This is very true. Alcoa used to make its CA-14 and CA-25 calcium aluminate cements at a U.S. plant. These are high purity cements, used mainly in refractories (furnace linings) -- a very different breed from your standard Type III Portland cement. Anyway, a couple of years ago, Alcoa decided to shut down U.S. production and make the stuff in Europe (Netherlands, I think).
I was working for a refractories company at the time and we had a few products which used CA-25 as a binder. The European stuff turned out to be slightly different from the American stuff in terms of workability and set time. If I recall correctly, it took slightly more water to get the same workability and had a significantly longer set time. There were also some small differences in strength. The end result was that, even though it was supposedly the same cement, we couldn't get the properties we wanted anymore, and we wound up having to reformulate our products with another manufacturer's cement. Eventually we moved away from cements altogether and switched to a phosphate binder, but that's another story.
The moral of the story is that, not only are there many different types of cements (varying proportions of alumina, silica, calcia, and other stuff), but the same cement varies from manufacturer to manufacturer, and for the same manufacturer, from plant to plant. If that weren't enough, Portland cements often vary significantly from batch to batch. Obviously, depending on your application, these variations may not matter much. But Dave is absolutely right; you can't say that cement is cement is cement and leave it at that.
Then there are aggregates. Good luck finding a "uniform" grade of anything. At best, you can hope for 65-70% to be somewhere around the specified mesh size when you run a screen. Moisture content may vary from lot to lot, too; one week, we suddenly discovered that all of our test batches were taking a few percent less water than we expected. It turned out that one of the aggregates we were using came off of a barge which had been sitting uncovered during heavy rains.
Theory is great, but don't forget that it's always based on the rules which we've made up to help us conceptualize the real world, and that the "real" real world is often far more complicated.
Dave Palmer
(773) 955-2223 snipped-for-privacy@iit.edu