Effect of molecular structure of polycarboxylate superplasticizer on its properties

In order to adapt to the construction of concrete of high strength, high performance and high working stability requirements of concrete, the author studied a new type of polyester type carboxy acid kind high efficiency water reducing agent, the optimum condition for synthesis of this kind of water reducing agent and the ratio of raw materials.

Through the study of the molecular structure of water reducing agent, polycarboxylic acid slump keeping performance and large monomer used chain length (degree of polymerization of EO), chain length longer initial dispersion is good, water reducing agent and chain length is shorter, conducive to water reducing agent dispersing performance. In this article, and the synthesis of polycarboxylic acid high efficiency water reducing agent in the concrete were tested, show that the products is a kind of small dosage, good dispersion performance, flow to keep good performance, and the adaptability of different cement good high efficiency water reducing agent, and heat and the dosage of water conservancy project in the larger study on the adaptability.

Key words: polycarboxylic acid water-reducing agent; High efficiency water reducing agent; Dispersion; Fluidity; Adaptability.

As traffic, water conservancy and hydropower, the development of urban construction, hundreds of layers and high span Bridges, high-rise buildings, large hydropower station, tunnel, etc Process of the construction of the gradual rise of concrete construction, refers to the international strength, durability and corrosion resistance performance demand is higher and higher, almost all of the concrete are required to use pumping concrete, and with the specialization of commercial concrete is more and more high, concrete mixing plant and construction site often have a certain distance, so it also improved the workability of concrete keep capacity requirements. In the process of high performance concrete production, concrete admixture becomes the fifth indispensable component of coagulant soil. Currently widely used additive mainly lignin sulfonate, naphthalene series, melamine, poly carboxylic acid, amino sulfonic acid salt (PCE), such as one PCE represents the development direction of high efficiency water reducing agent.

Polycarboxylic acid water reducing agent is first appeared in Japan in the 1980 s a new type of high efficiency water reducing agent, it has a small dosage, high water reducing rate, collapse and working performance of concrete, so it just appeared to cause the attention of the concrete admixture industry at home and abroad. Experts predict that in the 21st century, the world will be primarily due to the use of concrete admixture Polycarboxylic acid high efficiency water reducing agent, currently used in the high efficiency water reducing agent of concrete main should have naphthalene series water reducing agent, although this water reducer high water reducing rate, but has confirmed slumping sex difference, the production process has the disadvantages such as the pollution to the environment. This article from the chemical structure, action mechanism of high performance water reducer, on the basis of leading functional groups, through copolymerization, the design and synthesis of its carboxyl – ether bond as the leading functional groups of comb structure of Polycarboxylic acid high efficiency water reducing agent. In this paper, synthesis of Polycarboxylic acid water reducing agent groups play a role in each are not identical, the carboxyl group not only have water reducing effect, also have retarding effect, but also key of polyoxyethylene ether base can form hydrogen bonding with the water in the cement paste, outside the cement particles form three-dimensional hydrophilic film so as to maintain the effect of the dispersion.

Preparation and performance test of polycarboxylic acid water reducing agent Preparation of polycarboxylic acid water reducing agent

Main raw materials: methoxy polyethylene glycol methyl acrylate (mpeg (- n) ma, n is the number of moles of ethylene oxide EO repeating unit), industrial grade; Acrylic acid (AA), analysis pure; Methacrylate (MAA), analytical purity; Sodium hydroxide, industrial grade; Chain transfer agent; The initiator; Deionized water.

Test method: adding to a four flask which installed with thermometer, mixer, dropping funnel, reflux condenser,adding monomer and a moderate amount of deionized water stirring up to about 90 ℃,add monomer water solution, chain transfer agent and initiator, control the adding time within 2 hours, heat preservation for 30 min, then cooled to below 40 ℃, add sodium hydroxide neutralizing to pH 7 or so, then we can get deep red transparent liquid, add water diluted to the concentration of 22%.

Ratio of poly carboxylic acid water reducing agent in the research of initiator, chain transfer agent and sequence of feeding, reaction temperature, reaction time and the concentration of total material under the condition of fixed, by adjusting the reaction monomer mole ratio and large monomer polymerization degree, the preparation of a series of poly (carboxylic acid water reducing agent with different structure. The influence of various factors on the properties of polycarboxylic acid water reducer was studied by testing its working performance and working retention in cement, mortar and concrete.

Performance testing:

Raw materials: hot cement; Ordinary Portland cement; Benchmark cement. The stone is gravel with a grain size of 5~ 26mm, continuous grain size and a void rate of 43%. Quartz sand, fine modulus 29, mud content 0.2%.

Test method: the flow of clean slurry was measured according to GBT 8077-2000 concrete admixture homogeneity test method. The water reduction rate and slump loss of concrete were measured in accordance with JC473-2001. Determination of water reduction rate and compressive strength ratio of concrete admixture according to GB8076-1997; Infrared spectroscopy was determined by Nicolet Magna- IR550 infrared spectrometer. The Scanning Electron Microscope used Japanese company jsm-6360lv SEM Scanning Microscope.

Test results and discussion Effect of molecular structure of polycarboxylic acid on its properties

There are other reasons for the difference in the properties of water reducers besides the molecular structure. The structure of the three basic same, namely synthetic products raw materials contain group is roughly same, have carboxyl, ether, just peak is not the same as the size of each functional group show the degree of polymerization, raw material ratio of reaction and reaction conditions. The above several aspects of different in the three potential of cement hydration, on the net slurry and concrete workability and creates a difference in the reaction efficiency of minimum, thereby causing it on concrete workability than the combined length of chain is poor; The reaction efficiency is high, and the working performance is better on the clean slurry and concrete.

The influence of molecular structure of water reducing agent on the fluidity of cement slurry

According to the test method of the homogeneity test method of concrete admixture (GB/T80772000), the measurement of the net slurry mobility of the composite sample of high efficiency water reducing agent was carried out. The solid content of ska-20 polycarboxylic acid water-reducing agent is 22%, and the content is 1.0% of the cement quality. Test the initial flow of cement paste, 60, 90min.

N is the number of moles of ethylene oxide EO repeating unit. Different side chain length of monomer Mpegma with AA and MAA synthesis on the net cement slurry has good initial fluidity and spread within 90 min is not reduced, and with the increase of side chain length, the ratio of the two is becoming more and more big, namely with the increase of the length of the side chain, side chain gradually thinning, net paste fluidity increased. The products polymerized by Mpegraa and MAA are in slightly better condition on the net pulp, with better mobility and less bleeding. By Mpegma and MAA polymerization products than Mpegma and AA polymerization products on the main chain of molecules added a methyl (CH3), increase the water retention of cement is strong, illustrate the main chain structure change also has great influence on the performance of the product.

Effect of molecular structure of water – reducing agent on working performance of concrete

According to JC473-2001 concrete pumping agent, determination of concrete fluidity and slump Mpegma with AA polymerization products in the initial degree of extension and concrete slump while good, but the stability is poor, 60 min is almost completely lost, air content is higher, and the bleeding and segregation. Mpegma and MAA polymerized products have good expansion and slump in concrete, and good retention. At n= 90, the initial expansion and slump are poor and the retention is poor. This is because when n= 90, the side chain is the most sparse and the longest, resulting in insufficient mechanical strength of the comb structure. When concrete is stirred and cement hydration, the comb structure of the high-efficiency water-reducing agent is deformed and damaged. When n = 9, the side chain is short and dense, with less effective adsorption points and basically no flow of concrete. When the ratio is further adjusted and the length of the chain is combined, the extensibility and slump of the composite water-reducing agent concrete are good.

Table 1: the effect of different polymerization degree of polyoxyethylene (EO) on the performance of polycarboxylic acid water reducer

note: the first half of the table is Mpegma and MAA polymerization, while the second half is Mpegma and AA polymerization. N in the figure represents the degree of polymerization of polyoxyethylene in the molecule.

Test results of performance and adaptability of water reducing agent

Results of water reduction measurement

In this test, the water reduction rate of mortar was measured by the more commonly used flow test method. After fixing the dosage of cement (base cement) and standard sand, the flow diameter of mortar mixed with and without water reducing agent was up to 180mm 5mm by adjusting the water consumption. The difference in water consumption is water reduction. The water reduction rate is calculated according to equation (1) :

        W0 – W1

WR  = ————

        W0

In the formula, W0 is the amount of water used for mortar to flow up to 180mm when no water reducing agent is added. W1 is the amount of water used for mortar to flow up to 180mm when water reducing agent is added. WR is the water reduction rate. This method is simple to operate, with small error, and the error rate is generally no more than 2%, so the water reduction rate of this series of water reducing agents is 30%~ 35%

Table 2:  Test of mortar water reduction rate

Strength and strength ratio of concrete mixed with water – reducing agent

The dosage is 1.0% 0.22% (solid), under the condition of the water reducing performance of the sample performed well (water reducing rate was 28% ~ 33%) and good strength growth, 3 d intensity ratio reaches 190%, 7 d strength about 200%, the 28 d strength about 200%. This shows that the concrete using this water – reducing agent has the characteristics of rapid strength development.

The adaptability of water reducing agent to different cement

This experiment selected the hot cement (hongta) and ordinary Portland cement (Taihang, Zhenxing,Lafaji), the test results show that all kinds of cement mortar on the initial fluidity and the fluidity is mostly good, 90 min fluidity retention in 90%.

Table 3: ratio of compressive strength of polycarboxylic acid high performance water reducer

conclusion

1. the test use water as solvent, using catalyst, initiator and chain transfer agent, synthesising methoxy polyethylene glycol methyl acrylate- (methyl) binary copolymerization of acrylic acid water reducing agent  successful; Copolymerized carboxylic acid containing long side chain polyether group and short side chain polyether group can be designed and synthesized as long as the ratio of various functional groups is appropriate.
2. polycarboxylic acid high-efficiency water reducer has the characteristics of high water reduction rate (28-35%) and low gas content (around 3%);
3. the influence of molecular structure and side chain length of copolymer on the workability of cement concrete was studied in this article. The results showed that polycarboxylic acid’s slump keeping performance and large monomer used chain length (degree of polymerization of EO), chain length longer initial dispersion is good, water reducing agent and chain length is shorter, conducive to water reducing agent dispersing performance. The side chain is too long and too short to produce the water-reducing agent with good working performance. The middle chain is longer and the different chain length is mixed with a certain proportion.
4. this series of water reducing agents have good adaptability. It has good adaptability in middle heat and ordinary Portland cement.
5. with the same extension degrees compared to without water reducing agent of concrete admixture of concrete mixed with the water reducing agent with fast development, can significantly improve the strength of concrete strength, low dosage, etc, is preferred for the preparation of high strength material.