The earlier answers provide generally good points based on common knowledge. But the question is what is the difference between your cements PC40 and PC30. Yes, PC 30 is a lower class, but what makes it so? It sounds like these are Mexican cements. I am not so familiar with these. Is PC30 ground coarser? is it blended with a high proportion of limestone filler or slowly reactive fly ash? why is it a lower class? Knowing the answer can allow you to use it adequately to reach the strength you desire. Beyond that, while the strength depends on the type of cement, it depends greatly on the mix design. So You cannot just say we can't change the mix design because we need to pump. You can pump low w/cm concrete if you design it properly and achieve high strength. We pump concrete hundreds of meters in altitude in hot weather and achieve in excess of 100 MPa. As an engineer, you shall enforce the proper sequence of approving the mix design for your project, which includes proper mix design, testing trial batches and verifying that you meet all performance requirements. Do not get mislead by the general answers you got here. A new cement can influence rheology, setting time, strength development, and many other performance issues. You need to test your own mix in the lab and do a trial pumping in the field. Do not trust short cuts.

Compressive Strength of concrete F´c is function of compressive strength of cement ; quantity of cement; water to cement ratio; shape of coarse aggregate ;  maximum aggregate size; ratio of fine aggregate to coarse aggregate ; fineness modulus of fine aggregate (very important parameter )..

I think this concrete could achieve 450 kg/cm2. Sometimes cements are stronger than their nominal (minimal?) value in class.  Hakim S Abdelgader has right in thinks he wrote. I must say that a quite big amount (around 400 kg/m3) of cement can improve strength.

The use of cement with lower strength at 28 days, requiere more quantity of cement per cubic meter of concrete to obtain the similar strength at 28 days. The really results and the exact quantity only is possible to know making different proportions in the laboratory. It is neccesary the previous practices in the laboratory and is very important the use of superplasticizers admixtures to reduce the water cement ratio!!

The earlier answers provide generally good points based on common knowledge. But the question is what is the difference between your cements PC40 and PC30. Yes, PC 30 is a lower class, but what makes it so? It sounds like these are Mexican cements. I am not so familiar with these. Is PC30 ground coarser? is it blended with a high proportion of limestone filler or slowly reactive fly ash? why is it a lower class? Knowing the answer can allow you to use it adequately to reach the strength you desire. Beyond that, while the strength depends on the type of cement, it depends greatly on the mix design. So You cannot just say we can't change the mix design because we need to pump. You can pump low w/cm concrete if you design it properly and achieve high strength. We pump concrete hundreds of meters in altitude in hot weather and achieve in excess of 100 MPa. As an engineer, you shall enforce the proper sequence of approving the mix design for your project, which includes proper mix design, testing trial batches and verifying that you meet all performance requirements. Do not get mislead by the general answers you got here. A new cement can influence rheology, setting time, strength development, and many other performance issues. You need to test your own mix in the lab and do a trial pumping in the field. Do not trust short cuts.

In South Africa, cement classes are determined not only by the grinding of the cement particles but by the blends they have in them. Typically, as all comments indicate, the required compressive strength of concrete can always be easily designed for. Considering that, several parameters such as w/c ratio, aggregate types, and admixtures can influence the final strength developed.

Tricalcium silicate (C3S) and dicalcium silicate (C2S) are responsible for the strength development and strength gain of concrete. Both chemical compositions vary within certain range which might affect the rate of the strength gain. Several research efforts indicated that concrete compressive strength should not be affected by the cement type as long as there is no change in the cement quantity, w/c, aggregate source and no addition of supplementary cementitious material

Basically the compressive strength is mainly related to W/B and if W/B less than 0.4 it is supposed f'c will reach 350kgf/cm2. However in the mixture design the water content and cement content are too high and bleeding and segregation is liable to happen so as large slump loss. In order to solve this problem it is suggested to densely pack the granulate materials including coarse aggregate, sand and fly ash and at the end use slightly cement paste to glue the solid material. Superplasticizer is used to control the workability with water content less than 160kg/m3. Of course trial batch is necessary to conduct to check the performance of the mix.