Optimization of Cement Sulfate Part 1 – Cement without Admixture

Introduction

Sulfate is added to clinker in the manufacture of Portland cement for the purpose of retarding the hydration of the aluminate phase. Codes such as ASTM C150 and EN197 regulate the maximum sulfate expressed as SO₃ with respect to durability, but no SO₃ minimums are assigned.

An extensive study by William Lerch /1/ used isothermal calorimetry to study the influence of gypsum addition rates to a range of commercial clinkers on the hydration performance of corresponding Portland cements in terms of strength development and dimensional stability. It was concluded that the optimum sulfate content for maximum strength usually correlates with minimum dimensional change during water storage.

The optimum sulfate content was easily identified by a calorimetry heat profile. Optimum sulfate with respect to strength development and dimensional stability occurred when the depletion of soluble sulfate used up by aluminate hydration occurred at a time later than the maximum heat evolution from the main silicate hydration peak, Figure 1.

Figure 1 shows an example of hydrating Portland cement with slightly higher than optimum sulfate content monitored at room temperature by an isothermal conduction calorimeter. Portland cement mixed with water initially displays a strong exotherm caused (A) by rapid dissolution and initial hydration of mainly the aluminate phase. If sufficient sulfate is available in solution, the hydration rate rapidly decreases (B) as aluminate reacts with calcium and sulfate to form ettringite. The formation of ettringite prevents so called “Flash Set” and allows the concrete to be transported and placed while it is still fluid. After some time the strength giving Alite hydration takes off, which results in a broad exotherm (C). “Set” usually occurs at the initial part of the Alite exotherm. The Alite and aluminate hydration continue in parallel until the mixture runs out of soluble sulfate (D), which initiates the formation of aluminates with less sulfate than ettringite.

Laboratory Screening of Optimum SO₃ for a Portland Cement by Isothermal Calorimetry

Figure 2 shows how Lerch’s criteria – sulfate depletion peak to occur after the main silicate peak – can be used for a rapid indication of the optimum sulfate content of a laboratory ground clinker.

Evaluation of Laboratory Results and Further Field Tests for Sulfate Optimum

The results of the laboratory screening shown in Fig. 2 indicate that 2.5% added SO₃ might be sufficient to bring the resulting Portland cement to optimum SO₃ level. At this stage a field trial would be recommended, since several factors may cause cement manufactured in the field to perform different as compared to the laboratory ground cement. Such factors include:

• Sulfate becomes more effective if interground (as compared to simple addition to cement)
• The mill circuit in the field is probably very different from the lab. This results in a different particle size distribution, and hence a different reactivity of the cement. The net result is likely a different optimum SO₃ for the field cement.
• The solubility of different sulfate forms depends on the manufacturing conditions.

Figure 3 shows the results of a field trial in which cements where produced at increasingly higher dosages of gypsum into the mill, which resulted in a range of total SO₃ contents. The variation in response reflects the natural variation in cement composition.