Sand consists of weathered rock fragments, typically silica (quartz) or calcium carbonate. The presence of salt is not due to the sand material itself being sodium chloride, but whether salt residue has been deposited onto the grains. Whether sand contains salt depends heavily on its environment, distinguishing sharply between marine and inland locations.
Salt Content in Coastal and Beach Sand
Coastal and beach sand contains salt due to continuous interaction with seawater, which has an average salinity of about 3.5%. Seawater contains dissolved salts, predominantly sodium chloride, along with other ionic compounds like magnesium and calcium salts. Waves and tides repeatedly saturate the upper layers of the beach with this brine solution.
When saltwater saturates the sand and evaporates, the dissolved ions are left behind as a crystalline coating. This residue is deposited interstitially, trapped in the small spaces between the sand grains. The mass of salt residue can be measurable in sea sand, making chloride content a significant factor for certain uses.
Why Inland and Desert Sand is Different
Sand found in inland environments, such as riverbeds and quarries, is typically salt-free. This absence is due to leaching, the removal of soluble materials by water. Freshwater rain and flowing river water are highly effective at dissolving any trace salts present, as sodium chloride is highly soluble.
This constant washing ensures the sand grains remain composed of durable, insoluble minerals like silica and quartz. Even in deserts, infrequent flash floods flush the upper layers, carrying away soluble compounds. However, in specific desert areas where ancient salt lakes evaporated or high evaporation draws saline groundwater to the surface, the sand may contain significant salt layers or be composed of salt minerals like gypsum.
Impact of Salt Residue in Sand
The presence of salt residue in sand has significant practical consequences, particularly in construction. Using salt-laden beach sand for concrete and mortar is problematic because chloride ions accelerate the corrosion of steel reinforcement bars (rebar). The resulting rust takes up more volume than the steel, causing the concrete to crack and compromise structural integrity.
Construction standards require the use of washed or river sand, which has a very low chloride content. In gardening and agriculture, salt creates a condition known as physiological drought for most plants. High concentrations of salt ions make it difficult for roots to absorb water, even in moist soil. Additionally, sodium and chloride ions can be toxic, interfering with photosynthesis and displacing necessary nutrients, leading to reduced growth.