I should make it clear, I was not a rubber or chemical tech, but I did have to know the background of what we were making changes into. I was an electro-mechanical tech and troubleshooter for most of those years.
The silica (which is what sand is made of) is not ground beach sand but a chemically precipitated form which is micro fine and physically rough and chemically treated to bond with a specifically designed rubber. I believe the cost is on par with carbon black so the main reasons for using it is its physical advantages. It doesn't add strength but it does reduce rolling resistance and improves wear resistance. I worked on very large tires where the silica compounds were mainly used in the tread, the rest of the tire was conventional construction, which is pretty high tech in a Michelin.
I do believe clays are used in some rubber formulations, but not as cheap replacement for carbon black. The rubber compounds use kerosene, rosin, oils, plastics, fats, and many other substances to get the qualities needed for a certain area of the tire. I am used to seeing 6 to 12 different rubber compounds used in the production of a single tire. The big tires use steel cords rather than kevlar, and some are "bullet-proof".
Michelin's bicycle and motorcycle tires are just as cutting edge as any of their products, and do use innovative compounds and techniques to achieve amazing results. They had a racing snow tire that had no tread. It worked in specific conditions by having a layer of snow adhere to the rubber!
Early in my career I was a machinist and I made a ball mould for doing "bounce tests" in the lab. The lab tech brought me back 2 identical black rubber balls. One would pretty much return to your hand when dropped, the other would hit the floor and stay there, no bounce at all. It was kind of amazing.
Over the years I saw changes, but constant dedication to quality and innovation impressed me. Often it infuriated me, when I was asked to constantly improve accuracy beyond what we had accomplished in the past on the same machines, but it was needed and accomplished. It was an interesting career.