Method for improving early strength of high-volume fly ash cement
By combining seawater with CLDH/SiO2@CLDH as a chemical activator, the problem of insufficient early strength in high fly ash concrete was solved, achieving improved early strength and durability, and reducing corrosion risk and carbon dioxide emissions.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- HARBIN INST OF TECH
- Filing Date
- 2025-03-17
- Publication Date
- 2026-05-19
AI Technical Summary
In existing technologies, the early mechanical properties of high fly ash concrete decline and the setting time is prolonged, which limits its application in concrete. Furthermore, chemical activators pose operational risks and problems with later strength reduction.
Seawater combined with CLDH/SiO2@CLDH is used as a chemical activator to promote the activity of fly ash, increase the pH value by releasing OH- ions, enhance early strength, and promote cement hydration through the uniform dispersion of SiO2@CLDH, generating CSH gel to improve density.
It effectively improves the early strength of cement paste with high fly ash content, reduces carbon dioxide emissions, lowers corrosion risk, and enhances the early performance and durability of the material.
Smart Images
Figure CN120136472B_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of building materials manufacturing. Specifically, it relates to the application of CLDH / SiO2@CLDH; it also relates to an activator for improving the early strength of high-volume fly ash cement paste; and it also relates to a method for preparing fly ash cement paste. Background Technology
[0002] Cement production accounts for approximately 7.5% of global anthropogenic carbon dioxide emissions. With the rapid development of the construction industry, the demand for cement continues to grow. Increasing the use of mineral admixtures (SCMs) in concrete is one of the effective ways to mitigate this environmental impact, while also improving the durability of concrete. Fly ash, as one of the most widely used mineral admixtures, saw the concept of high fly ash concrete proposed and put into practical application in the late 1980s, which involves replacing 50% or more of cement with fly ash. However, due to the low reactivity of fly ash, the application of high fly ash cementitious composite materials is limited by setting time and early mechanical property degradation. The kinetics of fly ash reaction are affected by a variety of factors, such as particle fineness, curing temperature, and pH of the pore solution. Currently, the main methods for activating cement and fly ash cementitious materials include: (1) mechanical activation, usually achieved through grinding; (2) activation by introducing small amounts of components with higher reactivity than fly ash; and (3) chemical activation. When the pH of the solution exceeds 13.2, the glass structure of fly ash is significantly damaged, making chemical activation a more effective means. Adding strong alkalis such as sodium hydroxide can increase the pH of the pore solution and enhance the activity of fly ash. However, this leads to a decrease in later strength and an increase in porosity. Furthermore, the high alkalinity of the strong alkali solution increases operational risks, making its widespread use difficult. Therefore, many researchers are actively searching for mild chemical activators suitable for high-volume fly ash concrete. Summary of the Invention
[0003] This invention combines seawater with CLDH / SiO2@CLDH as a chemical activator to improve the early strength of cement paste with high fly ash content. Replacing CLDH with SiO2@CLDH not only serves as a chemical activator but also as a highly active ingredient, further enhancing the early strength of pozzolanic concrete from multiple perspectives. Seawater promotes the hydration reaction of CLDH and releases more OH- ions. - Seawater and CLDH / SiO2@CLDH act as a "chemical activator" for high pozzolanic concrete, helping to reduce carbon dioxide emissions and alleviate pressure on freshwater resources. Furthermore, fly ash and CLDH / SiO2@CLDH can bind some ions in seawater (such as Cl- ions), thereby increasing the pH value and enhancing the early strength of high pozzolanic concrete. -This reduces the risk of corrosion to some extent.
[0004] To address the aforementioned technical problems, the present invention adopts the following technical solution:
[0005] The purpose of this invention is to provide an application of CLDH / SiO2@CLDH, which is the combined use of SiO2@CLDH and seawater to activate the activity of fly ash and increase the fly ash content, wherein the fly ash content is 40wt% to 60wt%.
[0006] Further specifying, the SiO2@CLDH is prepared according to the following steps:
[0007] Step 1: At room temperature, stir 30 ml of anhydrous ethanol and 0.5 ml of ammonia in a water bath for at least 30 min, then add 3 ml of ethyl silicate (TEOs) and stir in a 70°C water bath for at least 1 h. This solution is denoted as solution A.
[0008] Step 2: Mix 20ml of anhydrous ethanol and 40ml of deionized water, then add 10g of hydrotalcite (LDH), and ultrasonically disperse until homogeneous. This solution is labeled as solution B.
[0009] Step 3: Pour solution B into solution A, heat and stir in a 70°C water bath for 3 minutes, filter and dry.
[0010] Step 4: Then grind it into powder, heat it at 500℃ for 5 hours, and then grind it further to obtain the activator SiO2@CLDH.
[0011] The present invention also provides an activator for improving the early strength of cement paste with high fly ash content, which is CLDH / SiO2@CLDH and seawater.
[0012] The present invention also provides a method for preparing fly ash cement paste, comprising the following steps: mixing 250g fly ash, 250g cement and 1000g sand thoroughly in a mixer; then adding 10g CLDH / SiO2@CLDH and stirring until uniform; finally, adding 250g seawater and stirring until uniform.
[0013] This invention utilizes seawater and CLDH as chemical activators. Seawater can promote the release of additional hydroxides (OH-) during the rehydration of CLDH. - This improves the activity of fly ash. Since the main limitation of nanomaterials in cement is agglomeration, if the composite material SiO2@CLDH is used in combination with seawater instead of CLDH, SiO2 will be uniformly dispersed in the cement paste, further promoting cement hydration. Through the pozzolanic reaction, Ca(OH)2 is consumed to generate CSH gel, which increases the material density and early strength.
[0014] After replacing CLDH with SiO2@CLDH, the remaining Cl - The concentration further decreased, which is attributed to the fact that the nano-SiO2 on the SiO2@CLDH surface alters the chemical properties of the CLDH surface, giving it a larger positive potential and exhibiting stronger surface adsorption of Cl. - The ability.
[0015] To further understand the features and technical content of this invention, please refer to the following detailed description and accompanying drawings. However, the accompanying drawings are for reference and illustration only and are not intended to limit the invention. Attached Figure Description
[0016] Figure 1 This is a flowchart of the synthesis of SiO2@CLDH materials;
[0017] Figure 2 It is the ratio of the compressive strength of cement mortar with high fly ash content at 1 day, 7 days and 28 days to the strength ratio of cement mortar with high fly ash content.
[0018] Figure 3 The changes in the hydration heat release rate and cumulative heat of the slurry under different conditions;
[0019] Figure 4 This is a graph showing the content of free chloride ions. Detailed Implementation
[0020] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, while not limiting the invention in any way. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.
[0021] Example 1
[0022] Raw materials: Synthetic seawater (sodium chloride, magnesium chloride hexahydrate, sodium sulfate, calcium chloride, potassium chloride, sodium bicarbonate, potassium bromide, boric acid, strontium chloride hexahydrate, sodium fluoride), tetraethyl orthosilicate (TEOS), ethanol, ammonia (28% by mass) and commercial MgAl-CO3 LDH (Mg4Al2(OH)2). 12 CO32mH2O was provided by McLean. All reagents were analytical grade and could be used directly without purification. Deionized water was used throughout the experiment.
[0023] Synthesis steps:
[0024] According to ASTM standards, each liter of synthetic seawater contains the following components: NaCl: 24.53g, MgCl2·6H2O: 5.20g, Na2SO4: 4.09g, CaCl2: 1.16g, KCl: 0.695g, NaHCO3: 0.201g, KBr: 0.101g, H3BO3: 0.027g, SrCl2·6H2O: 0.025g, NaF: 0.003g.
[0025] Synthesis of the composite material: At room temperature, 30 ml of ethanol and 0.5 ml of ammonia were added to a beaker and stirred in a 70°C water bath for half an hour. Then, 3 ml of TEOs was added, and the mixture was stirred in a 70°C water bath for one hour, denoted as solution A. Next, 20 ml of ethanol and 40 ml of deionized water were mixed, and then 10 g of LDH was added. After ultrasonic dispersion for ten minutes, this was denoted as solution B. Solution B was poured into solution A, and the mixture was stirred in a 70°C water bath for three hours, then filtered and dried. After drying, the mixture was ground into powder, placed in a high-temperature furnace, heated at 500°C for 5 hours, and further ground to obtain the obtained composite material SiO2@CLDH. Figure 1 As shown.
[0026] Mixing process: First, mix the fly ash, cement, and sand in a mixer for 2 minutes according to the mix proportions in Table 1 to ensure thorough mixing. Then, add CLDH or SiO2@CLDH to the mixed powder and stir for 5 minutes to achieve uniformity. Finally, add seawater and mix.
[0027] Table 1. Mixing ratio
[0028]
[0029] Figure 2 The compressive strength of high-volume fly ash cement mortar at 1 day, 7 days, and 28 days, as well as the strength ratio of high-volume fly ash mortar to cement mortar, are shown. Clearly, the compressive strength decreases significantly after a large amount of cement is replaced by fly ash. The compressive strength of fly ash (FA) at 1 day and 7 days is only 40.5% and 50.2% of the cement mortar strength, respectively. The combined use of seawater and CLDH can significantly improve the early compressive strength of high-volume fly ash cement mortar, reaching 74% and 85.3% of the cement mortar strength at 1 day and 7 days, respectively, and 88.1% of the cement mortar strength in later stages. The combined use of SiO2@CLDH composite material and seawater further enhances the early compressive strength of high-volume fly ash cement mortar, reaching 84.5% of the cement mortar strength at 1 day. Therefore, the combined use of seawater with CLDH or SiO2@CLDH can effectively alleviate the problem of insufficient early strength, thereby improving the feasibility of high-volume fly ash cement mortar.
[0030] Figure 3The changes in hydration heat release rate and cumulative heat of the slurry under different conditions are shown. For the SC group, the synergistic effect of seawater and CLDH significantly enhanced the hydration rate, causing the main exothermic peak to appear earlier. This phenomenon indicates that the simultaneous use of seawater and CLDH has a better promoting effect on the early hydration of cement slurry with high fly ash admixture. Due to the nucleation effect, the good dispersion of nano-SiO2 on the CLDH surface further accelerated the hydration process, resulting in a higher hydration heat release rate in the SSC group. Correspondingly, the SC and SSC groups also showed higher total heat release. The reason for using seawater and CLDH as chemical activators is that seawater can promote the release of additional hydroxides (OH-) during the rehydration of CLDH. - This improves the activity of fly ash. Since the main limitation of nanomaterials in cement is agglomeration, if the composite material SiO2@CLDH is used in conjunction with seawater instead of CLDH, SiO2 will be uniformly dispersed in the cement paste, further promoting cement hydration. Through the pozzolanic reaction, Ca(OH)2 is consumed to generate CSH gel, which increases the material density and early strength.
[0031] Seawater may cause Cl in the pore solution - The concentration is too high, while both fly ash and CLDH can effectively bind Cl. - .Depend on Figure 4 It can be seen that when fly ash and CLDH coexist, the Cl in the pore solution after 28 days... - The concentration further decreased. After replacing CLDH with SiO2@CLDH, the free Cl... - The lowest concentration is attributed to the fact that the nano-SiO2 on the SiO2@CLDH surface alters the chemical properties of the CLDH surface, giving it a larger positive potential and exhibiting stronger surface adsorption of Cl. - This ability helps to alleviate corrosion problems caused by chloride ions.
[0032] The specific embodiments of the present invention have been described above. It should be noted that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.
Claims
1. A method for preparing fly ash cement paste, characterized in that, Includes the following steps: Mix 250g fly ash, 250g cement, and 1000g sand thoroughly in a mixer; then add 10g CLDH or SiO2@CLDH and mix until homogeneous; finally, add 250g seawater and mix until homogeneous.
2. The method for preparing fly ash cement paste according to claim 1, characterized in that, The SiO2@CLDH is prepared according to the following steps: Step 1: At room temperature, stir 30 ml of anhydrous ethanol and 0.5 ml of ammonia in a water bath for at least 30 min, then add 3 ml of tetraethyl orthosilicate (TEOs) and stir in a 70°C water bath for at least 1 h. This solution is denoted as solution A. Step 2: Mix 20 ml of anhydrous ethanol and 40 ml of deionized water, then add 10 g of hydrotalcite (LDH), and ultrasonically disperse until homogeneous. This mixture is labeled as solution B. Step 3: Pour solution B into solution A, heat and stir in a 70°C water bath for 3 minutes, filter and dry. Step 4: Then grind it into powder, heat it at 500℃ for 5 hours, and then grind it further to obtain the activator SiO2@CLDH.