Proper curing and testing is the only accurate method for determining how much compressive strength is contributed for each pound of portland cement. However, rules of thumb are useful in estimating the properties of trial mixes. Here are some rules of thumb that can help estimate strength contributions for different cementitious materials and water reducers. Portland cement--many experienced concrete mix specialists use 10 psi per pound of cement per cubic yard of concrete at 28 days. Water reducers--the use of a water reducer produces higher compressive strengths than can be attributed to the reduced water to cement ratio resulting from the removal of mix water. Concrete mix specialists sometimes use a rule of thumb of 10% to 15% increase in compressive strength above that expected at the lowered water to cement ratio. Pozzolans--pozzolans vary greatly, even when meeting the applicable ASTM standards. It's difficult to estimate the contribution of each pozzolan to compressive strength. However, in a mix in which 20% by weight of the portland cement was replaced by an equal volume of class C fly ash, the ash produced a compressive strength of 18.8 psi per pound of pozzolan per cubic yard after 28 days; as opposed to 9.7 psi per pound of regular portland type I cement. Precautions--rules of thumb are always violated; they only work on the average. Cement, water reducers and pozzolans vary.