The loss of concrete slump is the result of a combination of factors. Understanding the mechanism of concrete slump loss is helpful to understand the role of polycarboxylate superplasticizer in reducing slump loss.
The slump loss is essentially a loss of fluidity of the concrete slurry.Many scholars have explained the situation of concrete slump loss from various angles. There are three main reasons for the current loss of slump:
1、Increase in viscosity of cement paste
On the one hand, as the hydration progresses, the free water in the slurry gradually decreases, and the "lubricant" effect exerted by the free water also decreases, and the cement slurry thus exhibits an increase in viscosity.Macroscopically, it shows the phenomenon of reduced fluidity of the slurry.
In concrete, the reasons for the decrease in free water include the hydration reaction consumption of water and cement, and the chemical combination in hydration products including calcium silicate hydrate gel (C-S-H) and ettringite (AFt).This in turn leads to a loss of concrete slump.In addition, as the hydration progresses, the hydration products generated gradually increase, which also causes the viscosity of the concrete to increase due to the increase of the solid phase ratio, causing the slump of the concrete to decrease with time.
2、Water reducing agent consumption
When the polycarboxylic acid-based water reducing agent is added to the concrete, it will adsorb on the surface of the cement particles, thereby exerting the function of dispersing the cement particles, so that the cement slurry has better fluidity.The adsorption of the water reducing agent causes a decrease in the number of free water reducing agent molecules in the slurry, and a decrease in the molecular weight of the free water reducing agent causes a decrease in the fluidity retention of the slurry.In addition, the polycarboxylic acid-based water reducing agent molecules are also adsorbed on the surface of the early hydration product, and cannot be dispersed.And as the hydration progresses, the hydration product gradually covers the surface of the cement particles, and finally the polycarboxylic acid-based water reducing agent molecules adsorbed on the surface of the cement particles are covered.The dispersibility of the coated polycarboxylic acid-based water reducing agent molecule is decreased.The role of these two aspects will eventually lead to concrete slump loss
3、Water evaporation and bubble escape in the slurry
The moisture of the cement slurry in the concrete evaporates with time, resulting in an increase in the viscosity of the slurry and a loss of concrete slump.In addition, the concrete to which the polycarboxylic acid-based water reducing agent is added tends to introduce a certain amount of bubbles, and the gas content of the concrete is 3% to 6%.With the continuation of time, combined with the initial fluidity of concrete, the internal bubbles gradually undulate and rupture, which is characterized by a decrease in gas content, which is also a great challenge to the retention of concrete slump.In general, high temperature, high wind speed, and low relative humidity of the air will cause a rapid slump loss of concrete due to evaporation of water in the slurry.The bubble instability introduced into the concrete due to the addition of the polycarboxylic acid-based water reducing agent and the low viscosity of the concrete, the loss of the gas content of the concrete is too fast, and the loss of concrete slump is further aggravated.