Does hot cement mean higher mix temperatures?

Jun 1, 2005 12:00 PM, BY PCA

As clinker gains additional energy stored in the form of heat through the process of grinding, the result is “hot cement.” Once the clinker is ground, this hot cement is stockpiled in storage silos where the elevated temperature, especially in warm climates, is not readily abated. Summer months exacerbate the situation since ambient temperatures are generally high and cement consumption increases. Warmer temperatures limit the tendency of hot cement to release large quantities of heat; and, increased consumption means freshly ground cement is stored only briefly before shipment to ready-mixed plants for use in batching.

Slump loss, strength reduction, and other concrete-related problems are often attributed to the temperature of cement upon batching. Research, however, has demonstrated that the ultimate effect of cement on the concrete mixture's temperature is minimal. Initial studies by Lerch, 1955, (see reference following) and duplicated research have shown that lowering the temperature of normal concrete by 1°C requires reducing the temperature of the cement by 8.2°C, lowering the water temperature by 4.9°C, and reducing the temperature of the aggregates by only 1.5°C. Thus, the influence of cement temperature on overall concrete batch temperature is less than that of the other concrete ingredients.

This effect can be traced to mix proportions and intrinsic thermal properties of concrete constituents. Among materials added to normal concrete, cement accounts for only 7-15 percent of the mix volume; water and aggregates generally comprise approximately 70-90 percent. When compared to the weight of the other ingredients, cement's potential influence on temperature is further diminished. In addition, cement has a low average specific-heat capacity, i.e., it gains and loses heat readily when exposed to high temperatures or when supplied with energy. On average, aggregates have a specific-heat capacity equivalent to cement, but water's specific heat is high — it can store large amounts of heat.

Overall, the importance of controlling concrete mix temperature is uncontested. Temperature affects the properties of fresh and hardened concrete as well as thermal cracking potential. Accordingly, close attention must be paid to batch temperatures in hot- and cold-weather concreting. The relative effect of “hot cement,” however, on the batch temperature is demonstrably negligible.

HOT RESOURCE

Available through the Portland Cement Association, Hot Cement and Hot Weather Concrete Tests provides a review of data showing the effect of hot cement on the mix temperature and properties of hot-weather concrete. Calculations for heat capacity of the mix ingredients are included, as well as a discussion of how the temperatures of ingredient materials influence concrete mix temperature. Additionally, the study details the impact of freshly mixed concrete temperature on slump, compressive and flexural strengths.

Results indicate that concrete properties depend on the fresh mix temperature, not on the temperature of its various constituents. Thus, specifying a maximum temperature for freshly mixed concrete is recommended, rather than limiting the temperature of individual ingredients. Accordingly, procedures are described for controlling the temperature of freshly mixed concrete during hot weather.

Hot Cement and Hot Weather Concrete Tests by W. Lerch (Item Code: IS015) can be obtained by visiting PCA's online bookstore at www.cement.org or contacting Customer Service at 800/868.6733. Bulk quantities are available at $1.00 each for orders of 25 or more.