Desert sand is plentiful, yet useless for making concrete
Construction consumes about fifty billion tons of beach and fossil sand every year, and on human timescales that sand does not come back. The world's deserts hold vast amounts, but their grains are unsuitable for concrete. Sand, it turns out, is a limited resource defined by nothing more than grain size.
Sand is not a single substance but a size class: mineral grains coarser than silt and finer than gravel. Geologists set the range at 0.0625 to 2 millimetres, while the engineering Unified Soil Classification System uses 0.074 to 4.75 mm. The lower limit has shifted over time. Albert Atterberg's early 20th-century standard counted grains as small as 0.02 mm, the same size Archimedes assumed in The Sand Reckoner around 240 BCE; the US Department of Agriculture chose 0.05 mm in 1938. A simple test survives all of these: sand feels gritty between the fingers, whereas silt feels like flour.
Finer divisions exist too. ISO 14688 splits sand into fine, medium and coarse, and American practice uses five grades from very fine to very coarse, pegged to the logarithmic Krumbein phi scale. A soil counts as sandy when sand-sized particles make up more than 85 percent of its mass.
Most inland and temperate coastal sand is quartz, silicon dioxide, which is hard and chemically inert enough to outlast other minerals under weathering. Calcium carbonate is next, such as aragonite made over the past 500 million years by corals and shellfish; it dominates reef regions like the Caribbean, and the dazzling white sands of tropical coasts are eroded limestone mixed with shell and coral fragments, meaning living things help make beaches.
Local rock sets the colour. Gypsum forms the famous white dunes of White Sands National Park in New Mexico. Magnetite, volcanic basalt and obsidian yield dark or black sands, while chlorite, glauconite or olivine-rich lava give green ones. Iron impurities inside quartz crystals lend many southern European sands a deep yellow. Arkose, rich in feldspar, comes from nearby granite, and some deposits even contain garnets and small gemstones.
Source: Sand
