Modified Cellulose ether for mortar

The types of cellulose ether and its main functions in mixed mortar and the evaluation methods of properties such as water retention, viscosity and bond strength are analyzed. The retarding mechanism and microstructure of cellulose ether in dry mixed mortar and the relationship between the formation of the structure of some specific thin layer cellulose ether modified mortar and the hydration process are expounded. On this basis, it is suggested that it is necessary to accelerate the study on the condition of rapid loss of water. The layered hydration mechanism of cellulose ether modified mortar in the thin layer structure and the spatial distribution law of polymer in the mortar layer. In the future practical application, the effect of cellulose ether modified mortar on temperature change and compatibility with other admixtures should be fully considered. This study will promote the development of application technology of CE modified mortar such as external wall plastering mortar, putty, joint mortar and other thin layer mortar.

Key words: cellulose ether; Dry mixed mortar; mechanism

1. Introduction

Ordinary dry mortar, exterior wall insulation mortar, self-calming mortar, waterproof sand and other dry mortar has become important part of building materials based in our country, and cellulose ether is the derivatives of natural cellulose ether, and important additive additive of various kinds of dry mortar, retarding, water retention, thickening, air absorption, adhesion and other functions.

The role of CE in mortar is mainly reflected in improving the workability of mortar and ensuring the hydration of cement in mortar. The improvement of mortar workability is mainly reflected in water retention, anti-hanging and opening time, especially in ensuring thin layer mortar carding, plastering mortar spreading and improving the construction speed of special bonding mortar has important social and economic benefits.

Although a large number of studies on CE modified mortar have been carried out and important achievements have been made in the application technology research of CE modified mortar, there are still obvious deficiencies in the mechanism research of CE modified mortar, especially the interaction between CE and cement, aggregate and matrix under special use environment. Therefore, Based on the summary of relevant research results, this paper proposes that further research on temperature and compatibility with other admixtures should be carried out.

2、the role and classification of cellulose ether

2.1 Classification of cellulose ether

Many varieties of cellulose ether, there are nearly a thousand, in general, according to the ionization performance can be divided into ionic and non-ionic type 2 categories, in cement-based materials due to ionic cellulose ether (such as carboxymethyl cellulose, CMC) will precipitate with Ca2+ and unstable, so rarely used. Nonionic cellulose ether can be in accordance with (1) the viscosity of standard aqueous solution; (2) the type of substituents; (3) degree of substitution; (4) physical structure; (5) Classification of solubility, etc.

The properties of CE depend mainly on the type, quantity and distribution of substituents, so CE is usually divided according to the type of substituents. Such as methyl cellulose ether is a natural cellulose glucose unit on the hydroxyl is replaced by methoxy products, hydroxypropyl methyl cellulose ether HPMC is hydroxyl by methoxy, hydroxypropyl respectively replaced products. At present, more than 90% of the cellulose ethers used are mainly methyl hydroxypropyl cellulose ether (MHPC) and methyl hydroxyethyl cellulose ether (MHEC).

2.2 The role of cellulose ether in mortar

The role of CE in mortar is mainly reflected in the following three aspects: excellent water retention ability, influence on the consistency and thixotropy of mortar and adjusting rheology.

The water retention of CE can not only adjust the opening time and setting process of the mortar system, so as to adjust the operating time of the system, but also prevent the base material from absorbing too much and too fast water and prevent the evaporation of water, so as to ensure the gradual release of water during the hydration of cement. The water retention of CE is mainly related to the amount of CE, viscosity, fineness and the ambient temperature. The water retention effect of CE modified mortar depends on the water absorption of the base, the composition of the mortar, the thickness of the layer, the water requirement, the setting time of the cementing material, etc. Studies show that in the actual use of some ceramic tile binders, due to the dry porous substrate will quickly absorb a large amount of water from the slurry, the cement layer near the substrate loss of water leads to the hydration degree of cement below 30%, which not only cannot form cement gel with bonding strength on the surface of the substrate, but also easy to cause cracking and water seepage.

The water requirement of mortar system is an important parameter. The basic water requirement and the associated mortar yield depend on the mortar formulation, i.e. the amount of cementing material, aggregate and aggregate added, but the incorporation of CE can effectively adjust the water requirement and mortar yield. In many building material systems, CE is used as a thickener to adjust the consistency of the system. The thickening effect of CE depends on the degree of polymerization of CE, solution concentration, shear rate, temperature and other conditions. CE aqueous solution with high viscosity has high thixotropy. When the temperature increases, structural gel is formed and high thixotropy flow occurs, which is also a major characteristic of CE.

The addition of CE can effectively adjust the rheological property of the building material system, so as to improve the working performance, so that the mortar has better workability, better anti-hanging performance, and does not adhere to the construction tools. These properties make the mortar easier to level and cure.

2.3 Performance evaluation of cellulose ether modified mortar

The performance evaluation of CE modified mortar mainly includes water retention, viscosity, bond strength, etc.

Water retention is an important performance index which is directly related to the performance of CE modified mortar. At present, there are many relevant test methods, but most of them use vacuum pump method to directly extract the moisture. For example, foreign countries mainly use DIN 18555(test method of inorganic cementation material mortar), and French aerated concrete production enterprises use filter paper method. The domestic standard involving water retention test method has JC/T 517-2004(plaster plaster), its basic principle and calculation method and foreign standards are consistent, all through the determination of mortar water absorption rate said mortar water retention.

Viscosity is another important performance index directly related to the performance of CE modified mortar. There are four commonly used viscosity test methods: Brookileld, Hakke, Hoppler and rotary viscometer method. The four methods use different instruments, solution concentration, testing environment, so the same solution tested by the four methods are not the same results. At the same time, the viscosity of CE varies with temperature and humidity, so the viscosity of the same CE modified mortar changes dynamically, which is also an important direction to be studied on CE modified mortar at present.

Bond strength test is determined according to the direction of mortar use, such as ceramic bond mortar mainly refer to “ceramic wall tile adhesive” (JC/T 547-2005), Protective mortar mainly refer to “external wall insulation mortar technical requirements” (DB 31 / T 366-2006) and “external wall insulation with expanded polystyrene board plaster mortar” (JC/T 993-2006). In foreign countries, the adhesive strength is characterized by the flexural strength recommended by the Japanese Association of Materials Science (the test adopts the prismatic ordinary mortar cut in two halves with the size of 160mm×40mm×40mm and modified mortar made into samples after curing, with reference to the test method of the flexural strength of cement mortar).

3. Theoretical research progress of cellulose ether modified mortar

The theoretical research of CE modified mortar mainly focuses on the interaction between CE and various substances in mortar system. The chemical action inside the cement-based material modified by CE can be basically shown as CE and water, hydration action of cement itself, CE and cement particle interaction, CE and cement hydration products. The interaction between CE and cement particles/hydration products is mainly manifested in the adsorption between CE and cement particles.

The interaction between CE and cement particles has been reported at home and abroad. For example, Liu Guanghua et al. measured the Zeta potential of CE modified cement slurry colloid when studying the action mechanism of CE in underwater non-discrete concrete. The results showed that: The Zeta potential (-12.6mV) of cement-doped slurry is smaller than that of cement paste (-21.84mV), indicating that the cement particles in cement-doped slurry are coated with non-ionic polymer layer, which makes the double electric layer diffusion thinner and the repulsive force between colloid weaker.

3.1 Retarding theory of cellulose ether modified mortar

In the theoretical study of CE modified mortar, it is generally believed that CE not only endows mortar with good working performance, but also reduces the early hydration heat release of cement and delays the hydration dynamic process of cement.

The retarding effect of CE is mainly related to its concentration and molecular structure in mineral cementing material system, but has little relationship with its molecular weight. It can be seen from the effect of the chemical structure of CE on the hydration kinetics of cement that the higher the CE content, the smaller the alkyl substitution degree, the larger the hydroxyl content, the stronger the hydration delay effect. In terms of molecular structure, hydrophilic substitution (e.g., HEC) has stronger retarding effect than hydrophobic substitution (e.g., MH, HEMC, HMPC).

From the perspective of the interaction between CE and cement particles, the retarding mechanism is manifested in two aspects. On the one hand, the adsorption of CE molecule on the hydration products such as c – s –H and Ca(OH)2 prevents further cement mineral hydration; on the other hand, the viscosity of pore solution increases due to CE, which reduces the ions (Ca2+, so42-…). The activity in the pore solution further retards the hydration process.

CE not only delays setting, but also delays the hardening process of the cement mortar system. It is found that CE affects the hydration kinetics of C3S and C3A in cement clinker in different ways. CE mainly decreased the reaction rate of C3s acceleration phase, and prolonged the induction period of C3A/CaSO4. The retardation of c3s hydration will delay the hardening process of mortar, while the extension of induction period of C3A/CaSO4 system will delay the setting of mortar.

3.2 Microstructure of cellulose ether modified mortar

The influence mechanism of CE on the microstructure of modified mortar has attracted extensive attention. It is mainly reflected in the following aspects:

Firstly, the research focus is on the film forming mechanism and morphology of CE in mortar. Since CE is commonly used with other polymers, it is an important research focus to distinguish its state from that of other polymers in mortar.

Secondly, the effect of CE on the microstructure of cement hydration products is also an important research direction. As can be seen from the film forming state of CE to hydration products, hydration products form a continuous structure at the interface of cE connected to different hydration products. In 2008, K.Pen et al. used isothermal calorimetry, thermal analysis, FTIR, SEM and BSE to study the lignification process and hydration products of 1% PVAA, MC and HEC modified mortar. The results showed that although the polymer delayed the initial hydration degree of cement, it showed a better hydration structure at 90 days. In particular, MC also affects the crystal morphology of Ca(OH)2. The direct evidence is that the bridge function of polymer is detected in the layered crystals, MC plays a role in bonding crystals, reducing microscopic cracks and strengthening the microstructure.

The microstructure evolution of CE in mortar has also attracted a lot of attention. For example, Jenni used various analytical techniques to study the interactions between materials within polymer mortar, combining quantitative and qualitative experiments to reconstruct the entire process of mortar fresh mixing to hardening, including polymer film formation, cement hydration and water migration.

In addition, the micro-analysis of different time points in the mortar development process, and can not be in situ from the mortar mixing to hardening of the entire process of continuous micro-analysis. Therefore, it is necessary to combine the whole quantitative experiment to analyze some special stages and trace the microstructure formation process of key stages. In China, Qian Baowei, Ma Baoguo et al. directly described the hydration process by using resistivity, heat of hydration and other test methods. However, due to few experiments and failure to combine resistivity and heat of hydration with the microstructure at various time points, no corresponding research system has been formed. In general, until now, there has been no direct means to quantitatively and qualitatively describe the presence of different polymer microstructure in mortar.

3.3 Study on cellulose ether modified thin layer mortar

Although people have carried out more technical and theoretical studies on the application of CE in cement mortar. But he has to pay attention to is that CE modified mortar in the daily dry mixed mortar (such as brick binder, putty, thin layer plastering mortar, etc.) are applied in the form of thin layer mortar, this unique structure is usually accompanied by the mortar rapid water loss problem.

For example, ceramic tile bonding mortar is a typical thin layer mortar (the thin layer CE modified mortar model of ceramic tile bonding agent), and its hydration process has been studied at home and abroad. In China, Coptis rhizoma used different kinds and amounts of CE to improve the performance of ceramic tile bonding mortar. X-ray method was used to confirm that the hydration degree of cement at the interface between cement mortar and ceramic tile after mixing CE was increased. By observing the interface with a microscope, it was found that the cement-bridge strength of ceramic tile was mainly improved by mixing CE paste instead of density. For example,Jenni observed enrichment of polymer and Ca(OH)2 near the surface. Jenni believes that the coexistence of cement and polymer drives the interaction between polymer film formation and cement hydration. The main characteristic of CE modified cement mortars compared to ordinary cement systems is a high water-cement ratio (usually at or above 0. 8), but because of their high area/volume, they also harden rapidly, so that cement hydration is usually less than 30%, rather than more than 90% as is usually the case. In the use of XRD technology to study the development law of the surface microstructure of ceramic tile adhesive mortar in the hardening process, it was found that some small cement particles were “transported” to the outer surface of the sample with the drying of the pore solution. To support this hypothesis, further tests were carried out using coarse cement or better limestone instead of the previously used cement, which was further supported by the simultaneous mass loss XRD absorption of each sample and the limestone/silica sand particle size distribution of the final hardened body. Environmental scanning electron microscopy (SEM) tests revealed that CE and PVA migrated during wet and dry cycles, while rubber emulsions did not. Based on this, he also designed an unproven hydration model of thin layer CE modified mortar for ceramic tile binder.

The relevant literature has not reported how the layered structure hydration of polymer mortar is carried out in the thin layer structure, nor has the spatial distribution of different polymers in the mortar layer been visualized and quantified by different means. Obviously, the hydration mechanism and microstructure formation mechanism of CE-mortar system under the condition of rapid water loss are significantly different from the existing ordinary mortar. The study of the unique hydration mechanism and microstructure formation mechanism of thin layer CE modified mortar will promote the application technology of thin layer CE modified mortar, such as external wall plastering mortar, putty, joint mortar and so on.

4. There are problems

4.1 Influence of temperature change on cellulose ether modified mortar

CE solution of different kinds will gel at their specific temperature, the gel process is completely reversible. The reversible thermal gelation of CE is very unique. In many cement products, the main use of the viscosity of CE and the corresponding water retention and lubrication properties, and the viscosity and gel temperature has a direct relationship, under the gel temperature, the lower the temperature, the higher the viscosity of CE, the better the corresponding water retention performance.

At the same time, the solubility of different kinds of CE at different temperatures is not completely the same. Such as methyl cellulose soluble in cold water, insoluble in hot water; Methyl hydroxyethyl cellulose is soluble in cold water, not hot water. But when the aqueous solution of methyl cellulose and methyl hydroxyethyl cellulose is heated, the methyl cellulose and methyl hydroxyethyl cellulose will precipitate out. Methyl cellulose precipitated at 45 ~ 60℃, and mixed etherized methyl hydroxyethyl cellulose precipitated when the temperature increased to 65 ~ 80℃ and the temperature decreased, precipitated re dissolved. Hydroxyethyl cellulose and sodium hydroxyethyl cellulose are soluble in water at any temperature.

In the actual use of CE, the author also found that the water retention capacity of CE decreases rapidly at low temperatures (5℃), which is usually reflected in the rapid decline of workability during construction in winter, and more CE has to be added. The reason for this phenomenon is not clear at present. The analysis may be caused by the change of the solubility of some CE in low temperature water, which needs to be carried out to ensure the quality of construction in winter.

4.2 Bubble and elimination of cellulose ether

CE usually introduces a large number of bubbles. On the one hand, uniform and stable small bubbles are helpful to the performance of mortar, such as improving the constructability of mortar and enhancing the frost resistance and durability of mortar. Instead, larger bubbles degrade the mortar’s frost resistance and durability.

In the mixing process of mortar with water, the mortar is stirred, and the air is brought into the newly mixed mortar, and the air is wrapped by the wet mortar to form bubbles. Normally, under the condition of low viscosity of the solution, the bubbles formed rise due to buoyancy and rush to the surface of the solution. The bubbles escape from the surface to the outside air, and the liquid film moved to the surface will produce pressure difference due to the action of gravity. The thickness of the film will become thinner with time, and finally the bubbles will burst. However, due to the high viscosity of the newly mixed mortar after adding CE, the average rate of liquid seepage in the liquid film is slowed down, so that the liquid film is not easy to become thin; At the same time, the increase of mortar viscosity will slow down the diffusion rate of surfactant molecules, which is beneficial to foam stability. This causes a large number of bubbles introduced into the mortar to stay in the mortar.

Surface tension and interfacial tension of aqueous solution culminating Al brand CE at 1% mass concentration at 20℃. CE has air entraining effect on cement mortar. The air entraining effect of CE has negative effect on mechanical strength when large bubbles are introduced.

The defoamer in mortar can inhibit the foam formation caused by CE use, and destroy the foam that has been formed. Its action mechanism is: the defoaming agent enters the liquid film, reduces the viscosity of the liquid, forms a new interface with low surface viscosity, makes the liquid film lose its elasticity, accelerates the process of liquid exudation, and finally makes the liquid film thin and crack. The powder defoamer can reduce the gas content of the newly mixed mortar, and there are hydrocarbons, stearic acid and its ester, trietyl phosphate, polyethylene glycol or polysiloxane adsorbed on the inorganic carrier. At present, the powder defoamer used in dry mixed mortar is mainly polyols and polysiloxane.

Although it is reported that in addition to adjusting the bubble content, the application of defoamer can also reduce shrinkage, but different kinds of defoamer also have compatibility problems and temperature changes when used in combination with CE, these are the basic conditions to be solved in the use of CE modified mortar fashion.

4.3 Compatibility between cellulose ether and other materials in mortar

CE is usually used together with other admixtures in dry mixed mortar, such as defoamer, water reducing agent, adhesive powder, etc. These components play different roles in mortar respectively. To study the compatibility of CE with other admixtures is the premise of efficient utilization of these components.

Dry mixed mortar mainly used water reducing agents are: casein, lignin series water reducing agent, naphthalene series water reducing agent, melamine formaldehyde condensation, polycarboxylic acid. Casein is an excellent superplasticizer, especially for thin mortars, but because it is a natural product, the quality and price often fluctuate. Lignin water-reducing agents include sodium lignosulfonate (wood sodium), wood calcium, wood magnesium. Naphthalene series water reducer commonly used Lou. Naphthalene sulfonate formaldehyde condensates, melamine formaldehyde condensates are good superplasticizers, but the effect on thin mortar is limited. Polycarboxylic acid is a newly developed technology with high efficiency and no formaldehyde emission. Because CE and common naphthalene series superplasticizer will cause coagulation to make concrete mixture lose workability, so it is necessary to choose non-naphthalene series superplasticizer in engineering. Although there have been studies on the compound effect of CE modified mortar and different admixtures, there are still many misunderstandings in use due to the variety of various admixtures and CE and few studies on the interaction mechanism, and a large number of tests are needed to optimize it.

5. Conclusion

The role of CE in mortar is mainly reflected in the excellent water retention capacity, the influence on the consistency and thixotropic properties of mortar and the adjustment of rheological properties. In addition to giving mortar good working performance, CE can also reduce the early hydration heat release of cement and delay the hydration dynamic process of cement. The performance evaluation methods of mortar are different based on the different application occasions.

A large number of studies on the microstructure of CE in mortar such as film forming mechanism and film forming morphology have been carried out abroad, but up to now, there is no direct means to quantitatively and qualitatively describe the existence of different polymer microstructure in mortar.

CE modified mortar is applied in the form of thin layer mortar in daily dry mixing mortar (such as face brick binder, putty, thin layer mortar, etc.). This unique structure is usually accompanied by the problem of rapid water loss of mortar. At present, the main research focuses on the face brick binder, and there are few studies on other types of thin layer CE modified mortar.

Therefore, in the future, it is necessary to accelerate the research on the layered hydration mechanism of cellulose ether modified mortar in the thin layer structure and the spatial distribution law of polymer in the mortar layer under the condition of rapid water loss. In practical application, the influence of cellulose ether modified mortar on temperature change and its compatibility with other admixtures should be fully considered. Related research work will promote the application technology development of CE modified mortar such as external wall plastering mortar, putty, joint mortar and other thin layer mortar.