The use of plastering mortar should be determined by the facade dimensional precision and wall structure of buildings. For example, since the brick structure wall has many mortar joints between blocks and large dimensional deviation of blocks, ordinary plastering mortar is usually used. As for reinforced concrete structure, if the dimensional accuracy deviation of template is small, or the size of concrete wall panel and beam section is inconsistent, then ordinary plastering must be applied; however, if the dimensional accuracy deviation of base course is small, then thin-layer mortar plastering can be applied. For instance, the overlay mortar of external thermal insulation EPS boards can only be thin-layer plastering mortar. If ordinary plastering mortar is applied, the shear loads generated by its self-weight of mortar will make the shear stress increase, affecting the safety of structure. Since the on-site mixed plastering mortar can only take control of raw materials and roughly of mortar proportion, therefore, it is relatively easy to have quality problems; however, ready-mixed plastering mortar will not be easy to have quality problems due to its industrialized production.
The consistency of plastering mortar can be determined by base materials. Selecting ordinary plastering mortar, we should also take its feasibility into consideration at the same time, including the water retention, cohesion, thixotropy, as well as the initial cracking resistance, bonding strength, compressive strength, the later cracking resistance and many other indicators. Emphasis should not be laid on one particular indicator to affect other properties of mortar. For instance, increasing the amount of cement to improve the mortar strength may result in shrinkage cracks of mortar in the later period; increasing the proportion of powdered materials to improve the water retention or operability of mortar may lead to plastic cracking of mortar; affecting the thixotropy of mortar to improve the water retention of mortar may result in difficulty for press polish of plaster; using a large amount of fly ash to reduce the cost may result in peeling of mortar; using the fine sand or adding the air-entraining agent may result in the low strength of mortar, thus peeling and cracking. Therefore, in the selection of ordinary plastering mortar, we should first conduct the small-area plastering operation on the entity of project and then finally determine the economically reasonable plastering mortar.
Attention should also be paid to the amount of cement, organic cementitious materials, water-retaining thickening materials, and the proportion problems of aggregates. If the amount of cement is small but that of organic cementitious material is large, then the mortar is poor in water resistance; if the amount of cement is large but that of organic cementitious material is small, then the mortar is great in brittleness and easy to crack. If the amount of water-retaining thickening material is small, the mortar is prone to early cracks; a large amount of water-retaining thickening material can reduce the compressive strength, water resistance and durability of mortar; a large content of mud and mica in aggregate can reduce the bonding strength of mortar.