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What are the factors affecting the performance of polycarboxylate superplasticizer in concrete?
Blog what are the factors
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Dry-mixed mortar is widely used in fields such as building plastering, bonding, and masonry, and its carbonation resistance is a key indicator for measuring long-term durability. As high-performance admixtures such as polycarboxylate superplasticiser (PCE) powder become increasingly standard in dry-mixed formulations, a crucial question arises: what is the impact of PCE powder on the carbonation depth of dry-mixed mortar? This article provides a detailed analysis of the mechanism behind this effect.
Although this reaction slightly increases the surface compressive strength, it has a major negative consequence: it reduces alkalinity.
Healthy, uncarbonated mortar has a high pH value (usually above 12.5). This high alkalinity forms a passive protective layer around embedded steel reinforcement (steel bars) to prevent rusting.
The carbonation process reduces the pH value to around 8.5-9. Once the carbonation front reaches the steel reinforcement, this protective layer is destroyed. Under the influence of moisture and oxygen, the steel begins to corrode (rust).
Corroded steel will expand, generating significant internal pressure that can lead to cracking, spalling (chipping), and ultimately structural failure.
Carbonation depth is the measurable distance from the mortar surface to the critical point at which the pH is still sufficient to protect steel. A lower carbonation depth indicates better long-term protection and higher durability.
PCE powder is a high-performance water reducer (superplasticiser). Its primary function is to enhance the workability and fluidity of mortar while significantly reducing water consumption. This water-reducing capability is the main reason for its effective inhibition of carbonation.
Here are two key mechanisms that play a role:
This is the most important factor.
Mechanism: PCE molecules adsorb onto the surface of cement particles. Through steric hindrance, its polymer side chains can effectively prevent particle aggregation. This “lubricating” effect significantly improves the fluidity of the mixture, enabling formulators to achieve the desired workability with less water.
Impact on Porosity: The initial moisture content of the mixture directly determines the final porosity of the hardened mortar. The moisture that does not participate in the chemical hydration of cement will eventually evaporate, leaving behind a network of capillary pores. By reducing the initial moisture content (lowering the water-cement ratio), PCE powder can help reduce the excess moisture that forms these pores.
Result: The capillary pore volume of hardened mortar is significantly reduced. This structure, with lower porosity and permeability, makes it more difficult for carbon dioxide molecules to pass through, thereby significantly slowing the carbonation rate.
In addition to reducing water consumption, PCE can also enhance the quality of the cement slurry itself.
Mechanism of action: PCE effectively disperses cement particles, ensuring that each particle is more easily exposed to water, thereby making the cement hydration process more efficient and thorough.
Impact on hydration products: Thorough hydration produces more calcium silicate hydrate (CSH) gel, the “binder” that imparts strength to mortar. CSH gel can more effectively fill the voids between unhydrated cement particles and aggregates.
Result: The final mortar matrix is denser, tighter, and has lower permeability. This dense microstructure acts as a physical barrier, further preventing the infiltration of carbon dioxide and other corrosive substances such as chlorides and sulfates.
In summary, PCE powder combats carbonation from two aspects: it reduces the number of “channels” (capillaries) through which CO₂ enters, and makes the “walls” (cement paste matrix) thicker and more impermeable.
Although PCE powder has a significant overall effect in reducing carbonation, it is necessary to consider the potential problems caused by incorrect use:
Excessive use: Using too much PCE powder can lead to excessive air incorporation. Although a small amount of air improves freeze-thaw resistance, a large number of unstable air voids can increase the mortar’s overall porosity, potentially forming permeable channels for carbon dioxide and partially offsetting the benefits of reduced water usage.
Segregation and bleeding: Excessive cement content can also lead to segregation (separation of aggregates from the cement paste) and bleeding (water rising to the surface). This results in uneven microstructure, forming fragile, porous areas that are highly susceptible to carbonation.
These issues are not due to inherent defects in PCE but rather arise from improper formulation design or incorrect dosing.
Q1: Does PCE powder always reduce carbonation depth?
A1: No. It occurs only when the optimal dosage (0.1%–0.3%) is used, and proper curing is applied. Excessive dosage (>0.5%) or improper curing may increase the depth of carbonation.
Q2: What is the difference in carbonation resistance between PCE and naphthalene-based superplasticisers?
A2: PCE exhibits superior performance. Due to its superior pore-refining effect, PCE can reduce the carbonation depth by 20-40%, whereas naphthalene-based products can only reduce it by 10-15%.
Q3: Can mineral admixtures enhance the impact of PCE on carbonation?
A3: Yes. The synergistic effect of fly ash, slag, or metakaolin with PCE further refines the pores and reduces the carbonation depth by an additional 10-20%.
The impact of PCE powder on the carbonation depth of dry mixed mortar is clear and scientifically proven: it can significantly reduce it.
PCE powder can reduce the carbonation depth of dry-mixed mortar by 20% to 40% by refining pores, promoting hydration reactions, and improving interfacial bonding. This is the key to ensuring its long-term durability. This directly translates into better protection of steel reinforcement and significantly extends the structure’s service life.
By integrating these strategies, manufacturers and construction workers can produce dry-mixed mortar with excellent carbonation resistance, thereby extending its service life in urban, industrial, and harsh environments. With the increasing demand for durable, low-maintenance materials in green buildings, PCE-modified dry-mixed mortar has emerged as a reliable solution.
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