The strength of cement is a function of its porosity. As said above, you need a certain amount of water for the formation of the hydration products. But if you add more and more water (increase the w/c ratio), you dilute the cement paste and create more water-filled pore space between the grains, i.e. you find less nuclei for the hydrates in each volume unit. Hydrates have to grow larger and larger to cover the spatial gap (the water) between them and to interact and to develop strength - either physically (interlocked growth) or chemically (e.g. van-der-Waals attraction).

That's easy ... There is an optimum in water/cement-ratio for each cement-containing mix. The cement needs a specific portion of water to hydrate and thereby develop strength, but excessive water will decrease the strength.

The strength of cement is a function of its porosity. As said above, you need a certain amount of water for the formation of the hydration products. But if you add more and more water (increase the w/c ratio), you dilute the cement paste and create more water-filled pore space between the grains, i.e. you find less nuclei for the hydrates in each volume unit. Hydrates have to grow larger and larger to cover the spatial gap (the water) between them and to interact and to develop strength - either physically (interlocked growth) or chemically (e.g. van-der-Waals attraction).

I agree that porosity is an important factor and that for a specific ratio w/c the mixture performs the best as far as mechanical characteristics is concerned. Do anyone knows this optimum ratio?

There is no general optimum w/c ratio. There is a theoretical ration of water which can be chemically bound by the cement. It depends on the chemical composition of the cement For Portland cement it is somewhere around w/c 0.23. But that is just a theoretical figure. Look for "Powers-Brownyard model" to find literature about it.

Industrially manufactured Portland cements are not entirely identical. Each cement has its own optimum, each mixture has its own optimum, ...

As discussed above, the ratio w / c has a great influence on the resistance since this ratio is greater i.e after evaporation of excess water leaves voids (porosity) and subsequently causes decrease in strength.

The 0,23 w/c ratio is only a theoretical value (depending of course on composition) in that this is the amount of water that truly reacts with the components of the cement grain after en infinite amount of time has occurred.

However, if the sample is sealed the dessication caused by reacting the water will reduce the humidity below a critical level (~80%) whereupon the hydration reaction stops due to a lack in driving force as though the reaction were reversible. Outside humidity is required for the reaction to continue. In theory a 0.42 w/c is required for full reaction in a sealed system (again dependent on clinker composition).

Additionaly, a paste prepared with the minimum 0,23 w/c would not be placeable (workable) and requires excess water to permit proper consolidation. This excess water is what causes the porosity in the system (whether or not it eventually evaporates) and the weakness of the concrete. The other variables stated in Mme Tsam's response will affect workability and define the amount of water required for proper placement and thus the strength of the concrete. Generally speaking, without additives, you would need to increase the w/c to 0,6 or more to get a workable inexpensive concrete without much strength, ~20 MPa.

Adding additives and controlling the mix design permits us to easily make concrete up to 80-90 MPa. After that, again look at Mme Tsam's response for durability problems caused by stress cracking due to temperature, shrinkage etc.

Your concrete is only as good as the weakest link.

Dear all, Thank you for your answers, which I find very interesting.

Dear Aria, I totally agree with you. The only I would like to emphasize is that by increasing cement quantity you definitely do not always improve strength and durability. Even in cement's quantity, I think there is an optimum percentage used, for a certain ratio w/c.

Moreover, as far as w/c ratio is concerned regarding my question I read somewhere that " for w/c

Any paste with a w/c above 0.42 will have large capillary pores, pastes bellow 0.42 are supposed to be free of such pores nevertheless the strength keep on increasing as the w/c decreases if the paste doesn't have consolidation pores, this has been seen on Ultra High Performance Concretes.