Preventive Measures for Sanding of Cement-Sand Mortar Floor Surfaces
Release Time:
2021-02-22
Source:
Causes and Prevention of Sanding in Cement-Sand Mortar Floor Surfaces
Cement-sand mortar screeds are commonly used in construction projects, particularly in resettlement programs for relocated residents, new rural development initiatives, renovation of dilapidated housing, and the construction of affordable and low-rent housing—projects that directly address people’s livelihoods. This type of flooring system is simple in design, durable and wear-resistant, and cost-effective; however, improper construction often leads to three common quality defects: cracking, hollowing, and sanding. These issues compromise both the aesthetic appearance and the functional performance of the floor, eliciting strong dissatisfaction and complaints from residents. Therefore, they must be given due attention and thoroughly addressed.
Analysis of the causes of sanding:
1. The water-cement ratio of the cement-sand mortar mix is too high, resulting in excessive consistency. The water-cement ratio is inversely proportional to the strength of the cement-sand mortar; as the water-cement ratio increases, the mortar strength decreases. If too much water is used during construction, the mortar strength will be significantly reduced. Moreover, excess water can lead to bleeding in the mortar, further diminishing the surface strength of the floor. Once the finished floor is subjected to foot traffic and abrasion, it will begin to dust.
2. Improper sequencing of construction operations. A lack of understanding of the cement hardening process, coupled with inappropriate scheduling of the troweling operation and conditions such as an excessively dry or overly wet underlying layer, can result in troweling being carried out either too early or too late. If troweling is performed too early, cement hydration has just begun, the gel structure has not yet fully developed, and there is still a significant amount of free water; even after troweling, a film of water will remain on the surface, which is detrimental to the strength and wear resistance of the finishing mortar layer. Conversely, if troweling is delayed until after final set and hardening have occurred, not only does the work become difficult, making it impossible to eliminate surface capillarity and trowel marks, but the already hardened surface may also be disturbed, thereby substantially reducing the strength and wear resistance of the finishing mortar layer.
3. Improper curing of the surface layer. After completion of a cement floor, failure to cure or insufficient curing duration can result in rapid evaporation of surface moisture in dry conditions, thereby impairing the hydration of cement, slowing down the hardening process, and in severe cases even halting hardening altogether. This leads to dehydration of the cement mortar, compromising its strength and wear resistance. Furthermore, if the floor is watered for curing less than 24 hours after troweling, the relatively “fresh” surface may peel off over large areas, exposing the sand particles and causing dusting upon use.
4. Inadequate protection of the finished floor. If a cement floor is put into use or subjected to subsequent construction operations before it has attained sufficient strength, the surface may be subjected to friction and other forms of wear, which can easily lead to sanding. This problem is particularly pronounced in low-temperature conditions.
5. Raw materials do not meet the required specifications. First, the cement has a low strength grade, or expired, damp, and caked cement is used; such cement exhibits poor activity, which adversely affects the strength and wear resistance of the floor finish. Second, the sand particles are excessively fine, requiring a large amount of mixing water and resulting in an increased water-cement ratio and reduced strength. Moreover, excessive silt content in the sand impairs the bond between the cement and sand, making the floor prone to sanding.
Preventive Measures for Sanding
1. Strictly control the water-cement ratio. The consistency of cement mortar used for floor finishes shall not exceed 35 mm, and the slump of concrete and fine-aggregate concrete used for flooring shall not exceed 30 mm. The leveling layer must be thoroughly wetted, the cement slurry applied evenly, and the screed compacted and leveled immediately after spreading. The concrete finish shall be vibrated to achieve full compaction; for fine-aggregate concrete, rolling with a roller or tamping with a wooden float is recommended to bring up a cement paste on the surface, thereby ensuring the strength and density of the finish.
2. Master the timing of surface troweling. For cement floors, troweling should generally be performed no fewer than three times. The first pass should be carried out immediately after the surface layer has been laid. Begin by using a wooden trowel to evenly rub and pat the surface, ensuring uniformity and compactness of the material, and achieving a smooth, level finish—ideally with no visible water film on the surface. The second troweling should be completed after the cement has reached initial set but before final set (typically when a light footprint can be left by stepping on the surface without noticeable sinking), to further compact and smooth the surface. The third troweling is primarily intended to eliminate trowel marks and seal fine pores, while further compacting and polishing the surface (the timing should be such that no footprints are left upon stepping on the surface, or only very faint ones remain). Under no circumstances should troweling be performed after the cement has fully set.
3. After the cement floor has been troweled to a smooth finish, water curing should be carried out after approximately one day and one night, depending on the ambient temperature; alternatively, the surface may be covered with straw mats or sawdust and then watered for curing. Where conditions permit, a threshold made of yellow clay or lime paste can be constructed at the doorway to create a water-retaining barrier for curing. For floors using ordinary Portland cement, the curing period shall be no less than seven days and nights; for floors using slag Portland cement, the curing period shall be no less than ten days and nights.
4. Arrange the construction sequence reasonably to avoid premature foot traffic. Cement flooring should, whenever possible, be laid only after finishing work such as wall and ceiling painting and other decorative works, in order to prevent contamination and damage to the finished surface. If it must be installed prior to other decorative works, effective protective measures shall be implemented, such as laying straw mats, straw curtains, or roofing felt, and a curing period of 7 to 10 days and nights must be ensured.
5. Cement shall preferably be high-early-strength Portland cement or ordinary Portland cement, with a strength grade of not less than 32.5. Cement that is past its shelf life or has become caked due to moisture absorption shall not be used. Sand shall preferably be coarse or medium sand, with a silt content not exceeding 3%. For the surface layer, the nominal size of fine aggregate and crushed stone shall not exceed 15 mm, nor exceed two-thirds of the thickness of the surface layer, and the silt content shall not exceed 2%.
