Currently, many buildings exhibit cracking in their exterior cement mortar shortly after being put into service. What, exactly, causes this phenomenon? To investigate, the Marketing Department at Shijiazhuang Qingquan Cellulose Co., Ltd.conducted a study and discovered that there is more than one reason for mortar cracking:
1. During construction, some contractors select high-quality cement and aggregates for structural components such as beams, slabs, and columns; however, assuming that structural safety is not a concern during the plastering phase, they opt for inexpensive cement, fine aggregates, and additives. This leads to excessive shrinkage in the cement mortar, resulting in cracking.
2. The simple addition of gypsum or lime powder to the cement mortar mix—while improving workability—often occurs without the inclusion of redispersible polymer powder. This omission leads to a reduction in the mortar's overall strength, as well as diminished shear resistance and bond strength, making it prone to delamination (hollowing) and cracking.
3. Improper construction techniques or a failure to adhere to regulatory specifications are often to blame; many critical precautions are frequently overlooked.
4. Factors related to changes in temperature and humidity:
① Temperature fluctuations cause building materials to expand or contract. Since exterior cement mortar is largely exposed to direct sunlight, if the mortar mix does not incorporate a high-water-retention agent—such as Hydroxypropyl Methylcellulose (HPMC)—its water retention capacity will be insufficient. This leads to temperature-induced shrinkage cracks in the exterior mortar. Although the cracks resulting from thermal stress may initially appear minute, repeated cycles of expansion and contraction will cause these fissures to progressively widen.
② Exterior cement mortar is constantly exposed to the atmosphere and frequently expands or contracts in response to changes in humidity. Redispersible polymer powder is known for imparting excellent flexural strength and impact resistance to mortar. The shrinkage caused by humidity fluctuations constitutes a reversible process driven by wet-dry cycles. When the shrinkage stress exceeds the mortar's tensile strength, mortar mixes that lack redispersible polymer powder are inevitably prone to cracking.
