Polycarboxylic Acid Water-Reducing Agents and Concrete
Polycarboxylic acid water-reducing agents are surfactants with molecular structures containing two distinct functional groups: hydrophilic groups and hydrophobic groups. Under normal conditions, polycarboxylic acid water-reducing agents reduce interparticle forces by adsorbing onto the surfaces of cement particles, thereby increasing the flowability of concrete.
However, another important function of polycarboxylic acid water-reducing agents is their retarding effect.
Retarding Mechanism of Polycarboxylic Acid Water-Reducing Agents
According to relevant studies, the primary mechanism of polycarboxylic acid water-reducing agent’s retarding effect is to encapsulate the calcium hydroxide generated during the cement hydration reaction within its molecular structure, forming a high-molecular-weight membrane. This membrane prevents the rapid release of calcium hydroxide, thereby “sealing” the surface of cement particles, inhibiting their re-crystallization, reducing nucleation between particles, and slowing down the rate of cement hydration, thus achieving a retarding effect.
When polycarboxylic acid water-reducing agents are used, their hydrophobic groups on the molecular surface strongly attract the hydrophilic groups of cement particles, enabling them to adsorb onto the cement surface relatively easily. During the hydration reaction of cement particles, the polymer membrane hinders the release of calcium hydroxide while increasing the distance between cement particles, thereby forming a porous structure that maintains the retarding effect while ensuring workability. However, the dispersion effect of polycarboxylic acid water-reducing agents weakens over time, accelerating the cement hydration reaction and thereby reducing the retarding effect.
Additionally, the retarding mechanism of polycarboxylic acid water-reducing agents is closely related to their molecular weight and chemical structure. Generally, polycarboxylic acid water-reducing agents with higher molecular weights increase their adsorption force with cement particles due to their high molecular weight, thereby effectively slowing down the hydration reaction rate. Additionally, the chemical structure of polycarboxylic acid water-reducing agents contains active groups such as hydroxyl and carboxyl groups, which can form subtle coordination interactions with the cement surface, thereby achieving a retarding effect.
The retarding effect of polycarboxylic acid water-reducing agents significantly influences the strength development process of concrete, and the extent of this influence is also related to factors such as the degree of cement hydration and temperature. When polycarboxylic acid water-reducing agents are fully utilized in concrete, they can control the age-dependent strength development of concrete, shorten the hardening time of concrete, thereby enhancing early strength and improving concrete workability. Additionally, the retarding effect of polycarboxylic acid water-reducing agents can effectively prevent concrete shrinkage and improve concrete durability.
However, it is important to note that excessive use of polycarboxylic acid water-reducing agents may have some negative effects, such as prolonging concrete setting time, delaying construction progress, and causing instability in concrete age-related strength, particularly in high-temperature and high-humidity environments.
