Cement: What are Reactive Powder Mortars with Calcium Aluminate Cement?

Mortars composed of fiber and silica fume reinforced Calcium Aluminate Cement (CAC) perform better at low water/cement ratios. These mortars contain metal fiber reinforced CAC and silica fume under constant workability. Moreover, the admixture also consists of fine aggregate (0.1-1 mm) and copolymer acrylic polyester-based plasticizer. Thanks to using very fine aggregate in the mixture, the micro-voids in the structure are reduced and compressive strength is increased. Such admixtures composed of the aforementioned ingredients are referred to as “Reactive Powder Mortars” (RPMs). RPMs are preferred in the traditional construction and building industry. However, there are also new areas of use. These include:

• Light containers for the transportation of gaseous or liquid substances,
• Impermeable facilities built for the disposal of nuclear or thermal wastes,
• The automotive industry, where wear and impact damping are required during the production of metal or polymer parts.

Testing:

Mortar recipes are developed at optimum ratios considering the effect of silica fume and quartz and fiber amount on thermal strength, 28-day compressive and tensile strength.

• 5 separate admixtures developed were coded based on the amount of silica fume added.
• Two separate ratios of dry acrylic polymer, i.e., 0.5% and 1% of the cement in the admixture, were used.

Table 1. Test Kits:a) Ingredients of 0.5% dry acrylic polymer admixtures containing silica fume at different ratios, b) Ingredients of 1.0% dry acrylic polymer admixtures containing silica fume at different ratios

Purpose of the Test:

The purpose of the study was to obtain a high-performance RPM that has the below-mentioned properties by using CAC instead of Portland cement in the system consisting of CAE-based plasticizer:

• Very high early strength,
• Non-shrinking,
• Improved durability,
• Controllable setting time.

Things to Consider While Developing a Recipe

There are some issues to consider while determining the compositions of RPM admixtures.

These include:

• The risk of unstable aluminate hydrates losing strength in the long term while transitioning to a stable phase,
• Difficulty in finding effective water reducer-plasticizer for cements with high alumina content.

These two problems are due to the ineffective performance of melamine and naphthalene added polymers on CAC. The root cause of the first problem is that high water/cement ratios trigger the formation of unstable hydrates. However, plasticizers reinforced by copolymer acrylic polyester, a recently discovered chemical, were observed to have high performance in calcium aluminate cements. Thus, it has been possible to have CAC systems at low water/cement ratios. The reason for using CAE-based plasticizer in this study was to overcome the aforementioned problems and to have CAC systems at low water/cement ratios.

Test Results:

1. The graph presenting the required water/cement+silica fume ratio in two different plasticizer ratios shows the effect of using 1% and 0.5% on the water requirement.

Figure 1. Required Water/Cement + Silica Fume Ratio in Two Different Plasticizer Ratios

• We can observe below the effect of silica fume addition on compressive strength. While the strength increases up to the ratio of 10%, replacing it with 20% resulted in loss of strength. (in 0.5% ACE use)

Figure 2. Effect of Silica Fume Additive on Compression Strength over Time

• The loss of strength in mortars which are exposed to 50°C heat for 7 days is presented in the table below.

Table 2. Loss of Strength in Mortars with Different Compositions Exposed to 50°C Heat for 7 Days

• No loss was observed in the tensile and compression strengths in the study until 0.25 water-cement ratio was reached.
• Using 10% silica fume and 0.5% plasticizer, it is possible to reach the following figures;

1 day = 45 MPa

28 days = 160 MPa.

• High plasticizer dosages caused a decrease in 1-day strengths and delayed early hydration reactions.
• The thermal strengths of the obtained RPM systems were very high due to their alumina content.
• At low water-cement ratios, no strength loss was observed even at high temperatures.

The Significance of Using Reactive Powder Mortars with Calcium Aluminate Cement

In terms of durability, it is possible to say that RPMs perform very well against chemical and environmental impacts due to their voidless dense structure. The high performance is due to having a very low water/cement ratio, silica fume content and fiber usage. The ductile fracture mechanism of RPMs does not require excessive use of reinforced concrete reinforcement. The decrease in the dead loads due to the thin sections reduces the earthquake forces acting on the structure and increases the earthquake resistance of the structure. Thanks to the low porosity, concrete with RPM has almost no permeability to liquid/gas and radioactive elements. These admixtures should be used particularly where significant weight reduction and high strength are required. Thus, they are preferred in the construction and building sectors. These mortars are often prepared with Portland cement. However, as a result of this study;

We can conclude that preparation of high strength and ductile mortars with active powder technique is also possible with CAC. Moreover, mortars prepared in this manner present higher performance than the Portland cement due to the CAC content. For further details on CAC please check this article.