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Lignin is both a natural resource rich in natural resources on the earth and a very important renewable resource. With the increasing global resource crisis and increasing requirements for the ecological environment, lignin and its derivatives are characterized by abundant raw materials, low price, non-toxicity, diversified molecular structure and easy chemical modification. Applications are increasingly being valued at home and abroad.
As a renewable natural terpene molecular compound, lignin is increasingly being developed and utilized by scholars around the world. At present, lignin, as a by-product of the wood hydrolysis industry and the paper industry, has not been fully utilized and has caused a great load on the environment. According to estimates by the United Nations Environmental Organization, the world produces 30 million tons of industrial lignin per year, and currently only 6% of industrial lignin (mainly lignosulfonate) is being used.
Lignin is another major component of plant fiber raw materials (other major chemical components are cellulose, semi-fibres), which are mostly present in the intercellular layer, scattered around the fibers, allowing the fibers to adhere to each other and consolidate. Fibers and fibers accumulate with each other to form plants. The lignin has a complex molecular structure and contains various functional groups and chemical bonds, so its reaction ability is quite strong. When the plant material is subjected to cooking delignification, the treatment method is different, and the mechanism and product of delignification are also different. The solution obtained by treating with sodium hydroxide solution is “alkali lignin”; the solution obtained by sodium sulfide or sulfate method is “sulfate lignin” (containing sulphonated lignin); and the treatment with sulfite method is “Lignin sulfonate”, the main reaction is that propyl phenyl group is sulfonated under sulfite paddle conditions, and the sulfonic acid group replaces the hydroxyl group at the a position to form a water-soluble sulfonate, see formula (8-1). ). Its molecular weight ranges from 20,000 to 50,000, its nature is anionic surfactant, it has a certain water reducing function, and can be used as a cement concrete water reducing agent.
In the formula (8-1), M is Na+, Ca2′ Mg2+, NH4+, K+ or the like.
8.1 Lignosulfonate water reducing agent
Lignosulfonate water reducer has been used for more than 70 years and can be used for cast-in-place and precast concrete and reinforced concrete, mass concrete, dam concrete, pumping concrete for water conservancy, port, transportation, industrial and civil construction. , concrete for large formwork construction, concrete for slipform construction and waterproof concrete. Since the lignosulfonate water reducing agent is a by-product of the pulp industry and has a certain water reducing effect, its production and use are expanding. The development and application of lignosulfonate water reducer is a typical ** turning waste into treasure, which has made an indelible contribution to environmental protection. The application of lignosulfonate water reducer in concrete engineering , saving a lot of resources for the country.
The basic component of lignosulfonate is a benzylmethylpropane derivative, which contains a certain sulfonic acid group, which determines that it has good water solubility, is soluble in various aqueous solutions of different pH values, but is insoluble in ethanol. , organic solvents such as acetone. A typical softwood lignosulfonate can be represented by the following chemical formula, and the molecular structural unit is represented by the formula (8-2).
The main source of lignosulfonate is sulphite papermaking waste. After the wood is cooked together with sodium sulfite under high temperature and high pressure, the cellulose is separated from the lignin, and the cellulose pulp is used for papermaking, production of rayon, and the like. The remaining lignin sulfonate waste liquid is fermented and de-sweetened. After being extracted with alcohol, it can be used as a water reducing agent, and concentrated and spray-dried to obtain a brown powdery lignosulfonate having a molecular domain of 2000 to 100,000. The largest output of this process is Bao Lige, with a global annual output of 300,000 tons. The domestic use of this process mainly includes Shanghai Carnell, Jilin Kaishan, Jilin Shijie and Guangzhou Jiangmen. The sulfite papermaking waste liquid is different in the introduction of alkaline substances, and the cations in the lignosulfonate have various forms, such as calcium lignosulfonate, magnesium lignosulfonate and sodium lignosulfonate. , referred to as wood calcium, wood magnesium and wood sodium products.
The performance comparisons of wood calcium, wood sodium and wood magnesium are shown in Table 8-1.
|Project||Wood calcium||Wood sodium||Magnesium|
|pH value||4〜6||9 〜9. 5||4〜6|
|Exterior||Brown or brown color powder||Brown powder||Brown powder|
|Water reduction rate /%||5〜10||5〜10||5〜10|
|Air entrainment /%||About 3||About 2.5||About 2.5|
|Compressive strength ratio / K||3d||90 〜100||95 〜105||约100|
|Condensation time difference / min||Initial condensation||+ 270||+ 30||+0|
|Final condensation||+ 275||+60||+ 30|
8.2 Application status of lignin sulfonate water reducer
Lignosulfonates are mainly used as concrete water reducers, cement grinding aids, chemical grouting materials and asphalt emulsifiers. As early as 1935, the United States began to use lignosulfonate as a concrete water reducer, and many countries began to research and promote it. However, China began to produce such products from the 1970s. The sulfonate-based water reducing agent mainly includes a M-type water reducing agent containing calcium lignosulfonate as a component, an MZ-type water reducing agent obtained by sulfonation of alkali lignin, and a CS-produced by concentrated black liquor of Taiyuan Paper Mill. 1 Water reducing agent and MY type water reducing agent developed by Guangzhou Paper Mill and the Fourth Navigation Bureau of the Ministry of Communications.
Lignosulfonate as a water reducing agent has its advantages and disadvantages, which limits the application of lignosulfonate in concrete and also affects its own value. Therefore, so far, although lignosulfonate is still a widely used concrete water reducer, it cannot meet the requirements when formulating high performance concrete. Moreover, the lignosulfonate water reducing agent is still mainly used in concrete construction in summer as a concrete retarder; or used in the construction of large-scale concrete to reduce the hydration heat of cement and avoid cracking of concrete. In view of this situation, at present, all countries are committed to the research on the modification of lignosulfonate, and many achievements have been made, but in general, the effect is not as good as that of naphthalene and so on. Therefore, the future development direction is still It is a modification of wood calcium, and on the basis of not destroying its original advantages, it achieves the performance requirements of high-efficiency water reducing agent.
8.3 Mechanism of action of lignin-based water reducer
The mechanism of water reducing action of lignin-based water reducing agent can be summarized into three points, namely, the rejection of the same kind of charge, the inhibition of initial hydration of cement and the lubrication of small bubbles. Lignosulfonates are 髙 molecular surfactants, so the magnitude of their surface tension also affects the dispersion properties. Since the lignosulfonate has the inhibition of dispersing, venting and initial hydration, it has a good water reducing effect at low dosage.
As an anionic polymer surfactant, lignosulfonate exhibits different macroscopic properties when it is incorporated into concrete as a water reducing agent due to its different molecular weight, as shown in Figure 8-1.
When the molecular weight of lignosulfonate is above 10,000, it mainly exhibits adsorption. When the concrete is mixed with lignosulfonate, the bound water of the initial hydration of the cement is reduced to some extent, and the content of the mixture is increased. Free water to achieve water reduction. It is precisely because of the adsorption of lignin sulfonate by cement mineral components that the hydration of cement minerals, that is, retardation, is delayed. However, if excessively added with wood calcium, the excessive adsorption of the macromolecular lignin sulfonate by the cement mineral composition will isolate the hydration of the cement mineral component, which will cause the concrete to not condense for several days, and even cause engineering quality. ACCIDENT. Since the molecular weight distribution of the lignosulfonate water reducing agent is wide and the macroscopic properties of the different molecular weights in the cement mixture are different, the amount of the water reducing agent is limited as a concrete water reducing agent. In general, the amount of ordinary lignosulfonate is 0.20%~0.30% of the amount of the gelling material, and the optimum amount is 0-25%. The water reduction rate of the dosage is generally between 8% and 10%. Ordinary water reducer.