The process of concrete production, transportation and construction will encounter adaptability problems. It will affect the application effect and promotion of polycarboxylic acid admixtures, the adaptability problem has become a difficult problem for concrete workers, it is related to the local raw materials, and the climate and environmental conditions around, but also involves cement chemistry, polymer materials science, surface physical chemistry and electrochemistry and other aspects of knowledge.
The following introduces several small problems encountered in the process of using GQ brand polycarboxylic acid superplasticizer in Shandong high-speed project in the past two years and the solution ideas.
The dosage is sensitive to the adaptability of various materials, and the optimal dosage is not easy to grasp and difficult to control in production. The admixture content will fluctuate with the fluctuation of sand, cement and various concrete admixtures, and the admixture content is not unchanged in the actual production process of concrete. Among them, the more common factors affecting the admixture content are the following:
(1) The grain composition, stone powder content, mud content and other factors affect the admixture content
(2) Cement factors: mineral composition, alkali content, admixtures, fineness, grinding AIDS, etc., all have an impact on the compatibility of cement and carboxylic acid admixtures.
(3) fly ash: the burning loss, fineness and water requirement of fly ash also seriously affect the amount of admixtures.
Through the following sets of experimental data, it can be seen that the influence of the quality of machine-made sand on the admixture content is for reference.
Various mechanical sand technical indicators:
Test results:
Note: A# is clean machine-made sand; B# is dirty bad mechanism sand; The dry sand is C# dry stone powder (stone powder content 11.2%).
Through the above experimental data, it can be found that the mud content of machine-made sand and methylene blue value have a very obvious effect on the admixture content.
The admixture content is sensitive to temperature change
The external temperature directly affects the hydration rate of cement, which determines that the slump loss of concrete is affected by temperature. High temperature will cause slump loss too fast. However, too low temperature will prolong the condensation time and affect the construction progress. In the same season, the change of admixture content during the day and at night will not exceed 0.1. If the change of admixture during the day and at night is too large, the main consideration is the change of raw materials.
Effect of fineness and mud content of sand on admixtures
(1) The mud content seriously affects the admixture content, and the specific influence results refer to the experimental data in question
(2) The washing machine sand is generally poor in coarse particle grading, resulting in poor workability. The adjustment process should be to reduce the total amount of stones and the proportion of large stones under the condition of appropriately increasing the gelling material.
The specific adjustment examples are as follows:
Raw material
Cement; China United Cement P.C42.5.
Fly ash: Grade II of a power plant in Shandong.
Machine-made sand: the fineness modulus of washed wet machine-made sand is about 3.9.
Admixture: polycarboxylic acid water reducing agent -GQ201.
The gas content of machine-made sand is not easy to introduce, resulting in low gas content of concrete. The reason why machine-made sand is not easy to introduce stable bubbles is that machine-made sand has many angular and flaky particles. It is not easy to form stable tiny bubbles in the process of forced mechanical mixing. GQ201 polycarboxylic acid system itself has a certain air entrainment effect, in addition to air entrainment agent (high strength self-mixing) has a better air entrainment effect, through the introduction of uniform distribution, stable and closed micro bubbles, greatly improve the concrete’s work, significantly improve the flow performance of concrete, reduce the occurrence of concrete segregation and bleeding phenomenon. Customers can determine the dosage of air entrainment agent through specific experiments, the recommended dosage (0.3% ~ 0.5%). Effect of mixing ratio of dry and wet sand on admixtures
Washed wet sand is relatively clean, but generally coarse and fine particles are missing, and the particle grading is not good, while dry sand is not washed and has a high content of stone powder, which will adsorb polycarboxylate admixtures to a certain extent, affecting the play of water reduction effect of admixtures, and the admixture content will increase. Moreover, when the concrete strength grade is low and the gelling material is insufficient, Excessive addition of dry stone silt with high stone powder content is also prone to the phenomenon of “floating ash” after concrete demoulding. The use of two kinds of sand can complement each other to a certain extent, but as for what kind of collocation ratio is most appropriate, it needs to be adjusted according to the actual situation of production and testing.
The following is a group of Shandong high-speed test data for reference:
Conclusion: The content of dry stone powder is higher (11.2%), but the value of methylene blue is not higher (1.25). The increase of stone powder content reduces the overall fineness modulus of sand, improves the workability of concrete, but affects the time loss of concrete.
The viscosity of concrete is large, it is not easy to pump construction, and it is not easy to receive the surface
The admixture content is high, the water consumption is low, the concrete viscosity will increase, and it is easy to produce late slump “back” phenomenon, it is not easy to pump. Therefore, in ensuring the reasonable water consumption and admixture content of concrete, do not deliberately reduce the water consumption or admixture content. In addition, it should be noted that the cementing material or other fine powder particles such as stone powder will also cause the viscosity of the concrete to be large, affecting the construction.
retraction
The concrete is in good condition, and the slump increases after the time, the occurrence of this phenomenon should mainly consider the following reasons:
(1) The admixture in the concrete admixture too much slow-setting components, or one of the polycarboxylic acid slow-release effect is too strong, the concrete is easy to produce water after a period of time, directly leading to the collapse of the concrete in the later period of growth or more serious to its precipitation, resulting in a significant lag back phenomenon (this phenomenon is easy to appear in the case of climate change or the water requirement of the cementing material is reduced). Solution: Adjust the admixture components and reduce the amount of slow-release components.
(2) The influence of concrete strength grade In concrete, high strength grade is prone to hysteresis phenomenon, the main reason is that the proportion of gelling material in high strength grade is large, the concrete admixture in the case of high mixing ratio is slow to disperse, in the case of insufficient mixing, the general production is easy to judge the admixture content is not enough, so it is easy to overadd the admixture. A hysteresis occurs. Solution: Ensure the ratio of water to cement, and properly extend the mixing time during production.
(3) The influence of sand In production, coarse sand is easy to produce hysteresis. This phenomenon is mainly due to the coarse sand in the coagulation, resulting in the concrete slurry can not wrap the stone, the concrete in the production and transportation in the state of motion, the surface effect is good, but after the pressure pumping or pouring is completed, there will be obvious stone settlement, running slurry phenomenon, we can be classified as hysteresis phenomenon. Solution: Appropriate amount of sand with small fineness modulus is added, the mix ratio is adjusted or the gas content of concrete is increased appropriately, and the workability of concrete is changed. In addition, the state of concrete can not only look at the dynamic, but also pay attention to the static. Appropriate amount of concrete should be taken to stand for a period of time and then observed, if there are problems such as bleeding, running pulp should be adjusted in time.
In short, in order to solve the various problems encountered in the use of polycarboxylic acid high performance water reducer, raw material suppliers, admixtures and commercial concrete manufacturers need to solve together. First of all, we must realize that no matter what kind of admixture has adaptability problems; Secondly, it is necessary to adjust the admixture formula and determine the admixture amount for different raw materials on the basis of many tests, and finally, it must be further optimized and adjusted in the concrete production and construction process. Strengthening the control of all kinds of raw materials into the plant is the most important thing to control the quality of concrete! (For reference only, purely personal views, welcome to exchange and learn)
Post time: Aug-12-2024