The Benefits of Using White Cement in the Manufacture of Aerated Concrete – II

The production of Autoclaved Aerated Concrete (AAC) is growing rapidly because it is fire resistant, lightweight and a better insulator. 

In a previous article, we discussed in detail the areas where aerated concrete can be used and how it is preferred because it is so much lighter than traditional concrete. We also touched on the raw materials it contains and the manufacturing process. Now, we will discuss the advantages of using white cement in the production of aerated concrete instead of gray cement. You can read the first article in the series here  

What are the Benefits of Using White Cement in the Manufacture of Aerated Concrete?

The preliminary curing process is one that consumes a significant amount of time and energy in the production of autoclaved aerated concrete. In this study, white cement was used as one of the raw materials to manufacture autoclaved aerated concrete instead of gray Portland cement. The aim was to reduce the preliminary curing time, obtain a lighter and more economical product and increase production capacity. The experiments were conducted at the Çimsa R&D Center Formülhane. The goal was to improve the mechanical properties of the product by using white cement.  In addition to improving product strength, we also aimed to reduce product density, achieve higher insulation values and a product that is whiter and easier to work with.

Characteristics of Cement Used in the Production of Aerated Concrete 

In the manufacture of autoclaved aerated concrete, the chemical reaction (expansion) begins when the paste is poured into the forms, which is when the preliminary curing process begins. The preliminary curing process takes 4-5 hours with autoclaved aerated concrete made using gray Portland cement. By using Cem I 52.5 R white cement, our aim was to reduce the curing time and obtain a concrete which hardens enough to cut in a shorter time. The results of the gray and white cements used in the study are provided in Table 1. 

Table 1. Strength under Pressure and Time to Onset of Hardening

Aerated Concrete Experiments 

Autoclaved aerated concrete paste mixtures were prepared with two different types of cement. The reference gray Portland cement was used, and the performance of mixtures with white cement in different proportions (100%, 90%, 80% and 60%) was evaluated. The water/additive ratio was kept constant for mixtures with a reduced amount of cement. All of the mixtures were moved to the preliminary curing areas and measured until they reached a pre-specified level of hardness suitable for cutting.

Table 2. Preliminary Curing Process for Aerated Concrete Cake 

Cake height was similar to the reference mixture when cutting hardness was reached. 

The amount of cement in the mixtures was reduced, and the effect that this reduction had on mechanical performance was examined. A sample was taken from the cake when it reached the desired cutting hardness in order to identify the phases that affect preliminary curing time, and phase analysis was performed using the XRD-Rietveld method. 

The most important phase that provides strength in the production of autoclaved aerated concrete is the tobermorite phase. All of the mixtures that contained white cement achieved higher Tobermorite + Portlandite phases than that made with gray cement.

After reaching cutting hardness, the mixtures were put in the autoclave. Then, all of the mixtures were measured for dry density and strength under pressure.  The A factor which demonstrates the relationship between dry density and strength under pressure was calculated.  

Table 3. Strength and Dry Density Values of the Final Aerated Concrete Product

The final strength of autoclaved aerated concrete is also related to the cement phases and the hydrates of the final product. The amount of hydration was determined using the XRD-Rietveld method in order to identify the relationship between strength and final product hydration analysis resulting from the use of white cement.

Table 4. Aerated Concrete Final Product XRD Phase Amount (%)

Whiteness was also measured in the final autoclaved aerated concrete block product.

Table 5. Analysis of Whiteness (%) in Aerated Concrete Final Product

Results of Aerated Concrete Experiments:

In the mixture where white cement was reduced by 20%, the preliminary curing process was reduced by approximately 30%. In addition, the whiteness value of the final product increased 11% with the use of white cement. The strength of the products made using white cement was higher than those made with the reference product even though less cement was used. The XRD analysis supported the results for the ratio of phases that affect strength.   

Advantage of Using White Cement in the Manufacture of Aerated Concrete 

In conclusion, using white cement instead of gray cement in the manufacture of autoclaved aerated concrete offers a number of advantages. Based on the parameters of preliminary curing and cutting hardness, using white cement to manufacture autoclaved aerated concrete provides advantages both in terms of the process and the final product. The preliminary curing process with autoclaved aerated concrete takes places between 50-55 ^oC. This temperature is achieved using natural gas. By reducing the amount of time required for preliminary curing, we also reduce the amount of natural gas used and therefore the amount of CO₂ released into the atmosphere. In addition, by reducing the amount of cement used by 20%, CO₂ emissions are reduced even further indirectly.

References Used:

• Turkish Autoclaved Aerated Concrete Association
• Portland Cement Association