cement-production-phases
Reading Time 3 minutes
Share:

Cement Production Phases: What Are Calcium Aluminate Cement Phases?

Calcium aluminate cement is a special cement with high performance and strength that is resistant against heat. In this article, we will answer such questions as how to produce calcium aluminate cements, what are the phases and effects.

What Are Cement Production Phases?

Raw material: The main raw materials to produce calcium aluminate cement are limestone and bauxite. Although alumina is commonly found in nature in a distributed state, it is generally made of silica (such as in clays), however bauxite is the only mineral suitable to produce cement in commercial scale. Standard CAC qualities (38-40% Al2O3) are made of ferrobauxites and they contain up to 20% iron oxide. And other impurities such as titanium, magnesium and alkali oxides reduce the quality level due to the occurrence of other phases. The silica content should be quite low (<6%) however a few percent of titanium dioxide does not impact the quality level. Limestone and especially bauxite is found in the market at different quality levels, and this has an impact on the price of the end product.

Calcium aluminate cements are obtained through cooking bauxite and limestone in a rotary furnace until they melt (at 1700°C). The alumina content in the cement is more than 30%. After it cools down, the melted cement clinker looks like a compact rock which is dark in color with fine grains such as basalt. The clinker is then put through a ball mill to be ground. This clinker is very strong and therefore it requires more energy to be milled compared to Portland cement. In contrast to Portland cements, no mineral additive is used during milling.

The oxidizing and reducing nature of the environment is an important point. Undesired complex phases occur when the environment is reducing. The occurrence of these complex phases would cause the phases that are responsible for strength and setting to be reduced. Therefore, the inside of the furnace should be oxidizing.

What Are Calcium Aluminate Cement Phases?

The reactivity of calcium aluminate cement is dependent on the amounts of different reactive phases. Calcium aluminate cement consists of the following phases:

  • Calcium monoaluminate (CA, 50-55%)
  • Mayenite (C12A7, 1-3%)
  • Gehlenite (C2AS, 1-15%)
  • Ferrite (C4AF, 15-25%)

CA – Calcium monoaluminate: The main hydraulic mineral in is calcium mono-aluminate (CA). The hydration of CA contributes to the initial strength of CACs. Since the aluminate phase is a very reactive phase, the setting occurs in a very short time after getting into contact with the water.

what-are-cement-production-phases

Figure 1. CA Phase SEM Image

C12A7 – Mayenite: It plays an active role in the setting time. The Mayenite phase exists as long as there is hydration in the environment. Because OH ion is added into the crystalline structure.

cement-production-phases-1

Figure 2. Mayenite phase SEM Image

C2AS- Gehlenite: This is the phase that occurs in the presence of SiO2 which comes from the raw materials. It has a negative impact on the early hydration of cement. The reactivity of the cement would be reduced if this occurs in high amounts.

2CaO + Al2O3 + SiO2           –             C2AS

cement-production-phases-calcium-aluminate-cement

Figure 3. Gehlenite Phase SEM Image

C4AF- (Brownmillerite): Ferrite phase is the phase that occurs when the inside of the furnace is oxidizing. It gives the cement its color.

4CaO + Al2O3+ Fe2O3           –            C4AF

Why Are Calcium Aluminate Cement Phases Important?

Calcium aluminate cements (CAC) which are different from Portland cements in terms of their chemical and physical properties have thermal resistance, wear and acid resistance and they have high performance in terms of gaining early strength. These cements allow for obtaining practical, fast, and long-living repair concretes and they are used with different purposes in many places across the world.

References:

  • Jason H. Ideker, Karen L. Scrivener, Herve Fryda, and Bruno Touzo. 2019. Calcium Aluminate Cements. Place of publication unknown : Lea’s Chemistry of Cement and Concrete, 2019.
  • Pöllmann, Herbert. 2012. Calcium Aluminate Cements –Raw Materials, Differences, Hydration and Properties. Germany : University of Halle-Wittenberg, 2012.

In this article, we will introduce the flux product produced at Çimsa to be used to increase efficiency in the Iron- steel …

By nature, concrete exhibits low tensile strength, brittleness, unstable crack propagation, low breaking strength, and inadequate ductility. These shortcomings are generally eliminated …