What sand:cement ratio is strongest? 1

[Tomfh]

In regular sand:cement grouts and mortars you see increasing compressive strength with increasing cement quantity. Eg a 3:1 sand to cement ratio gives you a stronger grout than 6:1. However I understand pure cement is weaker than a strong sand/cement grout (correct me if I’m wrong there)

At what point does extra cement result in diminishing returns?

Assume regular quartz sand and a normal water to cement ratio. Nothing fancy.

 

[Ron]

Compressive strength comes from the cement and intergranular friction of the aggregate. If you coat the sand particles sufficiently to bind them with a layer of cement paste, strength will be good. If you use too much cement, you'll get a lot of shrinkage, including loss of intergranular contact. If you use too much sand, you won't have enough cement paste to coat the particles, thus you only get intergranular friction which will reduce the strength.

A ratio of 2.5 thru 4 to 1 sand to cement will give you reasonable grout strength for structural applications. I would not use 6:1 except to set fence posts.

 

[miningman]

kinda depends on the dimensions one is trying to grout. If your playing with 0.25 inches gap , then the above is correct but if your trying to grout rock masses, forget the sand completely and go for an ulta fine grind grout cement.