Q.: At what temperature does concrete cure best?
A.: By "best" we assume that you mean "most thoroughly" and not "fastest." The higher the temperature the faster the concrete will cure. But concrete cured at high temperatures generally does not reach as high an ultimate strength.
Investigations by Paul Klieger reported in the Portland Cement Association Research Bulletin 103 show the strength of Type I portland cement concretes at various ages when cured for the first 28 days at temperatures of 120°, 105°, 90°, 73°, 55°, 40° and 25°F, and then cured for the remaining period at 73°F until an age of 365 days. At an age of 1 day the 120°F concrete was strongest and the 25°F concrete was weakest. By 7 days the high-temperature cured concretes had no more strength than the 73° concrete or even less. By the age of 28 days the high-temperature concretes were weaker than the 73° concrete. From 28 days to 1 year the 55° concrete was considerably stronger than the 73° concrete. From 90 days onward the 40° concrete was stronger than the 73° concrete and the 25° concrete was slightly stronger than the 73° concrete. All of this suggests that, provided there is continuous curing, concrete cured at about 55° for the first 28 days ultimately reaches the highest strength.
Klieger also investigated Type II and Type III cements under the same conditions and concretes made with each of these cements with 2 percent calcium chloride. Data for the other cements and admixtures resembled the results described for Type I but were not identical. Although the best curing temperature was 55°F for Type I and Type II cements, for Type III the best temperature was 40°F.