999 “cement mortar strength test method (ISO method)” made into 40mm×40mm×160mm prisms test block, under the condition of 20℃ sealed curing 28d. After weighing and calculating its apparent density, it was cracked open with a small hammer, and the macro hole condition of the central section of the test block was observed and photographed with a digital camera. At the same time, small pieces of 2.5 ~ 5.0mm were taken for observation by optical microscope (HIROX three-dimensional video microscope) and scanning electron microscope (JSM-5610LV).

2. Test results

2.1 Apparent density

According to the apparent density of cement slurry modified by different cellulose ethers, (1) the apparent density of pure cement slurry is the highest, which is 2044 kg/m³; The apparent density of the four kinds of cellulose ether modified slurry with the cement ratio of 0.60% was 74% ~ 88% of that of pure cement slurry, indicating that cellulose ether caused the increase in the porosity of cement slurry. (2) When the ratio of cement to cement is 0.60%, the effect of different cellulose ethers on the porosity of cement slurry is very different. The viscosity of HEC, HPMC2 and MC modified cement slurry is similar, but the apparent density of HEC modified cement slurry is the highest, indicating that the porosity of HEC modified cement slurry is smaller than that of HPMc2 and Mc modified cement slurry with similar viscosity. HPMc1 and HPMC2 have similar group content, but the viscosity of HPMCl is much lower than that of HPMC2, and the apparent density of HPMCl modified cement slurry is significantly higher than that of HPMC2 modified cement slurry, which indicates that when the group content is similar, the lower the viscosity of cellulose ether, the lower the porosity of the modified cement slurry. (3) When the cement-to-cement ratio is very small (0.05%), the apparent density of HPMC2-modified cement slurry is basically close to that of pure cement slurry, indicating that the effect of cellulose ether on the porosity of cement slurry is very small.

2.2 Macroscopic pore

According to the section photos of cellulose ether modified cement slurry taken by digital camera, pure cement slurry is very dense, almost no visible pores; The four kinds of cellulose ether modified slurry with 0.60% cement ratio all have more macroscopic pores, indicating that cellulose ether leads to the increase of cement slurry porosity. Similar to the results of the apparent density test, the effect of different cellulose ether types and contents on the porosity of cement slurry is quite different. The viscosity of HEC, HPMC2 and MC modified slurry is similar, but the porosity of HEC modified slurry is smaller than that of HPMC2 and MC modified slurry. Although HPMC1 and HPMC2 have similar group content, HPMC1 modified slurry with lower viscosity has smaller porosity. When the cement-to-cement ratio of HPMc2 modified slurry is very small (0.05%), the number of macroscopic pores is slightly increased than that of pure cement slurry, but greatly reduced than that of HPMC2 modified slurry with 0.60% cement-to-cement ratio.

2.3 Microscopic pore

4. Conclusion

(1) Cellulose ether can increase the porosity of cement slurry.

(2) The effect of cellulose ether on the porosity of cement slurry with different molecular structure parameters is different: when the viscosity of cellulose ether modified cement slurry is similar, the porosity of HEC modified cement slurry is smaller than that of HPMC and MC modified cement slurry; The lower the viscosity/relative molecular weight of HPMC cellulose ether with similar group content, the lower the porosity of its modified cement slurry.

(3) After adding cellulose ether into cement slurry, the surface tension of liquid phase is reduced, so that the cement slurry is easy to form bubbles i cellulose ether molecules directional adsorption in the bubble gas-liquid interface, improve the strength and toughness of the bubble liquid film adsorption in the bubble gas-liquid interface, improve the strength of the bubble liquid film and strengthen the ability of the tough mud to stabilize the bubble.