Q.: A mill test report from a manufacturer of Type I cement gives compressive strength at 3 and 7 days. Can this information be used to determine how much cement is needed for a cubic yard of 3000-psi concrete?
A.: No. The tests are made on 2-inch cubes of a standard mortar made with one part of cement to 2.75 parts of graded standard sand. The mortar water-cement ratio is 0.485. To meet ASTM's Standard Specification for Portland Cement (ASTM C 150), standard cubes made with Type I cement must have minimum 3-day and 7-day strengths of 1800 psi and 2800 psi respectively. Strengths shown on the mill test report should equal or exceed these values. But knowing the reported cube strength won't tell you how much cement is needed to produce a given strength concrete. The amount of cement needed depends primarily on the required slump and water-cement ratio, not on the cement properties.