Concrete

It may contain salt. So later it will result in shrinkage and cracking. And it will also not result in same strength as river sand gives.

Definitely the salts adsorbed (dissolution might not actually occur) on the sand are going to affect the durability in reinforced concrete structures but in plain concrete or mortar for plaster, it might not be that detrimental. It is not that hard to get rid of salts adhered to sand particles.

However, apart from salts, there are several factors that hinder its use, like aggregate shape and size. Sea sand, generally tends to very fine and rounded, which is not particularly advisable when designing mixes. A kind of blending is required with coarse and angular aggregate to develop the rheology and microstructure desired.

However, when the economy demands, it is used. Practically speaking, the aggregates are good to use as long as they are in compliance with respective codes of practice (like ASTM C33). If the aggregates make it through all the tests and clauses, then you can officially use the material in construction.

Edit: The possibility of the use of crushed gravel.
Crushed gravel would be siliceous in nature and could be a decent alternative. However, the process could be slightly demanding, since crushing operations are usually performed on larger boulders to “crush” them to required sizes and distribute for use.

There could also be an issue of salts. In my post, I said that they could be physically removed, which might not be completely true.

You see the water (which usually has dissolved salts) that is present will be absorbed (absorption capacity of aggregate) and in the process, the salts may get deposited in the porous microstructure of the aggregate, which ultimately cannot be removed by physical treatment completely.

So a careful investigation is recommended that is more than just tests on aggregate since the combination of water (in concrete) might release the salts back into solution.

Test for Determination of Specific Gravity

Indian Standard Specification IS: 2386 (Part III) of 1963 gives various procedures to find out the specific gravity of different sizes of aggregates. The following procedure is applicable to aggregate size larger than 10 mm. A sample of aggregate not less than 2 kg is taken. It is thoroughly washed to remove the finer particles and dust adhering to the aggregate. It is then placed in a wire basket and immersed in distilled water at a temperature between 22° to 32°C. Immediately after immersion, the entrapped air is removed from the sample by lifting the basket containing it 25 mm above the base of the tank and allowing it to drop 25 times at the rate of about one drop per sec. During the operation, care is taken that the basket and aggregate remain completely immersed in water. They are kept in water for a period of 24 ± 1/2 hours afterwards. The basket and aggregate are then jolted and weighed (weight A1) in water at a temperature 22° to 32° C. The basket and the aggregate are then removed from water and allowed to drain for a few minutes and then the aggregate is taken out from the basket and placed on dry cloth and the surface is gently dried with the cloth. The aggregate is transferred to the second dry cloth and further dried. The empty basket is again immersed in water, jolted 25 times and weighed in water (weight A2). The aggregate is exposed to atmosphere away from direct sunlight for not less than 10 minutes until it appears completely surface dry. Then the aggregate is weighed in air (weight B). Then the aggregate is kept in the oven at a temperature of 100 to 110°C and maintained at this temperature for 24 ± 1/2 hours. It is then cooled in the air-tight container, and weighed (weight C).

Specific Gravity = C / B − A; Apparent Sp. Gravity = C / C− A

Water absorption = 100 (B − C) / C

Where, A= the weight in gm of the saturated aggregate in water (A1 – A2),

B = the weight in gm of the saturated surface-dry aggregate in air, and

C = the weight in gm of oven-dried aggregate in air.