Causes and Control Measures of Slump Loss of Concrete Mixed with Superplasticizer

The practical problem of large slump loss caused by the addition of high-efficiency water reducer concrete brings great trouble to the construction and even causes quality accidents.

The reasons for the slump loss of concrete with high-efficiency water reducer are summarized. The main reasons for the excessive slump loss of concrete with superplasticizer are as follows: (1) due to the cement particles to the water reducing agent molecule The adsorption reduces the concentration of the water reducing agent in the liquid phase of the slurry. When the cement and the water reducing agent are stirred with water, more water-reducing agent molecules dissociate and accumulate on the surface of the cement particles, reinforcing the cement particles. The electrostatic repulsion between the two increases the dispersibility of the cement particles and increases the slump of the concrete. However, the water-reducing agent molecules (actually an anion with a relatively large molecular mass) adsorb a large amount on the surface of the cement particles. The concentration of the water reducing agent in the whole liquid phase is significantly decreased. Over time, the cement is initially hydrated, and the water reducing agent molecules adsorbed on the surface of the cement particles are covered by the generated cement hydration product, at this time, despite the slurry Some of the superplasticizer molecules in the body will also be adsorbed on the surface of the cement hydration product colloid layer, but the amount of water-reducing agent molecules remaining in the slurry is small, and the cement particles quickly coagulate and the viscosity of the slurry increases. Big, The concrete slump is reduced, that is, the slump is lost over time. (2) Gas escape and water evaporation. Even if a non-air-inducing type water reducing agent is added to the concrete, there is always a certain amount of gas. Introduction, if the bleed-type water reducing agent is added, the amount of bleed air is larger. Since the concrete may have gas escape during transportation and stacking, the polar microbubbles originally functioning as "ball bearings" in the fresh concrete Reduced, coupled with the evaporation of water, the slump value of concrete will decrease over time. For concrete mixed with superplasticizer, the water consumption is greatly reduced, and the amount of water evaporation is not mixed with water reducing agent. The concrete is roughly similar. Therefore, the evaporation of water per unit volume in concrete increases, and the slump loss increases. (3) The proportion of solid-liquid phase in the cement slurry system increases. After the addition of water reducing agent, due to the water reducing agent The dispersing effect, the wetting effect on the surface of cement particles is strengthened, and the hydration products containing crystal water such as AFt and CSH increase, the proportion of solid-liquid phase of the whole system increases, the amount decreases relatively, and the condensation tendency accelerates, resulting in concrete. The slump value drops faster. At high temperatures and This phenomenon is particularly noticeable under dry conditions.

According to the mechanism analysis of the slump loss of concrete with superplasticizer, it is considered that the following measures can be taken to effectively control: (1) post-mixing method or batch adding method; (2) appropriate increase of water reduction (3) using a carrier fluidizing agent; (4) compounding a certain amount of reactive high molecular material; (5) appropriately compounding the air entraining component; (6) properly compounding the water retaining and plasticizing components (7) Adopt new measures in the synthesis process of high-efficiency water reducer; (8) Appropriately compound the retarding components; (9) Take comprehensive measures.