Calcium aluminate cement (CAC) is obtained by sintering a mixture of calcareous and alumina materials in the proper ratios and then grinding the resulting product to a fine powder. Compared to Portland cements (OPC), the annual production of calcium aluminate cement is quite small. However, when the performance is compared to that of Portland cements, it has unique properties that are preferred in special applications. When used in traditional mortars, it has the following properties:
- Rapid strength development even at low temperatures
- Durable at high temperatures / excellent refractory performance
- Resistant to aggressive conditions
Calcium aluminate cement is also commonly used with other minerals and additives in different combinations including Portland cement and calcium sulfate. When used in this fashion, CAC’s offers the following advantages: rapid hardening, high initial strength, controlled expansion and resistance to contraction.
How Does CAC Age?
Basically, the cement slurry obtained by mixing cement and water creates a plastic-like texture, and the hydration reactions between them begin. These reactions impart a binding property, in other words, element of strength to the cement mortar. The hydration speed and degree of excellence is determined by the fineness of the cement, the amount of cement and the composition of the cement. However, it is also affected by the ambient temperature and moisture. All hydraulic binders exposed to high humidity undergo prehydration (aging). Calcium aluminate cements are more reactive than Portland cements and performance is expected at an earlier stage, so the effect of moisture is noticed more quickly. The effects calcium aluminate cements on aging can be reduced with proper mineralogy and recipe designs.
What are the Key Considerations When Using Calcium Aluminate Cement?
The Ternary System is a binding system that uses a combination of Portland Cement, Calcium Aluminate Cement and Calcium Sulfate for various needs in applications ranging from leveling screed, repair mortar and grout. Mixing these three components in different ratios gives the final product the following properties:
- Adjustable setting time
- High initial strength
- Early drying
- Crack prevention.
These advantages can also cause several negative effects if the recipe design is flawed.
Figure 1. Ternary binding systems
Region 1: OPC and CAC binary system
- Advantage – Rapid Hardening
Valid for OPC and CAC binary systems. These types of systems result in rapid setting in just a few minutes and are used for repair jobs in small areas. The advantage of these systems is that they consist of only two components, but their use is limited by the fact that strength is less than OPC and there is no compensation for contraction.
Region 2: OPC dominant ternary system
- Advantage: Early strength and Preventing Contraction
The strength that systems develop as the ratio of CAC increases is characterized in two steps. The first step occurs within the first 24 hours, essentially resulting from the hydration of CAC and CS* (plaster) and it is also related to the extensive precipitation of ettringite, which forms with the OPC phases that can contribute to calcium ions. The expectations for final strength in these blends should be precisely tested.
Region 3: CAC dominant ternary system
- Advantages: Early strength + Rapid Drying + Crack Prevention + Alkali Barrier
Moving from Region 2 to 3 results in reduced OPC content and an increase in CAC + CS*, which makes ettringite and hydrated alumina the primary hydrate phase. In these systems, calcium sulfate is generally added as hemi-hydrate (plaster) or gypsum.
Compared to Region 2, the properties of systems in Region 3 are significantly improved due to the formation of ettringite, resulting in high initial strength and rapid hardening.
How to Prevent Calcium Aluminate Cement Aging?
Aging in ternary systems prepared with calcium aluminate can occur due to poor recipe design or the cement being exposed to high moisture and temperature.
As will be seen in the leveling screed below, which was prepared using ternary binding systems, there is a mottled appearance on the application surface due to the fact that the cement blend was exposed to moisture.
Calcium Aluminate Cement Aging: Experiments and Examples
Figure 2. Mottling in leveling screed application
In order to observe the aging process, performance tests were conducted by exposing a dry blend of leveling screed prepared using the ternary system to high temperature and high humidity.
It is particularly important that a dry blend with the proper recipe design is prepared for the experimental process. A design consistent with primarily Region 3 calcium aluminate cement was selected in the study. The recipe included the following additives: cellulose ether, anti-foaming additive, set accelerating and set retarding admixtures.
Table 1: Recipe design of the ternary binding system
In the aging study, CAC samples were left at 35 degrees Celsius and 80% humidity for 1 day and 3 days.
In the second step, self-leveling screed systems were created with fresh cement and aged for 1 day and 3 days.
Figure 3. Leveling screed aging test
SEM EDX analyses showed a high percentage of calcium and sulfate elements in the white spots on the samples aged 1 day and 3 days.
Figure 4. SEM appearance of leveling screed
The effect of applications with mortars exposed to high humidity and temperature on the strength and setting time of the leveling screed was compared with the reference sample.
Table 2 Post-application strength test results
In the blend made with Recipro40, strength at 3 hours, 6 hours and 24 hours was higher than the reference.
When workability parameters are compared, end of setting values were longer in blends exposed to aging for 3 days.
Expectations for leveling screed are that setting time will be shorter than 6 hours, and that it will have high initial strength. Another important parameter is that surface integrity will be preserved after the application.
Table 3 Workability test results for leveling screed slurry
The plaster in the system caused mottling on the surface due to the fact that it remained free because the reactive phases that it might have reacted with were reduced as a result of prehydration, in other words aging. When these conditions and experimental results are evaluated, it is clear that measures must be taken to prevent mottling on the surface. In order to prevent prehydration, i.e. aging:
- The right ternary system must be designed.
- Proper packaging must be selected.
- Proper stock conditions must be maintained.
Using Calcium Aluminate Cement: Results
All hydraulic binders are by nature susceptible to aging. The moisture of materials in leveling screed and similar technical blends and the temperature they are exposed to have a negative effect on performance in terms of:
- Strength gain
- Setting time
- Occurrence of mottling/staining
The more prehydration there is, the lower the performance of the final product will be because moisture covers the surface of the binder and prevents the reactive phases from occurring. In ternary systems, the aging of any binder disrupts the system. As a result, black and white spots (CAC and CS*) can be seen on the surface. In order to achieve long-term reactivity, the phase structure and content of CAC are key. Recipro40 was designed to resist aging by reducing the effects of prehydration. The storage and packaging of cement and mortar is critical in terms of aging performance.