After plastering with cement mortar, quality accidents such as cracking, hollowing, and peeling often occur, which can lead to cracking or peeling of the outer decorative layer on top of the plaster layer, seriously affecting the decorative quality of the wall. The performance of plastering mortar and plastering construction technology are the main factors affecting the quality of plastering layer.
Plastering mortar is widely used for leveling construction before wall decoration. However, quality accidents such as cracking, hollowing, and peeling often occur in the plaster layer after using cement mortar plastering, which in turn can lead to cracking or peeling of the outer decorative layer on top of the plaster layer, seriously affecting the decorative quality of the wall. Mortar shrinks due to water loss, and the shrinkage is manifested as a large surface area and a small interior area, which in turn causes the plaster layer to warp outward and separate from the base layer, resulting in hollowing and ultimately detachment; Meanwhile, improper treatment of the base layer and plastering construction techniques can also lead to shrinkage of the mortar. So reducing the shrinkage of mortar and strictly adhering to the requirements of plastering construction can effectively control the quality problems of the plastering layer.
1. Raw materials and testing
Cement: slag cement with a strength grade of 32.5;
Artificial sand: Zone I medium sand with a fineness modulus of 2.6, where w (stone powder) is 6.40%;
Fly ash: Grade 1 fly ash, with a water requirement ratio of 99%;
Lime paste: made by ordinary ball milled quicklime powder and matured, with a viscosity of 58 mm;
Putty: Water resistant putty.
2. Experimental results
2.1 In general, the more water is used for mortar to reach the same consistency, the greater the corresponding shrinkage of the mortar.
2.2 Cube compressive strength test
The strength of mortar under standard curing conditions is greater than that under natural curing conditions; The strength of the mortar mixed with the mixture is lower than that of cement mortar, so the mortar mixed with the mixture is more suitable for surface plastering construction of lightweight walls with lower strength grades.
2.3 Mortar plastering technology test
Two walls were constructed outdoors using ceramic aggregate concrete hollow blocks and sintered porous bricks, and were separated and subjected to plastering tests using four different types of mortar. In plastering construction, tests are mainly conducted on the wetting degree of the base wall and the interval between the plastering time of the surface and bottom mortar. The degree of wetting of grassroots walls can be divided into complete wetting (i.e. watering thoroughly, the water absorption rate of the wall is close to saturation) and incomplete wetting. The time interval for plastering the bottom and surface layers is determined to be 1 hour after the final setting of the mortar begins (i.e. when the final setting degree reaches 70-80% and there is no indentation on the surface of the plastering layer when lightly pressed with fingers). The thickness of the bottom mortar is controlled at 6-7mm, and the thickness of the surface mortar is 8-9mm. 24 hours after the completion of the plastering work, the plastering layer shall be cured by watering twice a day, with each watering reaching the wetting of the plastering surface, and continuously watered for 7 days. Afterwards, the plaster surface of the wall is naturally cured outdoors. Within one month of the experiment, the outdoor ambient temperature is between 16-26C; The humidity is between 45% and 70%, with 3 instances of light to moderate rain during this period. After completing the plastering test for one month, it was observed that the cement mortar had cracks in all states except for complete wetting and long time intervals without significant cracks.
3. Analysis and Discussion of Experimental Results
The cracking of the mortar plastering layer is mainly due to uneven shrinkage caused by large surface water loss shrinkage and small internal shrinkage, resulting in tensile force on the mortar surface and internal pressure. When the tensile force on the surface of the mortar exceeds its tensile strength, cracks will occur. So reducing water consumption and increasing the toughness of mortar are the main methods to effectively control mortar shrinkage. A portion of the water in the mortar is used for the hydration of the cementitious material. The remaining portion is used to meet the consistency requirements, and most of the water used in this part will eventually evaporate, causing the mortar to shrink due to dehydration. Therefore, when preparing mortar, while meeting the requirements of hydration water and construction consistency, it is necessary to minimize the water cement ratio and reduce water consumption as much as possible, which can reduce the shrinkage caused by water loss and control the occurrence of cracks. Adding a certain amount of fly ash and lime to cement mortar can reduce water consumption while ensuring consistency. Fly ash contains large amounts of
The amount of glass microspheres used in fly ash reduces the internal specific surface area and water demand. When used in cement mortar, it has a water retention effect and can also have a certain water reducing effect to reduce drying shrinkage caused by water loss. In addition, fly ash can also improve the compactness of mortar, refine pores, and fill harmful capillary pores. When quicklime is matured into lime slurry, it can automatically form extremely fine (with a diameter of about 1 μ m) colloidal dispersed calcium hydroxide particles, which adsorb a thick water film on the surface and have good plasticity. Adding lime slurry to cement mortar can improve plasticity, reduce water consumption, and minimize shrinkage. Adding putty to cement mortar can also reduce shrinkage without changing the amount of water used. This is because during the setting and hardening process of mortar, the adhesive in putty dries and forms a film in the transition zone between sand and cement slurry, making the bond between the two more dense and firm; In addition to the combination with sand, the putty will be evenly dispersed into the cement paste. At the initial stage of cement hydration, the putty will adhere to gel and unhydrated cement particles. As the cement hydration progresses, the water in the cement paste will continue to consume, and the hydrates will increase. The putty will gather in the pores, form a close stack on the surface of gel and unhydrated particles, and gradually fill the pores. With the further reduction of hydrated and dry water, the putty binder accumulated in the gel and pores will agglomerate to form a polymer network. The elastic modulus of the polymer is lower than that of the cement, which improves the toughness and crack resistance of the putty mortar and effectively resists shrinkage. The purpose of completely wetting the base layer during plastering construction is to allow it to absorb enough moisture and avoid
Avoid absorbing moisture from plastering mortar, which may affect the normal hydration of mortar. The main purpose of controlling the interval between the bottom layer and the surface layer plastering is to ensure that the bottom layer has a certain strength and bonding force. If the interval is too long, the bottom layer mortar will completely harden, causing the surface layer mortar to form two layers with the bottom layer, resulting in cracking or peeling due to weak bonding. The interval time is too short, and the bottom mortar has not yet reached a certain degree of solidification, which may be bonded by the newly applied mortar and fall off.
4. Conclusion
a. Cement fly ash mortar, cement putty mortar, and cement lime mortar are suitable for use as plastering layers on wall materials with low strength grades such as ceramic aggregate concrete hollow blocks and sintered porous bricks; After using cement lime mortar for plastering, it is important to pay attention to controlling the curing conditions. Using cement mortar plastering can reduce the occurrence of cracks while ensuring appropriate construction techniques.
b. Adding a certain amount of mixture (such as fly ash, lime paste, putty, etc.) to cement mortar can reduce water consumption or improve mortar toughness, effectively controlling the occurrence of cracks on the surface of the plaster layer.
c. During plastering construction, the surface of the base wall should be completely wetted; The interval between plastering the surface layer and the bottom layer should not be too long or too short, and it is advisable for the final setting of the mortar to reach 70-80%.