Shortage and cost of River sand and M-sand is increasing day by day. Many engineers and builders are finally willing to turn there head towards other alternative sources. Many of them ask the question why we are not using soil for construction. It is proven that only some soil can be used for construction instead of sand, but if the soil you are looking forward to use is cohesive soil then you should avoid it.
Reasons to avoid Cohesive soil in concrete
The cohesive soil has a high amount of clay and silt content whose size is less than 75 microns. They also have high electrostatic charge and high plastic limit. Hence cohesive soil does not react and bonds well with cement matrix.
Because of its cohesive property, clay will surrounds and sticks to aggregate and prevent wetting of cement matrix to aggregate. This will decrease the strength of concrete in flexural bending and compression. Further, it will result in stripping of aggregate from the concrete.
Apart from silica, cohesive soil has a lot of other undesirable substances like sulfates, chlorides and salts which will cause corrosion in steel and concrete.
Since the size of cohesive soil is very small it has high surface area and high void content(air content). It is well known that the strength of concrete is directly proportional to the number of voids in it. Actually, 1% increase in the void content in concrete decreases the strength of concrete by 8% (Ref 1).
Because of size, cohesiveness, and adsorption, the cohesive soil has high adsorption, shrinkage and swelling(bulking) during wetting and drying which makes them not suitable for concrete, and we will not get reliable workability.
Most likely they will also have organic content which will decompose in time.
Sometimes cohesive soil can be good in concrete
Asper code silt content up to 5% is allowed in the sand(non-cohesive) because it increases the density of the concrete and does not have any other impact on the properties of the concrete. 5% silt in sand increases the compressive strength of concrete by 5%. But silt more than 5% percent affects other properties of the concrete and eventually decreases the compressive strength of concrete.