Leveling screed is a leveling material which is known as self-leveling screed as a cement-based polymer modified self-leveling material. Self-Leveling Screeds are used to correct deformities and level the floors. These are easily applied of floors. They create a substrate after they harden which is ideal for ceramic coating or using other coating options. We will give information about Self-Leveling Screeds in this post. Additionally, this article will address the question: “Calcium sulphate effect: What are self-leveling screeds?”
Characteristics of Self-Propagating Alums
Main characteristics of Self-Leveling Screeds are;
- High fluidity,
- High early strength,
- Low segregation,
- Dimensional stability.
In Self-Leveling Screeds made by using only Portland cement, the surface shows cracks, and distortions at the corners. The reason for that is Portland cements have low hydration rate and low strength and they lead to contraction over time. Therefore, calcium aluminate cements and sulphoaluminate cements are frequently preferred in Self-Leveling Screeds since they have high strength, and they are fast hardening and contraction preventing materials.
The Effects of Calcium Sulphate Amount and Its Varieties on Calcium Aluminate Cement Based Self-Propagating Alums
The kind and amount of calcium sulphate are quite effective in hydration products. Because if calcium sThe kind and amount of calcium sulphate are quite effective in hydration products. Because if calcium sulphate completely drains, the remaining aluminate phases enter into reaction with ettringite to create monosulphoaluminate phase. Monosulphate formation leads to porosity formation and causes strength decrease. To prevent this from happening, calcium sulphate may be added to the medium. Thus, it would be possible to decrease the rate of return from ettringite for monosulphoaluminates.
Calcium Sulphate varieties are α-hemihydrate (CaSO4. H2O), anhydrate (CaSO4) and dihydrate (CaSO4.2H2O). Dissolution rates of these varieties are α- hemihydrate > Dihydrate > Anhydrate.
- As ettringite formation amount is parallel to the dissolution rate, the highest ettringite formation is seen in mixtures made with α- hemihydrate.
- And the high dissolution rates of α- hemihydrates cause decrease in mortar fluidity and increase in hydration temperature.
- Ettringite formation is lower in anhydrate compared to hemihydrate due to its low dissolution rate.
Prevention of contraction is related to ettringite formation in calcium aluminate cements. Ettringite formation in calcium aluminate cement is provided by calcium sulphate or double Portland Cement/Calcium Aluminate and triple systems. You can check out our posts titled ‘‘What Is Ettringite?’’ and ‘‘Everything You Need to Know about Calcium Aluminate Cement’’ to learn more about ettringite formation and its effects in triple systems.
Tests:
Table 1. Chemical Compositions of Materials Used in The Test
Table 2. Self-Propagating Alum Formulation Used in The Test
Letters and figures found in Table 2 are related to below indicated calcium sulphate kinds and amounts;
A: α- hemihydrate, A2: 2%, A4: 4%, A6: 6%
N: Anhydrate, N2: 2%, N4: 4%, N6: 6%
T: Dihydrate, T2: 2%, T4: 4%, T6: 6%
Leveling and setting times, solubility rates and ettringite formation, compressive and flexural strengths by calcium sulphate kind as well as contraction results were assessed as a result of the tests conducted in scope of this study.
1. Leveling and Setting Time
Table 3. Leveling and Setting Time
- Propagation of formulations made with anhydrate was higher compared to others (approx. 160Mm) while propagation of the ones with dihydrate were the lowest (approx. 153 mm). And propagation of mortars prepared using α- hemihydrate was between the two types of plasters (approx.156mm).
- When propagation losses between 4 minutes and 20 minutes are analyzed, the rate of loss is seen to be not related to α- hemihydrate amount. In anhydrate, propagation loss varies between 14 and 29 mm depending on the dosage of anhydrate. In other words, propagation loss decreases as anhydrate dosage increases.
- In all mixtures, increasing calcium sulphate (plaster) amount was observed to cause the initial setting time to extend. The most extension in initial setting time was seen in anhydrate (110-115 minutes). In other words, anhydrate has a higher retardation effect compared to other plaster types. In contrast to other plasters, dihydrate caused initial setting time to shorten compared to the reference mixture (58-69 minutes).
- And the changes in completion of setting are similar to the ones in initial setting.
2. Solubility Rates and Ettringite Formation
Solubility rates of plasters at 20°C are α- hemihydrate: 6.7 g/L, dihydrate: 2.1 g/L, anhydrate: 2.7 g/L.
- Dissolution rates of α-hemihydrate and dihydrate are much higher compared to anhydrate. Thus, a faster ettringite formation is seen during hydration.
- Anhydrate systems show a slower ettringite formation due to lower dissolution rates. Free water molecules are present between cement particles. This causes setting time extensions.
3. Compressive and Flexural Strength:
For α- hemihydrate;
Figure 1. α- hemihydrate Compressive and Flexural Graphs
- For α-hemihydrate, 1-day compressive and flexural strengths are not dependent on plaster amount.
- The highest 3-day compressive and flexural strengths were obtained at 2% α- hemihydrate. (Flexural strength: 7.2 MPa, compressive strength: 34.1 MPa).
- 3-day compressive and flexural strengths were seen to decrease at over 2% plaster amount dosages. But it is still higher compared to the reference.
- 7-day and 28-day strength changes also show the same trend with 3-day strength.
For anhydrate;
Figure 2. Anhydrate Compressive and Flexural Graphs
- As distinct from α-hemihydrate, 1-day compressive and flexural strengths were increased with the addition of anhydrate. Maximum 1-day strength was obtained with 2% plaster amount. (Flexural strength: 2,6 MPa, Compressive strength: 11,7 MPa.)
- Strengths with 6% anhydrate mixtures decreased at a range between 35-38% compared to 2%. However, compressive, and flexural strengths increased compared to the reference in all plaster dosages.
- And 7-day and 28-day strengths also show the same trend.
For dihydrate;
Figure 3. Dihydrate Compressive and Flexural Graphs
- In mixtures made with dihydrate, compressive and flexural strengths all increase depending on 4% plaster dosage.
- In contrast to α-hemihydrate and anhydrate, the highest strength was obtained with 4% dihydrate amount.
- With 4% dihydrate plaster, 1-day flexural and compressive strengths increased to 4.1 MPa and 17.2 MPa.
- Early strength increase was observed with dihydrate. The reason for this is lesser ettringite formation due to lower amounts of SO3. Therefore, dihydrate plaster variety may be preferred is we want early strength in Self-Leveling Screeds.
4. Contraction
Figure 4. Contraction graphs at different dosages by calcium sulphate type: A) Addition of 2% different plaster type, b) Addition of 4% different plaster type, c) Addition of 6% different plaster type
- α- hemihydrate has a higher expansion rate compared to others Due to its high dissolution rate, it is better compared to other plasters in preventing contraction by enabling high ettringite formation.
- Contraction amount was decreased by increasing plaster amount.
- When plaster types are listed according to their ability to prevent contraction, the list goes as follows; α- hemihydrate> anhydrate > dihydrate.
Test Results
- There are maximum plaster types and amounts for high flexural and compressive strengths. Increasing calcium sulphate amount causes flexural and compressive strengths to rise to their maximum values first, to be followed by a decrease.
- Dihydrate gives the mortar a higher early strength compared to other plasters.
- Most preferred plaster type for its ability to prevent contraction is α-hemihydrate due to its high dissolution rate.
- Anhydrate leads to longer setting times and low amounts of propagation losses due to its low dissolution rate.
- As a result of this study, the best plaster type for Self-Leveling Screeds is found to be anhydrate. And optimum dosage amount was found to be 4% of bonding agent amount as a result of the tests.
What Is the Importance of Self-Propagating Alums?
Self-Leveling Screeds are polymer modified cements which have high fluidity properties that do not require addition of excessive water for placement. They are used to create a flat and smooth floor surface with a higher compressive strength. Therefore, they must have properties such as high fluidity, high early strength, low segregation, and dimensional stability. Self-Leveling Screeds can be made using Portland cement. However, Portland cement causes cracks on the surface and distortions at the corners due to its low hydration rate, its low strength and leads to contraction over time. Calcium aluminate cement is frequently preferred in Self-Leveling Screeds since they have high early strength, and they are fast hardening and contraction preventing materials. In this post, we examined various topics such as what Self-Leveling Screeds and their properties are. In addition, we talked about the effects of calcium sulphate’s amounts and types on calcium aluminate cement based Self-Leveling Screeds. And the tests conducted in this subject also indicated that different calcium sulphate (plaster) type and dosage amount should be used in Self-Leveling Screeds in order to obtain the desired results.
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