A low-temperature resistant and early-strength type alkali-free liquid accelerator and its preparation method

Abstract

The present invention discloses a low-temperature resistant and early-strength type alkali-free liquid accelerating agent and its preparation method, belonging to the technical field of alkali-free liquid accelerating agents. The composition includes 12-15 parts of aluminum sulfate, 2-5 parts of sodium thiocyanate embedded by β-cyclodextrin, 5-8 parts of calcium hydroxide deactivator, 3-5 parts of fiber, 5-10 parts of hydrophilic polymer substance, 3-5 parts of cationic surfactant, 6-10 parts of polyaluminum chloride, and 2-3 parts of air-entraining agent. The low-temperature resistant and early-strength type alkali-free liquid accelerating agent of the present invention can improve the frost resistance of sulfate alkali-free accelerating agents, reduce the viscosity of cement slurry, promote the hydration reaction of C3S, shorten the setting time of concrete, and improve the later strength of concrete.

Description

Technical Field

[0001] The present invention relates to the technical field of non-alkali liquid accelerating agents, and in particular to a low-temperature resistant and early-strength non-alkali liquid accelerating agent and a preparation method thereof. Background Art

[0002] In recent years, shotcrete has been widely used and recognized in various engineering fields, including the construction of tunnels, underground urban spaces, mines, and foundation pit support. Since shotcrete is convenient and fast in technical construction and can reduce the required manpower, it is widely used in tunnel construction in China. If the stability performance of shotcrete is insufficient, it will lead to the instability of the surrounding rock and pose a serious threat to the safety of tunnel construction. The accelerating agent is an essential component in shotcrete mortar.

[0003] Sulfate-free alkali accelerating agent is a kind of accelerating agent. The sulfate-free alkali accelerating agent cannot meet the requirements of the setting time of cement slurry, resulting in a large rebound of shotcrete. To solve this problem, a liquid accelerator containing fluoroaluminate is called a fluorine-containing non-alkali liquid accelerator (AF-hf). Fluoroaluminate is often mixed with aluminum sulfate to improve the coagulation property of the non-alkali liquid accelerator. It should be noted that fluoroaluminate is a liquid mixture produced by dissolving aluminum hydroxide in excess hydrofluoric acid. Although the addition of fluoroaluminate shortens the setting time, while the fluorine-containing non-alkali liquid accelerator prolongs the age of the cement paste by increasing the content of Al 3+ it also introduces a large amount of fluoride ions.

[0004] The fluoride-free alkali-free liquid accelerator inhibits the hydration of C(3)S, increases the number of harmful pores, reduces the pH value, decreases the chloride-binding ability of shotcrete. Corrosion product detection shows that shotcrete with fluoride-free alkali-free liquid accelerator is prone to the rapid formation of loose corrosion products, mainly Fe2O3, thus accelerating the corrosion rate of steel bars. The presence of fluorine may affect the compatibility between cement and admixtures (such as fly ash, slag powder), and then affect the performance of concrete. The sulfate-aluminate-based alkali-free liquid accelerator mainly provides early strength and shortens the setting time by hydrating to form hydration products such as ettringite (AFt) and calcium silicate hydrate (C-S-H) gel; while the fluoroaluminate-based alkali-free liquid accelerator provides early strength and shortens the setting time by consuming gypsum and forming hydration products such as calcium aluminate hydrate (C-A-H) gel and calcium hydroxide (CH). When the content of fluorosilicate slag is relatively high, fluoride ions react with calcium hydroxide to form excessive calcium fluoride (CaF2) precipitation, and these precipitates may coat the cement particles, delaying cement hydration. This coating effect reduces the contact area between cement particles and water, thus slowing down the rate of the hydration reaction. Although calcium fluoride precipitation may delay the late hydration of C3S, the fluoroaluminate-based alkali-free liquid accelerator can increase the strength of concrete at the early stage by promoting the hydration of C3A and the formation of AFt. And when C3S starts to hydrate, it will gradually increase the concentration of the colloidal solution. The increase in the concentration of the colloidal solution will lead to an increase in the viscosity of the cement paste, slowing down the rate of the cement hydration reaction, because the mutual collision and aggregation of colloidal particles reduce the reaction opportunity with water, which helps to delay the setting time of cement.

[0005] Fluoroaluminate (AAF)-based alkali-free liquid accelerators exhibit faster setting effects at low ambient temperatures and better adaptability to low ambient temperatures. In contrast, the performance of aluminum sulfate-based alkali-free liquid accelerators at low temperatures may be inferior to that of fluoroaluminate-based accelerators. Low ambient temperatures promote the hydrolysis of aluminum ions into aluminum hydroxide gel precipitates, reducing the dissolution stability of aluminum sulfate inorganic salts and making them prone to freezing and failure in low-temperature environments. The difference in frost resistance between calcium aluminohydrate (C-A-H) gel and calcium silicate hydrate (C-S-H) gel is as follows. Some studies have shown that even in the absence of pore water, ultra-low temperature erosion can damage the C-S-H structure, while the doping of aluminum atoms can effectively stabilize the C-S-H structure. Specifically, under ultra-low temperature erosion, the silicon-oxygen chains of aluminum-free C-S-H break, generating more defect vacancies, resulting in a decrease in its d002 interlayer space and the collapse of the basic component volume, further forming microcracks. In contrast, the aluminum-silicon-oxygen chains of C-A-S-H are more stable, with a longer average chain length and even slight polymerization. Different from the basic colloidal particle units of C-S-H, the basic colloidal particle units of C-A-S-H do not easily undergo structural collapse under ultra-low temperature erosion (Zhu X, Ren Q, He B, et al. Upscaling degradation of cementitious calcium(aluminate) silicate hydrate upon ultra-low temperature attack: A multiscale insight and a bottom-up enhancement route[J]. Composites Part B: Engineering, 2022, 243: 110122.). Therefore, the present invention proposes a low-temperature resistant early-strength alkali-free liquid accelerating agent and its preparation method. Summary of the Invention

[0006] In order to overcome the deficiencies of the prior art, one of the objectives of the present invention is to provide a low-temperature resistant early-strength alkali-free liquid accelerating agent, which can improve the frost resistance of sulfate-free alkali-free accelerating agents, reduce the viscosity of cement mortar, promote the hydration reaction of C3S, shorten the setting time of concrete, and improve the later strength of concrete.

[0007] Another objective of the present invention is to provide a preparation method for the low-temperature resistant early-strength alkali-free liquid accelerating agent, which is simple to prepare and convenient for on-site construction.

[0008] A third objective of the present invention is to provide a concrete, which uses the above-mentioned low-temperature resistant early-strength alkali-free liquid accelerating agent during the concrete construction process.

[0009] One of the objectives of the present invention is achieved by the following technical solutions:

[0010] A low-temperature resistant and early-strength type alkali-free liquid accelerating agent, comprising 12-15 parts of aluminum sulfate, 2-5 parts of sodium thiocyanate embedded by β-cyclodextrin, 5-8 parts of calcium hydroxide deactivator, 3-5 parts of fiber, 5-10 parts of hydrophilic polymer substance, 3-5 parts of cationic surfactant, 6-10 parts of polyaluminum chloride, and 2-3 parts of air-entraining agent.

[0011] Further, it comprises 15 parts of aluminum sulfate, 2 parts of sodium thiocyanate embedded by β-cyclodextrin, 8 parts of calcium hydroxide deactivator, 3 parts of fiber, 5 parts of hydrophilic polymer substance, 3 parts of cationic surfactant, 10 parts of polyaluminum chloride, and 2 parts of air-entraining agent.

[0012] Further, the calcium hydroxide deactivator is one or both of a carboxylic acid ester type erosion inhibitor and silica fume.

[0013] Further, the hydrophilic polymer substance is one or both of polyethylene oxide (PEO) and polyacrylamide (PAM).

[0014] Further, the cationic surfactant is one or both of dodecyldimethylamine oxide and polyoxyethylene laurylamine.

[0015] Further, the fiber is one or more of steel fiber, glass fiber, basalt fiber, polypropylene fiber, nano-carbon fiber, and polyacrylonitrile fiber.

[0016] Further, the air-entraining agent is one or more of coconut oil fatty acid diethanolamide, sodium rosinate, and triterpenoid saponin.

[0017] Further, the preparation method of the sodium thiocyanate embedded by β-cyclodextrin comprises the following steps:

[0018] S1. Weigh β-cyclodextrin, add it to distilled water at 70 °C and stir to dissolve, adjust the pH to 7.4 to obtain a β-cyclodextrin solution;

[0019] S2. Weigh sodium thiocyanate, prepare a sodium thiocyanate ethanol solution with a mass ratio of 1:1, slowly add the sodium thiocyanate ethanol solution to the β-cyclodextrin solution, and stir at a constant temperature of 60 °C - 65 °C to promote the entry of sodium thiocyanate molecules into the hydrophobic cavity of β-cyclodextrin to form an inclusion compound. Through suction filtration and drying, the sodium thiocyanate embedded by β-cyclodextrin is obtained.

[0020] One of the objectives of the present invention is achieved by the following technical solutions:

[0021] A preparation method of a low-temperature resistant and early-strength type alkali-free liquid accelerating agent, comprising the following steps:

[0022] S1. Weigh each component raw material of the low-temperature resistant and early-strength type alkali-free liquid accelerator according to the formulated amount;

[0023] S2. Heat deionized water to 70 - 85 °C, then add the aluminum sulfate and stir until completely dissolved; cool the solution to below 55 °C, then add the sodium thiocyanate embedded in β-cyclodextrin, the calcium hydroxide deactivator, the fiber, the hydrophilic polymer substance, the cationic surfactant, and the polyaluminum chloride, and stir evenly;

[0024] S3. Add the air-entraining agent to the solution in step S2, stir evenly, and cool to room temperature to obtain the low-temperature resistant and early-strength type alkali-free liquid accelerator.

[0025] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0026] The present invention provides a low-temperature resistant and early-strength type alkali-free liquid accelerator. Sodium thiocyanate (NaSCN) can lower the freezing point of water in concrete, enabling the normal hydration of cement at low temperatures. Sodium thiocyanate is embedded with β-cyclodextrin to avoid salting out of sodium thiocyanate due to its low solubility at low temperatures; the calcium hydroxide deactivator can react with calcium hydroxide, which helps to refine the pore structure of concrete, increases the gel content and gel porosity, but reduces the capillary porosity, enhances the compactness of concrete, thereby reducing the transport of water in concrete and improving the durability of concrete, thus enhancing the frost resistance of concrete; fibers can form a random structure in concrete, enhancing the internal binding force of concrete, offsetting part of the expansion stress, reducing deformation, and inhibiting the further expansion of cracks, thereby enhancing the frost resistance of concrete; the hydrophilic long side chains in the molecular structure of the hydrophilic polymer substance can adsorb and fix free water molecules, increasing the viscosity of the mixing water. This water retention effect can reduce the free water in the cement paste, thereby reducing the fluidity and viscosity of the free water; the cationic surfactant carries a positive charge. When they adsorb to the surface of cement particles, they can neutralize the negative charge on the surface of cement particles, reducing the repulsive force between cement particles. Therefore, they can be used as flocculants to promote the flocculation of cement particles in the cement paste, thereby reducing the number of free cement particles in the cement paste. Part of the free water is also bound inside the floccules, reducing the content of free water. At the same time, the internal friction of the cement paste is reduced, maintaining the dispersed state of cement particles, thereby reducing the viscosity of the cement paste. After the viscosity is reduced, the fluidity of the cement paste is improved. It can be seen that the hydrophilic polymer substance and the cationic surfactant can synergistically reduce the flow of free water in the cement paste, promote the late hydration reaction of C3S, reduce the setting time of the cement paste, and improve the setting strength of the later concrete, effectively solving the problem that as the hydration of C3S proceeds, the generated hydrated calcium silicate gel gradually increases, and these gel particles form a colloidal solution in the solution, and the increase in the concentration of the colloidal solution itself delays the setting time of the cement; polyaluminum chloride can achieve the doping of aluminum atoms in the hydrated calcium silicate (C-S-H) gel to form C-A-S-H. The C-A-S-H gel has good frost resistance, thereby improving the frost resistance of the sulfate-free alkali accelerator. Specific embodiments

[0027] The following further describes the present invention in combination with specific embodiments. It should be noted that, on the premise of no conflict, any combination of the following described embodiments or technical features can form a new embodiment.

[0028] Sodium-based bentonite was purchased from Hubei Kewode Chemical Co., Ltd.;

[0029] Gypsum (CaSO4·2H2O) was purchased from Shanghai Yian Chemical Technology Co., Ltd.

[0030] Example 1

[0031] This embodiment provides a low-temperature resistant and early-strength type alkali-free liquid accelerator, which includes 15 parts of aluminum sulfate, 2 parts of sodium thiocyanate embedded by β-cyclodextrin, 8 parts of calcium hydroxide deactivator carboxylic acid ester type erosion inhibitor, 3 parts of steel fiber, 5 parts of hydrophilic polymer poly(ethylene oxide), 3 parts of cationic surfactant dodecyldimethylamine oxide, 10 parts of polyaluminum chloride, and 2 parts of air-entraining agent coconut oil fatty acid diethanolamide.

[0032] Sodium thiocyanate (NaSCN) embedded by β-cyclodextrin can lower the freezing point of water in concrete, enabling the cement to hydrate normally at low temperatures. The sodium thiocyanate is coated with a nano-carrier to avoid the salting-out phenomenon of sodium thiocyanate due to low solubility at low temperatures.

[0033] The preparation method of sodium thiocyanate embedded by β-cyclodextrin includes the following steps:

[0034] S1. Weigh β-cyclodextrin, add it to distilled water at 70°C and stir to dissolve, adjust the pH to 7.4 to obtain a β-cyclodextrin solution;

[0035] S2. Weigh sodium thiocyanate, prepare a sodium thiocyanate ethanol solution with a mass ratio of 1:1, slowly add the sodium thiocyanate ethanol solution to the β-cyclodextrin solution, and stir at a constant temperature of 60°C - 65°C to promote the entry of sodium thiocyanate molecules into the hydrophobic cavity of β-cyclodextrin to form an inclusion complex. Through suction filtration and drying, sodium thiocyanate embedded by β-cyclodextrin is obtained.

[0036] Silica fume is an ultra-fine silica material, which can improve the early strength of concrete. The silica in the pozzolan reacts with the by-product calcium hydroxide of cement hydration to generate more calcium silicate hydrate (CSH), increasing the gel content and gel porosity, but reducing the capillary porosity, thereby reducing the transport of water in concrete and improving the durability and long-term strength.

[0037] The carboxylic acid ester type erosion inhibitor can react with calcium hydroxide (CH) in concrete to form calcium carboxylate, which helps to reduce the content of calcium hydroxide in concrete, avoid the reaction of calcium hydroxide with acidic substances in the environment, resulting in the corrosion of concrete, and improve the durability of concrete. The carboxylic acid ester type erosion inhibitor also reduces the chance of sulfate reacting with tricalcium aluminate (C3A) to form ettringite by reacting with the sulfate ions of gypsum, thereby reducing the content of ettringite, reducing the expansion risk of concrete, and causing cracking and damage of concrete.

[0038] Fibers can form a random structure in concrete, enhance the internal binding force of concrete, offset a part of the expansion stress, reduce deformation, and inhibit the further expansion of cracks, thereby improving the frost resistance of concrete.

[0039] The hydrophilic long side chains in the molecular structure of hydrophilic polymers can adsorb and fix free water molecules, increasing the viscosity of the mixing water. This water retention effect can reduce the free water in the cement paste, thereby reducing the fluidity and viscosity of the free water.

[0040] Surface graft modification of C-S-H is carried out using a silane coupling agent such as KH570 to improve its dispersion degree in the cement paste and enhance its early strength performance at low temperatures.

[0041] During the hydration process of cement particles, they react with water to form hydration products such as calcium silicate gel (C-S-H) and calcium hydroxide (Ca(OH)2). These hydration products usually carry negative charges. Cationic surfactants carry positive charges. When they adsorb onto the surface of cement particles, they can neutralize the negative charges on the surface of cement particles, reducing the repulsive force between cement particles. Therefore, they can be used as flocculants to promote the flocculation of cement particles in the cement paste, thereby reducing the number of free cement particles in the cement paste. Part of the free water is also trapped inside the flocs, reducing the content of free water. At the same time, the internal friction of the cement paste is reduced, maintaining the dispersed state of cement particles, thereby reducing the viscosity of the cement paste. After the viscosity is reduced, the fluidity of the cement paste is improved.

[0042] Example 2

[0043] This example provides a low-temperature resistant and early-strength type alkali-free liquid accelerator, including 12 parts of aluminum sulfate, 5 parts of β-cyclodextrin-embedded sodium thiocyanate (NaSCN), 5 parts of calcium hydroxide deactivator silica fume, 4 parts of glass fiber, 8 parts of hydrophilic polymer polyacrylamide, 5 parts of cationic surfactant dodecylamine polyoxyethylene ether, 8 parts of polyaluminum chloride, and 3 parts of air-entraining agent sodium rosinate.

[0044] Example 3

[0045] This example provides a low-temperature resistant and early-strength type alkali-free liquid accelerator, including 13 parts of aluminum sulfate, 3 parts of β-cyclodextrin-embedded sodium thiocyanate, 6 parts of calcium hydroxide deactivator silica fume, 5 parts of basalt fiber, 10 parts of hydrophilic polymer polyacrylamide, 4 parts of cationic surfactant dodecyldimethylamine oxide, 6 parts of polyaluminum chloride, and 3 parts of air-entraining agent triterpenoid saponin.

[0046] Comparative Example 1

[0047] Different from Example 1, this comparative example provides a low-temperature resistant and early-strength type alkali-free liquid accelerator, including 15 parts of aluminum sulfate, 2 parts of sodium thiocyanate, 8 parts of calcium hydroxide deactivator carboxylic acid ester type corrosion inhibitor, 3 parts of steel fiber, 5 parts of hydrophilic polymer polyethylene oxide, 3 parts of cationic surfactant dodecyldimethylamine oxide, 10 parts of polyaluminum chloride, and 2 parts of air-entraining agent coconut oil fatty acid diethanolamide.

[0048] Comparative Example 2

[0049] This comparative example provides a low-temperature resistant and early-strength alkali-free liquid accelerating agent. Different from Example 1, this comparative example provides a low-temperature resistant and early-strength alkali-free liquid accelerating agent, including 15 parts of aluminum sulfate, 2 parts of sodium thiocyanate, 8 parts of calcium hydroxide deactivator carboxylate-type erosion inhibitor, 3 parts of steel fiber, 5 parts of hydrophilic polymer poly(ethylene oxide), 3 parts of cationic surfactant dodecyldimethylamine oxide, and 2 parts of air-entraining agent coconut oil fatty acid diethanolamide.

[0050] Comparative Example 3

[0051] This comparative example provides a low-temperature resistant and early-strength alkali-free liquid accelerating agent, including 15 parts of aluminum sulfate, 2 parts of β-cyclodextrin-embedded sodium thiocyanate, 3 parts of steel fiber, 5 parts of hydrophilic polymer poly(ethylene oxide), 3 parts of cationic surfactant dodecyldimethylamine oxide, 10 parts of polyaluminum chloride, and 2 parts of air-entraining agent coconut oil fatty acid diethanolamide.

[0052] Comparative Example 4

[0053] This comparative example provides a low-temperature resistant and early-strength alkali-free liquid accelerating agent, including 15 parts of aluminum sulfate, 2 parts of β-cyclodextrin-embedded sodium thiocyanate, 8 parts of calcium hydroxide deactivator carboxylate-type erosion inhibitor, 3 parts of steel fiber, 3 parts of cationic surfactant dodecyldimethylamine oxide, 10 parts of polyaluminum chloride, and 2 parts of air-entraining agent coconut oil fatty acid diethanolamide.

[0054] Comparative Example 5

[0055] This comparative example provides a low-temperature resistant and early-strength alkali-free liquid accelerating agent, including 15 parts of aluminum sulfate, 2 parts of β-cyclodextrin-embedded sodium thiocyanate, 8 parts of calcium hydroxide deactivator carboxylate-type erosion inhibitor, 3 parts of steel fiber, 10 parts of polyaluminum chloride, and 2 parts of air-entraining agent coconut oil fatty acid diethanolamide.

[0056] Experimental Example

[0057] Synthesis of Portland Cement Raw Materials

[0058] Compact a mixture of reagent-grade Ca(OH)2 and SiO2 (molar ratio 3:1), calcine it once at 1500 °C for 2 h, then grind it, repeat 5 times, and calcine for 10 h to synthesize tricalcium silicate (C3S).

[0059] Compact a mixture of reagent-grade Ca(OH)2 and Al2O3 (molar ratio 6:1), calcine it at 1400 °C for 3 h, then grind it, repeat 5 times, and calcine for 10 h to synthesize tricalcium aluminate (C3A).

[0060] Mix 75% tricalcium silicate, 20% tricalcium aluminate, and 5% gypsum evenly by mass percentage of the total mass as the Portland cement sample.

[0061] Usage method of low-temperature resistant and early-strength type non-alkali liquid accelerating agent

[0062] Net paste with liquid accelerating agent: Put sand, sodium-based bentonite, and 400 g of the Portland cement sample into the mixing pot, stir at low speed for 30 s and then stop. Use a 50 mL syringe to add the low-temperature resistant and early-strength type non-alkali liquid accelerating agent provided in Examples 1-3 and Comparative Examples 1-5 at one time. The mass ratio of the low-temperature resistant and early-strength type non-alkali liquid accelerating agent to the Portland cement sample is 3.0%. Stir at low speed for 5 s, add the weighed water (140 g minus the water content in the low-temperature resistant and early-strength type non-alkali liquid accelerating agent), and then stir at high speed for 15 s. After the stirring is completed, immediately pour it into a mold with dimensions of 70 mm×70 mm×70 mm at 20±2 °C, insert it with a knife, vibrate gently several times, scrape off the excess net paste, and level the surface. Start timing from the final setting time of the cement paste after adding water to obtain the Portland cement mortar samples, denoted as the groups of Examples 1-3 and the groups of Comparative Examples 1-5. The Portland cement mortar samples of the groups of Examples 1-3 and the groups of Comparative Examples 1-5 are repeated 9 times. All the Portland cement mortar samples are placed in a humidity curing box at a temperature of -20±2 °C and a relative humidity of 95% HR, and the following experiments are carried out: the setting time experiment, the compressive strength test of concrete specimens, and the frost resistance test are repeated 3 times respectively.

[0063] The strength of masonry mortar is expressed by strength grade. The mortar strength grade is determined by the compressive strength value (unit: MPa) measured by the standard test method at the age of 28 d of a cube test block with a side length of 70.7 mm under the standard curing conditions (temperature (20±2) °C, relative humidity above 90%).

[0064] Experimental Example 1. Setting time

[0065] The setting time provided in Examples 1-3 and Comparative Examples 1-5 was measured using a Vicat apparatus (Wuxi Jianyi Experimental Equipment Co., Ltd.). The test method for the setting time of the cement paste was carried out according to the Chinese standard of GB / T35159-2017 "Accelerating agent for shotcrete". The initial setting time and final setting time of the cement paste were measured using a Vicat apparatus. The Vicat apparatus is an instrument used to measure the setting time of cement. It consists of a test needle with a specific diameter and weight and a scale. The usage method is as follows:

[0066] The steps for measuring the initial setting time of the cement paste are as follows:

[0067] The test piece (the mold filled with neat cement paste) was placed in a humidity curing box to maintain the humidity and temperature of the cement paste and simulate the actual curing conditions. At 30 minutes after adding water, the determination of the initial setting time was started. The mold filled with neat cement paste was taken out of the humidity curing box and placed under the needle of the Vicat apparatus. The needle of the Vicat apparatus was lowered to gently contact the surface of the neat cement paste. After the needle contacted the surface of the neat cement paste, the screw on the Vicat apparatus needed to be tightened to fix the position of the needle and ensure that the needle would not move during the test. This process took about 1 to 2 seconds. After tightening the screw, the screw was suddenly loosened, which would cause the needle to sink freely and vertically into the neat cement paste under the action of gravity. Observe the sinking situation of the needle in the neat cement paste. The sinking depth of the needle would be different due to the setting state of the cement paste. If the cement paste has not started to set, the needle will sink deeper; if it has started to set, the sinking depth of the needle will be shallower. Observe the scale reading when the needle stops sinking in the neat cement paste, which can reflect the setting state of the cement paste. After the needle sinks, wait for 30 seconds and then observe the reading of the needle on the scale. This reading can reflect the setting state of the cement paste after a certain period of time.

[0068] When the sinking depth of the needle reaches a certain standard (usually 1.3 mm ± 0.5 mm), it can be considered that the cement paste has started to set. This process needs to be repeated several times until the test results are the same for two consecutive times.

[0069] The steps for determining the final setting time of the cement paste are as follows:

[0070] After the initial setting time is determined, the mold containing the cement paste needs to be removed from under the glass plate and flipped 180 degrees in a translational manner. This is to prevent the cement paste at the bottom of the mold from undergoing early hardening due to prolonged contact with the glass plate, which may affect the determination of the final setting time. After flipping, the larger end of the mold is placed upward and the smaller end downward on the glass plate. The flipped mold needs to be placed back into the humidity curing box for continued curing at a temperature of -20 ± 2°C and a relative humidity of 95% HR to maintain the humidity and temperature of the cement paste and simulate actual curing conditions. Near the final setting time, measurements need to be taken every 15 minutes. This is because the final setting is a gradual process and frequent checks are required to determine the exact final setting time. When the depth of the test needle sinking into the specimen is 0.5 mm, it is considered that the cement has reached the final setting state. This standard means that the cement paste has completely lost its plasticity and become hard enough that the test needle cannot leave an obvious mark on the surface. To ensure the accuracy of the test results, tests need to be conducted at two other different points on the specimen. If the test results at all three points show that the depth of the test needle sinking is 0.5 mm, then it can be determined that the cement paste has reached the final setting state. Only when the results of the three tests are consistent can it be finally determined that the cement paste has reached the final setting state. This is to ensure the reliability of the test results and avoid misjudgment caused by uneven local hardening.

[0071] Through these steps, the final setting time of the cement paste can be accurately determined, which is very important for construction and engineering quality control. The final setting time is an important indicator of the transformation of the cement paste from a plastic state to a hard state and has guiding significance for determining the construction timing and subsequent construction steps.

[0072] The measurement results of the setting times of Examples 1 - 3 groups and Comparative Examples 1 - 5 groups are shown in Table 1.

[0073] Table 1

[0074]

[0075] The setting time of cement is divided into initial setting time and final setting time. The time from the addition of water until the cement paste begins to lose plasticity and its fluidity decreases is called the initial setting time. The time from the addition of water until the cement paste completely loses plasticity and begins to have a certain structural strength is called the final setting time. The initial setting and final setting of cement are specified through tests. The setting time required by the Chinese standard of GB / T 35159-2017 "Quick-setting admixture for shotcrete" is as follows: The initial setting time of the non-alkali liquid quick-setting admixture should be within 5 minutes, and the final setting time should be within 12 minutes. The results in Table 1 show that the initial setting time and final setting time of Examples 1-3 are lower than those of Comparative Examples 1-5. Compared with Comparative Example 1, the initial setting time and final setting time of Examples 1-3 are comparable to those of Comparative Example 1, and no salting-out phenomenon of sodium thiocyanate is found, indicating that sodium thiocyanate is coated with a nano-carrier to avoid the influence of low solubility at low temperatures on the effect; compared with Comparative Example 2, the initial setting time and final setting time of Examples 1-3 are significantly lower than those of Comparative Example 2, indicating that polyaluminum chloride can achieve the doping of aluminum atoms in the calcium silicate hydrate (C-S-H) gel to form C-A-S-H. The C-A-S-H gel has good frost resistance, thus improving the frost resistance of concrete. Therefore, low temperature has little influence on the initial setting time and final setting time of Examples 1-3; compared with Comparative Example 3, the initial setting time and final setting time of Examples 1-3 are comparable to those of Comparative Example 3, indicating that the calcium hydroxide deactivator carboxylic acid ester type erosion inhibitor reduces the content of calcium hydroxide and does not affect the initial setting time and final setting time of Examples 1-3; compared with Comparative Example 4, the initial setting time and final setting time of Examples 1-3 are significantly lower than those of Comparative Example 4, indicating that the hydrophilic polymer polyethylene oxide can reduce the free water in the cement paste, thereby reducing the fluidity and viscosity of the free water and promoting the hydration of C3S, significantly shortening the initial setting time and final setting time of the concrete; compared with Comparative Example 5, the initial setting time and final setting time of Examples 1-3 are significantly lower than those of Comparative Example 4, indicating that the cationic surfactant carries a positive charge, reducing the viscosity of the cement paste and the initial setting time and final setting time of the concrete.

[0076] Experimental Example 2. Compressive strength test of concrete specimens

[0077] For the silicate cement mortar specimens of Example 1-3 groups and Comparative Example 1-5 groups in Experimental Example 1, after setting for 1 day and 28 days, the compressive strength of the concrete specimens was measured. The test method for the compressive strength of the cement mortar was carried out according to the Chinese standard of GB / T 35159-2017 "Quick-setting admixtures for shotcrete". The compressive strength of the standard concrete specimens was detected by a compression testing machine. Cubic specimens were evaluated in groups of three, and the arithmetic mean of the three test values was taken as the average compressive strength of the mortar cubic specimens of this group of specimens (accurate to 0.1 MPa). When the difference between one of the maximum or minimum values and the intermediate value exceeds 15% of the intermediate value, both the maximum and minimum values are discarded, and the intermediate value is taken as the compressive strength value of this group of specimens; if the differences between the two values and the intermediate value both exceed 15% of the intermediate value, the test results of this group of specimens are invalid. The results are shown in Table 2.

[0078] Compressive strength test: The test of compressive strength is usually completed by placing the mortar test block on a pressure testing machine and recording the maximum load before the test block fails. The calculation formula for compressive strength is σ = P / S, where σ is the compressive strength of the specimen (in megapascals (MPa)), P is the failure load of the specimen (in newtons (N)), and S is the compressed area of the specimen (in square millimeters (mm²)).

[0079] The 1-day compressive strength of the mortar refers to the compressive strength measured after the mortar test block is cured for 1 day under standard curing conditions. This index is of great significance for evaluating the early strength development and construction performance of the mortar. Construction application: In actual construction, the data of 1-day compressive strength can be used to evaluate whether the mortar is suitable for rapid construction or needs to extend the curing time to ensure sufficient strength.

[0080] Table 2

[0081]

[0082] The 1-day compressive strength of the mortar required by the Chinese standard of GB / T 35159-2017 "Quick-setting admixture for shotcrete" is as follows: The 1-day compressive strength of the mortar with non-alkali liquid quick-setting admixture is greater than or equal to 7 MPa. The results in Table 2 show that the 1-day compressive strength and 28-day compressive strength of Examples 1-3 are significantly greater than those of Comparative Examples 2-5. Compared with Comparative Example 1, the 1-day compressive strength and 28-day compressive strength of Examples 1-3 are comparable to those of Comparative Example 1, indicating that polyaluminum chloride can achieve the doping of aluminum atoms in calcium silicate hydrate (C-S-H) gel to form C-A-S-H. The C-A-S-H gel has good frost resistance, thus improving the frost resistance of concrete, and sodium thiocyanate has little effect on the strength of concrete. Compared with Comparative Example 2, the 1-day compressive strength and 28-day compressive strength of Examples 1-3 are significantly lower than those of Comparative Example 2, indicating that polyaluminum chloride can achieve the doping of aluminum atoms in calcium silicate hydrate (C-S-H) gel to form C-A-S-H. The C-A-S-H gel has good frost resistance, thus improving the frost resistance of concrete. Compared with Comparative Example 3, the 1-day compressive strength and 28-day compressive strength of Examples 1-3 are significantly higher than those of Comparative Example 3, indicating that the calcium hydroxide deactivator carboxylic acid ester type erosion inhibitor reduces the content of calcium hydroxide, which helps to refine the pore structure of concrete, increases the gel content and gel porosity, but reduces the capillary porosity, enhances the compactness of concrete, thus reducing the transport of water in concrete and improving the strength of concrete. Compared with Comparative Example 4, the 1-day compressive strength of Examples 1-3 is comparable to that of Comparative Example 4, but the 28-day compressive strength of Examples 1-3 is significantly higher than that of Comparative Example 4, indicating that the hydrophilic polymer polyethylene oxide can reduce the free water in the cement paste, thus reducing the fluidity and viscosity of the free water, promoting the hydration of C3S, and significantly improving the later strength of concrete. Compared with Comparative Example 5, the 1-day compressive strength of Examples 1-3 is comparable to that of Comparative Example 5, but the 28-day compressive strength of Examples 1-3 is significantly higher than that of Comparative Example 5, indicating that the cationic surfactant carries a positive charge, reduces the viscosity of the cement paste, and improves the later strength of concrete.

[0083] Experimental Example 3. Frost Resistance Test

[0084] After curing for 28 days, in this experiment, a cylinder with a diameter of 10 cm and a height of 10 cm was poured, and a rapid freeze-thaw cycle test was carried out in accordance with GB / T 50082-2019 "Standard Test Method for Long-Term Performance and Durability of Ordinary Concrete". The mass loss reaching 5% was used as the basis for terminating the concrete frost resistance test. The test results are shown in Table 3.

[0085] Table 3

[0086]

[0087] As can be seen from Table 3, the number of freeze-thaw cycles of Examples 1-3 is significantly greater than that of Comparative Example 2, indicating that polyaluminum chloride can achieve the doping of aluminum atoms in calcium silicate hydrate (C-S-H) gel to form C-A-S-H. The C-A-S-H gel has good freeze resistance, thus improving the freeze resistance of concrete; compared with Comparative Example 3, the number of freeze-thaw cycles of Examples 1-3 is significantly higher than that of Comparative Example 3, indicating that the calcium hydroxide deactivator carboxylic acid ester type erosion inhibitor reduces the content of calcium hydroxide, which helps to refine the pore structure of concrete, increases the gel content and gel porosity, but reduces the capillary porosity, enhances the compactness of concrete, thereby reducing the transport of water in concrete, improving the strength of concrete, and helping to improve the freeze resistance of concrete; compared with Comparative Example 4, the number of freeze-thaw cycles of Examples 1-3 is higher than that of Comparative Example 4, indicating that the hydrophilic polymer polyethylene oxide can reduce the free water in the cement paste, thereby reducing the fluidity and viscosity of the free water, promoting the hydration of C3S, significantly improving the later strength of concrete, and helping to improve the freeze resistance of concrete; compared with Comparative Example 5, the number of freeze-thaw cycles of Examples 1-3 is higher than that of Comparative Example 5, indicating that the cationic surfactant carries a positive charge, reduces the viscosity of the cement paste, improves the later strength of concrete, and helps to improve the freeze resistance of concrete.

[0088] The above embodiments are only the preferred embodiments of the present invention and cannot be used to limit the scope of protection of the present invention. Any non-substantial changes and substitutions made by those skilled in the art on the basis of the present invention fall within the scope of protection required by the present invention.

Claims

1. A low temperature resistant early strength alkali-free liquid accelerating agent, characterized in that: The invention comprises 12 to 15 parts of aluminum sulfate, 2 to 5 parts of sodium thiocyanate embedded in β-cyclodextrin, 5 to 8 parts of calcium hydroxide deactivator, 3 to 5 parts of fiber, 5 to 10 parts of hydrophilic polymer, 3 to 5 parts of cationic surfactant, 6 to 10 parts of polyaluminum chloride and 2 to 3 parts of air entraining agent. The preparation method of the sodium thiocyanate embedded in β-cyclodextrin comprises the following steps: S1. Weigh β-cyclodextrin, add it into 70°C distilled water, stir and dissolve it, and adjust the pH value to 7.4 to obtain a β-cyclodextrin solution; S2, weighing sodium thiocyanate, preparing a sodium thiocyanate ethanol solution in a mass ratio of 1:1, slowly adding the sodium thiocyanate ethanol solution to the β-cyclodextrin solution, stirring at a constant temperature of 60° C. to 65° C. to promote the sodium thiocyanate molecules to enter the hydrophobic cavity of the β-cyclodextrin to form an inclusion complex, and then filtering and drying to obtain the β-cyclodextrin-encapsulated sodium thiocyanate; The calcium hydroxide deactivator is one or both of a carboxylate corrosion inhibitor and silica fume; The hydrophilic polymer substance is one or both of polyethylene oxide and polyacrylamide; The cationic surfactant is one or both of dodecyl dimethyl amine oxide and dodecylamine polyoxyethylene ether.

2. The low temperature resistant and early strength alkali-free liquid accelerating agent according to claim 1, characterized in that: The invention comprises 15 parts of aluminum sulfate, 2 parts of sodium thiocyanate embedded in beta-cyclodextrin, 8 parts of calcium hydroxide deactivator, 3 parts of fiber, 5 parts of hydrophilic polymer substance, 3 parts of cationic surfactant, 10 parts of polyaluminum chloride and 2 parts of air entraining agent.

3. The low temperature resistant and early strength alkali-free liquid accelerating agent according to claim 1, characterized in that: The fiber is one or more of fiber steel fiber, glass fiber, basalt fiber, polypropylene fiber, nano carbon fiber, and polyacrylonitrile fiber.

4. The low temperature resistant and early strength alkali-free liquid accelerating agent according to claim 1, characterized in that: The carboxylate corrosion inhibitor is a mixture of one or more of mono-aza-hybridized carboxylate, di-aza-hybridized di-carboxylate or fatty acid carboxylate.

5. The low temperature resistant and early strength alkali-free liquid accelerating agent according to claim 1, characterized in that: The air entraining agent is one or more of coconut oil fatty acid diethanolamide, sodium rosin acid, and triterpenoid saponin.

6. A method for preparing a low temperature resistant and early strength alkali-free liquid accelerating setting agent according to any one of claims 1 to 5, characterized in that: The following steps are involved: S1. Weigh the raw materials of each component of the low-temperature resistant and early-strength alkali-free liquid accelerating setting agent according to the formula amount; S2, heating the deionized water to 70-85°C, then adding the aluminum sulfate, stirring until completely dissolved; cooling the solution to below 55°C, then adding the β-cyclodextrin-encapsulated sodium thiocyanate, the calcium hydroxide deactivator, the fiber, the hydrophilic polymer, the cationic surfactant, and the polyaluminum chloride, and stirring evenly; S3, adding the air entraining agent to the solution of step S2, stirring evenly, and cooling to room temperature to obtain the low-temperature stable and early-strengthening alkali-free liquid accelerating agent.

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