为什么我们不用沙漠里的沙子来建造房屋？

About 20% of the earth's surface is desert, and the ground of these barren places is almost always covered with thick sand, such as the famous Sahara Desert, where the average thickness of sand can be as high as about 150m.

This situation makes people wonder: if there is so much sand in the desert, why don't we use the sand in the desert to build houses?

When we build a house, we do need a lot of sand. In professional terms, it is called "sand", and its main use is to prepare mortar and concrete.

Mortar is a mixture of water, cement, and sand, which is relatively low in strength. It is usually used for masonry, decoration, and plastering, while concrete is a mixture of water, cement, sand, and gravel. Concrete is relatively strong, can bear a lot of weight, and is mainly used in the main stress areas of building structures.

In simple terms, the basic theory of concrete is to fill the gaps of large stones with pebbles, fill the gaps of pebbles with sand, and then wrap the pebbles and sand with cementitious material while filling the gaps. Thus, the concrete can be dense enough to achieve high enough strength.

The stones and sand in concrete are also called "aggregates". As the name implies, "aggregate" is the substance that acts as the "skeleton", and sand is one of the finest particles. Therefore, sand is also known as "fine aggregate". Their role in concrete is to fill the gaps between the larger aggregates, and the larger aggregates and cement paste as a cementitious material tightly connected.

To produce qualified concrete, sand as "fine aggregate" needs to meet a series of conditions, of which three conditions are essential, namely:

1. Its strength should be high enough.

2. There should be no excessive impurities.

3. sand should not be too coarse or too fine. Generally, there are three specifications of sand for building houses: coarse sand, medium sand, and fine sand, and their corresponding fineness modulus are 3.7 ~ 3.1, 3.0 ~ 2.3, and 2.2 ~ 1.6 respectively.

However, the sand in the desert almost perfectly misses the above three conditions. Firstly, sand in the desert is made of loose rock weathering, which leads to its strength usually not up to the standard of building a house.

Second, desert sand usually contains a large number of impurities harmful to concrete, such as mica minerals, organic matter, sulfides, chlorides, etc. This will greatly reduce the strength of the concrete while weakening its durability of the concrete.

What's more, due to a long time weathering "polishing", the desert sand particles are generally very small, its fineness modulus is less than 1.5, and some desert sand is even less than 0.7, obviously not up to the standard of building houses.

On the other hand, desert areas on the earth's surface are generally far away from human habitation areas, so even if the sand of a certain desert can barely meet the standard for building houses, its relatively high mining and transportation costs usually discourage people from this idea.

Because of this, we usually do not use desert sand to build houses. Among other types of natural sands, mountain sands and sea sands are not preferred for building houses because they usually contain a lot of clay and silt, and sea sands also contain a lot of organic matter and salts.

Generally speaking, people will prefer river and lake sands to build houses, because these sands are relatively strong, they contain relatively few impurities under long-term hydraulic action, and their particle size also meets the standards for building houses.

By the way, even if we can't find enough natural sand, we can make suitable sand by hand. With the use of a stone crusher, we can crush the common stones in nature such as river pebbles, granite, and some construction waste with suitable conditions into the sand, also called mechanism sand.

It is worth mentioning that in case there is no other sand available, it is not impossible to build a house with desert sand. It is just very troublesome. For example, we can use inorganic bonding materials like epoxy resin to bond the sand together, and its strength is comparable to concrete.

For example, we can sift the sand in the desert, choose the sand with larger particles and fewer impurities, and then use this sand and cement as raw materials to make a mortar block with sufficient strength. Then the mortar block will be crushed and sieved, you can get the "fine aggregate" which can be used to build a house.

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