I know that concrete sets due to a chemical reaction between the cement and water. But does water-cement ratio also affect the setting time?
Setting of concrete involves both chemical and physical phenomena. Several studies have shown that when the water-cement ratio is reduced, the initial and final times of setting are also reduced. In one study, water-cement ratio was increased by increasing water content while keeping all other batch quantities constant (Ref. 1). The effect on time to reach initial and final set is shown in the following table: In this study, concrete temperature was 63 F, ambient temperature was 70 F, and ambient relative humidity was 70%. Another study showed similar results (Ref. 2). Concretes made with 517 pounds of Type I cement per cubic yard and water-cement ratios of 0.54 and 0.58 had initial setting times of 4 hours 40 minutes and 5 hours 10 minutes respectively. Final setting times were 6 hours 20 minutes and 6 hours 45 minutes. The tests were run at 73 F. Results of these studies support the rule of thumb that adding a gallon of water per cubic yard of concrete can be expected to increase setting time by about 15 minutes at moderate temperatures. References 1. G.D. Stefanou and Ch. Larsinos, "Influence of Mixing Water on the Setting Time of Concrete," The International Journal of Cement Composites and Lightweight Concrete, February 1981, pp. 45-48. 2. Vance Dodson, "Time of Setting," Significance of Tests and Properties of Concrete and Concrete-Making Materials, STP 169C, ASTM, West Conshohocken, Pa., 1994, p. 83.