With the continuous development of my country's national economy, the demand for concrete in basic industrial construction and the construction industry has increased. In order to solve the problems of concrete bleeding, segregation and workability, etc., researchers have developed admixtures that adapt to various concrete symptoms, and water-reducing agents are one of them. Water reducing agents have attracted much attention due to their advantages such as high dispersion performance, wide application range, small slump loss, and green and environmental protection. However, water-reducing agents have high requirements on sand and gravel materials and gradation, so they are mostly used in high-grade concrete, which greatly limits the promotion and application of high-performance water-reducing agents in low-grade concrete. To this end, a high-performance water-reducing agent used in low-grade concrete has been researched and developed. It has good adaptability to sand and gravel materials with high mud content and low design gradation requirements for low-grade concrete, and has good slump retention. And the workability is better than aliphatic water reducing agent.
1 Test
1.1 Main raw materials
1.1.1 Synthetic raw materials
Methacrylic acid, polyethylene glycol monomethyl ether 800 , polyethylene glycol monomethyl ether 1000, polyethylene glycol monomethyl ether 1200, polyethylene glycol monomethyl ether 2000, p-toluenesulfonic acid, dimethyl maleate, hydroquinone, phenothiazine, Ammonium sulfate, mercaptopropionic acid, 30% sodium hydroxide solution: all are industrial grade.
1.1.2 Materials for concrete testing
Cement: Yangzi P·O42.5 cement; fly ash: Grade II; medium sand: fineness modulus 2.6; fine sand: Fineness modulus 1.8; large stones: 10 ~ 20 mm; small stones: 5 ~ 10mm.
1.2 Main instruments and equipment
Waters gel permeation chromatograph, 1 set of slump testing equipment, SHB-IIIS circulating water multi-purpose vacuum pump, QHJ756B powerful constant speed mixer, 5 L Intelligent lifting oil bath.
1.3 Test method
1.3.1 Preparation of macromonomer
In a three-port laboratory equipped with a thermometer, oil bath, speed-adjustable stirrer and vacuum device Add polyethylene glycol monomethyl ether, methacrylic acid, p-toluenesulfonic acid, hydroquinone, and phenothiazine weighed according to the formula into the flask, and react to the optimal time under the set temperature and vacuum degree, and then Lower the temperature to 50°C and discharge the material to obtain the polyethylene glycol monomethyl ether methacrylate mixture.
1.3.2 Preparation of water-reducing agent
In a four-necked flask equipped with a thermometer, speed-regulating stirrer, reflux condenser and dripping device, add the formula amount of soft water , then stir and heat in a water bath to 80~83°C, add the aqueous solution composed of the above-prepared macromonomer and mercaptopropionic acid dropwise according to the formula amount (drip within [180±10)min], and at the same time, add the initiator ammonium persulfate dropwise Aqueous solution (drip within [210±10)min]. After the initiator liquid is dropped, keep it warm for 1 hour, then cool it to below 50°C, and adjust the pH value to 7 with 30% sodium hydroxide solution to obtain 40% water-reducing agent.
1.3.3 Performance test method
Test the performance of concrete according to JC 474-2001 "Concrete Pumping Agent" and JG/T 223-2007 "High-Performance Water-Reducing Agent" .
2 Results and Discussion
Tests were conducted on the effects of MPEG molecular weight, dimethyl maleate dosage, chain transfer agent dosage and ammonium persulfate dosage on the performance of C30 concrete. The test concrete mix ratio (kg/m3) is: m (cement): m (fly ash): m (medium sand): m (large gravel): m (small gravel): m (admixture: m (water) =255∶85∶770∶810∶260∶4.42∶158.
2.1 The influence of MPEG molecular weight on the properties of C30 concrete
It can be seen that low molecular weight MPEG is different from high molecular weight. Compared with the molecular weight MPEG, the 30-min slump loss of concrete mixed with synthetic water-reducing agent is smaller, but the initial expansion and compressive strength of concrete mixed with high-molecular-weight MPEG synthetic water-reducing agent are greater than those mixed with low molecular weight. The reason is that when the mass ratio of alkyd to acid is the same, the carboxyl group content of the water-reducing agent synthesized by low molecular weight MPEG is relatively slow, so the slump loss over time is small. In addition, the air entraining property of low molecular weight MPEG is better than that of high molecular weight MPEG. The mass of MPEG is large, so the strength of low molecular weight MPEG is lower than that of high molecular weight MPEG.
2.2 Effect of the amount of dimethyl maleate on the properties of C30 concrete
As the dosage of dimethyl maleate increases, the 30-minute slump and expansion of concrete mixed with synthetic water-reducing agent gradually increase, and when the dosage is 2.0% and 3.0%, the 30-minute expansion is larger than the initial one. ; But when the dosage is 4.0%, the initial and 30 min slump and expansion tend to decrease again. The reason is that dimethyl maleate undergoes a nucleophilic substitution reaction on the acyl carbon under strongly alkaline conditions. Causes the methoxy group to detach and form carboxyl groups. Carboxyl groups have a dispersing effect on cement, so as the dosage of dimethyl maleate increases, the 30 minute slump and expansion gradually increase. When the dosage is 4.0%, the slump and expansion of dimethyl maleate gradually increase. The increase in the dosage of dimethyl maleate leads to a decrease in the carboxyl density of the water-reducing agent, thereby reducing the initial slump and expansion of concrete under the same water consumption [4]. Therefore, the dosage of dimethyl maleate At 3.0%, the comprehensive performance of concrete is the best.
2.3 The impact of the amount of chain transfer agent on the performance of C30 concrete
It can be seen that as the amount of chain transfer agent increases, the amount of chain transfer agent mixed into The initial and 30-minute slumps of the water-reducing agent concrete increased first and then decreased, while the compressive strength was not much different. The amount of chain transfer agent was small, the molecular weight of the water-reducing agent was relatively large, the dispersion performance was poor, and the dispersion was maintained If the amount of chain transfer agent is too large, the molecular weight of the water-reducing agent will be small and the dispersion retention will decrease. When chain transfer agent is usedWhen the amount is 0.70%, the comprehensive performance of concrete is the best.
2.4 Effect of the dosage of ammonium persulfate on the performance of C30 concrete
It can be seen that with the increase of the dosage of ammonium persulfate, the initial and 30 min slump of concrete mixed with synthetic water-reducing agent The drop degree increases. This is because the increase in the amount of ammonium persulfate increases the conversion rate of the monomer and increases the proportion of the main peak area, causing the initial and 30 min slumps of concrete to also increase. However, when the amount of ammonium persulfate is large, the overall performance of concrete tends to be stable, so the optimal amount of ammonium persulfate is 1.4%.
3 Cost Analysis
The C20 and C25 concrete tests and cost analysis were carried out in a concrete mixing station on the water-reducing agent and aliphatic water-reducing agent synthesized in this study. The aliphatic water-reducing agent currently used in a mixing station is 1,800 yuan/t, the water-reducing agent developed in this research is 2,300 yuan/t, and Yangzi P·O42.5 cement is 380 yuan/t (price in December 2011). Fly ash is 150 yuan/(price in December 2011). Test C20 and C25 concrete mix proportions and concrete performance test results.
Based on the concrete mix ratio, the cost difference of concrete using different water reducing agents can be calculated. The cost of C20 concrete using water-reducing admixture is less than that of aliphatic water-reducing admixture: (177×0.38+55×0.15+3.4×1.8)-(167×0.38+65×0.15+3.4×2.3)=0.60 yuan/ m3;C25 The cost of using water-reducing admixture for concrete is lower than using aliphatic water-reducing admixture: (231×0.38+35×0.15+4.1×1.8)-(221×0.38+45×0.15+4.1×2.3)=0.25 Yuan/m3. The calculation results show that when C20 and C25 concrete uses the water-reducing agent developed by this research and development, it has an advantage in reducing costs compared with aliphatic water-reducing agents, especially when the price of cement is high. For example, when the cement price is 455 yuan/t, the raw material cost of preparing C20 concrete can be saved by 1.25 yuan/m3, and the raw material cost of preparing C25 concrete can be saved by 0.90 yuan/m3.
In the test of C20 and C25 concrete, the 30-minute slump and expansion of the developed water-reducing agent are better than those using aliphatic water-reducing agents. And when C20 and C25 concrete use water-reducing agents, the cement dosage can be reduced by 10 kg, while the compressive strength of concrete of the same period is basically the same.
4 Conclusion
(1) Under the same mass ratio of alkyd to acid, the water-reducing agent synthesized with low molecular weight MPEG has good slump retention, but the water-reducing agent mixed with it has good slump retention. The compressive strength of concrete is lower than when high molecular weight MPEG is used.
(2) When the dosages of dimethyl maleate, mercaptopropionic acid, and ammonium persulfate are 3.0%, 0.70%, and 1.4% respectively, the prepared water reducing agent has good overall performance.
(3) The comparative test results of applying the water-reducing agent synthesized in this study and the aliphatic water-reducing agent to low-grade concrete show that the developed water-reducing agent has good performance and is effective in reducing low-grade concrete. The cost of concrete has advantages over the use of aliphatic water-reducing agents. </p
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