Effect of Soil on Properties of Polycarboxylate Superplasticizer and Its Mechanism

Soil has different degrees of adverse effects on cement paste, cement mortar and concrete, and the degree of influence is related to the soil type and the structure of polycarboxylate superplasticizer.

The adsorption amount and adsorption rate of the soil to the polycarboxylate superplasticizer are far greater than the adsorption of the cement particles on the water reducing agent.This makes a large amount of polycarboxylic acid water reducing agent adsorbed by the soil, and the polycarboxylic acid water reducing agent which can react with the cement particles is greatly reduced, which seriously affects the working performance of the concrete.

However, it is one-sided to simply use the adsorption of clay on polycarboxylate superplasticizer as a factor affecting the performance of concrete.Because the soil itself has a strong water absorption, it will absorb the free water in the system and increase the viscosity of the concrete system.

Secondly, the calcium and magnesium ions contained in the soil may also coordinate with the carboxyl groups in the polycarboxylic acid water reducing agent, and the formation of the complex affects the performance of the polycarboxylic acid water reducing agent.

1 Physical adsorption of polycarboxylate superplasticizer on clay

The physical adsorption of the polycarboxylate superplasticizer on the clay is mainly achieved by Ca₂ + induced electrostatic attraction of the cement pore fluid.Due to the eutectic substitution in the crystal cell, a permanent negative charge appears inside the clay crystal layer, which causes the end face of the mineral crystal layer to adsorb cations (in the cement paste, the cation is mainly Ca₂ + ).The carboxyl group of the polycarboxylic acid water reducing agent can electrostatically react with Ca₂ + on the surface of the clay particles to realize physical adsorption of the polycarboxylic acid water reducing agent on the surface of the clay through the electrostatic attraction.

2 Chemical adsorption of polycarboxylate superplasticizer on clay

The chemisorption of the polycarboxylate superplasticizer on the clay is mainly achieved by entering the side chain of the polycarboxylic acid water reducing agent into the clay silicon oxide layer.Plank et al. believe that the polyethylene side chain of the polycarboxylate superplasticizer can enter the siloxane layer of montmorillonite through hydrogen bonding.The chain has an EO (oxyethylene) group,the oxygen atom can form a hydrogen bond with the hydrogen atom in the water adsorbed between the layers, and the adsorbed water generates a hydrogen bond with the hydroxyl group and the oxygen atom in the silicon oxide layer.Thereby, the polyoxyethylene side chain of the polycarboxylate superplasticizer enters the silicon oxide layer of the montmorillonite.

Taviot-Gueho et al. believe that water molecules in the hydrated Na+ of the montmorillonite layer are easily replaced by EO (oxyethylene) groups in the side chain of the polycarboxylate superplasticizer to form a network of EO units and Na+. Compound,This coordination allows the EO (oxyethylene) side chain of the polycarboxylate superplasticizer to enter the silicon-oxygen layer of the sodium-based montmorillonite to form a chemisorption.

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