ᐅ Concrete slab, insufficient cement content in the concrete mix

Hello, I am quite inexperienced with construction and very worried about the following issue:

I want to build a swimming pool in my garden, measuring 3x5x1.5 meters (10x16x5 feet), using Styrofoam formwork blocks that will be filled with concrete.

A friend who works in construction helped me pour the base slab (20cm (8 inches) thick) over the weekend, and for various reasons, we had to mix the concrete ourselves from cement and gravel.

I read online that the ideal mix ratio is 1 part cement to 4 parts gravel by weight, and this is exactly what my friend had told me before. He mixed the concrete, and I did the pouring.

Now that the slab is finished, I checked the delivery notes. When I calculate everything, we used exactly 8 tons of gravel and 1.25 tons of cement. But for a 1:4 ratio, we should have used 2 tons of cement.

He says this is not a problem and gave me explanations I don’t fully understand. (For example, there was some water in the gravel, so we actually used less gravel than was delivered. The gravel was indeed damp, but as a layperson, I am still very concerned if this will truly hold.

Does anyone have experience with this and could please share some advice?

No one will confirm that for you

There is not just one type of concrete and cement, but hundreds of different mixes.

I don’t want anyone to just guarantee that it will hold.
I would like to get an informal opinion from someone who is knowledgeable.
What would you do if you were in my position?

Ask for the type of cement and consult the manufacturer.

Is there at least rebar included, and has it been properly compacted?
If the rebar was installed correctly and the concrete was well compacted, I wouldn’t be worried.