How Does Calcium Aluminate Cement Age?

Calcium aluminate cements differ from Portland cements (OPC) in terms of both mineralogical and physical characteristics. The raw materials for Portland cement are primarily sourced from clay and limestone, so the primary oxides are CaO and SiO₂. Calcium aluminate cement, on the other hand, is rich in alumina because the source of the raw material is bauxite.  The primary oxide is Al₂O₃.

The most significant characteristic of calcium aluminate cement is the rapid development of strength. Calcium aluminate cement achieves more strength in just 24 hours than is developed by Portland cement in 28 days. It is also a cement that can withstand high temperatures, so it is also referred to as refractory cement. It can be used anywhere that Portland cement is used. 

How and Why Does Calcium Aluminate Cement Age?

When cement and water are mixed, hydration reactions take place. As a result of these reactions, a cement paste with plasticity and adhesion properties is created. Binding properties are what give cement its strength. The determining factors here are the fineness, amount and composition of the cement, the hydration rate and how efficient this process is. However, ambient temperature and humidity play an important role in these reactions. If hydraulic binders are exposed to high humidity, they suffer from the aging process known as prehydration. Calcium aluminate cements are more reactive than Portland cements, and their performance can be observed earlier. As a result, moisture plays a critical role and is noticed very quickly. The effects of aging on calcium aluminate cements can be minimized with proper mineralogy and recipe designs.

Experiments with Calcium Aluminate Cement Aging

The Ternary System is a binding system that uses a combination of Portland Cement, Calcium Aluminate Cement and Calcium Sulfate for the various needs in technical blend applications. The issue of aging in Ternary Systems has been analyzed in detail in the literature. A comprehensive look at the issue can be found in the article entitled “Investigation of the long-term stability during storage of dry-mix mortars” supported by Prof Dr. J. Plank and prepared by E. Dubina. The section of the article entitled, “Influence of Moisture Exposure on the Performance of Self-levelling mortars (SLUs)” concludes that formation of black and white stains is observed on the surface of some self-levelling mortars due to the aging of the cement in the mortar. Experiments were prepared with 2 different 40% alumina calcium aluminate cements (CAC1 and CAC2) in three different groups (fresh, aged 1 day and aged 3 days). The recipe used in the experiments for the self-levelling mortar is provided below.

Table 1. Recipe for Self-Levelling Mortar

Results obtained at the conclusion of the experiments:

Table 2. Chemical and Physical Analysis Results

Table 3. Phase Analysis Results

Table 4. Results for Strength under Pressure and Setting

Results of the Experiment with Calcium Aluminate Cement Aging

• No change in the cement phase was observed with aging.
• Loss of strength and longer setting time was observed with aging.

• The moisture and temperature the materials in the self-levelling mortar recipe are exposed to have a negative effect on the performance of the self-levelling mortar, strength gain, workability, setting time and spotting.
• The more prehydration there is, the more moisture coats the surface of the binders, resulting in unexpected effects on the performance of the self-levelling mortar.
• In ternary systems, the aging of any binder disrupts the system. Therefore, black or white (calcium aluminate cement and calcium sulfate) spots were observed on the surface.
• All calcium aluminate cements lack resistance to aging due to the high level of reactivity intrinsic to this cement.
• In order to achieve long-term reactivity, the phase structure and content of CAC are key.
• The storage and packaging of cement and mortar is critical in terms of slowing down the aging process.

What are the Key Considerations in Calcium Aluminate Cement Aging? 

Calcium aluminate cement is a hydraulic binder and, by its very nature, is subject to aging like all other hydraulic binders. As was demonstrated in the results from experiments with self-levelling mortar and similar technical blends, the more prehydration, i.e. aging, there is, the more moisture covers the surface of the binder, activating the reactive phases, all of which have a negative effect on performance parameters such as strength gain in the final product, workability, and setting time. The CAC phase structure and content plays an important role in ensuring long-term reactivity in calcium aluminate cements. The reason is that aging of any binder in ternary systems compromises the system and results in problems such as black and white spots on the surface. Therefore, in order to protect calcium aluminate cements from aging, inventory and packaging of the cement are critical steps in maintaining the high performance of the final product. 

References Used:

• Investigation of The Long-Term Stability During Storage of Cement in Drymix Mortars