Common descriptions of the performance of early-strength water-reducing agents are: high water-reducing rate, very good adaptability to cement, good concrete workability, and no slump in one hour. losses etc. In fact, the composition of cementitious materials is complex and changeable. From the perspective of adsorption-dispersion mechanism, it is impossible for any admixture to adapt to all situations. The early-strength admixture has good adaptability to cement compared with naphthalene-based water-reducing admixtures.
The workability of concrete can generally be divided into fluidity indicators and stability indicators. Concrete mixed with early-strength water-reducing admixture has better workability, and no obvious segregation or bleeding will occur at higher dosages or higher water consumption. The settlement of concrete in the formwork is also smaller, that is, In terms of stability indicators, the adaptability of early-strength superplasticizers to cement is significantly better than that of naphthalene-based superplasticizers. But in terms of liquidity indicators, this is not necessarily the case.
(1) The adaptability of early-strength superplasticizer is directly related to its dosage
We all know that high-grade concrete with a higher dosage of naphthalene-based water-reducing agent has better fluidity and smaller slump loss; however, medium and low-grade concrete often has poor fluidity and larger slump loss, and appropriately increasing the dosage is The most effective measures to improve adaptability. The same is true for early-strength admixtures. The author used cementitious materials and aggregates commonly used in Beijing to prepare C30 concrete, and used early-strength water-reducing admixtures as admixtures. It was found that the amount of water-reducing admixture (solidification) was between 0.13% and 0.15%. Over time, concrete can obtain better fluidity, but the slump loss is generally larger. No matter what kind of commonly used retarder is mixed and how much dosage is added, when the amount of water-reducing agent reaches 0.16%, most concrete will Good fluidity can be maintained after 1 hour.
(2) Cement with poor adaptability to naphthalene-based water-reducing agents generally has poor adaptability to early-strength water-reducing agents
Generally speaking, cement with high alkali content, high aluminate content or high fineness requires large amounts of water. The higher the dosage of naphthalene-based water-reducing admixture, the greater the slump loss. Similarly, the same rule applies to early-strength water-reducing admixtures. Some cements mixed with naphthalene-based water-reducing agents that have delayed bleeding will also bleed if early-strength water-reducing agents are used, but to a slightly lesser extent. If the cement has abnormal slump loss due to gypsum (the concrete loses fluidity a few minutes after leaving the machine), the use of early-strength water-reducing admixture will not improve it. Only supplementing sulfate ions at the same time can fundamentally solve the problem. , which is consistent with naphthalene-based water reducing agent.
(3) The "adaptability" of a specific early strength product is not as good as that of naphthalene series products
;">Naphthalene series products are products with the same structural properties that are synthesized from the same raw materials under the same process conditions. Early-strength superplasticizers are products with similar molecular structures that are synthesized from different raw materials under different process conditions. The difference in naphthalene series products is mainly reflected in the quality of raw materials and the stability of process conditions, while the difference in early-strength products is based on the difference in chemical molecular structure. Specific to applications, the adaptability of naphthalene series products to different situations is more reflected in the fluctuation of the optimal dosage within a certain range or the relative size of the slump loss value. For a specific early-strength product, the situation is completely different: if the product can adapt to the concrete material, the concrete will be in good condition and the collapse loss will be small; if it cannot adapt to the concrete material, the result will not be a difference in degree, but may be completely different. If it fails, you must switch to another type of product to solve the problem. In fact, this happens often, especially when northern raw materials are used. The possible reasons are cement minerals, trace elements or grinding aids, etc. That is to say, in terms of "adaptability", the adaptability of a specific early-strength product is not as good as that of naphthalene series products.
Generally speaking, the higher the water-reducing rate of the water-reducing agent, the more sensitive the fluidity of the mixture will be to the dosage within its effective dosage range. Therefore, many engineering and technical workers intuitively believe that early-strength superplasticizers are too sensitive when applied, and thus emphasize the difficulty of metering, concrete production and control. The premise of this understanding is to convert the water-reducing agent into a pure solid and see how the increase in pure solid content improves the flow properties of concrete. For example: for ordinary grade pumped concrete, the dosage range of naphthalene-based water-reducing admixture is in the range of 0.65% to 0.85% to achieve the best working performance of the concrete, while the dosage range of early-strength water-reducing admixture is 0.14 %~0.18%. The dosage range of the naphthalene-based water-reducing agent is about 0.2%, and the dosage range of the early-strength water-reducing agent is about 0.04%. In this sense, the carboxylic acid water-reducing agent is indeed much more sensitive than the naphthalene-based water-reducing agent. . </p
微信扫一扫打赏
