1 The difference between dry mortar and traditional mortar

One of the significant differences between dry mortar and conventional mortar is the use of small amounts of chemical additives for modification. If one additive is added to a dry mortar, this is referred to as primary modification; if two or more additives are added, this is referred to as secondary modification. The quality of dry mortar depends on the correct choice of ingredients and their harmonization with each other. Since chemical additives are expensive and have a significant effect on the performance of dry mortar, the dosage should be prioritized when selecting additives. The following describes the selection of a commonly used chemical additive, hydroxypropyl methyl cellulose.

2 Advantages of hydroxypropyl methylcellulose

Hydroxypropyl methyl cellulose, also known as rheology modifier, is an admixture used to regulate the rheological properties of freshly mixed mortar and is used in almost all types of mortar. The following properties should be considered when selecting its species and dosage:

(1) Water retention capacity at different temperatures;

(2) Thickening effect and viscosity;

(3) The relationship between consistency and temperature, and the effect on consistency in the presence of electrolytes;

(4) The type and extent of etherification;

(5) Improvements in the thixotropy and localization ability of the mortar (especially for mortars requiring vertical surface application);

(6) Rate of dissolution, conditions of dissolution and completeness of dissolution.

3 practical application

In dry mortars, secondary modification can be achieved by adding polyvinyl acetate in addition to hydroxypropyl methyl cellulose. Inorganic binders (e.g., cement and gypsum) in mortars provide high compressive strength but contribute less to tensile and flexural strength. Polyvinyl acetate forms an elastic film within the cement stone pores, allowing the mortar to withstand large deformation loads, thus improving its abrasion resistance.

Practice has shown that by adding different amounts of methyl cellulose ether and polyvinyl acetate to dry mortar, it is possible to prepare mortars for a variety of purposes, such as thin-layer coated slab bonding mortar, plastering mortar, decorative stucco mortar, aerated concrete block masonry mortar, and self-smoothing mortar for pouring flooring. The combined use of the two can not only improve the quality of mortar, but also significantly improve the construction efficiency.

In practice, in order to improve the comprehensive performance of mortar, the compound use of multiple additives is usually required. There is an optimal matching ratio between each additive, as long as the dosage and ratio are appropriate, it can improve the performance of mortar from different aspects. However, the use of a single additive on the mortar modification effect is often limited, and sometimes even produce negative effects. For example, although the addition of hydroxypropyl methyl cellulose alone can increase the cohesion of the mortar and reduce delamination, it will significantly increase the water consumption of the mortar and make the water retained in the interior of the slurry, resulting in a significant reduction in compressive strength. And the use of air-entraining agent alone, although it can significantly reduce the degree of delamination and water consumption of the mortar, but due to excessive air bubbles, the compressive strength of the mortar will also be reduced.

In order to maximize the improvement of the performance of masonry mortar, while avoiding adverse effects on other properties, so that the consistency, delamination and strength of masonry mortar can meet the requirements of the project and the relevant technical specifications, and to avoid the use of lime paste, save cement and protect the environment, it is necessary to take comprehensive measures from the perspective of water reduction, viscosity, water retention and thickening, and air-entraining plasticization, to research and development and the use of composite additives. This approach is very necessary.