Q: Is there a difference between w/c and w/cm?
A:When someone refers to the water-cement ratio, or w/c, They are comparing the weight of the water to the weight of the cement in a certain unit of concrete.
Quantities typically are based on a cubic yard, but the ratio is the same regardless of what size batch you're considering.
For a straightforward plain mix, a lower w/c ratio yields higher strength concrete. But as that ratio goes down, it also makes the concrete stiffer and perhaps harder to work with and finish well.
Sometimes one or more supplementary cementitious materials (SCMs) are specified as part of a concrete mix in substitution for a given amount of portland cement, often on a 1:1 basis. SCMs include such things as slag, fly ash, silica fume, and natural pozzolans. The reasons for using SCMs are numerous, ranging from environmental concerns to the desire for a certain behavior in the concrete. However, rather than just providing filler, they actively contribute to the concrete's strength. Therefore, the familiar w/c term itself has been supplemented to acknowledge the role that SCMs play in concrete strength, such that the more inclusive term is the water-cementitious materials ratio (w/cm). In daily work, the terms are used interchangeably.