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How do concrete admixtures affect pore structure and impermeability?

Concrete admixtures not only change construction and workability, but also permanently solidify into a hardened slurry pore structure in the early stages of particle dispersion and hydration rate. The connectivity of pores directly determines the impermeability, frost resistance, erosion resistance and durability performance, connecting the complete chain of “fresh microstructure → pore structure → durability”.

Classification of three types of hardened pores and their durability hazards

The internal holes of hardened concrete can be divided into three categories: gel holes with a size of less than 40 nm, which basically do not bring adverse effects; Fine pores with a pore size exceeding 50 nm are prone to forming water permeation pathways; The large pores formed by the inclusion of air during the mixing process will significantly lower the overall strength of the concrete.

Fine pores are easily interconnected to form a network structure, and water and various corrosive ions mostly penetrate the interior of concrete through this interconnected pore network. To optimize the pore structure, the key is to interrupt the pathways through which capillary pores interconnect.

Mechanism of pore control of water-reducing agent in low-water-ash system

A water-cement ratio of 0.29 belongs to a low water-cement ratio. Relying on water-reducing agents to break apart cement clumps and allow cement particles to disperse evenly. The hydration reaction of cement in the 7-day curing stage is more complete, and a large amount of hydration products are generated to fill the capillary pores. The overall pore volume, critical pore size, and ink bottle-shaped pores are all reduced, and the interconnected pore structure is greatly improved.

When the curing age reaches 28 days, the materials that can participate in hydration are basically consumed, the hydration reaction is basically stagnant, and the difference in pore parameters between different samples decreases accordingly.

Double contradictory effect of high water system water reducing agent

A water-cement ratio of 0.4 belongs to a high water-cement ratio. The water reducing agent releases the free water encapsulated by cement coagulation, and the gaps between cement particles become larger. After the slurry hardens, the capillary pore size becomes larger, and the degree of interconnection between pores is also higher. Although the degree of hydration reaction is improved in this ratio, the adverse effects of pore connectivity are more prominent, resulting in an overall increase in the total number of pores in the hardened cement slurry.

This means that it is difficult to improve the impermeability of concrete with a high water-cement ratio only by adding water reducing agent. It needs to be used together with lotion or asphalt. Only by filling holes with such materials can the impermeability effect be optimized.

Pore filling mechanism of styrene acrylic lotion

The lotion particles can be evenly dispersed into the pores, and no chemical reaction occurs during the material’s entire hydration process. After the lotion solidifies, organic particles form, blocking the pores’ through channels and effectively cutting off the interconnected pore network structure.

If the amount of lotion is too small, it will aggravate the flocculation and agglomeration of cement particles and worsen the pore structure; when the mixing ratio is higher than 3%, the hole-filling effect of lotion becomes the main effect, and the pore-refinement effect is very prominent. In addition, compared with the lotion with large particle size, the lotion with smaller particle size has better filling effect.

Long-term waterproof advantages of emulsified asphalt

After hydration, the high-asphalt-content proportion forms an organic and inorganic interwoven overall structure. The outer layer of the hydration product is wrapped in an asphalt film, which has a hydrophobic effect and blocks water infiltration from the source. Anionic asphalt has better dispersion, fewer internal pores in the formed matrix, and better long-term waterproof and anti-seepage ability than cationic asphalt. It is often used in bridge deck paving, waterproof mortar and other projects.

Synchronous improvement of transition zone at mortar interface

Adding admixtures can improve aggregate arrangement, causing them to pile up more densely and in a more orderly manner. This can reduce the thickness of the interface transition zone and also reduce the pores in this area. The interface transition zone of mortar itself has the weakest structure and is also the location where water infiltration is most likely to occur.

On the premise of consistent additive addition, the improvement effect of mortar’s impermeability performance is more significant than that of pure cement slurry.

Conclusion

Low water and high-strength structure: Prioritize polycarboxylate superplasticizer to refine pores; Ordinary waterproof mortar: highly mixed styrene acrylic lotion; Road and anti-seepage surface layer: modified with anionic emulsified asphalt; For cast-in-place components with high water-cement content, lotion/asphalt must be mixed to offset the negative impact of the increase of gross pores.