At home and abroad, research on improving the early strength of polycarboxylate superplasticizers mainly focuses on the following two aspects：
1 Adding early strength agent to the polycarboxylic acid water reducing agent, so that the water reducing agent has early strength function
2 By designing the molecular structure of the polycarboxylate superplasticizer, an early strong polycarboxylate superplasticizer with early strength properties was synthesized.
The compound early-type polycarboxylic acid water reducing agent is added with a strong early component in the polycarboxylic acid water reducing agent.The two are mixed in a certain proportion to make them have both early strength and water reduction.
Early strength components can be divided into three categories according to chemical composition:
1 inorganic salts (including chloride salts, nitrates, sulfates, carbonates, silicates, etc.);
2 organic matter (including triethanolamine, calcium formate, calcium acetate, calcium oxalate, etc.);
3 composite type, composite organic and inorganic substances, such as triethanolamine and sodium sulfate complex.
The main mechanism for the early strength effect of the combination of traditional inorganic salts (sodium chloride, calcium chloride) and polycarboxylate superplasticizers is the action of C3A and chloride salts in cement to form water-insoluble chlorohydrates. ,Sodium sulphate can react with Ca(OH)2 to form calcium sulphate. This calcium sulphate has good dispersibility and is easy to form hydrated calcium sulphoaluminate crystal with tricalcium aluminate.At the same time, the hydration process of tricalcium silicate is accelerated, and the hardening of concrete is accelerated.
Studies have shown that the 3d strength of NaCl in 0.75% of the concrete is increased by 48%, and the fluidity of the concrete mixture can still be maintained;The 3d strength of concrete with NaNO3 increased by 38% in 0.75% dosage, and good protection effect can be obtained.
Triethanolamine is the most commonly used early strength component in organic matter,Triethanolamine accelerates the formation of ettringite in the C-S-H system and improves the early strength of concrete.
The N atom in triethanolamine is more likely to form a covalent bond with a metal ion to form a stable complex.The formation of the complex inevitably destroys the products such as the tricalcium aluminate hydrate and the calcium sulphoaluminate, which accelerates the dissolution rate of the tricalcium aluminate and the tetracalcium aluminoferrate.Accelerating the formation of calcium sulphoaluminate, the formation of calcium sulphoaluminate will inevitably reduce the concentration of calcium ions and aluminum ions, further promoting the hydration of tricalcium silicate.The results show that the compressive strength of the mortar at 1d, 3d and 7d increased by 145%, 190% and 135% at 0.06% of triethanolamine.
The composite type may be a composite of an inorganic substance and an inorganic substance, or may be a composite of an inorganic substance and an organic substance, or may be a combination of an organic substance and an organic substance.The composite type tends to have a better early strength effect than the single blend type, and the blending amount is also reduced.
The results show that the composite early-type polycarboxylic acid water reducing agent mixed with sodium thiocyanate (1%) + triethanolamine (0.04%) is compared with the mortar mixed with common polycarboxylic acid water reducing agent.The intensity of 1d, 3d, and 7d increased by 165%, 100%, and 39%, respectively.
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