Characteristics of Various Mortar Additives in Self-Leveling Cement
Release Time:
2015-07-29
Source:
Characteristics of Various Mortar Additives in Self-Leveling Cement
When cement mortar is mixed with water, a gradual dissolution process occurs, which can be accelerated by external stirring. By monitoring the real-time power consumption of the mortar mixer, we can track the dynamic evolution of the mixing process: initially, as cement first comes into contact with water, it tends to form agglomerates, increasing mixing resistance and causing a sharp rise in power consumption; subsequently, as water and dispersants effectively disperse the cement particles, mixing resistance decreases, water is progressively distributed among the cement grains, and the agglomerates are broken up; once the mortar reaches its liquefaction point, its viscosity drops markedly and then stabilizes. Shortening the time required for the mortar to reach its maximum liquefaction point is crucial for improving pumping efficiency. This can be achieved in several ways, such as enhancing mixing efficiency, increasing the amount of mixing water, or using high-efficiency water-reducing admixtures. However, at a given mixing power, increasing the mixing water significantly degrades the overall performance of the mortar, whereas adding a water-reducing admixture can substantially accelerate the wetting and dispersion of cement by water, thereby shortening the mixing time considerably—though at low water-to-cement ratios, the dispersion rate still tends to slow down. In addition, self-leveling mortar formulations typically include viscosity-modifying additives designed to prevent bleeding and segregation, which must also dissolve and become effective rapidly; only when these two types of additives work in concert can a uniform, stable, highly flowable mortar be produced. The dispersion and activation rates of these two admixtures ultimately determine the required mixing time for self-leveling mortar.
Self-leveling mortar is one of the most complex types of specialty mortar. To enhance its workability, water-reducing admixtures are an essential component, and different types of such admixtures exhibit significant variations in their ability to disperse the mortar. Polycarboxylate superplasticizers can rapidly adsorb onto the surface of cement particles, exerting electrostatic repulsion and steric hindrance to disperse the particles effectively. This rapid dispersion capability substantially shortens the time required for the mortar to reach its maximum flow after water is added.
Compared with traditional manual mortar mixing, continuous mechanized pumping construction can significantly enhance productivity on large-scale job sites; however, the mixing and blending time for such specialized mortar pumps is relatively short, typically only 5 to 15 seconds. This brief mixing duration does not allow sufficient time for the mortar to fully hydrate with water, and some admixtures may not dissolve completely or exert their intended effects. In self-leveling mortars, the number of admixtures often exceeds five; for conventional self-leveling mortars, after water is added and the mixture is stirred, a “maturation” period of several minutes is even required to ensure that water-reducing agents and viscosity-modifying additives are fully dissolved, thereby achieving the desired flow properties and stability. The use of fast-dispersing admixtures can substantially shorten the mixing time, rapidly disperse cement-based mortars while stabilizing their rheological behavior, reduce the occurrence of such issues, and improve the delivery efficiency of mortar pumps.
For self-leveling mortars, even rapid dispersion of cement by water-reducing admixtures is often insufficient to achieve optimal flowability, as the dispersion process can sometimes lead to bleeding and segregation. Therefore, viscosity-modifying agents are typically added to the mix to stabilize the mortar’s viscosity; such agents are thus also referred to as stabilizers. The thixotropic behavior of stabilizers should not be excessive, as this would impair the mortar’s workability; hence, it is important to select types with moderate viscosity and low thixotropy. Commonly used options include low-viscosity cellulose ethers, whose dosage must be carefully controlled.
Casein, a natural dispersing water-reducing agent derived from milk, has long been widely used in self-leveling cements due to its excellent workability and resistance to bleeding. However, driven by increasingly stringent environmental requirements and its relatively low water-reducing efficiency, the market share of casein has declined year by year.
Large-scale, mechanized self-leveling floor construction places higher demands on the workability of the mortar. Tianjianji admixture can significantly reduce mixing time, making it easier to control rapid, continuous pumping operations. The functions of admixtures in self-leveling mortars can be summarized as follows:
- Even at low water-to-cement ratios, it can rapidly disperse cement mortar and break up cement agglomerates and flocculated states;
- It can reduce the power consumption and wear of the agitator blades, complete the mixing process rapidly, and eliminate flow delay issues;
- Higher water-to-cement ratio tolerance, preventing bleeding and segregation caused by excess water;
- Rapid dissolution to minimize self-leveling surface defects caused by undispersed agglomerates;
- Quickly disperses and takes effect under both high and low temperatures.
