Our company is being urged to use Type IP cement on a job in place of Type I. We understand that the Type IP in our area is made by substituting fly ash for some of the cement. Although the concrete made with Type IP is said to be more workable we are worried about early strength. Is it true that the fly ash ingredient contributes to the cementing action?
Fly ash has virtually no cementing action by itself. However, it is true that fly ash reacts with the lime released during the hydration of the cement to produce cementitious material. This reaction is relatively slow and for the most part accounts for the comparable strength obtained by using concrete made with Type IP cement. When a cement manufacturer blends fly ash with portland cement to make Type IP cement he tailors it in such a way that it meets the same early test strengths required of Type I cement. Strengths of 1800 psi at 3 days and 2800 psi at 7 days are required for both Type I cement (ASTM C 150) and Type IP cement (ASTM C 595). It should be noted, however, that these test strengths are measured on 2-inch cubes of mortar. Even when a Type I and a Type IP cement produce mortar cube strengths that are equivalent, their concrete strengths may not be. It would be best to compare their strengths in 6- by 12-inch concrete cylinders. It should also be noted that the ASTM strength requirements are minimum values. Mortar cube strengths of various cements exceed these values by different amounts, varying from small to quite large. For this reason two types of cement should not be expected to produce the same concrete strengths just because they meet the same minimum acceptance test strengths. The mix proportions may have to be modified .