Interstice: a small gap that lies between things.
I really love words, and this is one of my favorites. To me, the word itself describes exactly what it is. Inter; meaning between, among; Stice; meaning, I don’t know, but it sounds like a ‘small space’ to me. I like saying the word.
Why, in the name of all things holy, would anyone know that word, much less care about it. Well if you are a Civil Engineer, and you have taken a Properties of Materials course, you had to be exposed to the word, if not; you probably, never in a lifetime, would run across it. So what is it all about.
Concrete is made up of 1.) Large aggregate, 2.) Small aggregate, 3.) Sand, 4.) Cement and 5.) Water (There are other things like ‘water reducers’, ‘accelerators’ and ‘retarders’; collectively ‘additives’ – not for discussion here). If you filled a bucket full of the first three ingredients you would have a fairly solid, dense mixture, strong enough to stand on, but if you turned the bucket upside down, you would be left with a pile of loose rocky materials that would ultimately blow away. The reason this happens is because the interstices between the various aggregates are void and filled only with air. Nothing holding the group together.
You need to add a slurry of the materials 4 and 5 to fill the tiny interstices between the aggregates to form a tight, solid mass as the water drys, or hydrates. Voila! Concrete. Who knew?