What are the implications of specifying low carbon concrete?

Does anyone know of good data on whether using conventional SCM (fly ash and slag) actually reduce carbon once transportation distances are accounted for? I’m based in Vancouver, Canada, and the nearest blast furnace is 1000s of miles away, the nearest coal fired power plant is 100s of miles away. I’ve found lots of good UK data, but everything is essentially down the road there. I ask in part because on one of my current projects the contractor is using imported slag from Australia - in this case to get the right mix properties rather than any consideration of carbon. But that got me thinking about transport distances generally. Thanks!