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At present, there are three main methods for the modification of lignosulfonate water-reducing agent: by combining two admixtures to overcome their respective shortcomings and achieve the desired performance of water reduction, moisture retention and durability; From the design point of view, the harmful groups are harmless or converted into favorable groups by chemical methods, and the properties are fundamentally changed to achieve the purpose of modification; different molecular weight fractions are obtained by physical adsorption, ultrafiltration, extraction, etc. Separation is carried out to remove the small molecular weight fraction and the large molecular weight fraction, while retaining the lignosulfonate of the intermediate molecular weight fraction which has a better dispersion and water reduction effect. Sometimes, in order to achieve a more ideal water-reducing effect, it can usually be used in three ways, especially through the combination of physical modification and chemical modification, and the lignin can be improved by adjusting the molecular and functional groups. The dispersion and water-reducing effect of sulfonate can improve the disadvantages of poor air entrainment and excessive retardation during super-doping.
8. 4.1 Composite modification method
At present, the commonly used naphthalene-based high-efficiency water reducing agent will cause the cement hydration heat to be released in a short time, which is easy to cause cracks in the concrete structure, and the slump loss is large, while the general lignosulfonate has a good retarding effect, but The water-reducing effect is not enough. After the combination of the two, the water-reducing effect similar to that of the naphthalene-based superplasticizer can be achieved, and the price is low, the early cement hydration heat, the slump loss is small, and the concrete water retention is achieved. It has the advantages of good cohesiveness and good pumpability, and has the superiority of improving various working properties of concrete. It is a good composite admixture.
Because the quality of the bubble caused by the lignosulfonate water reducinWater reducing agentg agent is poor and less, it is not enough to improve the durability of the concrete, and the addition of the air entraining agent alone can not achieve the purpose of saving cement. The acid-reducing agent is used in combination with the air-entraining agent to achieve sufficient durability. Usually, the air-entraining agent used in combination with the lignosulfonate water-reducing agent is a synthetic polymer air entraining agent such as dodecylsulfonate. Sodium, etc., anionic surfactants, and rosin-based air entraining agents, and the most commonly used ones are rosin-based air entraining agents, which give better durability when used in combination.
8. 4. 2 chemical modification method
At present, the methods for chemically modifying lignosulfonates to increase their surface activity can be mainly divided into two categories: functional chemical modification and graft copolymerization chemical modification. Functionalized chemical modification is the chemical reaction of lignosulfonate to impart its desired properties. Commonly used functional chemical modification methods include condensation polymerization, alkylation, alkoxylation, oxidation, etc.; The graft copolymerization chemical modification is carried out by graft copolymerization of a synthetic monomer with a lignosulfonate to produce a ruthenium molecular compound. All of these methods can, to a certain extent, increase the surface activity of the lignosulfonate by increasing the hydrophilic or lipophilic groups as needed.
The retarding group (mono-OH), the ether chain (mono-O-) in the lignosulfonate is oxidized to the sulfhydryl group C-COOH with weak retardation effect, or graft copolymerized with other chemicals to improve wood Application properties of sulfonate. In practice, the lignosulfonate can be separated first, and the larger and smaller molecules of the molecular oxime are removed, and the lignin sulfonate having a narrow molecular weight distribution and a large molecular weight is obtained, and then the composite is modified to prepare a water reducing agent. The activation and compounding process of modified wood calcium is shown in Figure 8-2.
Experiments have shown that when modified with different gasifying agents, the modified products have certain effects on the flow of cement paste. Electrochemical oxidation generally uses Ru, graphite, Ni, or the like as an anode to oxidize the lignosulfonate. The condensation polymerization method is carried out by a polycondensation reaction of a lignosulfonate with a monomer such as formaldehyde, phenol or isocyanate. Lignosulfonate can replace phenols with formaldehyde under basic catalysis; at the same time, it can be used as a polycondensation reaction of aldehydes with phenols under acidic catalysis.
The graft copolymerization method is to graft copolymerize the lignosulfonate with the ethylenic monomer under the action of the initiator. The results show that the fraction of the graft reaction product with molecular weight less than 50,000 is significantly reduced, and the fraction greater than 100000 is significantly reduced. Significantly increased, there is almost no part less than 5000; and only short grafts can be formed in the reaction, and the number of molecules in each molecular weight segment varies, so that the molecular weight of the grafted copolymerization reaction for the lignosulfonate can be seen. Have a certain contribution. In addition, the lignosulfonate can be graft copolymerized with acrylic acid, styrene, methyl methacrylate, acrylonitrile or the like.
8.4.3 Physical modification method
The physical modification method of lignosulfonate is to physically classify the lignosulfonate according to different molecular weights to obtain products with different properties.
By separating the reducing substances in the lignosulfonate by ultrafiltration or electrodialysis, the excessive retardation of the lignosulfonate can be reduced. D. Former Soviet scholars proposed to modify the lignosulfonate by an adsorption process. The reduction product in the lignosulfonate is reduced from 10.8% to 0.5% to 3.0%, thereby having the function of sputum plasticization and reducing the retardation, but there are problems such as excessive gassing, so that the method has certain Limitations. The former graduate student of Wuhan University of Technology in Beijing used the “foam-adsorption” separation method to modify the lignosulfonate to remove the lignosulfonate with smaller molecular weight and larger size, and the medium molecular weight lignin with strong residual dispersion. The acid salt can reduce the bleed air and retardation of the lignosulfonate, and the dosage can be increased to 0.5% to 0.6%, and the water reduction rate can reach 18%. The cost of the separation and purification method is relatively low, and the structure of the purified lignosulfonate has not changed, which limits the further improvement of the water-reducing performance.
Table 8-2 Quality standards for different molecular weight wood calcium
|Variety||Moisture /%||pH value||Reducing sugar /%||Water insoluble matter/ %|
|Macromolecular calcium||<7||4. 5 〜5. 5||<8||<2|
|Ordinary wood calcium||<7||4. 5 〜5. 5||<8||<2|
|High sugar wood calcium||<7||4. 5 〜5. 5||8〜12||<2|
In 1980, Soviet scholars proposed an adsorption modification process. The essence of the process was to classify the sulfite pulp waste liquid by molecular weight and take the most active component. The lignin contained in the sulfite pulp waste liquid is a polymer having a very wide molecular weight distribution. A molecular weight of from a few hundred to several thousand is usually referred to as a low molecular weight component, and a high molecular weight component has a molecular weight of about 100,000. The latter has a strong retarding effect and delays the hardening process of the cement; the former has a weaker retarding effect, but can increase the gas venting property.
The main reactions in the modification process are as follows.
1 Lignin sulfonation reaction: Weigh a certain amount of alkali lignin into a flask, add a certain volume of water, stir and slowly add sodium hydroxide solution to completely dissolve the lignin. First, 8% of the oxidant H202 is added, and an oxidation reaction is carried out at 80 ° C. Further, formaldehyde and sodium sulfate are added, and the reaction product is reacted at 90X: for 3 hours, and the reaction product is sulfonated lignin, and the lignosulfonate is isolated.
2 Lignosulfonate modification: Lignosulfonate has a complex molecular structure and a wide molecular weight distribution, which affects the performance of cement water reducer during use. When the molecular weight is too small, the air entrainment of the concrete is increased and the strength is lowered; when the molecular weight is too large, the concrete is excessively retarded. Therefore, the separation of the molecular enthalpy of the lignosulfonate was measured by gel permeation chromatography using a membrane separation apparatus. Adding a certain amount of modifier to the separated lignosulfonate, vigorously stirring under alkaline medium and 200X: heating, reacting for 1~1.5h, methoxy decomposition, ether bond, carbon-carbon bond cleavage, A modified lignosulfonate water reducing agent is obtained.
The HHJI-20 developed by the former Soviet Union is made of cement as a modifier. It is blended with cement (accounting for 50% of the dry weight of JICT) in a 10% sulphite pulp waste liquid aqueous solution, stirred vigorously, and then used. The method of precipitation is purified. In this case, the retarding component (saccharide, unsulfonated resin, high molecular weight component in the wood salt) is first adsorbed on the cement particles, and then the hydration product and the precipitate are removed together.
HHJI-20 is characterized by high plasticizing properties and less delay in the formation of concrete. The amount of HHJI-20 is 0.6% (by weight of cement), the slump of the mixture is from 4 to 6 cm to 14 to 16 cm; when the amount is 1%, the flow with a slump of 20 to 22 cm can be obtained. The concrete has a 28d strength that is the same as the strength of the standard (unmixed admixture) concrete.
In order to improve the effect and productivity of the modification, in 1981, the former Soviet Union used a CM-489A 8m3 propeller mixer (160r/inin) to modify the lignin. The modified product is HH9, wherein the reducing substance content is reduced from 10.8% in the original sulfite pulp waste liquid to 0_5% to 3%. The optimum mixing time in the propeller is 0.5h. The optimum dosage of HH9 is 0_5%~0.6%, the water reduction rate can reach 25%, and the strength of concrete at various ages is greatly improved. Under steaming conditions, HH9 should be combined with early strength agent (sodium sulfate, sodium formate, etc.). When HH9 is produced, the loach obtained in the sedimentation tank contains a resin reducing substance and cement, which is in the form of a paste, which accounts for about 10% to 11% of the volume of HH9. This sludge is used for the manufacture of putty and cold asphalt paste.
The Howard modification method originated in the United States and was later introduced by the former Soviet Union. In 1981, an additive containing code M-4 was produced. The optimum blending amount of M-4 is 0.5%~0.7%, which can increase the slump of concrete mixture from 4~6cm to 22~23cnu. The plasticizing effect of M-4 is close to that of high-efficiency water reducing agent.
The Howard Act stipulates that Ca (OH) 2 is used to separate lignin (usually wood calcium) from sulfite pulp waste liquid, and then moved to sodium alkali to achieve ion exchange, and the high efficiency of wood sodium as the main component is reduced. Aqueous agent.
KBM is a modified lignin salt concentrate produced by placing a sulfite pulp waste liquid with a Ca(OH)2 emulsion (in an amount controlled by a mixture pH = 12) in a reactor and using compressed air for intense Stir; stand for 2h to precipitate the sludge, send the mother liquor to the ion exchange reactor, and raise the temperature to (90 ± 10) ° C; when this temperature is reached, the Na2C03 (occupied) is input via the meter and vortex pump The 1/3) solution of the above mother liquor dry matter is treated under conditions of (90 ± 10) 1: for 3 to 4 hours, during which it is periodically stirred, and then sent to a sedimentation tank to obtain a product and remove the sludge.
Properties of modified lignin ylide
Effect on the fluidity of cement paste Liquor sulfonate water reducer adsorbs at the solid-liquid interface, reduces its surface energy, and makes the cement dispersion system relatively stable, resulting in inhibition of initial hydration of cement, thereby improving The amount of free water and the fluidity of the cement slurry. At the same time, the water reducing agent adsorbs on the surface of the cement particles. On the one hand, the hydrated film prevents the formation of agglomerated structure and creates a space protection effect; on the other hand, it increases the dynamic potential of the surface of the cement particles and increases the electrostatic repulsion between the cement particles. Thereby destroying the agglomerated structure of the cement particles and dispersing the cement particles. Table 8-3 lists the results of the comparison of the flow turbidity of the lignosulfonate water reducing agent before and after the modification into the cement.
Table 8-3 Comparison of fluidity of lignosulfonate cement paste before and after modification
|Project||Mixed with /K|
|0. 25||0. 50||1. 00|
|Unmodified water reducing agent / mm||85||128||137|
|Modified water reducing agent / mm||109||192||239|
As can be seen from Table 8-3, the modified lignosulfonate has a greater increase in the liquidity of the pulp than the unmodified lignosulfonate. The reason is that the unmodified lignosulfonate has a large molecular weight distribution and contains a large amount of large and small molecules of lignosulfonate in the molecule. When the molecular weight of the water reducing agent is too small, multi-point adsorption cannot be formed. It is beneficial to form a higher electrostatic repulsion potential between the cement particles; in addition, the lignosulfonate is compound-modified, and a modifier is added to improve the dispersion, thereby making the cement paste liquidity more comparable. Big mention. It can be seen from Table 8-3 that when the content of the modified lignosulfonate is 0.5% and 1_0%, the fluidity of the cement paste is not much changed, indicating that when the modified lignosulfonate is mixed, When it is more than 0.5%, even if the dosage is further increased, the liquidity of the slurry does not change much.
Effect on the setting time of cement paste Table 8-4 lists the setting time and setting time difference of the lignosulfonate cement paste before and after the modification under different dosage conditions. Lignosulfonates have a longer setting time.
Table 8-4 Effect of lignin sulfonate before and after modification on cement paste setting time
|Water reducing agent type||Initial setting time / min||Final setting time / min|
Effect on concrete compressive strength Table 8-5 lists the comparative results of compressive strength of lignosulfonate concrete before and after modification with different dosages. The compressive strength of the modified lignosulfonate with a content of 0 to 5% was significantly higher than that of the unmodified lignosulfonate at 3d, 7d and 28d.
Table 8 – Effect of Lignosulfonate on the Compressive Strength of Mortar before and after S Modification
|Water reducing agent sample||Dosage /%||Concrete compressive strength / MPa|
|Before modification||0. 25||37. 80||40. 87||48. 16|
|Before modification||0. 50||41.51||46. 92||53. 62|
|Before modification||1.00||40. 96||45.89||55. 91|
|Modified||0. 25||46. 30||52.61||63. 25|
|Modified||0. 50||50. 42||54. 87||65. 88|
|Modified||1.00||47.29||54. 57||69. 15|
Effect on water reduction rate Table 8-6 lists the effects of the lignosulfonate concrete water reducer before and after modification on the water reduction rate. It can be seen from Table 8-6 that the modified lignosulfonate has a water reduction rate much larger than that before the modification. Comparing the modified lignosulfonate with the quinone-based Naphthalene based superplasticizer under the same dosage, it is found that the water-reducing rate is equivalent to the naphthalene-based water-reducing rate.
Table 8-6 Effect of Lignosulfonate on Water Reduction Rate before and after Modification
|Water reducing agent sample||Water reducing agent doping sensitivity /%||Water reduction rate /%|
|Before modification||0. 50||8. 1|