What are the problems faced by polycarboxylic acid water reducers in application?

Introduction

The development of water-reducing agents can generally be divided into three stages: the first generation of ordinary water-reducing agents represented by wood calcium; the second generation of high-efficiency water-reducing agents represented by naphthalene series and the third generation of high-performance water-reducing agents represented by polycarboxylic acid series. Due to the limitations of the materials and performance of the first two generations of lignin sulfonate series ordinary water-reducing agents and naphthalene series high-efficiency water-reducing agents, they can no longer meet the actual requirements of modern construction projects. In the 1990s, the rise of polycarboxylic acid high-efficiency water-reducing agents was very suitable for the construction of modern concrete projects due to their good technical characteristics and environmental advantages. Polycarboxylic acid-based water reducers have low harmful substance content, low dosage but high water reduction rate, good collapse retention performance, reduced shrinkage and increased strength. These excellent properties have enabled them to quickly occupy the water reducer market and be widely used in actual projects. Polycarboxylic acid water reducers have great room for development in my country in the future. In the future, the application trend of polycarboxylic acid water reducers will inevitably shift from major projects in the past to general projects and ordinary projects.

Problems faced by polycarboxylic acid water reducers in application

Due to the differences in raw materials, different regional environments, and the limitations of technicians' understanding, usage habits and theoretical cognition levels in the process of ready-mixed concrete, some problems have also arisen in the process of using polycarboxylic acid water reducers, which directly affect the use effect of polycarboxylic acid water reducers. At present, the mechanism of action and self-sensitivity of polycarboxylic acid water reducers in China cannot be fully controlled. Especially in actual projects, due to various reasons such as the wide variety of cements and the mud content of sand and gravel in concrete raw materials, when polycarboxylate water reducers are directly applied to concrete, problems such as poor compatibility with cement, unsatisfactory concrete performance, and large collapse damage may occur, which make it impossible to meet the requirements of the construction site. Only by properly handling the following series of problems can polycarboxylate water reducers be stably, efficiently, and widely used in concrete projects.

1. Adaptability of cement

The compatibility between concrete admixtures and cement has always been a difficult problem to overcome, and polycarboxylic acid water reducers are no exception. The components, specific surface area, alkali content, type and dosage of cement will affect the compatibility of water reducers and cement. Studies have shown that the higher the C3A content in cement, the larger the specific surface area and the higher the alkali content, the worse the fluidity of concrete will be. Shang Yan et al.'s research shows that the gypsum dosage and crystal morphology have a great influence on the adsorption behavior of carboxylic acid water reducers on the surface of cement particles, thereby affecting the dispersion of cement particles. As the amount of gypsum increases, the soluble concentration in the solution increases, which will form a competitive adsorption situation with the polycarboxylate water reducer, sharply reduce the adsorption rate of polycarboxylic acid, and seriously affect the adsorption and dispersion effect of the polycarboxylate water reducer. In addition, the SO42- ions contained in cement also have a relatively large impact on its use effect. A large amount of SO42- ions will directly affect the adsorption amount of cement particles by the polycarboxylate water reducer molecules, resulting in a significant weakening of the water-reducing effect of the water reducer, and then the problem of water seepage occurs. Research and engineering practice have shown that after the water reducer is added to cement, it may usually encounter problems such as low water reduction rate, low net slurry fluidity, poor fluidity, and severe water seepage.

2. Sensitivity

With the rapid development of my country's infrastructure projects, a large amount of high-quality natural sand and gravel aggregates have gradually been consumed and cannot meet the amount required for the project. However, river sand with high mud content, weathered sand, and artificial sand with high powder content are bound to be used in large quantities in engineering. Due to the high sensitivity of polycarboxylate water reducers, it will be a test for the quality of concrete projects.

2.1. Clay sensitivity

A large number of studies have shown that high clay content has a huge impact on concrete performance. Due to the high mud content in clay, it has a strong adsorption capacity for water reducers and water. After being added, it seriously affects the fluidity and slump retention of concrete; at the same time, it reduces the fluidity of the slurry and the connection performance between clay particles is poor, weakens the interface between aggregate and slurry, and may form airspace in the concrete, which directly affects the strength and durability of the concrete. For concrete mixed with water reducers, compared with the effect of mud content on the fluidity loss of naphthalene and fat-based water reducers, the effect of polycarboxylic acid water reducers is more obvious. Its fluidity decreases rapidly with the increase of mud content, and when the mud content reaches 4%, it completely loses fluidity. It can be seen that polycarboxylic acid water reducers are extremely sensitive.

2.2.Stone powder sensitivity Stone powder

As an accessory of machine-made sand, it will inevitably be added to concrete. And stone powder also has a certain degree of uncontrollability on cementitious materials and water reducers. According to research, when the amount of stone powder is small, it fills the cement and reduces the pores between particles. This reduces the pore water content, increases the free water, and increases the fluidity of the slurry. At the same time, the nucleation effect of stone powder and its participation in hydration reaction are beneficial to enhancing the strength of concrete. However, when its dosage is high, the effect of stone powder adsorbing water reducer is significantly enhanced, and its adaptability to polycarboxylic acid water reducer is seriously reduced. In the process of premixed concrete, the strength of concrete will not reach the expected effect while maintaining the original mix ratio.

3.Water-cement ratio

In the actual premixed concrete process, the raw materials used in the concrete mixing station cannot remain unchanged. Sometimes, in order to ensure good fluidity of concrete, the method of changing the water consumption per cubic meter is usually adopted. However, it will be found in the process of use that this traditional empirical method is not applicable to polycarboxylic acid high-efficiency water reducer, mainly because polycarboxylic acid water reducer is more sensitive to water consumption than traditional water reducer. When the water consumption is reduced, the expected workability of concrete cannot be achieved; when the water consumption is high, although the slump becomes larger, a large amount of water seepage and even a little segregation will occur, which has a great impact on the overall working performance of concrete, thus causing many inconveniences in actual on-site construction.

4.Compatibility of polycarboxylic acid water reducer with other admixtures

In order to better optimize the performance of water reducers, compounding of admixtures is usually selected. Traditional lignin, naphthalene, aliphatic, etc. can be miscible and compounded in any proportion, but polycarboxylic acid water reducer, due to its own sensitivity, will be found to have stronger selectivity for compound components during use, and about half of domestic water reducers are incompatible with it. The research results of Sun Zhenping et al. show that: from the perspective of mutual solubility alone, in actual projects, polycarboxylic acid and aliphatic and melamine water reducers will produce precipitation when compounded, and they cannot be compounded together; from the perspective of plasticizing effect, polycarboxylic acid and naphthalene, melamine, etc. compounding will reduce their plasticizing effect, especially the negative effect on plasticizing effect and slump retention when carboxylic acid and naphthalene are compounded. After many water reducers are added to concrete, their fluidity will increase with the increase of the dosage, but polycarboxylic acid water reducer does have the problem that its fluidity will decrease sharply when the dosage is too large. When a certain amount of air content is required in concrete, defoamers and air entraining agents are usually added. When using traditional water reducers, it can have a good effect, but when using polycarboxylic acid water reducers, sometimes the bubble performance cannot be changed.