Research And Progress Of Modified Cellulose In Mortar

Dry-mixed mortars such as ordinary dry-mixed mortar, exterior wall insulation mortar, self-leveling mortar, and waterproof mortar have become an important part of cement-based building materials. Cellulose ether is a derivative of natural cellulose ether and is an important modified additive for various types of dry-mixed mortar. It plays the functions of retarding, water retention, thickening, air entrainment, and bonding.

The role of cellulose 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 aspects such as water retention, sag resistance and opening time. It has important social and economic benefits especially in ensuring thin-layer mortar carding, plastering mortar paving and increasing the construction speed of special bonding mortar.

Although a lot of research work has been carried out on cellulose in mortar, and significant results have been achieved in cellulose-modified mortar, there are still some shortcomings in the interaction between cellulose ethers and cement, aggregates, and matrix in special environments. To this end, this article, on the basis of summarizing relevant research, further studies the temperature and compatibility of modified cellulose ether with other additives.

Types of cellulose ethers

There are many varieties of cellulose ethers, and there are nearly all of them. Generally speaking, they can be divided into two categories: ionic and non-ionic according to their ionization properties. In cement-based materials, ionic cellulose ethers, such as carboxymethylcellulose (CMC), will precipitate with Ca2+ and become unstable, so they are rarely used. Nonionic cellulose ethers can be classified according to (1) viscosity of standard aqueous solution (2) type of substituent (3) degree of substitution (4) physical structure (5) solubility properties, etc.

The properties of cellulose ethers mainly depend on the type, number and distribution of substituents, so cellulose ethers are usually divided according to the type of substituents. For example, methylcellulose ether (MC) is a product in which the hydroxyl group on the glucose unit of natural cellulose is replaced by a methoxy group, and hydroxypropyl methylcellulose ether (HPMC) is a product in which the hydroxyl group is replaced by a methoxy group and a hydroxypropyl group respectively. More than 90% of the cellulose ethers currently used are mainly methylhydroxypropylcellulose ether (HPMC) and methylhydroxyethylcellulose ether (MHEC).

The role of cellulose in mortar

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

The water retention of cellulose ether can not only adjust the opening time and coagulation process of the mortar system, thereby adjusting the operable time of the system, but also prevent the base material from absorbing too much water too quickly and hinder the evaporation of water, so as to ensure the gradual release of water when the cement hydrates. The water retention of cellulose is mainly related to the amount of cellulose added, viscosity, fineness and the ambient temperature of use. The water retention effect of modified cellulose depends on the water absorption of the base layer, the composition layer thickness of the mortar, the water demand, and the setting time of the cementitious material.

The water requirement of the mortar system is an important parameter. The basic water demand and the associated mortar output depend on the mortar formulation, and the incorporation of cellulose can effectively adjust the water demand and mortar output. In many building material systems, cellulose is used as a thickener to adjust the consistency of the system. The thickening effect of cellulose depends on the degree of polymerization of cellulose, solution concentration, shear rate, temperature and other conditions. High-viscosity cellulose aqueous solutions have high thixotropy. When the temperature is increased, a structural gel is formed and high thixotropic flow occurs.

The addition of cellulose can effectively adjust the rheology of the building material system, thereby improving the working performance and making the mortar have better workability and better anti-sag properties. These properties make the mortar easier to level and facilitate curing.

In addition to giving the mortar good working performance, modified cellulose can also reduce the early hydration heat of cement and delay the hydration dynamics process of cement. Based on the different use occasions of mortar, there are also differences in its performance evaluation methods.

Compatibility of modified cellulose with other additives in mortar

Cellulose is usually used together with other admixtures in dry-mixed mortar, such as defoamer, water-reducing agent, rubber powder, etc. Each of these components plays a different role in the mortar. Studying the compatibility of modified cellulose with other additives is a prerequisite for efficient utilization of these components.

The main water-reducing agents used in dry-mixed mortar are: casein, lignin-based water-reducing agent, naphthalene-based water-reducing agent, melamine formaldehyde condensate, and polycarboxylic acid water-reducing agent. Polycarboxylate superplasticizer (PCE) is the latest developed technology with high efficiency and no formaldehyde emission. Since cellulose ethers will agglomerate when combined with commonly used naphthalene-based water-reducing agents, causing the concrete mixture to lose workability, it is necessary to use non-naphthalene-based high-efficiency water-reducing agents in engineering. Although there have been studies on the composite effect of modified cellulose and different additives, due to the large number of various additives and cellulose types and not much research on the interaction mechanism, there are still many misunderstandings during use, and a large number of experiments are needed to optimize them.