Issue 26/2012 - Research progress of polycarboxylic acid concrete water reducing agents

Concrete water reducing agent is a kind of concrete admixture, which is mainly divided into naphthalene high-efficiency superplasticizer, sulfamic acid high-efficiency superplasticizer, resin high-efficiency superplasticizer, polycarboxylic acid high-efficiency superplasticizer
.
Among them, polycarboxylic acid high-efficiency superplasticizer is another new class of high-efficiency superplasticizer after naphthalene-based high-efficiency superplasticizer
and melamine superplasticizer.

Synthesis mechanism and performance characteristics

Polycarboxylic acid superplasticizer can be designed according to the actual performance needs of concrete, and most of its molecules are comb-shaped structures, characterized by multiple active groups with strong polarity on the main chain; The side chain carries a large number of hydrophilic active groups and a short molecular chain and a small number of hydrophobic groups, which must appear on the
polymer backbone in the appropriate proportions.
The synthesis of polycarboxylic acid high-efficiency superplasticizer is carried out in two steps: firstly, polymer monomers with a certain side chain length are synthesized, and these monomers are grafted and copolymerized with carboxylic acids, sulfonic acids, etc.
to connect active groups; The second step is to further polymerize the polymer monomer generated by the first step reaction with carboxylic acid and sulfonic acid into binary or multivariate copolymers under the action of the initiator, and finally form a polymer water reducing agent
.
The general molecular structure of polycarboxylic acid superplasticizer is expressed as in formula (1).

Polycarboxylic acid high-efficiency superplasticizer has a series of significant performance characteristics:
low dosage and high water reduction rate Although polycarboxylic acid high-efficiency superplasticizer has less adsorption capacity on the surface of cement particles, it can produce steric hindrance effect because it has many branched chains, so it can achieve a better plasticizing effect
when the amount is small.
According to the amount of solids, the dosage is generally 0.
1%~0.
3%.

The solid content of polycarboxylic acid high-efficiency superplasticizer liquid products is generally 18%~25%, the water reduction rate is generally 25%~35%, and some can be as high as 40%.


Strong plastic retention, low slump loss Polycarboxylic acid high-efficiency water reducing agent can effectively control the slump loss of concrete mixtures over time, and has little effect on the hardening time
.

Low shrinkage The volume stability of polycarboxylic acid-doped high-efficiency superplasticizer concrete is greatly improved
compared with that of second-generation superplasticizer concrete such as naphthalene.
According to GB8076-1997, the average 28-day shrinkage ratio of polycarboxylic acid-doped high-efficiency superplasticizer concrete is 102%, and the lowest shrinkage ratio is only 91%.


In addition, polycarboxylic acid water reducing agent contains a series of hydrophilic groups, such as carboxyl group (-COOH), ether group (-O-), hydroxyl group (-OH), so it has a certain liquid-gas interface activity and a certain amount of air entrainment and slight retardation
.
Moreover, polycarboxylic acid is a high-performance water reducing agent that does not use formaldehyde and any other harmful raw materials in the production process, so there is no health hazard to the human body during production and use, and does not cause any pollution
to the environment.

Japan is at the forefront of foreign research

At present, polycarboxylic acid high-efficiency water reducing agents have been successfully promoted and applied mainly in Japan
.
In 1986, Japanese catalyst company took the lead in developing polycarboxylic acid high-efficiency superplasticizer with a certain proportion of hydrophilic functional groups, and then gradually applied in concrete engineering, after 1995, the use of polycarboxylic acid high-efficiency superplasticizer in Japan has greatly exceeded naphthalene-based products, reaching 80%
of the amount of superplasticizer.
Japan named the polycarboxylic acid high-efficiency superplasticizer as high-performance AE superplasticizer, which was included in the JISA6024 national standard in 1995 and the JASS5 Japan Architectural Institute standard
in 1997.

The research and application of polycarboxylic acid high-performance water reducing agents in European and American countries is later than that in Japan
.
At present, there are more than
100 US patents on the research and development of polycarboxylic acid water reducing agents.
The inventors of these patents are more concerned about the water reduction performance and slump retention performance of freshly mixed concrete after the use of water reducing agent and some comprehensive properties of concrete air entrainment, segregation and water secretion, condensation and pumpability, and the patented products of polycarboxylic acid admixtures are more prominent in water reduction, fluidity maintenance, early strength, shrinkage and other functions
.

In the US patent US20050182162A1 applied by Toshio Hayashiya et al.
in 2005, the polycarboxylic acid high-efficiency concrete admixture invented has the characteristics
of high water reduction, good slump retention and excellent fluidity.
In the Chinese patent CN1784369A applied by Tomoko Nishikawa et al.
, a polycarboxylic acid concrete admixture is introduced, and the side chain contains polyalkylene glycol with a specific weight-average molecular weight
.
This admixture has properties
such as reducing the viscosity of concrete, improving slump and inhibiting slurry.
T.
Hirata et al.
synthesized a special water-soluble amphoteric copolymer water reducing agent, which can be used for general high-performance concrete and ultra-high-performance concrete
.
Switzerland's Sika adopts a new type of polycarboxylic acid high-efficiency superplasticizer synthesized from polyamide, acrylic acid and polyethylene hexanediol, which still has good dispersion performance
when the W/C of concrete is lower than 0.
15.

Domestic product research and development is relatively lagging behind

China's polycarboxylic acid water reducing agent has been rapidly developed in the past one or two years, and several self-made products have been put on the market, but the overall performance is not very good, and the stability is poor
.
Therefore, the current varieties of water reducing agents in China are mainly based on the second generation of naphthalene products, accounting for more than
80% of the total.
From the development and application characteristics of polycarboxylic acid superplasticizers in recent years, the application trend is gradually shifting from the application of key parts of major projects in the past to the application of general major projects and ordinary projects, and from the application of high-strength grade and special functional concrete to the application of
ordinary concrete.

At present, there are few domestic polycarboxylic acid high-efficiency water reducing agent independent products, and there is still a certain gap
in various aspects such as technology.
Shanghai Academy of Science and Technology took the lead in successfully researching LEX-9 polycarboxylic acid water reducing agent, which is designed
by molecular design using olebenic acid and epoxy ether as raw materials through "molecular design theory".
Olelenic acids have a wide range of sources, low price, good polymerization controllability, and strong adsorption capacity with cement; Epoxy ether is used because the oxygen atoms of the ether form a strong hydrogen bond with water molecules in water, which can form a stable hydrophilic three-dimensional protective layer to ensure the stability and durability
of cement dispersion.
The performance of the water reducing agent has reached the level of internationally renowned products and has been put into mass production, and has been used in major projects
such as Shanghai "magnetic levitation" track beams.

Guo Baowen et al.
of Beijing Institute of Building Research disclosed a polycarboxylic acid aerated high-efficiency concrete water reducing agent in the published invention patent CN1316398 in 2001, which is a kind of gas-entrainment high-efficiency concrete water reducing agent
prepared by chemical reaction with simple synthesis method and easy control of reaction conditions.
When the amount of water reducing agent is 1.
5% of the weight of cement, the gas content of the prepared concrete is generally 4%~7%, the water reduction rate can reach 30%, and the compressive strength of the 28-day sample is 110% ~ 126%
of the blank sample.

Jiang Yuanru of Xi'an University of Architecture and Technology et al.
use polycarboxylic acid water reducing agent obtained by ternary copolymerization of acrylic acid, sodium methacrylate and allyl polyoxyalkyl ether, which has high water reduction rate and diffusion performance
.
When the solid content is 30%, the dosage is 1%, and the water-cement ratio is 0.
29, the net slurry flow degree can reach 270mm
.
Sun Yusheng et al.
of Nanchang University used maleic anhydride and polyethylene glycol for esterification reaction to generate polyethylene glycol carboxylate macromolecular monomer, and then added sodium 2-acrylamide-2-methacrylate and methacrylic acid, and copolymerized by aqueous solution polymerization method under the initiation of persulfate to obtain polycarboxylic acid water reducing agent
.

Liu Derong of Sichuan Institute of Technology synthesized an acrylate-acrylate-acrylamide terpolymer water reducing agent, when their molar ratio is 2.
5:0.
5:1, the slump degree of concrete mixed with 1.
5% in cement remains good, the water reduction rate can reach up to 26%, and the strength of concrete products is greatly improved
.

Although there are a large number of articles published in China on polycarboxylic acid water reducing agents, they rarely involve the mechanism of action, the influence of water reducing agents on cement hydration and slurry microstructure, etc.
, and only slightly discussed
in some university papers.

The market is broad and develops in the direction of high performance and multi-function

The rapid development of large-scale projects of national key projects such as high-speed railway passenger lines and dams has promoted the use of polycarboxylic acid high-efficiency superplasticizer in China, while the use of polycarboxylic acid superplasticizer in concrete batching plants is still in the wait-and-see stage
.
With the decline of cost, the accumulation of experience and the maturity of process conditions, polycarboxylic acid high-efficiency water reducing agent will become one of
the most important varieties of concrete water reducing agent industry in China in the near future.

At present, the parent performance of polycarboxylic acid water reducing agent is single and there are not many varieties, which cannot meet the requirements of
concrete with different performance in different fields.
The demand for different properties such as early strength, slow setting, pumping, etc.
, can not produce the corresponding polycarboxylic acid precursor, which is a potential reason
restricting the wide application and development of polycarboxylic acid water reducing agents 。 Therefore, at present, many admixture companies in order to save costs, generally use compound technology, on the basis of the introduction of the parent body, a large number of compound early strength agents, retarders, shrinkage reducers, etc.
, so that although can achieve the effect of early strength, slow coagulation, shrinkage, but a large number of compound other admixtures will cause a series of problems such as poor compatibility with cement, low strength or even shrinkage, high cost, environmental pollution, etc.
, so the research and development of polycarboxylic acid water reducing agents should be comprehensively considered from concrete performance, price, environmental protection, sustainable development and other aspects.
Further develop
in the direction of high performance, multi-function and ecology.