must form before particles become interlocked) and both the aluminate Water/cement ratio, 0.5, 23C and the Alite rate of hydration. Increased admixture mix water addition The aluminate and Alite compete dosage for water for their hydration. Since aluminate is much more reactive, it is critical to have enough sulfate Acc. aluminate available to retard the aluminate. Sulfate must fi rst dissolve before Sulfate it can react with aluminate to form depletion ettringite, which coats the surface and retards the aluminate hydration. If not enough sulfate is available to slow down the aluminate hydration, Figure 1. Effect of progressively increased admixture dosage on calorimetry response. loss of workability or even fl ash Note how the strength giving silicate hydration peak appeared to be retarded, both in size and timing, as the strong overdose of admixture accelerated the aluminate set may occur as the aluminate hy- hydration, which further retarded the silicate hydration. The cement used meets the 2 drates formed rapidly consume free specifi cation for ASTM Type I and II Portland cement, 2.9% SO3, BSA 380 m /kg. water. Since the aluminate hydrates also tend to coat the Alite, excess aluminate hydrates may retard Laboratory screening of the It would be impossible for a cement and partly prevent the onset of the stability of cement – admixture producer to test all their cements for strength giving Alite hydration. combinations all possible admixture combina- Based on the results shown in Fig. tions. Cement is used in many dif- Calorimetry heat profi les from 1, it was concluded that the tested ferent processes, cement contents, hydrating Portland cement in cement-admixture combination ap- and temperatures, making it impos- presence of admixture pears to be “robust” because a 50% sible to optimize the sulfate content Fig. 1 shows an example of the admixture overdose was not enough for all situations. Furthermore, re- effect of a typical water-reducing to signifi cantly alter the cement hy- strictions imposed by ASTM, states, admixture on the hydration of a dration profi le. As a rule of thumb, DOT’s, AAHSTO; etc often limits Portland monitored at room tem- one should always test a candidate the total SO allowed in cement. On perature by an isothermal conduc- 3 cement-admixture combination at the other hand, one can select a few tion calorimeter. The Portland ce- an overdose of admixture, because most common admixture combina- ment by itself had a slightly higher the result will indicate if a mixture tions that represent the local mar- than optimum sulfate content, as is close to an “out-of-balance” situ- ketplace, and then use these combi- seen by the sulfate conversion ation or not. If an intended mixture nations for screening cement. Such peak appearing several hours after is close to being “out-of-balance”, screening should be done whenever the maximum of the main silicate once should look for a more stable there is a known change in cement hydration peak. As the water-re- mixture design. Mixtures close to composition or process, or when ducing admixture was added to “out-of-balance” typically result concrete producers are developing the mixture at increasing dosages, in undesired high variability in new mixture designs. the aluminate reactivity increased concrete setting time, strength while the main strength giving development, and sometimes also Likewise, it would be impossible silicate hydration was retarded. As premature stiffening as a result of for concrete producers to test all a consequence, note how the sulfate not properly controlled aluminate incoming cement loads for all depletion peak appeared closer and hydration. Since cement has an admixture combinations used. closer to the maximum of the main inherent natural variability in com- However, screens would be rec- silicate hydration peak. Eventually, position and reactivity, and concrete ommended whenever there is a the sulfate depletion peak appeared conditions vary, it is very important known change in type or amount before the now strongly retarded to design “robust” mixtures that can of the materials used, or when there silicate hydration peak, when a tolerate minor variations in reactiv- is a signifi cant change in concrete strong overdose of admixture was ity without the mixture being put curing temperature. used. “out-of-balance”. Test for the effect of additional soluble sulfate Mix with admixture Unstable cement – admixture com- Insuffi cent and 0.5% SO3 added binations usually display a sulfate aluminate control in as calcium sulfate "unstable" mixture depletion peak appearing before hemihydrate Retarded or at the same time as the main silicate silicate hydration peak. It is then hydration in recommended to run a separate test "Unstable" series with additional soluble sul- mixture fate added to the cement. The added sulfate test will help to confi rm the location of the sulfate depletion peak, which is sometimes diffi- cult to identify if it is completely Figure 2. Soluble sulfate test on an “unstable” cement-admixture combination by overlapping the main silicate peak. monitoring the timing of the main silicate hydration peak. An ASTM Type A Water The added sulfate test would also reducing admixture was used according to the manufacturers recommendations (0.4% prevent an “unknown” hydration s/s). Green plot is the “unstable” mixture; Yellow plot is the same mixture with 0.5% SO added to the cement as plaster. The cement used meets the specifi cation for ASTM peaks (such as early aluminate 3 Type I Portland cement, 2.3% SO3, BSA 370 m2/kg. hydration induced by accelerating admixtures) from being mistaken for a sulfate depletion peak. Sulfate is usually added in the form of cal- tion before time of placing. Note cium sulfate hemihydrate (plaster), however that calorimetry results since this is the most soluble form refl ect chemical aspects only. Any of calcium sulfate. Fig. 2 shows an physical effects such as absorption example of a sulfate test on a ce- on aggregate fi nes cannot be as- ment-admixture combination that sessed by calorimetry tests only. appears to be unstable. Since the heat evolution measured in an externally mixed sample does Conclusion not become quantitative until 10-15 Calorimetry can be a very useful minutes after inserting the sample tool for screening cement-admix- into the calorimeter, one must use ture combinations with respect some caution when comparing heat to hydration stability. Repeated evolution during the fi rst 15 min- tests are recommended, as single utes. Slump loss occurring more tests can usually not tell if a given than 15 minutes after initial mixing mixture design is very sensitive to would be detectable as an increased changes or not. heat evolution only if it is induced by an acceleration of the aluminate Most aspects of cement hydration hydration. and performance with admixture in- volve heat evolution, and can there- fore be monitored by calorimetry. Certain processes however, such as excess air entrainment and false set, involve so little heat relative to the overall process, that calorimetry is not a suitable screening tool. Loss of workability or slump in concrete can to some extent be assessed by calorimetry on ce- ment paste, by monitoring the heat associated with aluminate hydra- must form before particles become interlocked) and both the aluminate Water/cement ratio, 0.5, 23C and the Alite rate of hydration. Increased admixture mix water addition The aluminate and Alite compete dosage for water for their hydration. Since aluminate is much more reactive, it is critical to have enough sulfate Acc. aluminate available to retard the aluminate. Sulfate must fi rst dissolve before Sulfate it can react with aluminate to form depletion ettringite, which coats the surface and retards the aluminate hydration. If not enough sulfate is available to slow down the aluminate hydration, Figure 1. Effect of progressively increased admixture dosage on calorimetry response. loss of workability or even fl ash Note how the strength giving silicate hydration peak appeared to be retarded, both in size and timing, as the strong overdose of admixture accelerated the aluminate set may occur as the aluminate hy- hydration, which further retarded the silicate hydration. The cement used meets the 2 drates formed rapidly consume free specifi cation for ASTM Type I and II Portland cement, 2.9% SO3, BSA 380 m /kg. water. Since the aluminate hydrates also tend to coat the Alite, excess aluminate hydrates may retard Laboratory screening of the It would be impossible for a cement and partly prevent the onset of the stability of cement – admixture producer to test all their cements for strength giving Alite hydration. combinations all possible admixture combina- Based on the results shown in Fig. tions. Cement is used in many dif- Calorimetry heat profi les from 1, it was concluded that the tested ferent processes, cement contents, hydrating Portland cement in cement-admixture combination ap- and temperatures, making it impos- presence of admixture pears to be “robust” because a 50% sible to optimize the sulfate content Fig. 1 shows an example of the admixture overdose was not enough for all situations. Furthermore, re- effect of a typical water-reducing to signifi cantly alter the cement hy- strictions imposed by ASTM, states, admixture on the hydration of a dration profi le. As a rule of thumb, DOT’s, AAHSTO; etc often limits Portland monitored at room tem- one should always test a candidate the total SO allowed in cement. On perature by an isothermal conduc- 3 cement-admixture combination at the other hand, one can select a few tion calorimeter. The Portland ce- an overdose of admixture, because most common admixture combina- ment by itself had a slightly higher the result will indicate if a mixture tions that represent the local mar- than optimum sulfate content, as is close to an “out-of-balance” situ- ketplace, and then use these combi- seen by the sulfate conversion ation or not. If an intended mixture nations for screening cement. Such peak appearing several hours after is close to being “out-of-balance”, screening should be done whenever the maximum of the main silicate once should look for a more stable there is a known change in cement hydration peak. As the water-re- mixture design. Mixtures close to composition or process, or when ducing admixture was added to “out-of-balance” typically result concrete producers are developing the mixture at increasing dosages, in undesired high variability in new mixture designs. the aluminate reactivity increased concrete setting time, strength while the main strength giving development, and sometimes also Likewise, it would be impossible silicate hydration was retarded. As premature stiffening as a result of for concrete producers to test all a consequence, note how the sulfate not properly controlled aluminate incoming cement loads for all depletion peak appeared closer and hydration. Since cement has an admixture combinations used. closer to the maximum of the main inherent natural variability in com- However, screens would be rec- silicate hydration peak. Eventually, position and reactivity, and concrete ommended whenever there is a the sulfate depletion peak appeared conditions vary, it is very important known change in type or amount before the now strongly retarded to design “robust” mixtures that can of the materials used, or when there silicate hydration peak, when a tolerate minor variations in reactiv- is a signifi cant change in concrete strong overdose of admixture was ity without the mixture being put curing temperature. used. “out-of-balance”. Test for the effect of additional soluble sulfate Mix with admixture Unstable cement – admixture com- Insuffi cent and 0.5% SO3 added binations usually display a sulfate aluminate control in as calcium sulfate "unstable" mixture depletion peak appearing before hemihydrate Retarded or at the same time as the main silicate silicate hydration peak. It is then hydration in recommended to run a separate test "Unstable" series with additional soluble sul- mixture fate added to the cement. The added sulfate test will help to confi rm the location of the sulfate depletion peak, which is sometimes diffi- cult to identify if it is completely Figure 2. Soluble sulfate test on an “unstable” cement-admixture combination by overlapping the main silicate peak. monitoring the timing of the main silicate hydration peak. An ASTM Type A Water The added sulfate test would also reducing admixture was used according to the manufacturers recommendations (0.4% prevent an “unknown” hydration s/s). Green plot is the “unstable” mixture; Yellow plot is the same mixture with 0.5% SO added to the cement as plaster. The cement used meets the specifi cation for ASTM peaks (such as early aluminate 3 Type I Portland cement, 2.3% SO3, BSA 370 m2/kg. hydration induced by accelerating admixtures) from being mistaken for a sulfate depletion peak. Sulfate is usually added in the form of cal- tion before time of placing. Note cium sulfate hemihydrate (plaster), however that calorimetry results since this is the most soluble form refl ect chemical aspects only. Any of calcium sulfate. Fig. 2 shows an physical effects such as absorption example of a sulfate test on a ce- on aggregate fi nes cannot be as- ment-admixture combination that sessed by calorimetry tests only. appears to be unstable. Since the heat evolution measured in an externally mixed sample does Conclusion not become quantitative until 10-15 Calorimetry can be a very useful minutes after inserting the sample tool for screening cement-admix- into the calorimeter, one must use ture combinations with respect some caution when comparing heat to hydration stability. Repeated evolution during the fi rst 15 min- tests are recommended, as single utes. Slump loss occurring more tests can usually not tell if a given than 15 minutes after initial mixing mixture design is very sensitive to would be detectable as an increased changes or not. heat evolution only if it is induced by an acceleration of the aluminate Most aspects of cement hydration hydration. and performance with admixture in- volve heat evolution, and can there- fore be monitored by calorimetry. Certain processes however, such as excess air entrainment and false set, involve so little heat relative to the overall process, that calorimetry is not a suitable screening tool. Loss of workability or slump in concrete can to some extent be assessed by calorimetry on ce- ment paste, by monitoring the heat associated with aluminate hydra-