ASTM C595/C595M
Standard Specification for Blended Hydraulic Cements
Cements that combine portland clinker with limestone, slag, pozzolans, or a mix of them — including Type IL portland-limestone cement, now the default bag in much of the country.
Who needs it
Batch plants, mix designers, specifiers with carbon-reduction targets, and contractors noticing their cement designation changed without their mix changing.
What the test involves
It sets requirements for blended cements by category, most visibly Type IL for portland-limestone cement and Type IS for slag-blended cement, with the constituent percentage often shown in parentheses in the designation. Blended cements are made to perform comparably to conventional portland cement while grinding in a supplementary material at the plant, which lowers the clinker content and the embodied carbon of each pound of cement.
Common misconceptions
- Type IL is not a weaker or cheaper substitute. It is engineered to deliver comparable performance, and most jobs see no practical difference in the finished slab.
- Setting and finishing behavior can shift slightly with a blended cement, especially in cold weather, so crews sometimes blame the mix design when the change was actually at the cement silo.
- Blended cement is not the same as adding fly ash at the plant. In a blended cement the supplementary material is part of the cement itself, which changes how a mix is proportioned and documented.
Official source
We summarize scope and purpose only. The full text belongs to the issuing body — read it at the official source. ASTM International