Study on the influence of different molecular structure concrete slump retaining agents under high temperature

In this paper, the free radical polymerization method was used to design different molecular structures of polycarboxylate slump-preserving agents. The effects of different functional monomers, acid-to-ether ratio, ester density, side chain length, and other factors on the dispersion and slump-retaining properties of concrete under high temperatures were compared and analyzed. The optimal process of high-temperature resistance was screened, before and after the alkali hydrolysis treatment, and the dispersing and retention performance of the slump-retaining agent was tested. The high-temperature slump-preserving agent was verified by gel chromatography and an infrared spectrum analyzer, which provided technical support for solving the rapid collapse of concrete in a high-temperature environment in summer.


Introduction
The increase in ambient temperature has a significant impact on the maintenance performance of concrete slumps.When the ambient temperature is high, the cement hydration reaction speed of concrete is accelerated, the consumption of polycarboxylic acid water-reducing agent is accelerated in the hydration process of concrete, and the evaporation loss of water in concrete is accelerated, which leads to the loss of concrete slump is greater.Theoretically, when the ambient temperature increases by 10℃, the time loss of concrete slump increases by 10% ~ 40% [1 ～ 3] .In the commercial concrete industry, the concrete mixer truck is often used to transport ready-mixed concrete, and the loss of concrete slump is often too fast when the high-temperature exposure and long transportation in summer.Therefore, polycarboxylate superplasticizer needs to be combined with a slump preserving agent in the use process to improve its slump retention ability in high-temperature environments.
At present, the existing slump retaining agent on the market used in concrete generally has a good slump retaining effect within 1 ~ 2 h, but the slump retaining effect after 2 h is poor.With the continuous expansion of the urban scale and the aggravation of traffic congestion, the transportation time required for concrete from mixing to on-site pouring is constantly extended, and 3 h is a common phenomenon, and even 4～5 h is required in special cases.Especially in the Middle East market, such as Saudi Arabia, the highest temperature in summer can reach more than 50 ° C, the average temperature is about 40 ° C, there is little wind during the period, and the long transport distance of the ready mixed concrete is about 3 hours [4 ～ 10] .Therefore, to meet the practical application needs of engineering, it is particularly important to develop a high-performance slump preserving agent that can maintain the slump of concrete for a long time under higher ambient temperatures.

Performance test material
Water-reducing polycarboxylic acid water reducer PCE, self-made; Cement is produced by Fujian Minfu Company, with the model of ordinary Portland cement P•O42.5 (R); Sand: Mx=2.4～2.8;Tone: gravel with a diameter of 10 to 31.5 mm; Fly ash: Class II fly ash.

Synthesis of Slump Retainer
Add a certain amount of water and polyether monomer into the reaction kettle, stir until the polyether is completely dissolved, add hydrogen peroxide, and then add the mixed solution of acrylic acid and acrylate, the aqueous solution of vitamin C, and the aqueous solution of thioglycolic acid, respectively.Control the drop time within the specified time, and continue the reaction for 1 hour after the drop is completed.Adjust the pH value to 6.0-7.0, and then we have the required slump-retaining agent

Evaluation method
The performance of concrete shall be tested according to the methods in GB/T 50080-2016.The fluidity of cement paste is tested according to the method in GB/T 8077-2012.Tests were performed using a gel permeation chromatograph (1515, Waters, USA) equipped with a multi-angle laser light scattering (LS) detector and refractive index (RI) detector.A 0.1mol/L NaNO3 aqueous solution with pH 7 was used as the eluent and its flow rate was controlled at 0.5ml/min.Using dextran with different molecular weights as calibration standard, the PCE polymer was diluted to a concentration of 5 mg/mL with 0.1 mol/L sodium nitrate solution.The diluted solution is then injected directly into the 0.2 mL loop at a temperature of 35 ° C. Infrared spectrum analysis, using Avatar 360 spectrometer, manufactured by PerkinElmer Company of the United States, scanning range 5000 ~ 500 cm-1, spectral resolution 4 cm -1 .As can be seen from Figure 1, the fluidity of the paste synthesized by different slump-retaining monomers first increased and then decreased.At four temperatures, the better slump-preserving properties of the five functional monomers are HPA, especially after 1 h.

The influence of the acid ether than preserved influential agent
At high temperatures, the optimal slump retention monomer is HPA, other synthesis factors remain unchanged, and AA is adjusted by a single factor.The effects of different ratios of acid to ether on slump retention of concrete were studied at (40±3) ℃.The concrete test was carried out according to the equal admixture, and the concrete test results are shown in Figure 2. As can be seen from Figure 2, with the increase of carboxyl group density, the water reduction rate of slump slump-preserving agent increases accordingly, and the slump-preserving performance shows a trend of first increasing and then decreasing.When the acid-ether ratio is 1.47, the slump-preserving agent has a better slump-preserving effect.

The influence of the ester base density preserved the influential agent
The influence of the ester-to-ether ratio on the slump retention performance of the slump retaining agent was studied by adjusting the amount of HPA with other synthetic factors unchanged.When the ambient temperature is (40±3) ℃, the concrete test is carried out according to the equal dosage, and the concrete test results are shown in Figure 3.As can be seen from the figure above, with the increase in ester density, the water reduction rate does not change much, but the slump protection performance shows a trend of first increasing and then stabilizing.The ester-to-ether ratio is 3.61 ~ 4.6, and the slump protection performance is equivalent.

The influence of the acid ester against collapse agent
The effect of the acid-ester ratio on the slump retention performance of concrete was studied by adjusting the acid-ester ratio with a single factor.When the ambient temperature is (40±3) ℃, the concrete test is carried out according to the equal dosage, and the concrete test results are shown in Figure 4.According to the above test results, at (40±3) ℃, with the increase of the acid-ester ratio, the water reduction rate will also increase.The slump retention performance at 2 h and 3 h of the acid-to-ester ratio 0.41 was significantly better than that of other samples.

The influence of the side chain length of the bridge collapse agent
The influence of different lengths of side chains on slump retention of mixed soil was studied by single-factor adjustment of side chain length.EPEG2000, EPEG3000, and EPEG4000 were selected for experiment respectively.When the ambient temperature is (40±3) ℃, the concrete test is carried out according to the equal dosage, and the concrete test results are shown in Figure 5.According to the above test results, the concrete slump retention performance of EPEG3000 is the best.This is because when the side chain increases, a large number of long side chains extend into the liquid phase, providing spatial three-dimensional water film protection, effectively blocking the hydration reaction, and enhancing the slump retention effect.However, if the side chain is too long, the long side chain is easy to wrap, which affects the polymerization reaction and slump protection performance.

Performance analysis of slump retaining agent before and after alkaline hydrolysis at high temperature
The slump retaining agent was put into an aqueous solution with a pH value of 12~13, placed in a water bath at 70℃ for water bath, alkaline hydrolysis for 2 h, and the flow of the paste was tested, and the slump retaining performance before and after alkali hydrolysis was compared.The test results of the paste are shown in Figure 6.According to the above test results, the initial dispersion of the slump retaining agent after alkali hydrolysis at 70℃ is significantly greater than that before alkali hydrolysis, and the fluidity of net slurry reaches its peak at 1 h and starts to lose after 1 h.The peak value of the net slurry fluidity before alkali hydrolysis was 2 h, and it began to decrease slowly after 2 h, and the peak value was significantly higher than that of the sample after alkali hydrolysis, indicating that the slump preservation property decreased after alkali hydrolysis.

Gel chromatography
The molecular weight of S507 (acid/ether ratio 1.47, ester/ether ratio 3.61, acid/ester ratio 0.41) prepared in this paper was tested.The test results are shown in Figure 7.The Mp value of S507 is 78314, the width of molecular weight distribution is 1.66, and the conversion rate is 84.92%.The molecular weight distribution is more concentrated and the uniformity is good, which indicates that the process effectively controls the molecular weight distribution of polycarboxylic acid preservative, high conversion rate, and ideal structure, consistent with the expected idea.

Infrared spectral analysis
The infrared spectrum analysis of the high-temperature resistant slump preserving agent S507 is carried out, and the test results are shown in Figure 8.As can be seen from Figure 8, the stretching vibration absorption peak of ether bond -C-O-C-is at 1105.56cm -1 , 34374.92cm - is the stretching vibration characteristic absorption peak of hydroxy-OH, 2869.54 cm -1 is the stretching vibration characteristic absorption peak of methyl and methylene, 1728.50 cm -1 is the stretching vibration characteristic absorption peak of C=O in ester bond, and 1454.68 cm -1 is the characteristic absorption peak of -CH2-stretching vibration of methyl and methylene.It can be seen that the prepared high-temperature resistant slump preserving agent S507 contains a carboxyl group, ether group, ester group, hydroxyl group, and other functional groups in the molecule, which is consistent with the designed structure.

Conclusion
(1) In the high-temperature environment, the slump preserving performance of HPA is better than that of HEA, HPMA, MA, and EA.
(2) The high-temperature resistant slump preserving agent S507 was synthesized with EPEG3000, acid an ether ratio of 1.47, ester-to-ether ratio of 3.61, and acid-to-ester ratio 0.41.(3) The infrared spectrum showed that the long-acting tan-preserving agent S507 contained carboxyl, ether, ester, and hydroxyl groups in the molecule, which was consistent with the designed structure.The gel chromatography showed that the Mp value of S507 was 78314, the molecular weight distribution width was 1.66, and the conversion rate was 84.92%.

Figure 6 .
Figure 6.Test results of net slurry fluidity before and after alkali hydrolysis.