One of the biggest differences between dry mixed mortar and traditional mortar is to modify dry powder mortar with a small amount of chemical additives. Adding an additive to dry powder mortar is called primary modification, and adding two or more additives is called secondary modification. The quality of dry powder mortar depends on the correct selection of components and the coordination and matching of various components. Because the price of chemical additives is relatively expensive, and it has a great impact on the performance of dry powder mortar. Therefore, when selecting additives, the amount of additives should be put in the first place. The selection method of chemical additive cellulose ether is briefly introduced below.
Cellulose ether, also known as rheological modifier, is an additive used to adjust the rheological properties of fresh mortar, which is used in almost every kind of mortar. The following properties shall be considered when selecting its variety and dosage:
(1) Water retention at different temperatures;
(2) Thickening, viscosity;
(3) The relationship between consistency and temperature, and the influence on consistency in the presence of electrolyte;
(4) The form and degree of etherification;
(5) Improvement of thixotropy and positioning ability of mortar (this is necessary for mortar painted on vertical surface);
(6) Dissolution rate, conditions and completeness of dissolution.
In addition to adding cellulose ether (such as methylcellulose ether), vinyl polyvinyl acetate can also be added to the dry powder mortar, that is, secondary modification. Inorganic binders (cement and gypsum) in mortar can ensure high compressive strength, but have little effect on tensile strength and flexural strength. Polyvinyl acetate establishes an elastic film in the cement stone hole, so that the mortar can bear high deformation load and improve wear resistance. Practice has proved that adding different amounts of methyl cellulose ether and polyvinyl acetate into dry powder mortar can prepare thin-layer coating plate bonding mortar, plastering mortar, decorative plastering mortar, masonry mortar of aerated concrete block and self leveling mortar for pouring floor. Mixing the two can not only improve the quality of mortar, but also greatly improve the construction efficiency.
In practical application, in order to improve the comprehensive performance, a variety of admixtures need to be used in combination. There is the best matching proportion among the additives. As long as the mixing range and proportion are appropriate, they can improve the performance of mortar from different sides. However, when they are used alone, they have limited modification effect on mortar, and sometimes even have negative effects. For example, adding fiber alone can greatly increase the water consumption of mortar while increasing the cohesion of mortar and reducing the stratification degree, On the contrary, it leads to a large reduction in compressive strength; When air entraining agent is added alone, although the stratification of mortar can be greatly reduced and the water consumption can be greatly reduced, the compressive strength of mortar will decrease due to more bubbles. In order to improve the performance of masonry mortar to the greatest extent, while avoiding harm to other properties of mortar, make the consistency, layering and strength of masonry mortar meet the engineering requirements and the provisions of relevant technical specifications, do not use lime paste, save cement and protect the environment, it is very necessary to take comprehensive measures, develop and use composite admixtures from the perspective of water reduction, viscosity increase, water retention and thickening and air entrainment plasticization.