Sand as fine aggregate is mainly used to configure mortar (cement, sand and water) or concrete (cement, sand, stone and water) in construction projects. Among them, the mortar has low strength and is easy to form. It is generally used for non-important stress-bearing parts such as masonry, plastering, and decoration. The strength of concrete is very high, and each square meter of concrete can bear thousands of tons of weight. It is mainly used for the main stress parts of structures such as beams, slabs, bridge piers, pavements and dams.
In concrete materials, sand is used to fill the gaps between the stones and form the basic skeleton with the stones. The cement paste wraps the stones and sand and fills the pores between the aggregates to bond the skeleton composed of the stones and sand into a whole.
To make high-quality concrete, sand must meet certain quality requirements: the thicker the sand, the larger the volume, and the smaller the amount of cement slurry consumed. However, the large amount of cement slurry required for fine sand results in a lot of heat release from concrete hydration, which is prone to cracking due to uneven temperature. Therefore, the sand particle gradation is reasonable, which can make the porosity between the aggregates (sand and stones) small, require less cement slurry, and make the concrete easier to compact.
Harmful substances in sand. Harmful substances such as mica minerals, clay, silt, organic matter, sulfides, sulfates and chlorides in sand will weaken the strength and durability of concrete. In actual engineering applications, other social and economic factors need to be considered, such as the difficulty of sand mining, transportation and storage costs, and so on.
Therefore, most of the projects use river sand or lake sand. Due to the long-term erosion of water, this kind of sand has clean particles, smooth surface, abundant resources, and low price. However, sea sand often contains harmful impurities such as organic matter and salt, while mountain sand contains more mud and impurities and is less used.
As for desert sand, the reasons why it is not used in concrete are:
Because the desert is far away from the city, mining and transportation costs are high.
Desert sand itself is weathered from loose parent rock, and its strength is less than that of mined river sand or machine-made sand.
Desert sand has been polished by wind for a long time, with very small average particle size and high water absorption rate. It needs to consume more cement and reduce the strength of concrete.
Generally, the thickness of sand is determined by screening test, and its index is called “fineness modulus”. According to the fineness modulus, sand can be divided into three specifications, namely coarse sand (3.7~3.1), medium sand (3.0~2.3), and fine sand (2.2~1.6).
When configuring concrete, coarse and medium river sand should be preferred for fine aggregate. The fineness modulus of desert sand is usually less than 1.5, and the finer one has a modulus less than 0.7, which is ultra-fine sand. Concrete that uses desert sand alone will consume a large amount of cement, reduce strength, and weaken frost resistance and wear resistance. Neither has economic benefits, nor is it conducive to environmental protection. Coarse and medium sand are now more scarce than fine sand. In order to protect the environment, many national and regional government departments have also restricted the mining of river sand and lake sand, but the market has no significant demand for fine desert sand.
Therefore, the single particle size of desert sand does not meet the requirements of concrete use (standards and specifications). If desert sand must be used, it can be mixed with other sizes of sand to form a good particle gradation and improve the performance attributes of desert sand.