Concrete

Mud content refers to the content of fine particles with a particle size of less than 75μm in sand, its mineral composition and chemical composition are different from those of the parent rock, and with relatively strong adsorption.

The mud in natural river sand mainly comes from the clay at the bottom of the river. The mud in the artificial sand comes from the clay on the surface of the rock that has not been cleaned, and is mixed into the machine-made sand after being broken. The main mineral components of mud are kaolin, montmorillonite, and illite. Most of these mineral components are layered silicate minerals, crystalline hydrates composed of aluminosilicates. Montmorillonite has strong water absorption and swelling properties. Kaolinite does not swell when it absorbs water.

Influence of mud content on concrete workability

The concrete contains a large amount of layered cement-soil mineral components, which will absorb a large amount of mixing water, which reduces the free water in the mixture and deteriorates the fluidity of the concrete. Mud mineral particles such as kaolin and montmorillonite are easy to absorb water, resulting in a decrease in free water in the concrete mixture. After the montmorillonite absorbs water, its volume expands, which causes the viscosity of the concrete to increase, and the fluidity of the concrete becomes lower.

In addition, the surface of the mud particles is relatively rough, which causes the friction between the solid particles to increase when the concrete mixture flows, and the fluidity becomes worse.

In the production process, in order to meet the workability requirements of concrete, an extra amount of water is often added to increase the initial slump of the concrete. The increase in the water-binder ratio causes the strength of the concrete to decrease. Montmorillonite itself does not have hydration properties. After the concrete is hardened, the water in the montmorillonite evaporates or participates in the hydration of other substances, which causes the volume of the montmorillonite to shrink, resulting in poor volume stability and many tiny cracks.

Influence of mud content on water reducing agent

Clay refers to a multi-mineral aggregate of layered or layered chain silicate with a content of clay particles (particles ≤2μm) greater than 50%, with cohesiveness and plasticity, generally montmorillonite, kaolinite, and Yili A mixture of stone and mica.

Mud contains more layered silicate minerals. The characteristic of this type of minerals is that there are interlayer domains between the lamellae constituting the unit structure. It is easy to expand after absorbing water, and the size will become larger, sucking water molecules and polycarboxylic acid molecules into it. Resulting in a further reduction in the amount of water in the system, and the polycarboxylic acid water-reducing agent molecules used to disperse cement particles are occupied by clay, which affects the workability of the entire concrete system.

Taking montmorillonite as an example, the interlayer spacing in the fully dehydrated state is only about 1 nm, while the interlayer spacing can reach 2.14 nm after fully absorbing water and expanding. The side chains in the comb-shaped molecular structure of polycarboxylic acid water reducers are mostly fine. Long shape, easily stretched in water environment and absorbed into the clay layer, resulting in the entire polycarboxylate water-reducing agent molecule being anchored on the clay particles, and the dispersion efficiency is reduced. Polycarboxylic acid water reducer is very sensitive to the mud content. When the mud content in concrete is less than 3%, the mud content has little effect on the water reducer. The mud content is greater than 3%. The effect of the water reducing agent decreases rapidly.

Influence of mud content on the mechanical properties of concrete

In the process of concrete hardening, on the one hand, the presence of mud will hinder the bonding between cement stone and aggregate, and it is easy to form a weak area of ​​the structure, which reduces the strength of the concrete. On the other hand, the finer mud particles have a large specific surface area and will not be hydrated. After the concrete is stirred, it absorbs a large amount of free water. As the concrete hydrates or the free water evaporates, it will form in the area where the mud exists. Serious weakness. When the mud content is less than or equal to 3%, the concrete strength is less affected. The compressive strength of the same-strength concrete, whether it is 3d, 7d or 28d, decreases significantly with the increase of the mud content in the sand. When>3, the concrete 28d strength will decrease by about 3% for every 1% increase in mud content. The increase in the later strength of concrete decreases with the increase of the mud content. The larger the mud content, the smaller the later strength increase.

The influence of mud content on the durability of concrete

The mud content in concrete also seriously affects the durability of concrete. The evaporation of free water in the concrete during the hardening process causes the water-swelling soil to shrink greatly, which reduces the volume stability, and further leads to impermeability and impermeability to chloride ions. The greater the mud content, the higher the chloride ion diffusion coefficient of the concrete, the worse the impermeability, and the greater the shrinkage value, which will intensify as the concrete strength level increases.

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