Cellulose ether on self-leveling mortar

The effects of hydroxypropyl methyl cellulose ether on the fluidity, water retention and bonding strength of self-leveling mortar were studied. The results show that HPMC can effectively improve the water retention of self-leveling mortar and reduce the consistency of mortar. The introduction of HPMC can improve the bonding strength of mortar, but the compressive strength, flexural strength and fluidity are reduced. SEM contrast test was carried out on the samples, and the effect of HPMC on the retarding effect, water retention effect and strength of mortar was further explained from the hydration course of cement at 3 and 28 days.

Key words: self-leveling mortar; Cellulose ether; Fluidity; Water retention

0. Introduction

Self-leveling mortar can rely on its own weight to form a flat, smooth and strong foundation on the substrate, so as to lay or bond other materials, and can carry out a large area of high efficiency construction, therefore, high liquidity is a very significant feature of self-leveling mortar; Especially as a large volume, reinforced dense or gap less than 10 mm backfill or reinforcing use of grouting material. In addition to good fluidity, self-leveling mortar must have certain water retention and bond strength, no bleeding segregation phenomenon, and have the characteristics of adiabatic and low temperature rise.

Generally, self-leveling mortar requires good fluidity, but the actual fluidity of cement slurry is usually only 10 ~ 12 cm. Self-leveling mortar can be self-compacting, and the initial setting time is long and the final setting time is short. Cellulose ether is one of the main additives of ready-mixed mortar, although the addition amount is very low, but can significantly improve the performance of mortar, it can improve the consistency of mortar, working performance, bonding performance and water retention performance, has a very important role in the field of ready-mixed mortar.

1. Raw materials and research methods

1.1 Raw Materials

(1) Ordinary P·O 42.5 grade cement.

(2) Sand material: Xiamen washed sea sand, particle size is 0.3 ~ 0.6mm, water content is 1% ~ 2%, artificial drying.

(3) Cellulose ether: hydroxypropyl methyl cellulose ether is the product of hydroxyl replaced by methoxy and hydroxypropyl, respectively, with a viscosity of 300mpa·s. At present, most of the cellulose ether used is hydroxypropyl methyl cellulose ether and hydroxyethyl methyl cellulose ether.

(4) superplasticizer: polycarboxylic acid superplasticizer.

(5) Redispersible latex powder: HW5115 series produced by Henan Tiansheng Chemical Co., Ltd. is a redispersible latex powder copolymerized by VAC/VeoVa.

1.2 Test methods

The test was carried out in accordance with the industry standard JC/T 985-2005 “Cement-based Self-leveling Mortar for Ground Use”. The setting time was determined by referring to the standard consistency and setting time of JC/T 727 cement paste. Self-leveling mortar specimen forming, bending and compressive strength test refer to GB/T 17671. Test method of bond strength: The 80mmx80mmx20mm mortar test block is prepared in advance, and its age is over 28d. The surface is roughened, and the saturated water on the surface is wiped off after 10min wetting. The mortar test piece is poured on the polished surface with the size of 40mmx40mmx10mm. Bond strength is tested at design age.

Scanning electron microscopy (SEM) was used to analyze the morphology of cementified materials in slurry. In the study, the mixing method of all powder materials is: first, the powder materials of each component are evenly mixed, and then added to the proposed water for uniform mixing. The effect of cellulose ether on self-leveling mortar was analyzed by strength, water retention, fluidity and SEM microscopic tests.

2. Results and analysis

2.1 Mobility

Cellulose ether has an important effect on water retention, consistency and construction performance of self leveling mortar. Especially as a self-leveling mortar, fluidity is one of the main indexes to evaluate the performance of self-leveling mortar. On the premise of ensuring the normal composition of mortar, the fluidity of mortar can be adjusted by changing the content of cellulose ether.

With the increase of cellulose ether content. The fluidity of mortar decreases gradually. When the dosage is 0.06%, the fluidity of mortar decreases by more than 8%, and when the dosage is 0.08%, the fluidity decreases by more than 13.5%. At the same time, with the extension of the age, the high dosage indicates that the amount of cellulose ether must be controlled within a certain range, too high dosage will bring negative effects on the mortar fluidity. The water and cement in the mortar make up the clean slurry to fill the sand gap, and wrap around the sand to play a lubricating role, so that the mortar has a certain fluidity. With the introduction of cellulose ether, the content of free water in the system is relatively reduced, and the coating layer on the outer wall of sand is reduced, thus reducing the flow of mortar. Due to the requirement of self-leveling mortar with high fluidity, the amount of cellulose ether should be controlled in a reasonable range.

2.2 Water Retention

Water retention of mortar is an important index to measure the stability of components in freshly mixed cement mortar. Adding appropriate amount of cellulose ether can improve the water retention of mortar. In order to make the hydration reaction of the cementing material fully, a reasonable amount of cellulose ether can keep the water in the mortar for a long time to ensure that the hydration reaction of the cementing material can be fully carried out.

Cellulose ether can be used as a water-retaining agent because the oxygen atoms on the hydroxyl and ether bonds are associated with water molecules to form hydrogen bonds, making free water become combined water. It can be seen from the relationship between the content of cellulose ether and the water retention rate of mortar that the water retention rate of mortar increases with the increase of the content of cellulose ether. The water-retaining effect of cellulose ether can prevent the substrate from absorbing too much and too fast water, and prevent water evaporation, thus ensuring that the slurry environment provides sufficient water for cement hydration. There are also studies that show that in addition to the amount of cellulose ether, its viscosity (molecular weight) also has a greater impact on mortar water retention, the greater the viscosity, the better the water retention. Cellulose ether with a viscosity of 400 MPa·S is generally used for self-leveling mortar, which can improve the leveling performance of mortar and improve the compactness of mortar. When the viscosity exceeds 40000 MPa·S, the water retention performance is no longer significantly improved, and it is not suitable for self-leveling mortar.

In this study, samples of mortar with cellulose ether and mortar without cellulose ether were taken. Part of the samples were 3d age samples, and the other part of the 3d age samples were standard cured for 28d, and then the formation of cement hydration products in the samples was tested by SEM.

The hydration products of cement in the blank sample of mortar sample at 3d age are more than those in the sample with cellulose ether, and at 28d age, the hydration products in the sample with cellulose ether are far more than those in the blank sample. The early hydration of water is delayed because there is a complex film layer formed by cellulose ether on the surface of cement particles in the early stage. However, with the extension of the age, the hydration process proceeds slowly. At this time, the water retention of cellulose ether on the slurry makes there is enough water in the slurry to meet the demand of hydration reaction, which is conducive to the full progress of hydration reaction. Therefore, there are more hydration products in the slurry at the later stage. Relatively speaking, there is more free water in the blank sample, which can satisfy the water required by the early cement reaction. However, with the progress of hydration process, part of the water in the sample is consumed by the early hydration reaction, and the other part is lost by evaporation, resulting in insufficient water in the later slurry. Therefore, the 3d hydration products in the blank sample are relatively more. The amount of hydration products is much less than the amount of hydration products in the sample containing cellulose ether. Therefore, from the perspective of hydration products, it is again explained that adding an appropriate amount of cellulose ether to mortar can indeed improve the water retention of slurry.

2.3 Setting time

Cellulose ether has certain retarding effect on mortar, with the increase of cellulose ether content. The setting time of the mortar is then prolonged. The retarding effect of cellulose ether is directly related to its structural characteristics. Cellulose ether has dehydrated glucose ring structure, which can form sugar calcium molecular complex gate with calcium ions in cement hydration solution, reduce the concentration of calcium ions in cement hydration induction period, prevent the formation and precipitation of Ca(OH)2 and calcium salt crystals, so as to delay the hydration process of cement. The retarding effect of cellulose ether on cement slurry mainly depends on the degree of substitution of alkyl and has little relationship with its molecular weight. The smaller the substitution degree of alkyl, the larger the content of hydroxyl, the more obvious the retarding effect. L. Semitz et al. believed that cellulose ether molecules were mainly adsorbed on hydration products such as C — S — H and Ca(OH)2, and rarely adsorbed on clinker original minerals. Combined with the SEM analysis of cement hydration process, it is found that cellulose ether has certain retarding effect, and the higher the content of cellulose ether, the more obvious the retarding effect of complex film layer on the early hydration of cement, therefore, the more obvious the retarding effect.

2.4 Flexural strength and compressive strength

Generally, strength is one of the important evaluation indexes of cement-based cementitious materials curing effect of mixtures. In addition to high flow performance, self-leveling mortar should also have a certain compressive strength and flexural strength. In this study, the 7 and 28 days compressive strength and flexural strength of blank mortar mixed with cellulose ether were tested.

With the increase of cellulose ether content, mortar compressive strength and flexural strength are reduced in different amplitude, the content is small, the influence on strength is not obvious, but with the content of more than 0.02%, strength loss rate growth is more obvious, therefore, in the use of cellulose ether to improve mortar water retention, but also take into account the change of strength.

Causes of mortar compressive and flexural strength decline. It can be analyzed from the following aspects. First of all, early strength and fast hardening cement was not used in the study. When the dry mortar was mixed with water, some cellulose ether rubber powder particles were first adsorbed on the surface of the cement particles to form a latex film, which delayed the hydration of the cement and reduced the early strength of the mortar matrix. Secondly, in order to simulate the working environment of preparing self-leveling mortar on site, all specimens in the study did not undergo vibration in the process of preparation and molding, and relied on self-weight leveling. Due to the strong water retention performance of cellulose ether in mortar, a large number of pores were left in the matrix after mortar hardening. The increase of porosity in mortar is also an important reason for the decrease of compressive and flexural strength of mortar. In addition, after adding cellulose ether into mortar, the content of flexible polymer in the pores of mortar increases. When the matrix is pressed, the flexible polymer is difficult to play a rigid supporting role, which also affects the strength performance of the matrix to a certain extent.

2.5 Bonding strength

Cellulose ether has great effect on the bonding property of mortar and is widely used in the research and preparation of self-leveling mortar.

When the content of cellulose ether is between 0.02% and 0.10%, the bond strength of mortar is obviously improved, and the bond strength at 28 days is much higher than that at 7 days. Cellulose ether forms a closed polymer film between cement hydration particles and the liquid phase system, which promotes more water in the polymer film outside the cement particles, which is conducive to the complete hydration of cement, so as to improve the bond strength of the paste after hardening. At the same time, the appropriate amount of cellulose ether enhances the plasticity and flexibility of mortar, reduces the rigidity of the transition zone between mortar and substrate interface, reduces the slip stress between the interface, and enhances the bonding effect between mortar and substrate in a certain degree. Due to the presence of cellulose ether in cement slurry, a special interfacial transition zone and interfacial layer are formed between mortar particles and hydration products. This interfacial layer makes the interfacial transition zone more flexible and less rigid, so that mortar has strong bonding strength.

3. Conclusion and Discussion

Cellulose ether can improve the water retention of self-leveling mortar. With the increase of the amount of cellulose ether, the water retention of mortar is gradually enhanced, and the mortar fluidity and setting time are reduced to a certain extent. Too high water retention will increase the porosity of hardened slurry, which may make the compressive and flexural strength of hardened mortar have obvious loss. In the study, the strength decreased significantly when the dosage was between 0.02% and 0.04%, and the more the amount of cellulose ether, the more obvious the retarding effect. Therefore, when using cellulose ether, it is also necessary to comprehensively consider the mechanical properties of self-leveling mortar, reasonable selection of the dosage and the synergistic effect between it and other chemical materials.

The use of cellulose ether can reduce the compressive strength and flexural strength of cement slurry, and improve the bonding strength of mortar. Analysis of the reasons for the change of strength, mainly caused by the change of micro products and structure, on the one hand, cellulose ether rubber powder particles first adsorbed on the surface of cement particles, the formation of latex film, delay the hydration of cement, which will cause the loss of early strength of slurry; On the other hand, due to the film forming effect and water retention effect, it is conducive to the complete hydration of cement and the improvement of bond strength. The author believes that these two kinds of strength changes mainly exist in the limit of setting period, and the advance and delay of this limit may be the critical point that causes the magnitude of the two kinds of strength. A more in-depth and systematic study of this critical point will be conducive to better regulation and analysis of the hydration process of the cementified material in the slurry. It is helpful to adjust the amount of cellulose ether and curing time according to the demand of mortar mechanical properties, so as to improve the performance of mortar.