As an indispensable additive in polymer cement, nonionic cellulose ether has received extensive attention and research. Based on the relevant literature at home and abroad, the law and mechanism of non-ionic cellulose ether modified cement mortar were discussed from the aspects of the types and selection of non-ionic cellulose ether, its effect on the physical properties of polymer cement, its effect on the micromorphology and mechanical properties, and the shortcomings of the current research were put forward. This work will promote the application of cellulose ether in polymer cement.
Key words: nonionic cellulose ether, polymer cement, physical properties, mechanical properties, microstructure
1. Overview
With the increasing demand and performance requirements of polymer cement in the construction industry, adding additives to its modification has become a research hotspot, among which, cellulose ether has been widely used because of its effect on cement mortar water retention, thickening, retarding, air and so on. In this paper, the types of cellulose ether, the effects on the physical and mechanical properties of polymer cement and the micromorphology of polymer cement are described, which provides a theoretical reference for the application of cellulose ether in polymer cement.
2. Types of nonionic cellulose ether
Cellulose ether is a kind of polymer compound with ether structure made from cellulose. There are many kinds of cellulose ether, which has great influence on the properties of cement-based materials and is difficult to choose. According to the chemical structure of substituents, they can be divided into anionic, cationic and nonionic ethers. Nonionic cellulose ether with side chain substituent of H, cH3, c2H5, (cH2cH20)nH, [cH2cH(cH3)0]nH and other non-dissociable groups is the most widely used in cement, typical representatives are methyl cellulose ether, hydroxypropyl methyl cellulose ether, hydroxyethyl methyl cellulose ether, hydroxyethyl cellulose ether and so on. Different kinds of cellulose ethers have different effects on the setting time of cement. According to previous literature reports, HEC has the strongest retarding ability for cement, followed by HPMc and HEMc, and Mc has the worst. For the same kind of cellulose ether, molecular weight or viscosity, methyl, hydroxyethyl, hydroxypropyl content of these groups are different, its retarding effect is also different. Generally speaking, the greater the viscosity and the higher the content of non-dissociable groups, the worse the delay ability. Therefore, in the actual production process, according to the requirements of commercial mortar coagulation, the appropriate functional group content of cellulose ether can be selected. Or in the production of cellulose ether at the same time, adjust the content of functional groups, make it meet the requirements of different mortar.
3、the influence of nonionic cellulose ether on the physical properties of polymer cement
3.1 Slow coagulation
In order to prolong the hydration hardening time of cement, so that the newly mixed mortar in a long time to remain plastic, so as to adjust the setting time of the newly mixed mortar, improve its operability, usually add retarder in mortar, non-ionic cellulose ether is suitable for polymer cement is a common retarder.
The retarding effect of nonionic cellulose ether on cement is mainly affected by its own type, viscosity, dosage, different composition of cement minerals and other factors. Pourchez J et al. showed that the higher the degree of cellulose ether methylation, the worse the retarding effect, while the molecular weight of cellulose ether and hydroxypropoxy content had a weak effect on the retarding of cement hydration. With the increase of viscosity and doping amount of non-ionic cellulose ether, the adsorption layer on the surface of cement particles is thickened, and the initial and final setting time of cement are extended, and the retarding effect is more obvious. Studies have shown that the early heat release of cement slurries with different HEMC content is about 15% lower than that of pure cement slurries, but there is no significant difference in the later hydration process. Singh NK et al. showed that with the increase of HEc doping amount, the hydration heat release of modified cement mortar showed a trend of first increasing and then decreasing, and the HEC content when reaching the maximum hydration heat release was related to the curing age.
In addition, it is found that the retarding effect of nonionic cellulose ether is closely related to the composition of cement. Peschard et al. found that the lower the content of tricalcium aluminate (C3A) in cement, the more obvious the retarding effect of cellulose ether. schmitz L et al. believed that this was caused by the different ways of cellulose ether to the hydration kinetics of tricalcium silicate (C3S) and tricalcium aluminate (C3A). Cellulose ether could reduce the reaction rate in the acceleration period of C3S, while for C3A, it could prolong the induction period, and finally delay the solidification and hardening process of mortar.
There are different opinions on the mechanism of non-ionic cellulose ether delaying cement hydration. Silva et al. Liu believed that the introduction of cellulose ether would cause the viscosity of pore solution to increase, thus blocking the movement of ions and delaying the condensation. However, Pourchez et al. believed that there was an obvious relationship between the delay of cellulose ether to cement hydration and the viscosity of cement slurry. Another theory is that the retarding effect of cellulose ether is closely related to alkali degradation. Polysaccharides tend to degrade easily to produce hydroxyl carboxylic acid which can delay the hydration of cement under alkaline conditions. However, studies have found that cellulose ether is very stable under alkaline conditions and only degrades slightly, and the degradation has little effect on the delay of cement hydration. At present, the more consistent view is that the retarding effect is mainly caused by adsorption. Specifically, the hydroxyl group on the molecular surface of cellulose ether is acidic, the ca(0H) in the hydration cement system, and other mineral phases are alkaline. Under the synergistic action of hydrogen bonding, complexing and hydrophobic, acidic cellulose ether molecules will be adsorbed on the surface of alkaline cement particles and hydration products. In addition, a thin film is formed on its surface, which hinders the further growth of these mineral phase crystal nuclei and delays the hydration and setting of cement. The stronger the adsorption capacity between cement hydration products and cellulose ether, the more obvious the hydration delay of cement. On the one hand, the size of steric hindrance plays a decisive role in adsorption capacity, such as the small steric hindrance of hydroxyl group, its strong acidity, adsorption is also strong. On the other hand, the adsorption capacity also depends on the composition of hydration products of cement. Pourchez et al. found that cellulose ether is easily adsorbed to the surface of hydration products such as ca(0H)2, c.s.H gel and calcium aluminate hydrate, but it is not easy to be adsorbed by ettringite and unhydrated phase. Mullert’s study also showed that cellulose ether had a strong adsorption on c3s and its hydration products, so the hydration of silicate phase was significantly delayed. The adsorption of ettringite was low, but the formation of ettringite was significantly delayed. This was because the delay in the formation of ettringite was affected by the ca2+ balance in solution, which was the continuation of the delay of cellulose ether in silicate hydration.
3.2 Water Preservation
Another important modification effect of cellulose ether in cement mortar is to appear as a water-retaining agent, which can prevent the moisture in wet mortar from evaporating prematurely or being absorbed by the base, and delay the hydration of cement while extending the operating time of wet mortar, so as to ensure that thin mortar can be combed, plastered mortar can be spread, and easy to absorb mortar does not need to be pre-wet.
The water-holding capacity of cellulose ether is closely related to its viscosity, dosage, type and ambient temperature. Other conditions are the same, the greater the viscosity of cellulose ether, the better the water retention effect, a small amount of cellulose ether can make the water retention rate of mortar greatly improved; For the same cellulose ether, the higher the amount added, the higher the water retention rate of modified mortar, but there is an optimal value, beyond which the water retention rate increases slowly. For different kinds of cellulose ether, there are also differences in water retention, such as HPMc under the same conditions than Mc better water retention. In addition, the water retention performance of cellulose ether decreases with the increase of ambient temperature.
It is generally believed that the reason why cellulose ether has the function of water retention is mainly due to the 0H on the molecule and the 0 atom on the ether bond will be associated with water molecules to synthesize hydrogen bond, so that free water becomes binding water, so as to play a good role of water retention; It is also believed that the cellulose ether macromolecular chain plays a restrictive role in the diffusion of water molecules, so as to effectively control water evaporation, to achieve high water retention; Pourchez J argued that cellulose ether achieved the water retention effect by improving the rheological properties of the newly mixed cement slurry, the structure of porous network and the formation of cellulose ether film which hindered the diffusion of water. Laetitia P et al. also believe that rheological property of mortar is a key factor, but also believe that viscosity is not the only factor determining the excellent water retention performance of mortar. It is worth noting that although cellulose ether has good water retention performance, but its modified hardened cement mortar water absorption will be reduced, the reason is that cellulose ether in the mortar film, and in the mortar a large number of small closed pores, blocking the mortar inside the capillary.
3.3 Thickening
The consistency of mortar is one of the important indexes to measure its working performance. Cellulose ether is often introduced to increase the consistency. “Consistency” represents the ability of freshly mixed mortar to flow and deform under the action of gravity or external forces. The two properties of thickening and water retention complement each other. Adding an appropriate amount of cellulose ether can not only improve the water retention performance of mortar, ensure smooth construction, but also increase the consistency of mortar, significantly increase the anti-dispersion ability of cement, improve the bond performance between mortar and matrix, and reduce the sagging phenomenon of mortar.
The thickening effect of cellulose ether mainly comes from its own viscosity, the greater the viscosity, the better the thickening effect, but if the viscosity is too large, it will reduce the fluidity of mortar, affecting the construction. The factors that affect viscosity change, such as molecular weight (or degree of polymerization) and concentration of cellulose ether, solution temperature, shear rate, will affect the final thickening effect.
The thickening mechanism of cellulose ether mainly comes from hydration and entanglement between molecules. On the one hand, the polymer chain of cellulose ether is easy to form hydrogen bond with water in water, hydrogen bond makes it have high hydration; On the other hand, when cellulose ether is added to the mortar, it will absorb a lot of water, so that its own volume is greatly expanded, reducing the free space of particles, at the same time cellulose ether molecular chains intertwine with each other to form a three-dimensional network structure, mortar particles are surrounded in which, not free flow. In other words, under these two actions, the viscosity of the system is improved, thus achieving the desired thickening effect.
4. Effect of nonionic cellulose ether on the morphology and pore structure of polymer cement
As can be seen from the above, non-ionic cellulose ether plays a vital role in polymer cement, and its addition will certainly affect the microstructure of the entire cement mortar. The results show that non-ionic cellulose ether usually increases the porosity of cement mortar, and the number of pores in the size of 3nm ~ 350um increases, among which the number of pores in the range of 100nm ~ 500nm increases the most. The influence on the pore structure of cement mortar is closely related to the type and viscosity of non-ionic cellulose ether added. Ou Zhihua et al. believed that when the viscosity is the same, the porosity of cement mortar modified by HEC is smaller than that of HPMc and Mc added as modifiers. For the same cellulose ether, the smaller the viscosity, the smaller the porosity of the modified cement mortar. By studying the effect of HPMc on the aperture of foamed cement insulation board, Wang Yanru et al. found that the addition of HPMC does not significantly change the porosity, but can significantly reduce the aperture. However, Zhang Guodian et al. found that the greater the HEMc content, the more obvious the influence on the pore structure of cement slurry. The addition of HEMc can significantly increase the porosity, total pore volume and average pore radius of cement slurry, but the specific surface area of the pore decreases, and the number of large capillary pores larger than 50nm in diameter increases significantly, and the introduced pores are mainly closed pores.
The effect of nonionic cellulose ether on the formation process of cement slurry pore structure was analyzed. It was found that the addition of cellulose ether mainly changed the properties of liquid phase. On the one hand, the liquid phase surface tension decreases, making it easy to form bubbles in cement mortar, and will slow the liquid phase drainage and bubble diffusion, so that small bubbles are difficult to gather into large bubbles and discharge, so the voidage is greatly increased; On the other hand, the viscosity of the liquid phase increases, which also inhibits drainage, bubble diffusion and bubble merger, and enhances the ability to stabilize bubbles. Therefore, the influence mode of cellulose ether on the pore size distribution of cement mortar can be obtained: in the pore size range of more than 100nm, bubbles can be introduced by reducing the surface tension of liquid phase, and bubble diffusion can be inhibited by increasing the liquid viscosity; in the region of 30nm ~ 60nm, the number of pores in the region can be affected by inhibiting the merger of smaller bubbles.
5. Influence of nonionic cellulose ether on mechanical properties of polymer cement
The mechanical properties of polymer cement are closely related to its morphology. With the addition of nonionic cellulose ether, the porosity increases, which is bound to have an adverse effect on its strength, especially the compressive strength and flexural strength. The reduction of compressive strength of cement mortar is significantly greater than the flexural strength. Ou Zhihua et al. studied the influence of different types of non-ionic cellulose ether on the mechanical properties of cement mortar, and found that the strength of cellulose ether modified cement mortar was lower than that of pure cement mortar, and the lowest 28d compressive strength was only 44.3% of that of pure cement slurry. The compressive strength and flexural strength of HPMc, HEMC and MC cellulose ether modified are similar, while the compressive strength and flexural strength of HEc modified cement slurry in each age are significantly higher. This is closely related to their viscosity or molecular weight, the higher the viscosity or molecular weight of cellulose ether, or the greater the surface activity, the lower the strength of its modified cement mortar.
However, it has also been shown that nonionic cellulose ether can enhance the tensile strength, flexibility and cohesibility of cement mortar. Huang Liangen et al. found that, contrary to the change law of compressive strength, the shear strength and tensile strength of slurry increased with the increase of the content of cellulose ether in cement mortar. Analysis of the reason, after the addition of cellulose ether, and polymer emulsion together to form a large number of dense polymer film, greatly improve the flexibility of the slurry, and cement hydration products, unhydrated cement, fillers and other materials filled in this film, to ensure the tensile strength of the coating system.
In order to improve the performance of non-ionic cellulose ether modified polymer cement, improve the physical properties of cement mortar at the same time, does not significantly reduce its mechanical properties, the usual practice is to match cellulose ether and other admixtures, added to the cement mortar. Li Tao-wen et al. found that the composite additive composed of cellulose ether and polymer glue powder not only slightly improved the bending strength and compressive strength of mortar, so that the cohesiveness and viscosity of cement mortar are more suitable for the coating construction, but also significantly improved the water retention capacity of mortar compared with single cellulose ether. Xu Qi et al. added slag powder, water reducing agent and HEMc, and found that water reducing agent and mineral powder can increase the density of mortar, reduce the number of holes, so as to improve the strength and elastic modulus of mortar. HEMc can increase the tensile bond strength of mortar, but it is not good for the compressive strength and elastic modulus of mortar. Yang Xiaojie et al. found that the plastic shrinkage cracking of cement mortar can be significantly reduced after mixing HEMc and PP fiber.
6. Conclusion
Nonionic cellulose ether plays an important role in polymer cement, which can significantly improve the physical properties (including retarding coagulation, water retention, thickening), microscopic morphology and mechanical properties of cement mortar. A lot of work has been done on the modification of cement-based materials by cellulose ether, but there are still some problems that need further study. For example, in practical engineering applications, little attention is paid to the rheology, deformation properties, volume stability and durability of modified cement-based materials, and a regular corresponding relationship has not been established with added cellulose ether. The research on the migration mechanism of cellulose ether polymer and cement hydration products in hydration reaction is still insufficient. The action process and mechanism of the compound additives composed of cellulose ether and other admixtures are not clear enough. The composite addition of cellulose ether and inorganic reinforced materials such as glass fiber has not been perfected. All these will be the focus of future research to provide theoretical guidance for further improving the performance of polymer cement.