1. Introduction
The high durability and long life of concrete structure can significantly reduce the cost and environmental impact, which is the trend and hotspot of international concrete research [
1]. Polycarboxylic acid water-reducing agent is considered to be the third breakthrough in concrete technology and an important means to achieve high durability and long life of concrete structures. Because polycarboxylic acid high-performance water-reducing agents have the characteristics of a high rate of water reduction, high performance at a low content, and a small slump loss over time, they have been rapidly developed in regions such as Japan, the United States, and China and are widely used. In recent years, polycarboxylic acid-based high-performance water-reducing agents have been applied in many key projects such as high-speed railways, port and day terminals, hydropower dams, and municipal engineering. They have broad application prospects and will further move toward high-performance and multifunctional. However, many problems have arisen in the application of polycarboxylic acid water reducers, such as insufficient concrete water reduction rate, rapid loss of concrete fluidity, sensitive admixture content and easy segregation of bleeding, slow concrete early strength development, etc. Therefore, the further optimization of the composition and structure of the polycarboxylic acid-based high-performance water-reducing agent is an important research direction to improve its performance and expand its application.
Scholars have conducted a lot of research on the properties of cement-based materials modified by admixtures to improve the mechanical properties and rheological properties of mortar or concrete. Girum et al. [
2] investigated the effect of internal curing (IC) by superabsorbent polymers (SAP) on the internal relative humidity (IRH), autogenous shrinkage, coefficient of thermal expansion (CTE), and strength characteristics of low water-cement ratio (
w/c) mortars. Kim et al. [
3] assessed durability and rheological characteristics of binary blended mixtures incorporating high-volume ground-granulated blast-furnace slag (GGBFS). Yamada et al. [
4] studied the dispersion performance of polycarboxylate superplasticizers on cement particles and evaluated the fluidity, plastic viscosity, and shear yield stress of cement slurry under different water-cement ratios (
w/c). The relative effectiveness of them as dispersants in cement slurry was also studied. Feys et al. [
5] evaluated the effects of mixing time, mixing speed, and superplasticizer addition time on the rheological properties of cement slurry with self-compacting concrete consistency. The results show that changing the water consumption is the main factor affecting the rheology of cement slurry with low water-cement ratio (
w/p). Ma et al. [
6] studied the effect of polycarboxylate superplasticizers on Fubelite sulfoaluminate cement, and the results showed that the cement setting time depends on the amount of water-reducing agent. Through mercury intrusion method (MIP) and scanning electron microscope (SEM) images and other methods, the compatibility mechanism between polycarboxylate superplasticizer and cement was studied. Uchikawa et al. [
7] proved that the polycarboxylate superplasticizer is adsorbed on cement particles and acts through electrostatic and/or steric repulsion. The working performance of cement paste, mortar, and concrete containing polycarboxylic acid superplasticizer depends on various parameters. On the one hand, the type, chemical composition and molecular structure of polycarboxylic acid superplasticizers [
8] affect the rheological properties (such as apparent yield stress, viscosity, concrete slump); on the other hand, the chemistry of cement Composition (especially C
3 A content and soluble sulfate content), specific surface area, the presence of mineral additives or other types of mixtures, all affect the role of superplasticizers in cement paste, mortar, and concrete [
9,
10].
Good rheological properties help to ensure the working performance of concrete and the stability of the engineering structure, thereby extending the service life of the project. Although traditional working performance testing methods (such as slump and expansion) are simple, their characterization methods are rough, and it is difficult to comprehensively reflect the rheological properties of mortar mixtures. The rheological properties of mortar are the reaction of its structural characteristics. The rheological parameters can conveniently and accurately determine the working performance and fluidity loss of concrete. The structural characteristics of the mortar can be analyzed through the measurement of rheological parameters and the characteristics of rheological curves. This paper regards mortar as a complex fluid and establishes the correlation between working performance and rheological parameters, characterizes mortar performance with rheological parameters, and uses bubble characteristic parameters such as pore structure and distribution to accurately characterize the pore morphology and structure of mortar.
In this paper, three functional polycarboxylic acid water reducers are prepared, namely, water reducing polycarboxylic acid water reducing agent J, slow-release polycarboxylic acid water reducing agent H, and early strength polycarboxylic acid water reducing agent Z. These use functional polycarboxylic acid water reducer to control the working performance and mechanical properties of the mortar. Compare the cement mortar without and with different functional polycarboxylic acid superplasticizers, examine the fluidity, time loss and compressive strength of the mortar, analyze different functional polycarboxylic acid superplasticizers and naphthalene. The effect of water reducing agent on the yield stress and plastic viscosity of mortar and the rheology of mortar with functional polycarboxylic acid water reducing agent was studied, and the rheological parameters and bubble characteristic parameters were used to characterize the different effects of water-reducing agents to provide reference and basis for engineering design and construction.
4. Conclusions
Functional polycarboxylic acid high-performance superplasticizers function as a water-reducing and cement-dispersing agent to a certain extent. Different functional polycarboxylic acid high-performance water-reducing agents have various effects on the rheological and mechanical properties of cement pastes and mortars. Through the determination of rheological parameters such as shear yield stress, plastic viscosity, and fluidity, the functional polycarboxylic acid superplasticizers proved to have better water-reducing, cement dispersion, and slump retention features than the naphthalene-based superplasticizer. In terms of rheological and mechanical properties, the cement samples formulated with functional polycarboxylic acid water-reducing agents are more likely to achieve improved rheological properties and higher compressive strength than the cement-based materials containing naphthalene-based superplasticizers. Sustained-release functional polycarboxylic acid high-performance superplasticizer H gradually produces a water-reducing effect, which proves that it performs an excellent slump retention function. Early-strength functional polycarboxylic acid superplasticizer Z can significantly enhance the early strength of cement paste and mortar.
The findings of the current work confirm that the functional groups available in the molecular structure of functional polycarboxylic acid superplasticizers play an important role in the performance of cement-based materials. Hence, a variety of functional polycarboxylic acid water-reducing agents can be designed to improve the quality of different types of cement and thus meet the requirements of various real engineering applications.