Efflorescence, the unsightly white or grayish powder on concrete surfaces, is caused by internal and external factors. Internal causes include moisture penetration due to concrete porosity, inadequate waterproofing, or poor drainage, while material properties like concrete porosity and composition play a role. External causes include salt exposure, air pollution, agricultural activities, and biological growth. Additionally, design and construction errors like insufficient ventilation or improper waterproofing contribute to its development.