High slump loss resistant polycarboxylic acid water reducer and preparation method thereof

Abstract

The invention provides a high slump loss resistant polycarboxylate superplasticizer and a preparation method thereof, and the method comprises the following steps: adding monounsaturated polyether and water into a reaction device, stirring and dissolving, adding an oxidant, and stirring and dissolving; the material A and the material B are dropwise added into the reaction device at a uniform speed, the dropwise adding time is not less than 90 minutes, and the material A is dropwise added earlier than the material B; the material A comprises water, monounsaturated carboxylic acid and monounsaturated carboxylic ester, and the material B comprises water, sodium hypophosphite and a reducing agent; after dropwise adding, continuously stirring for at least 30 minutes to enable the reaction to be fully carried out; the molar ratio of the monounsaturated polyether to the sodium hypophosphite to the monounsaturated carboxylic acid is 1.21: (1.0-1.5): (3.0-3.4); the amount-of-substance ratio of the sodium hypophosphite to the monounsaturated carboxylic ester is 1: (3.1-4.8). The high slump retaining type polycarboxylic acid water reducing agent provided by the invention can realize slump retaining for more than 2 hours in a fine aggregate system with relatively high silt content.

Description

Technical Field

[0001] The present invention relates to the technical field of concrete admixtures, and in particular, to a high slump-retention polycarboxylate water reducer and a preparation method thereof. Background Art

[0002] A water reducer is a chemical additive commonly used in cement-based building materials that can reduce the water-binder ratio and increase the strength. Polycarboxylate water reducer is the most widely used water reducer variety at present and is a kind of high molecular compound. Polycarboxylate water reducer is obtained by free radical copolymerization of various unsaturated monomers such as unsaturated vinyl polyether macromonomers (such as methallyl alcohol polyoxyethylene ether, HPEG) and unsaturated small monomers (such as acrylic acid, hydroxyethyl acrylate, maleic anhydride, etc.).

[0003] There is a common technical difficulty with polycarboxylate water reducers, that is, when using sand and gravel with a high mud content in concrete, a large amount of polycarboxylate molecules will be adsorbed, resulting in a rapid loss of concrete fluidity. However, at present, due to the promotion of environmental protection policies, the supply of river sand and pebbles is extremely small, and concrete enterprises use a large amount of manufactured sand and crushed stones, inevitably resulting in a high mud content problem.

[0004] Therefore, there is an urgent need in the industry for a new technology for polycarboxylate water reducers and their preparation methods. Summary of the Invention

[0005] Aiming at the problem that the existing slump-retention polycarboxylate water reducer is difficult to achieve a slump retention of more than 2 hours for a fine aggregate system with a high mud content, the present invention provides a high slump-retention polycarboxylate water reducer and a preparation method thereof, which can not only achieve a slump retention of more than 2 hours, but also have a lower usage amount than the existing slump-retention polycarboxylate water reducers. The present invention combines sodium hypophosphite with acrylic acid and hydroxyethyl acrylate, and obtains a technical effect significantly superior to that of the existing commercially available slump-retention mother liquor, with an effect improvement of up to 100%.

[0006] To achieve the above object, the present invention provides a preparation method of a high slump-retention polycarboxylate water reducer, comprising the following steps: Step 1: Add monounsaturated polyether and water to a reaction device, stir to dissolve, and then add an oxidant, and stir until fully dissolved; Step 2: Dropwise add Material A and Material B into the reaction device at a uniform speed, with a dropping time of not less than 90 minutes, and Material A is dropped first and then Material B; wherein, Material A includes water, monounsaturated carboxylic acid and monounsaturated carboxylic acid ester, and Material B includes water, sodium hypophosphite and a reducing agent; Step 3: After the dropping is completed, continue to stir for at least 30 minutes to allow the reaction to proceed fully, and thus obtain the high slump-retention polycarboxylate water reducer; Among them, the molar ratio of the mono-unsaturated polyether, sodium hypophosphite and mono-unsaturated carboxylic acid is 1.21:1.0 - 1.5:3.0 - 3.4; The molar ratio of sodium hypophosphite and mono-unsaturated carboxylic acid ester is 1:3.1 - 4.8.

[0007] Further, in step 1, the mono-unsaturated polyether is methyl allyl alcohol polyoxyethylene ether and / or isopentenol polyoxyethylene ether; the molecular weight range of the mono-unsaturated polyether is 1500 - 4000.

[0008] Further, in step 1, the oxidant is ammonium persulfate and / or hydrogen peroxide; the dosage of the oxidant is 0.15% - 0.3% of the mass of the mono-unsaturated polyether.

[0009] Further, in step 2, the mono-unsaturated carboxylic acid is acrylic acid.

[0010] Further, in step 2, the mono-unsaturated carboxylic acid ester is hydroxyethyl acrylate and / or hydroxypropyl acrylate.

[0011] Further, when the mono-unsaturated carboxylic acid ester is hydroxyethyl acrylate and hydroxypropyl acrylate, the molar ratio of hydroxyethyl acrylate and hydroxypropyl acrylate is 1 - 1.5:1.

[0012] Further, in step 2, the reducing agent is vitamin C; the dosage of the reducing agent is 0.3% - 0.6% of the mass of the mono-unsaturated polyether.

[0013] Further, in step 2, the B material includes a thiol-containing chain transfer agent, and its mass is 0.1% - 5% of the mass of sodium hypophosphite.

[0014] The present invention also provides a high slump-retention type polycarboxylate water reducer, which is prepared by the above preparation method.

[0015] Further, the raw materials of the high slump-retention type polycarboxylate water reducer include methyl allyl alcohol polyoxyethylene ether, acrylic acid, hydroxyethyl acrylate, sodium hypophosphite, oxidant, reducing agent; among them, the molar ratio of the mono-unsaturated polyether, sodium hypophosphite and mono-unsaturated carboxylic acid is 1.21:1.0 - 1.5:3.0 - 3.4, the molar ratio of sodium hypophosphite and mono-unsaturated carboxylic acid ester is 1:3.1 - 4.8, and the thiol-containing chain transfer agent in the raw materials of the high slump-retention type polycarboxylate water reducer has a mass of 0.1% - 5% of the mass of sodium hypophosphite.

[0016] The present invention has the following beneficial effects: The present invention provides a preparation method of a high slump-retention polycarboxylate water reducer, comprising: first adding a mono-unsaturated polyether and water into a reaction device, stirring to dissolve, and then adding an oxidant and stirring to dissolve; dropping A material and B material into the reaction device at a uniform speed, with the dropping time being not less than 90 minutes, and A material being dropped completely before B material; A material comprises water, a mono-unsaturated carboxylic acid and a mono-unsaturated carboxylic acid ester, and B material comprises water, sodium hypophosphite and a reducing agent; after the dropping is completed, continue to stir for at least 30 minutes to allow the reaction to proceed fully, thus obtaining the high slump-retention polycarboxylate water reducer; wherein, the molar ratio of the mono-unsaturated polyether, sodium hypophosphite and the mono-unsaturated carboxylic acid is 1.21:(1.0 - 1.5):(3.0 - 3.4); the molar ratio of sodium hypophosphite and the mono-unsaturated carboxylic acid ester is 1:3.1 - 4.8. The high slump-retention polycarboxylate water reducer prepared by the method of the present invention can not only achieve a slump retention of more than 2 hours, but also has a lower usage amount than the existing slump-retention polycarboxylate water reducer, and the effect is significantly better than the technical effect of the existing commercially available slump-retention mother liquor.

[0017] The core of the technical solution of the present invention is the strong synergistic effect between sodium hypophosphite and the mono-unsaturated carboxylic acid ester. In the present invention, sodium hypophosphite and the mono-unsaturated carboxylic acid ester (such as hydroxyethyl acrylate) have extremely excellent synergistic effects, far exceeding the simple synergistic effect of sodium hypophosphite and acrylic acid. In addition, the synergistic effect between sodium hypophosphite and hydroxyethyl acrylate is nearly 100% higher than that without adding sodium hypophosphite, while the synergistic effect between sodium hypophosphite and acrylic acid is only 15 - 20% higher than that without adding sodium hypophosphite.

[0018] Because a relatively large amount of sodium hypophosphite not only functions as a chain transfer agent, but is also directly incorporated into the main chain of the polycarboxylic acid molecule and exists in the form of a -P(=O)- subunit. The sodium hypophosphite of the present invention has three functions: chain transfer (controlling the synthesized molecular weight), polymerizing small monomers (incorporating into the main chain), and reducing agent (participating in the redox initiation process). Introducing a phosphate group greatly enhances the anionic charge density of the polycarboxylic acid main chain, and simultaneously plays the following three key roles: (1) The negative charge of the polycarboxylic acid main chain prevents it from being adsorbed by clay; (2) The relatively high anionic charge density of the polycarboxylic acid main chain protects the ester group to a certain extent, delays the release of acrylate in the alkaline cement paste liquid phase, extends the release time, and improves the slump retention effect; (3) The phosphate group not only enhances the complexing ability of the original polycarboxylic acid molecule with calcium ions, but also greatly improves the complexing ability of the hydroxyl group in hydroxyethyl acrylate with calcium ions, enabling the ethylene glycol molecule after the hydrolysis of hydroxyethyl acrylate to directly adsorb on the surface of cement particles and continuously maintain the dispersing effect, rather than being released. Under normal conditions, the released polyethylene glycol molecule will be adsorbed by clay and become a sacrificial agent. However, due to the action of the sodium hypophosphite residue, the binding between polyethylene glycol and cement particles is tighter, and it is more difficult to be released and adsorbed by clay after hydrolysis.

[0019] And the above effects can be achieved only with a sufficient amount of sodium hypophosphite, that is, a large amount of sodium hypophosphite must be incorporated into the main molecular chain of polycarboxylic acid. The molar ratio of sodium hypophosphite to hydroxyethyl acrylate used in the present invention is 1:3.1 - 4.8, which means that one sodium hypophosphite can act on about 3 - 5 hydroxyethyl acrylate molecules. In addition, through comparison, it is found that using only sodium hypophosphite without hydroxyethyl acrylate has limited effect improvement. Using only hydroxyethyl acrylate without sodium hypophosphite (using conventional mercaptopropionic acid as a chain transfer agent) is a conventional commercially available product. Moreover, the synergistic effect of sodium hypophosphite and hydroxyethyl acrylate far exceeds the conventional slump retention mother liquor optimization schemes in the market (such as pentacarbon monomers, hexacarbon monomers, small sulfonate monomers, etc.).

[0020] In addition to the purposes, features, and advantages described above, the present invention has other purposes, features, and advantages. The following will refer to the drawings to further elaborate on the present invention in detail. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The drawings forming a part of this application are used to provide a further understanding of the present invention. The schematic embodiments and descriptions of the present invention are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings: Figure 1 is the infrared spectrum of the mother liquor product obtained in the preferred Embodiment 1 of the present invention; Figure 2 is the infrared spectrum of the mother liquor product obtained in Comparative Example 4. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0022] The following will elaborate on the embodiments of the present invention in detail with reference to the drawings.

[0023] All reagents used in the examples of the present invention are analytical pure reagents with a purity of more than 99% without special instructions, and the sodium hypophosphite used is sodium hypophosphite monohydrate (molecular weight 105.99).

[0024] Example 1: (The molar ratio of sodium hypophosphite to monounsaturated carboxylic ester is 1:3.5) In this embodiment, polycarboxylate slump retention mother liquor is synthesized by mixing allyl alcohol polyoxyethylene ether with a molecular weight of 2400 (HPEG-2400, a monounsaturated polyether), sodium hypophosphite (chain transfer agent), acrylic acid (small monomer), hydroxyethyl acrylate (small monomer), ammonium persulfate (oxidant), and vitamin C (reductant) in a certain proportion and following a certain procedure. The synthesis steps for 1 kg of mother liquor are as follows: (1) Add 300 g of hot water at 80°C to the reaction kettle, start stirring, add 290 g of HPEG, and stir until dissolved. The temperature after dissolution should be between 35 - 45°C, and then add 1.1 g of ammonium persulfate.

[0025] (2) Ingredients in Bucket A: 40 g of water, 22 g of acrylic acid, 55 g of hydroxyethyl acrylate; Ingredients in Bucket B: 80 g of water, 14.3 g of sodium hypophosphite, 0.5 g of VC.

[0026] (3) Slowly drip the contents of Buckets A and B into the reaction kettle at a uniform speed. The dripping time should be controlled within 160 - 190 min, and Bucket A should finish dripping 20 - 30 min earlier than Bucket B. During the dripping process, the temperature should rise slowly and should not drop at least. When the dripping is completed, the temperature should be between 45 - 50°C.

[0027] (4) After the dripping is completed, rinse the dripping bucket with hot pure water and slowly drip it into the reaction kettle (within no more than 30 min). Keep stirring during the dripping of the cleaning water. The reaction is completed after 1 h. Add water to make the total amount reach 1 kg, which is the finished mother liquor product.

[0028] Example 2: (Reducing the proportion of sodium hypophosphite) The difference between Example 2 and Example 1 is that the molar ratio of sodium hypophosphite to hydroxyethyl acrylate is 1:4.8, and the others are the same as in Example 1.

[0029] Example 3: (Increasing the proportion of sodium hypophosphite) The difference between Example 3 and Example 1 is that the molar ratio of sodium hypophosphite to hydroxyethyl acrylate is 1:3.1, and the others are the same as in Example 1.

[0030] Example 4: (Adding a chain transfer agent with 0.1% mercapto group) Based on Example 1, in Example 4, the B material includes thiol propionic acid, and its dosage is 0.1% of the mass of sodium hypophosphite, and the others are the same as in Example 1.

[0031] Example 5: (Adding a chain transfer agent with 5% mercapto group) Based on Example 1, in Example 5, the B material includes thiol propionic acid, and its dosage is 5% of the mass of sodium hypophosphite, and the others are the same as in Example 1.

[0032] Example 6: (Partially replacing hydroxyethyl acrylate with hydroxypropyl acrylate) Example 6 is different from Example 1 in that hydroxypropyl acrylate is used to partially replace hydroxyethyl acrylate, and the molar ratio of hydroxyethyl acrylate to hydroxypropyl acrylate is 1.5:1, and the others are the same as in Example 1.

[0033] Example 7: (Using hydroxypropyl acrylate to partially replace hydroxyethyl acrylate) Example 7 is different from Example 1 in that hydroxypropyl acrylate is used to partially replace hydroxyethyl acrylate, and the molar ratio of hydroxyethyl acrylate to hydroxypropyl acrylate is 1:1, and the others are the same as in Example 1.

[0034] Example 8: (Using hydroxypropyl acrylate to completely replace an equimolar amount of hydroxyethyl acrylate) Example 8 is different from Example 1 in that an equimolar amount of hydroxypropyl acrylate is used to completely replace hydroxyethyl acrylate, and the others are the same as in Example 1.

[0035] Example 9: (Adding a chain transfer agent with 1.5% mercapto group) Based on Example 1, in Example 9, the B component includes mercaptopropionic acid, and its dosage is 1.5% of the mass of sodium hypophosphite, and the others are the same as in Example 1.

[0036] Comparative Example 1: (Without adding hydroxyethyl acrylate, that is, sodium hypophosphite simply reacts with acrylic acid) A preparation method of a high slump retention type polycarboxylate water reducer includes the following steps: (1) Add 300 g of hot water at 80 °C to the reaction kettle, start stirring, add 310 g of HPEG, stir and dissolve it. After dissolution, the temperature should be within 35 - 45 °C, and then add 1.1 g of ammonium persulfate.

[0037] (2) Preparation of component A: 40 g of water, 44 g of acrylic acid; Preparation of component B: 80 g of water, 13.7 g of sodium hypophosphite, 0.5 g of VC.

[0038] (3) Slowly add components A and B to the reaction kettle at a uniform speed. The dropping time should be controlled within 160 - 190 min, and component A should be dropped completely 20 - 30 min earlier than component B. During the dropping process, the temperature should rise slowly and should not drop at least. When the dropping is completed, the temperature should be within 45 - 50 °C.

[0039] (4) After the dropping is completed, rinse the dropping bucket with hot pure water and slowly drop it into the reaction kettle (not exceeding 30 min). Keep stirring during the dropping of the rinsing water. The reaction is completed after 1 h. Add water to make the total amount reach 1 kg to obtain the mother liquor product.

[0040] Comparative Example 2: (Without adding sodium hypophosphite and adding all mercaptopropionic acid) A preparation method of a high slump retention type polycarboxylate water reducer includes the following steps: (1) Add 300 g of hot water at 80 °C to the reaction kettle, start stirring, add 290 g of HPEG, stir and dissolve it. After dissolution, the temperature should be between 35 - 45 °C, and then add 1.1 g of ammonium persulfate.

[0041] (2) Ingredients for bucket A: 40 g of water, 22 g of acrylic acid, 55 g of hydroxyethyl acrylate; Ingredients for bucket B: 80 g of water, 1.8 g of mercaptopropionic acid, 0.5 g of VC.

[0042] (3) Slowly drip the ingredients in buckets A and B into the reaction kettle at a uniform speed. The dripping time should be controlled within 160 - 190 min, and bucket A should finish dripping 20 - 30 min earlier than bucket B. During the dripping process, the temperature should rise slowly and should not drop at least. When the dripping is completed, the temperature should be between 45 - 50 °C.

[0043] (4) After dripping is completed, rinse the dripping bucket with hot pure water and slowly drip it into the reaction kettle (not exceeding 30 min). Keep stirring during the dripping of the cleaning water. The reaction is completed after 1 h. Make up the water to a total of 1 kg to obtain the finished mother liquor.

[0044] Comparative Example 3: (Sodium hypophosphite and mercaptopropionic acid are added simultaneously, and the addition amount of mercaptopropionic acid is 6%) A preparation method of a high slump - retaining polycarboxylate water - reducing agent includes the following steps: (1) Add 300 g of hot water at 80 °C to the reaction kettle, start stirring, add 290 g of HPEG, stir and dissolve it. After dissolution, the temperature should be between 35 - 45 °C, and then add 1.1 g of ammonium persulfate.

[0045] (2) Ingredients for bucket A: 40 g of water, 22 g of acrylic acid, 55 g of hydroxyethyl acrylate; Ingredients for bucket B: 80 g of water, 14.3 g of sodium hypophosphite, 0.85 g of mercaptopropionic acid, 0.5 g of VC.

[0046] (3) Slowly drip the ingredients in buckets A and B into the reaction kettle at a uniform speed. The dripping time should be controlled within 160 - 190 min, and bucket A should finish dripping 20 - 30 min earlier. During the dripping process, the temperature should rise slowly and should not drop at least. When the dripping is completed, the temperature should be between 45 - 50 °C.

[0047] (4) After dripping is completed, rinse the dripping bucket with hot pure water and slowly drip it into the reaction kettle (not exceeding 30 min). Keep stirring during the dripping of the cleaning water. The reaction is completed after 1 h. Make up the water to a total of 1 kg to obtain the finished mother liquor.

[0048] Comparative Example 4: (The proportion of sodium hypophosphite is too low) The difference between Comparative Example 4 and Example 1 is that the molar ratio of sodium hypophosphite to hydroxyethyl acrylate is 1:5, and the others are the same as in Example 1.

[0049] Comparative Example 5: (Excessive proportion of sodium hypophosphite) The difference between Comparative Example 5 and Example 1 lies in that the molar ratio of sodium hypophosphite to hydroxyethyl acrylate is 1:2.8, and the others are the same as in Example 1.

[0050] Comparative Example 6: Commercially available slump retention mother liquor product (SBT PCA®-HS polycarboxylate slow-release water-reducing agent mother liquor) For the specific formulations of Examples 1-9 and Comparative Examples 1-6, see Table 1.

[0051] Table 1

[0052] Note: * is hydroxypropyl acrylate, which means using hydroxypropyl acrylate to completely or partially replace the dosage of hydroxyethyl acrylate. JS is one of the conventional synthetic formulations of water-reducing agent mother liquor, and Comparative Example 2 is one of the conventional synthetic formulations of slump retention mother liquor. The temperature range refers to the starting dropping temperature and the ending dropping temperature during the small-scale test. Due to poor heat preservation during the small-scale test, the starting temperature in large-scale production is generally lower, but the ending temperature is higher.

[0053] The mother liquor product obtained in Example 1 was freeze-dried, and then infrared spectroscopy was performed using a KBr tablet to obtain the infrared spectrum as shown Figure 1 below. Analyzing it gives the possible explanations for each infrared peak position, as shown in Table 2.

[0054] Table 2

[0055] From Figure 1 the infrared spectrum shown above, it can be seen that the phosphinic acid group was effectively introduced into the molecule.

[0056] The mother liquor product obtained in Comparative Example 4 was freeze-dried, and then infrared spectroscopy was performed using a KBr tablet to obtain the infrared spectrum as shown Figure 2 below. Analyzing it gives the possible explanations for each infrared peak position, as shown in Table 3: Table 3

[0057] From Figure 2 the infrared spectrum shown above and Table 3, it can be seen that no obvious phosphinic acid group was observed in the molecule.

[0058] The mother liquors obtained in Example 1 and Comparative Example 4 were refined, placed in a dialysis bag with a molecular weight cut-off of 5000 Da, and repeatedly washed to remove unreacted small molecules to ensure that only polycarboxylic acid molecules remained in the mother liquor. Then, X-ray fluorescence spectroscopy (XRF) was performed on the treated mother liquor to measure the mass fraction of phosphorus element content. The test results are shown in Table 4: Table 4

[0059] It can be seen from the test results in Table 4 that the phosphorus element content in Example 1 is 0.61%, while the phosphorus element content in Comparative Example 4 is 0.02%, which is equivalent to almost no phosphorus element. Thus, it can be judged that when the dosage of sodium hypophosphite is too low, the phosphorus element does not enter the main chain of the polycarboxylic acid molecule.

[0060] Test Example 1: Using a mortar spread cylinder, a rapid compatibility test of concrete admixtures was carried out in accordance with GB 50119-2013 "Technical Specification for Application of Concrete Admixtures". The mortar mix ratios used in the test are shown in Table 5, and all materials are from Yichun City, Jiangxi Province: Table 5

[0061] Among them, the mud content of the sand is 5.0% and the MB value is 5.7, which is sand with a mud content exceeding the national standard GB / T 14684-2011 "Sand for Construction". The remaining materials are conventional materials. The cement is P.O 42.5 cement, the fly ash is Class II fly ash, and the slag powder is S95 grade slag powder. During the experiment, the room temperature was 25°C. The test results of the mortar performance are shown in Table 6: Table 6

[0062] Note: Since GB 50119-2013 stipulates that the initial fluidity of the mortar should be between 330-370 mm, the dosage of the water-reducing mother liquor was adjusted in Experiments 15-17 to ensure compliance.

[0063] Analyzing the experimental data in Table 6, it can be clearly found that Experiments 1-9 are relatively excellent. Among them, Example 1 has the best economy and cost performance, Example 8 has the best slump retention performance, Example 3 has the largest initial spread, and Example 6 is a better balance point between hydroxyethyl acrylate and hydroxypropyl acrylate. Example 9 is the optimal addition amount of mercaptopropionic acid, and Example 5 is the maximum addition amount of mercaptopropionic acid. Exceeding this value will cause a significant decrease in the slump retention performance. The performance of Example 7 is similar to that of Example 6. Example 8 is the best among the examples containing hydroxypropyl acrylate, but the improvement compared to Examples 6 and 7 is not very obvious.

[0064] Experiment 9 proves that an appropriate amount of mercapto chain transfer agent (mercapto propionic acid) helps to improve the slump retention effect, and the slump loss value at 3 hours is close to that of the example with hydroxypropyl acrylate added. This may be because sodium hypophosphite, as a strong reducing chain transfer agent, assists in regulating the main chain molecular weight through its reducibility, forming a longer main chain structure with a high charge density. The addition of a small amount of mercapto propionic acid (usually the dosage is lower than that of sodium hypophosphite), through its stronger chain transfer ability, moderately cuts the molecular side chain, making the molecular weight distribution more uniform. This synergistic effect not only retains the slow-release ability of the main chain (dominated by sodium hypophosphite), but also optimizes the dispersion efficiency of the side chain (assisted by mercapto propionic acid), thus providing dispersing force both in the initial stage and the later stage. It is precisely because of such a synergistic effect that sodium hypophosphite can be more evenly distributed on the polycarboxylic acid main chain, further increasing the effect of sodium hypophosphite. It can be seen from Experiment 11 that the excessive addition of mercapto propionic acid will affect the performance of sodium hypophosphite. This may be due to the mutual competition between the two chain transfer agents, resulting in the ineffective binding of sodium hypophosphite to the polycarboxylic acid molecular chain. The limit value for the beneficial effect of a small amount of mercapto propionic acid to the adverse effect of a larger amount of mercapto propionic acid can be seen in Example 5. When the dosage of mercapto propionic acid reaches 5% of sodium hypophosphite, the slump retention performance begins to decline. Therefore, the addition amount of mercapto propionic acid should not be greater than 5% of the mass of sodium hypophosphite. When it reaches 6%, the slump retention performance is already significantly poor. A more appropriate addition amount is selected near 1.5% in Example 9.

[0065] Examples 2 and 3 show the higher and lower values of sodium hypophosphite addition. It can be seen that when the addition of sodium hypophosphite reaches the level of Example 3, the performance cannot be further improved, the ineffective economic cost will increase significantly, and it has a negative impact on the mortar strength (see Table 7). When sodium hypophosphite is reduced to the level of Example 2, the performance begins to decline. If the dosage of sodium hypophosphite is further reduced to the level of Comparative Example 4, the performance is less different from that of the ordinary slump retention mother liquor. If the dosage of sodium hypophosphite is too small, sufficient anionic charge density cannot be achieved, and the complexing property of the terminal hydroxyl group of hydroxyethyl acrylate cannot be increased either. At the same time, sodium hypophosphite has reducibility. Since in the synthesis process of polycarboxylate superplasticizer, in order to ensure the full progress of the reaction, generally an excessive amount of oxidant is used. At this time, if the amount of sodium hypophosphite is small, it will be oxidized to sodium phosphate and cannot be integrated into the polycarboxylic acid molecular main chain. Therefore, no obvious phosphinic acid group is observed in the infrared spectrum of the superplasticizer prepared in Comparative Example 4.

[0066] It can be seen from Experiment 10 and Experiment 14 that the conventional ordinary slump retention mother liquors shown in Comparative Example 2 and Comparative Example 6 are far inferior to each example in terms of slump retention performance, reflecting the advantages of the present invention in slump retention performance for high silt content. In Comparative Example 2, since a large amount of polycarboxylic acid molecules are adsorbed by the mud powder, and the ethylene glycol produced by the hydrolysis of the ester substance is also adsorbed by the mud powder, resulting in obvious fluidity loss.

[0067] It can be seen from Experiment 15, Experiment 16, and Experiment 19 that for the mother liquor synthesized in Comparative Example 1 using sodium hypophosphite without using hydroxyethyl acrylate, no matter which slump retention mother liquor it is combined with, the technical effects of the examples cannot be achieved, and the slump retention performance is not good. However, it has a slight advantage over the ordinary water-reducing mother liquor JS. This is also because sodium hypophosphite exists in its molecular chain, reducing the adsorption of mud powder on the molecules of the water-reducing mother liquor.

[0068] Comparing Experiment 18 with Experiment 14, it can be seen that when using the slump retention mother liquor of Example 1, its dosage only needs to reach 60% of that in Experiment 14 to achieve a slump retention performance close to that in Experiment 14. It can be understood that the performance of the slump retention mother liquor in Example 1 is improved by about 67% compared with that in Comparative Example 6.

[0069] According to GB / T 17671-2021 "Test Method for Strength of Cement Mortar (ISO Method)", after the tests of the mortars numbered 1, 2, 3, 11, 12, 8, and 7 in Test Example 1 were completed, they were put into molds to test their 3d and 28d strengths; at the same time, according to JGJ 70-2009 "Standard Test Method for Basic Properties of Building Mortar", their impermeability was tested. The results are shown in Table 7 as follows: Table 7

[0070] It can be seen from the strength test data in Table 7 that using an excessive amount of hypophosphite groups in Comparative Example 5 will significantly reduce the 3d strength. This is because the excess sodium hypophosphite or the phosphate produced by reacting with the excessive oxidant but not entering the main chain is a strong retarder, which will seriously inhibit the hydration rate of cement. In Comparative Example 4, using an insufficient amount of sodium hypophosphite shows a significant reduction in the 28d strength of the mortar. This may be due to insufficient chain transfer agent, resulting in too large a molecular weight of polycarboxylic acid, and then causing entanglement of molecular chains; long molecular chains are prone to form entanglements in the solution, reducing their adsorption efficiency on the surface of cement particles, resulting in poor dispersion effect. Some cement particles form clusters due to insufficient dispersion, affecting the compactness of the mortar and resulting in a significant reduction in the later strength.

[0071] In addition, the impermeability test data of Experiment 12 and Experiment 13 show that both excessive and insufficient sodium hypophosphite will significantly reduce the impermeability of mortar. Among them, the reduction in the compactness of mortar caused by insufficient sodium hypophosphite will of course significantly reduce the impermeability performance. Excessive sodium hypophosphite will remain in the water reducer and gradually react with the excessive oxidant to form sodium phosphate. The calcium phosphate precipitate formed after entering the cement paste is dispersed in the cement paste, which may block the continuity between C-S-H gel and ettringite (AFt), form microcrack channels, and weaken the impermeability barrier effect. It should be noted that the reducing property of sodium hypophosphite as a reducing agent is not as good as that of VC. The two compete to react with the oxidant. VC has strong reducing property and fast reaction rate; sodium hypophosphite has weak reducing property and slow reaction rate, but it will still react continuously. Based on this consideration, the appropriate dosage of sodium hypophosphite depends on its reaction rate with the oxidant. When the amount of sodium hypophosphite is too small, it has been oxidized before it can be integrated into the polycarboxylic acid main chain and cannot enter the polycarboxylic acid main chain; when it is too much, there is still free sodium hypophosphite remaining after the reaction is completed, which continuously reacts with the excessive remaining oxidant to produce sodium phosphate.

[0072] For Experiment 8 and Experiment 7, we can find that the introduction of hydroxypropyl acrylate has no obvious effect on the impermeability performance. However, excessive hydroxypropyl acrylate has limited improvement in the slump retention performance and will, to a certain extent, reduce the 3-day strength of the mortar. This may be because when the amount of hydroxypropyl acrylate is relatively large, its release rate is significantly slower than that of hydroxyethyl acrylate. Although it prolongs the slump retention time of the mortar, the remaining un-released hydroxypropyl acrylate still exists after the mortar hardens, affecting the early hydration rate of the cement and resulting in a decrease in the 3-day strength. Generally speaking, when the dosage of hydroxypropyl acrylate is small, its improvement in the slump retention time is limited; when the dosage is too large, it will cause a decrease in the early (3-day) strength of the mortar.

[0073] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A preparation method of a high slump-retention polycarboxylate water reducer, characterized in that, It includes the following steps: Step 1: Add monounsaturated polyether and water into the reaction device, stir to dissolve, and then add an oxidant, and stir until fully dissolved; Step 2: Dropwise add Material A and Material B into the reaction device at a uniform speed. The dropping time is not less than 90 minutes, and Material A is dropped completely before Material B; wherein, Material A includes water, monounsaturated carboxylic acid and monounsaturated carboxylic acid ester, and Material B includes water, sodium hypophosphite and a reducing agent; Step 3: After the dropping is completed, continue to stir for at least 30 minutes to make the reaction proceed fully, and then the high slump-retention polycarboxylate superplasticizer is obtained; Wherein, the molar ratio of monounsaturated polyether, sodium hypophosphite and monounsaturated carboxylic acid is 1.21:1.0~1.5:3.0~3.4; The molar ratio of sodium hypophosphite and monounsaturated carboxylic acid ester is 1:3.1~4.

8.

2. The preparation method of a high slump-retention type polycarboxylate water reducer according to claim 1, characterized in that, In Step 1, the monounsaturated polyether is methyl allyl alcohol polyoxyethylene ether and / or isopentenol polyoxyethylene ether; the molecular weight range of the monounsaturated polyether is 1500~4000.

3. The preparation method of a high slump retention type polycarboxylate water reducer according to claim 1, characterized in that, In Step 1, the oxidant is ammonium persulfate and / or hydrogen peroxide; the dosage of the oxidant is 0.15%~0.3% of the mass of the monounsaturated polyether.

4. The preparation method of a high slump retention type polycarboxylate water reducing agent according to claim 1, characterized in that In Step 2, the monounsaturated carboxylic acid is acrylic acid.

5. The preparation method of a high slump retention type polycarboxylate water reducer according to claim 1, characterized in that, In Step 2, the monounsaturated carboxylic acid ester is hydroxyethyl acrylate and / or hydroxypropyl acrylate.

6. The preparation method of a high slump retention type polycarboxylate water reducer according to claim 5, characterized in that, When the monounsaturated carboxylic acid ester is hydroxyethyl acrylate and hydroxypropyl acrylate, the molar ratio of hydroxyethyl acrylate and hydroxypropyl acrylate is 1~1.5:

1.

7. The preparation method of a high slump-retention type polycarboxylate water reducer according to claim 1, characterized in that, In Step 2, the reducing agent is vitamin C; the dosage of the reducing agent is 0.3%~0.6% of the mass of the monounsaturated polyether.

8. The preparation method of a high slump retention type polycarboxylate water reducer according to claim 1, characterized in that, In Step 2, Material B includes a mercapto-containing chain transfer agent, and its mass is 0.1%~5% of the mass of sodium hypophosphite.

9. A high slump-retention type polycarboxylate water reducer, characterized in that, It is prepared by using the preparation method described in any one of Claims 1~8.

10. A high slump retention type polycarboxylate water reducing agent according to claim 9, characterized in that, The raw materials of the high slump-retention polycarboxylate superplasticizer include methyl allyl alcohol polyoxyethylene ether, acrylic acid, hydroxyethyl acrylate, sodium hypophosphite, an oxidant, and a reducing agent; wherein, the molar ratio of monounsaturated polyether, sodium hypophosphite and monounsaturated carboxylic acid is 1.21:1.0~1.5:3.0~3.4, the molar ratio of sodium hypophosphite and monounsaturated carboxylic acid ester is 1:3.1~4.8, and the mercapto-containing chain transfer agent in the raw materials of the high slump-retention polycarboxylate superplasticizer has a mass of 0.1%~5% of the mass of sodium hypophosphite.

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