Portland Cement and Its Types

Type 1 Portland cement is the most widely used grade in Tehran's urban projects, serving slab and column pours as well as ordinary mortars. Type 2 offers moderate sulphate resistance and is the sensible choice for foundations where the geotechnical report indicates moderate sulphate content. Type 5 is reserved for soils and groundwater with high sulphate levels, typical of the southern provinces and desert margins. Pozzolanic and slag cements release less heat of hydration and suit mass pours or situations where low permeability matters. White cement belongs to facade work, pointing and decorative detailing, never to structural concrete.

The strength class printed on the bag refers to the 28-day compressive strength of standard mortar in kg/cm2, so 425-1 means a Type 1 cement of class 425. This figure is frequently confused with the cement content of concrete. Cement content is the mass of cement per cubic metre of concrete; strength class describes the quality of the cement itself. What actually governs concrete strength is the water-to-cement ratio, and adding extra water on site to improve workability cuts strength sharply. No amount of additional cement compensates for it.

The most common site error is raising the cement content in the belief that more cement means more strength. High cement content increases plastic shrinkage, surface cracking and internal heat. The second error is storing bags on damp floors or against wet walls, which turns cement into lumps with little binding value. The third is leaving fresh concrete without curing during the first days; cement deprived of water for hydration never reaches its nominal strength.

When purchasing, check the production date and the national standard mark on the bag. For larger projects, bulk delivery to a silo accompanied by the plant's analysis certificate is safer than buying bagged cement on the open market. If cement has been stored for more than three months, sampling and a mortar strength test before use in structural members is a reasonable precaution.