Why doesn't river gravel make good concrete?

Question

People used it a lot 50 years ago, but now it is breaking apart. Some say they did not use enough cement, it has shale in it (scorio), it has coal in it. I think the sand may be too fine.Why doesn't it hold up over the years?

Answer 1

River gravel is harder because any materials
that were soft have already been ground down.
Also, cracks in the rocks over time will break
leaving only pieces that have no cracks or damage.
That means when you add it to cement composite
you are increasing the strength by adding harder material
that is less likely to shear because it has no flaws.

Answer 2

Concrete is made of three types of ingradients (1) cementitious material (2) fine aggregate (3) course aggregate.
Cementitious materials used in modern days are cement, lime (very rare), fly ash, microsilica, etc.
Fine aggregates are basically sand.
Course aggregates are crushed stones, gravels (in old days)

River gravels are well rounded, in other words, it has all smooth sides and round corners due to friction and abbration. This makes it's bond with cement a lot weaker. Crushed stones (currently widely used) have sharp corners and rough sides. This makes it's bond with cement a lot stronger. Also the compressive strength of stone counts. The crushed stones did not experience frictional or abbrasion forces. They had only weathering effects. They have more strength than gravels. Well graded (mix of all sizes) are more suitable for concrete. Too much flaky material (even well graded) is also not good for higher strength concrete.
Cement is expensive material. So to fill the voids between stones (or course aggregates), we use fine aggregates or sand. It also has goo bonding strength with cement. Also too much use of cement is not good either. Cement generates a lot of heat when reacts with water, called heat of hydration. If this heat is not controlled with cold water or any method, it will make concrete crack. Use of cement shall be according to its application. So basically sand takes place between all voids between stones. Sand is cheap, available everywhere, it's fineness and it's good bond with cement makes it more favorable.
These are the main reasons. If gravels are good then we can eliminate shale from it. But it is not main reason why we don't use gravel.

Answer 3

As a rule River gravel is one of the best types to use in concrete, but it is very expensive, so it's use is limited for that reason.

It is very hard and very smooth, and some of the oldest sections of highways still in existence were made using river gravel. It is still used where a decorative appearance is required to the finished concrete surface, mostly in landscaping applications.

Answer 4

"Good" aggregate is sharp edged and insoluble.
(Better 'grip` for the cement and resistant to water damage.)
'River gravel` is round edged, and likely to contain
water sensitive materials, (carbonate rock,
non -silica sand etc.), that can break down over time. `Good` river gravel, (mostly silicates), is not
too bad.

You seem to have pretty much answered your own question.
If the mix was 'poor` it could be the main cause of the deficiency.