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How can concrete admixtures help increase the efficiency of mixing plants in 2026?
Blog How can concrete adm
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The rheological properties of fresh concrete, as a core indicator of workability, directly determine construction pouring efficiency, forming quality, and subsequent mechanical performance development. Multiple factors such as raw material characteristics, mix design, and environmental conditions influence its stability. In engineering practice, temperature, humidity, and diurnal temperature differences caused by seasonal changes are key environmental factors that lead to fluctuations in the rheological properties of concrete.
The high temperature in summer accelerates the cement hydration reaction, which can easily cause rapid loss of slump; The low temperature in winter may suppress the adsorption efficiency of additives, which may induce the risk of slump rebound and segregation; The significant temperature difference between day and night in autumn leads to a dynamic fluctuation of “early strength and late weakness” in concrete; The temperature and humidity in spring are moderate, but there are still issues of rheological instability caused by short-term climate fluctuations.
As the core regulating material for concrete rheology, the mechanism of action of concrete admixtures depends on the adsorption dispersion effect of molecules on the surface of cement particles, which is particularly sensitive to environmental temperature. Polycarboxylate-based water-reducing agents are widely used due to their high water-reduction rate and good environmental friendliness.
However, at low temperatures, the degree of ionization of carboxyl groups decreases, and adsorption efficiency decreases by 20% to 30%; naphthalene-based water-reducing agents have a simple molecular structure and stronger low-temperature adaptability, but their water-reduction rate and plastic-retention performance are relatively weak.
Therefore, conducting systematic testing and mechanism analysis of the influence of admixtures on the rheological properties of concrete in different seasons, clarifying the optimal selection and dosage range of admixtures in each season, and establishing a collaborative regulation system of “season admixture rheological properties” have important theoretical value and practical significance for improving construction quality and reducing cost losses.
Using P · O42.5 ordinary Portland cement produced by a certain building materials group, its physical and mechanical performance indicators are as follows: specific surface area of 345m2/kg, initial setting time of 158min, final setting time of 242min, 3d compressive strength of 28.6MPa, 28d compressive strength of 46.8MPa; Mineral admixtures are selected from S95 grade slag powder (activity index 98%) and grade I fly ash (activity index 82%), with dosages of 20% and 10% of the total cementitious material, respectively.
Aggregate: Fine aggregate is made of river sand with a fineness modulus of 2.6, an apparent density of 2650kg/m3, a mud content of 1.2%, and a mud block content of 0.3%; The coarse aggregate is selected as 5-25mm continuous graded crushed stone, with an apparent density of 2700kg/m3, a crushing index of 8.5%, and a mud content of 0.5%.
Admixtures and mixing water: Three commonly used admixtures were selected for the experiment.
A is an early strength polycarboxylate superplasticizer (with a solid content of 40%, a water reduction rate of 32%, and containing calcium formate as an early strength component);
B is a slow-setting and plastic-retaining polycarboxylate water reducer (with a solid content of 40%, a water reduction rate of 35%, and containing sodium gluconate as a retarding component);
C is a naphthalene-based water-reducing agent (with a solid content of 30% and a water reduction rate of 25%), which meets the technical requirements of GB/T8076-2025 “Concrete Admixtures”.
The mixing water is tap water, pH7.2， Comply with the water standards for concrete mixing.
The experimental environment simulation uses an environmental climate chamber to simulate typical environmental parameters in four seasons, with specific settings as follows.
Summer: temperature (35 ± 2) ℃, relative humidity 60% ± 5%, no day night temperature difference; Autumn: daytime temperature (25 ± 2) ℃, nighttime temperature (10 ± 2) ℃, day night temperature difference of 15 ℃, relative humidity of 70% ± 5%; Winter: temperature (5 ± 2) ℃, relative humidity 50% ± 5%, no day night temperature difference; Spring: Temperature (15 ± 2) ℃, relative humidity 75% ± 5%, no day night temperature difference.
The testing plan adopts macroscopic rheological testing, according to GB/T50080-2016, to test the initial (0min), 30, 60, and 90min slump (S) and expansion (D) of concrete. Weigh the raw materials according to the mix proportion, stir evenly at the corresponding seasonal ambient temperature (stirring time of 3 minutes), and complete macroscopic rheological and rheological parameter tests separately.
Set up 3 parallel tests for each group, and take the average as the test result. In autumn, additional tests were conducted to alternate day and night temperatures. Samples were prepared at 25 ℃ during the day and 10 ℃ at night to compare the rheological properties of concrete at different time periods.
Slump, expansion, yield stress, plastic viscosity, etc., are all based on three sets of parallel test data in each group. The test results are presented as “mean ± standard deviation”, and the reliability of the data is ensured by removing outliers (data deviating from the mean by ±3 times the standard deviation). Statistical significance testing is performed using SPSS 26.0 software for independent-samples t-tests, with P<0.05 indicating statistical significance.
The Bingham plasticity model was used to linearly fit the rheological curve of concrete. The yield stress (τ0), plastic viscosity (μ), and goodness-of-fit correlation coefficient (r2) were calculated using Origin2023 software. r2 ≥ 0.95 was considered to have a good fitting effect and could accurately characterize the rheological properties of concrete.
The organization and tabulation of experimental data were carried out using Microsoft Excel 2021 software to ensure uniform data format and consistent accuracy; The trend chart of the rheological properties of concrete with the addition of admixtures and time, as well as the processing and drawing of SEM microstructure images, were all completed using Origin2023 software. The chart format complies with the requirements of GB/T7714-2015 “Information and Reference citation rules”.
The slump loss rate of CK in the summer benchmark group reached 41.7%, while in the winter benchmark group. However, the 1-hour loss rate was only 8.2%, there was a significant slump rebound phenomenon (rebound value -9mm), and the yield stress was 92Pa, the plastic viscosity was 68Pa · s, and the rheological parameters were much worse than in spring; The yield stress of the spring benchmark group is 60Pa, and the plastic viscosity adaptability is better than that of the winter and summer benchmark groups. Only the slump loss rate does not meet the low loss requirements for pumping construction.
All three types of additives can effectively improve rheological properties, among which polycarboxylate-based additives (A, B) perform better than naphthalene-based additives (C). The initial slump of the 0.8% dosage group of early strength polycarboxylate superplasticizer (Class A) is 200mm, and the slumps at 30, 60, and 90 minutes are 190, 175, and 160mm, respectively. The 1-hour loss rate is 12.5%, the initial expansion is 450mm, and the 1-hour expansion is 380mm, with no bleeding or segregation. The initial slump of the 1.0% dosage group is 215mm, the 1-hour loss rate is 11.2%, the rheological parameters are τ0 = 26 Pa, μ = 24 Pa·s, and the flowability is better.
The performance of the 0.8% dosage group of slow-setting and plastic-retaining polycarboxylate water reducer (Class B) is similar to that of Class A. Still, the initial setting time is extended to 10 hours, which may affect the construction progress. The initial slump of the 1.0% dosage group of naphthalene-based water reducing agent (Class C) is 185mm, with a 1-hour loss rate of 18.4%. The flowability and plasticity are lower than those of the polycarboxylate-based water reducing agent.
In spring, the goodness of fit of concrete rheological parameters is ≥ 0.96, and the Bingham model can accurately characterize its rheological properties. Considering comprehensive fluidity, plasticity, and construction efficiency, early strength or conventional polycarboxylate superplasticizer should be selected in spring, with a dosage of 0.8%~1.0%, which can ensure good workability and promote early strength development without the need for additional dosage adjustment.
In the high-temperature environment of 35 ℃ in summer, the hydration reaction rate of cement is fast, and the free water in the slurry is rapidly consumed, resulting in significant loss of concrete slump. The initial slump of the benchmark group CK (control group) was only 120mm, and the 1-hour loss rate reached 41.7%. The expansion decreased from 320mm to 180mm, which no longer meets the requirements of pumping construction.
After adding admixtures, the rheological properties of concrete are significantly improved, and the plastic retention effect of different types of admixtures varies significantly. The type B polycarboxylate superplasticizer with retarding and plasticizing properties performs the best, and its sodium gluconate retarding component can effectively delay the hydration process of cement and reduce water consumption.
When the dosage is 1.0%, the initial slump reaches 220mm, and the slumps at 30, 60, and 90 minutes are 205, 185, and 170mm, respectively. The 1-hour loss rate is only 15.9%; When the dosage was increased to 1.2%, the 1-hour loss rate further decreased to 12.2%, but the expansion reached 520mm and slight bleeding occurred. The early strength polycarboxylate superplasticizer (Class A) has the second best plastic retention effect, with a slump loss rate of 19.5% after 1 hour at a dosage of 1.2%. Due to the accelerated early hydration of the early strength component, the loss rate is slightly higher than that of Class B.
Overall, in summer environments, a dosage of 1.0% to 1.2% for the retarding and plasticizing polycarboxylate superplasticizer (Class B) is the optimal choice, which can control slump loss while ensuring fluidity.
The core characteristic of the autumn environment is the large temperature difference between day and night (15 ℃), which leads to significant temporal differences in the rheological properties of concrete. At 25 ℃ during the day, the cement hydration rate is faster, and the slump loss is significant; At 10 ℃ at night, hydration slows down, and the slump tends to increase, which puts higher demands on the adaptability of additives.
According to the SEM microscopic analysis, the slurry microstructure of the day group with 1.0% of Class A admixture is dense, the hydrated calcium silicate gel is distributed in a network shape, and the aggregate is closely bonded with the slurry; The gel structure of the night time 0.7% dosage group is slightly loose, but there are no obvious pores and cracks, while the phenomenon of back enlargement causes the slurry bleeding of the night time group B, and there are obvious capillary channels.
Overall, in autumn, early strength polycarboxylate superplasticizer (Class A) should be selected. A dynamic dosage strategy should be adopted: 0.9% to 1.0% dosage during high temperature periods (25 ℃) during the day, and 0.7% to 0.8% dosage during low temperature periods (10 ℃) at night, which can control the fluctuation range of slump between day and night within 15mm.
Under the low-temperature environment of 5 ℃ in winter, the rheological properties of concrete show characteristics opposite to those in summer. The initial slump of the benchmark group CK is 110mm, and the 1-hour loss rate is only 8.2%, but it is prone to a slump rebound phenomenon (Table 2). This is because low temperature inhibits the adsorption efficiency of water-reducing agent molecules and the hydration rate of cement. Free water-reducing agent molecules accumulate in the slurry, while hydration consumes insufficient water, resulting in an abnormal increase in fluidity.
The performance differences of different types of additives are significant. Naphthalene-based water-reducing agents (Class C) perform best due to their simple molecular structure, with a decrease in adsorption efficiency of only 10% to 15% at low temperatures and a low risk of free-molecule enrichment. The initial slump of the Class C 0.8% dosage group was 190mm, which increased to 185mm after 1 hour, with a maximum value of only -5mm. There was no segregation or bleeding, and the rheological parameters were τ0 = 35 Pa, μ = 32 Pa·s, which met the construction requirements.
Polycarboxylic acid-based water-reducing agents (Class A and Class B) are sensitive to low temperatures. The initial slump of the Class A 0.8% dosage group is 200mm, which increases to 215mm after 1 hour and returns to a maximum value of 15mm. The Class B 0.8% dosage group returns to a maximum value of 22mm. When the dosage is increased to 1.0%, the maximum value exceeds 30mm, indicating significant bleeding and separation of the slurry and aggregate.
Early-strength polycarboxylate superplasticizer (Class A) can promote a small amount of hydration reaction due to the presence of early-strength components. The rebound phenomenon is slightly lighter than Class B. Considering both rheological properties and early strength, the optimal dosage for naphthalene-based water reducers (Class C) in winter environments is 0.6% to 0.8%. If early strength is required, early strength polycarboxylate water reducers (Class A) with a dosage of 0.7% to 0.8% can be selected.
Different seasonal environments significantly affect the rheological properties of concrete by regulating the adsorption efficiency of admixture molecules and the rate of cement hydration. Therefore, targeted selection of admixture types and dosages is necessary.
At a high temperature of 35 ℃ in summer, the optimal dosage of 1.0% to 1.2% retarding and plastic retaining polycarboxylate superplasticizer can control the 1-hour slump loss rate at 12.5% to 15.8%. The Bingham model fits a yield stress of 22-28 Pa and a plastic viscosity of 21-26 Pa·s, balancing fluidity and anti-bleeding properties.
At a low temperature of 5 ℃ in winter, the addition of 0.6% to 0.8% naphthalene-based water reducing agent can prevent the slump from increasing (≤ 5mm) and maintain a plastic viscosity of 32- 38 Pa · s. If early strength is required, an early-strength polycarboxylate water-reducing agent with a dosage of 0.7% to 0.8% can be selected.
Under the temperature difference of 15 ℃ between day and night in autumn, the early strength polycarboxylate superplasticizer adopts a dynamic dosage strategy (0.9% to 1.0% during the day and 0.7% to 0.8% at night), which can control the slump fluctuation within 15mm and ensure the compactness of the slurry microstructure.
At a normal temperature of 15 ℃ in spring, a dosage of 0.8% to 1.0% polycarboxylate superplasticizer is sufficient to meet the demand. The slump loss rate within 1 hour is less than or equal to 12.5%, making it suitable for pumping construction. The goodness-of-fit r2 of the Bingham model is greater than or equal to 0.95, and τ0=25-40 Pa and μ=22-38 Pa · s can be used as pumping thresholds.
In the project, it is necessary to optimize construction according to the season, shorten transportation time in summer, and preheat raw materials in winter. This study does not cover the effects of extreme weather and composite admixtures. In the future, a dynamic prediction model can be established and adapted to the new standard GB/T8076-2025 to improve the accuracy of regulation and provide technical support for seasonal construction of concrete engineering.
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