You cannot have concrete without sand. River beds are being dug up across Africa to fuel a building boom, with little thought for what this means for the health of the river, and those who depend on it, as Harriet Constable found in Kenya.
Sand. The word conjures happy holiday memories: building castles from it; watching nervous crabs scuttle across it; digging giant holes in it, and then hiding in them and leaping out at opportune moments to terrify unknowing relatives. Sand is the make up of glittering beaches, hundreds of thousands of years of weathering to create millions and millions of tiny, sparkling, and yet seemingly insignificant particles. Sand is infinite, surely. And yet the world is running out.
Image caption A beach in Tiwi, along Kenya's coast, where sand dredging has caused the beach to subside and start to disappear - it's a key nesting ground for sea turtles
It's obvious when you think about it. All the major building materials - concrete, bricks, glass, are made using sand. Exploding population numbers and the knock on need for development have made sand the second most used natural commodity on the planet after water. Billions and billions of tonnes are being used across the globe.
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