A low-alkali, high-strength cementitious material for ecological porous concrete and its preparation method

Abstract

This invention relates to the field of concrete technology, and discloses a low-alkali, high-strength cementitious material for ecological porous concrete and its preparation method. The raw materials include diatomaceous earth, cement, a water-reducing agent, oxalic acid, ferric sulfate, and water. The cementitious material has a 28-day compressive strength of 70–90 MPa. After adding aggregate, the resulting ecological porous concrete exhibits reduced alkalinity and stable strength, and is plant-friendly. The cementitious material comprises the following raw materials and their mass fractions: 2–10 parts diatomaceous earth, 68–76 parts cement, 20–30 parts water, 0.1–0.2 parts water-reducing agent, 1–5 parts oxalic acid, and 1–5 parts ferric sulfate. This cement-based cementitious material features low alkali and high strength, a simple and low-cost preparation method, and is friendly to plant growth and the aquatic environment, providing theoretical support for its vegetative performance and possessing high social value.

Description

Technical Field

[0001] This invention relates to the field of concrete technology, and in particular to a low-alkali, high-strength cementitious material for ecological porous concrete and its preparation method. Background Technology

[0002] With the advancement of urbanization and the expansion of land resource utilization, the excavation for infrastructure projects has resulted in the exposure of large amounts of soil and rock slopes. Traditional slope protection methods include dry-laid rubble masonry, masonry cages, plastering, and grass planting. However, traditional slope protection methods have disadvantages such as long construction periods, high costs, monotonous appearance, poor erosion resistance, and the need for regular maintenance. In particular, they can lead to the destruction of native vegetation, soil erosion, and water pollution, which runs counter to the development trend of protecting the ecological environment. Due to increasing concerns about environmental sustainability, people are updating and improving various slope protection methods to maintain slope stability and the ecological environment. Ecological porous concrete, as a new type of building material with vegetation-growth properties, is gradually being applied to green projects such as slope protection, riverbank protection, sidewalk greening, parking lot greening, rooftop greening, and infrastructure in ecological scenic areas.

[0003] Ecological porous concrete is a new type of ecological slope protection material that combines vegetation-promoting and mechanical properties. It is mainly composed of cementitious materials (ordinary silicate cement), coarse aggregates (9mm-31mm), water, and admixtures. The cementitious material coats and bonds the aggregates together, and the pores formed between the aggregates provide an environment for the growth of numerous plant roots. Since the suitable pH value for plant root environments is typically between 5.5 and 9, and ecological porous concrete uses common silicate cement as its cementitious material, the cement hydration process produces alkaline products such as Ca(OH)2, resulting in a pore environment pH value as high as 12.0. This cannot meet the environmental requirements for plant seed germination and normal growth. Furthermore, high-alkalinity concrete often produces OH- ions when immersed in water. - Leakage causes an increase in the pH value of the surrounding water, leading to water pollution. This is detrimental to the growth of aquatic organisms and makes the area unsuitable for the construction of fishponds, crab ponds, and fishing dams. To solve this problem, it is necessary to modify the ecological porous concrete or the cementitious materials used to reduce alkali content. Commonly used methods for reducing alkali content include: using low-alkali cement, replacing cement with an equal amount of mineral admixtures, spraying or soaking with chemical solutions, and adding chemical admixtures.

[0004] To date, research on ecological porous concrete in my country is still in its early stages, particularly regarding the cementitious materials used in ecological porous concrete, where there are no relevant technical regulations or application standards. Chinese patent CN116081967A discloses a low-carbon vegetation concrete cementitious material and its preparation method. This invention mainly uses active mineral admixtures and high-strength sulfoaluminate cement to reduce alkali, maintaining the pH value of the vegetation concrete cementitious material below 9; its 3-day compressive strength reaches 25MPa–28MPa; and its 28-day compressive strength reaches 46.8MPa–53MPa, exhibiting good performance in all aspects. However, while the pH value of this invention reaches below 9, the strength of the cementitious material is insufficient and cannot meet the requirements of engineering applications. Chinese patent CN115321925A discloses a cementitious material for vegetation concrete. This invention mainly uses low-alkali cement and phosphogypsum to reduce alkali, achieving a 7-day pH value of 9.8–10.22 and a 28-day strength of 45.7MPa–55.2MPa. The pH value of this invention cannot meet the environmental requirements for normal plant growth, and its strength is not ideal, making it difficult to achieve the effect of low alkali and high strength.

[0005] Most existing methods for reducing alkalinity in eco-friendly concrete are cumbersome, costly, and ineffective, typically resulting in a pH level of 10.0–12.0 after treatment, making plant survival difficult. A few methods achieve better alkalinity reduction, but these significantly decrease the strength of the eco-friendly porous concrete, failing to balance low alkalinity and high strength, and thus failing to meet the requirements of a vegetated environment. Therefore, there is an urgent need to develop key alkalinity-reducing materials suitable for eco-friendly porous concrete. Summary of the Invention

[0006] To address the lack of low-cost, high-strength cementitious materials for eco-friendly porous concrete in existing technologies, this invention proposes a low-alkali, high-strength cementitious material for eco-friendly porous concrete and its preparation method. This material features low alkalinity and high strength, and the resulting eco-friendly concrete exhibits low alkalinity, stable strength, excellent durability, and is plant-friendly. This invention also provides a method for preparing the low-alkali, high-strength cementitious material for eco-friendly porous concrete, which has low raw material costs, simple curing conditions, and meets environmentally friendly requirements. To achieve the above objectives, this invention provides the following technical solutions:

[0007] A low-alkali, high-strength cementitious material for ecological porous concrete is characterized by comprising the following raw materials and their mass fractions: 2-10 parts diatomaceous earth, 68-76 parts cement, 20-30 parts water, 0.1-0.2 parts water-reducing agent, 1-5 parts oxalic acid, and 1-5 parts ferric sulfate.

[0008] Preferably, the diatomaceous earth contains more than 87% silica, is calcined diatomaceous earth of 200-400 mesh, has a pH value of less than 10, and a loss on ignition of 1.0%.

[0009] Preferably, the cement used is P·O 42.5 ordinary Portland cement, with a specific surface area of ​​327 m². 2 ·kg -1 The compressive and flexural strengths after 3 days were 28.1 MPa and 5.3 MPa, respectively.

[0010] Preferably, the water is tap water.

[0011] Preferably, the water-reducing agent is a high-performance polycarboxylate water-reducing agent with a water reduction rate of 30%.

[0012] Preferably, the oxalic acid is dihydrate and has a molecular weight of 126.07.

[0013] Preferably, the ferric sulfate is hydrated ferric sulfate with a molecular weight of 399.88.

[0014] The present invention also aims to provide a method for preparing the low-alkali high-strength cementitious material for ecological porous concrete, comprising the following steps: dissolving oxalic acid and ferric sulfate in water and stirring until there are no particles to obtain an aqueous solution; mixing diatomaceous earth, cement, and water-reducing agent until there are no obvious white particles to obtain a mixture; adding the aqueous solution to the mixture at a water-cement ratio of 0.25 to 0.35, stirring at a stirring speed of 200 r / min until uniform, and stirring for 120 s to obtain the low-alkali high-strength cementitious material for ecological porous concrete.

[0015] Compared with the prior art, the present invention has the following beneficial effects:

[0016] 1. The low-alkali, high-strength cementitious material for ecological porous concrete provided by this invention has the characteristics of low alkalinity and high strength, and is not prone to leaching OH-. It is friendly to plant growth and water environment, and the ecological porous concrete made from it is conducive to the growth of various plants.

[0017] 2. The low-alkali, high-strength cementitious material components for ecological porous concrete provided by this invention, comprising diatomaceous earth, oxalic acid, and ferric sulfate, interact and participate in the entire cement hydration process. Diatomaceous earth, through adsorption, potential pozzolanic effect, and micro-aggregate effect, can reduce the pore alkalinity of cement-based materials and improve their compressive strength. Oxalic acid and ferric sulfate, through acid-base neutralization reaction, can consume Ca(OH)2 in the matrix, generating water, oxalate, and Fe(OH)3 colloids that can fill the pores of the cement matrix. The combined use of these three components can significantly reduce the alkalinity of the cement matrix, providing favorable environmental conditions for normal plant growth, while also improving the strength and stability of the cement matrix. Ecological porous concrete prepared using the cementitious material of this invention has low alkalinity, excellent strength, and high durability, making it suitable for engineering construction in various environments, such as windy sand environments, freshwater environments, and coastal environments.

[0018] The low-alkali, high-strength cementitious material of this invention for ecological porous concrete can be molded at room temperature, has simple curing conditions, a simple alkali reduction process, can effectively reduce carbon emissions, has low raw material costs, and meets environmentally friendly requirements, which is conducive to its widespread industrial application. Attached Figure Description

[0019] Figure 1 This is a flowchart illustrating the preparation process of low-alkali, high-strength cementitious materials for use in ecological porous concrete.

[0020] Figure 2 and Figure 3 This is a SEM image of the cementitious material after it was crushed in Example 4. Specific implementation methods

[0021] It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention; the terms “comprising,” “including,” or any other variations thereof as used herein are intended to cover non-exclusive inclusion; for example, a composition, step, method, article, or apparatus that includes the listed elements is not necessarily limited to those elements, but may include other elements not expressly listed or elements inherent to such composition, step, method, article, or apparatus.

[0022] The technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0023] Example 1

[0024] A low-alkali, high-strength cementitious material for ecological porous concrete and its preparation method, wherein the raw material ratio by mass percentage is: diatomaceous earth: cement: water: water-reducing agent = 6.4%: 73.6%: 20%: 0.1%, and the water-cement ratio of this ratio is 0.25.

[0025] The preparation method of the low-alkali, high-strength cementitious material for the ecological porous concrete is as follows:

[0026] (1) Weigh out the raw materials mentioned above.

[0027] (2) Mix diatomaceous earth, ordinary silicate cement and water-reducing agent evenly until there are no obvious white particles.

[0028] (3) Add the aqueous solution at a water-cement ratio of 0.25, mix evenly at a stirring speed of 200r / min, and stir for 120s to obtain a low-alkali high-strength cementitious material for ecological porous concrete.

[0029] Example 2

[0030] A low-alkali, high-strength cementitious material for ecological porous concrete and its preparation method, wherein the raw material ratio by mass percentage is: cement: water: oxalic acid: water-reducing agent = 76%: 23%: 1%: 0.1%, and the water-cement ratio of this ratio is 0.3.

[0031] The preparation method of the low-alkali, high-strength cementitious material for the ecological porous concrete is as follows:

[0032] (1) Weigh out the above-mentioned raw materials, dissolve the oxalic acid in water and stir until there are no particles.

[0033] (2) Ordinary silicate cement and water-reducing agent are mixed evenly.

[0034] (3) Add an aqueous solution at a water-cement ratio of 0.3, mix evenly at a stirring speed of 200 r / min, and stir for 120 s to obtain a low-alkali high-strength cementitious material for ecological porous concrete.

[0035] Example 3

[0036] A low-alkali, high-strength cementitious material for ecological porous concrete and its preparation method, wherein the raw material ratio by mass percentage is: cement: water: ferric sulfate: water-reducing agent = 76%: 23%: 1%: 0.1%, and the water-cement ratio of this ratio is 0.3.

[0037] The preparation method of the low-alkali, high-strength cementitious material for the ecological porous concrete is as follows:

[0038] (1) Weigh the above-mentioned raw materials, dissolve the ferric sulfate in water and stir until there are no particles.

[0039] (2) Mix ordinary silicate cement and water-reducing agent evenly.

[0040] (3) Add an aqueous solution at a water-cement ratio of 0.3, mix evenly at a stirring speed of 200 r / min, and stir for 120 s to obtain a low-alkali high-strength cementitious material for ecological porous concrete.

[0041] Example 4

[0042] A low-alkali, high-strength cementitious material for ecological porous concrete and its preparation method, wherein the raw material ratio by mass percentage is: diatomaceous earth: cement: water: oxalic acid: ferric sulfate: water-reducing agent = 6%: 72%: 20%: 1%: 1%: 0.1%, and the water-cement ratio of this ratio is 0.25.

[0043] The preparation method of the low-alkali, high-strength cementitious material for the ecological porous concrete is as follows:

[0044] (1) Weigh the above-mentioned raw materials, dissolve oxalic acid and ferric sulfate in water and stir until there are no particles.

[0045] (2) Mix diatomaceous earth, ordinary silicate cement and water-reducing agent evenly until there are no obvious white particles.

[0046] (3) Add the aqueous solution at a water-cement ratio of 0.25, mix evenly at a stirring speed of 200r / min, and stir for 120s to obtain a low-alkali high-strength cementitious material for ecological porous concrete.

[0047] Comparative Example 1

[0048] Raw material ratio by weight percentage: Cement: Water: Water-reducing agent = 75%: 25%: 0.1%

[0049] The comparative sample was prepared by mixing cement and water-reducing agent evenly, adding an aqueous solution at a water-cement ratio of 0.25, mixing evenly at a stirring speed of 200 r / min, and stirring for 120 s.

[0050] Comparative Example 2

[0051] Raw material ratio by weight percentage: Cement: Water: Water-reducing agent = 70%: 30%: 0.1%

[0052] The comparative example is prepared by mixing cement and water-reducing agent evenly, adding an aqueous solution at a water-cement ratio of 0.3, mixing evenly at a stirring speed of 200 r / min, and stirring for 120 s.

[0053] To verify the effectiveness and advantages of the present invention, the above embodiments and comparative examples used the aqueous alkalinity release method for testing. Cement paste cured for 3 days, 7 days, 28 days, and 56 days was immersed in 500 ml of aqueous solution, and the container was sealed to prevent water evaporation. The pH value was then tested after 24 hours. The pH meter used was a PH818 pH meter from Xima Instruments. Before each test, the aqueous solution used to soak the cement paste sample was stirred evenly, and the reading was taken after it stabilized. Three tests were performed, and the average value was taken. The test results are shown in Table 1.

[0054] The compressive strength test methods in the above embodiments and comparative examples all refer to "GB / T 17671-2021 Cement Mortar Strength Test Method (ISO Method)". Cement paste was made into prism specimens with a size of 40mm×40mm×160mm. After curing at room temperature for 24 hours, the specimens were demolded. The compressive strength was tested again after curing for 3d, 7d, 28d and 56d. The broken samples were scanned by electron microscopy to obtain SEM images of the hardened paste of low-alkali high-strength cementitious material used in ecological porous concrete. The compressive strength test results are shown in Table 2.

[0055] Table 1: pH test results of the low-alkali, high-strength cementitious material of the present invention used in ecological porous concrete

[0056]

[0057]

[0058] According to the results in Table 1, the pH value of the cement matrix in Examples 1 and 4 was significantly lower than that in Comparative Example 1, decreasing from 10.48-11.56 to 8.56-8.62 in 3-56 days. The pH value of the cement matrix in Examples 2 and 3 was also significantly lower than that in Comparative Example 2, decreasing from 11.10-11.20 to 8.49-8.50 in 3-56 days.

[0059] At 3 days, Examples 1 and 4 showed a significant alkalinity reduction effect compared to Comparative Example 1, with pH values ​​decreasing by 3.7% and 12.7% respectively. Example 4 showed the most significant effect in the early stage, which could improve the survival rate of plant seeds. The pH values ​​of Examples 2 and 3 at 3 days were also reduced by 5.9% and 5% respectively compared to Comparative Example 2.

[0060] By day 7, except for Example 3, the pH values ​​of the other examples all showed a significant decrease compared to the comparative examples. Example 1 showed the largest decrease, with the pH value of the cement-based matrix decreasing by 17% after 7 days compared to day 3. Example 2 also showed a significant decrease, reaching 12.8%. However, the pH value of Example 3 at day 7 was 11.2, which was 0.25 units higher than that of Comparative Example 2. Although the pH value decrease of Example 4 at day 7 was not significant, it was still 13% lower than that of Comparative Example 1. By day 28, the pH values ​​of the cement-based matrices in Examples 1-4 were all lower than those in Comparative Examples 1 and 2, and the pH values ​​of Examples 1, 2, and 4 were all less than 8.7, which met the environmental requirements for normal plant growth.

[0061] By day 56, except for Example 3, the pH value of each example did not decrease significantly. Example 3 showed the largest decrease, with a pH value reduction of 7.1% compared to day 28. The pH values ​​of Examples 1-4 were all significantly lower than those of the comparative example, demonstrating excellent alkalinity reduction effects.

[0062] In summary, Example 4 showed the best alkalinity reduction effect. The low alkalinity in the early stage can improve the survival rate of plant seeds, and the low alkalinity in the later stage can provide a good environment for normal plant growth.

[0063] Table 2: Compressive strength test results of the low-alkali, high-strength cementitious material of the present invention used in ecological porous concrete.

[0064]

[0065] As shown in Table 2, the 3-day strength of the cement matrix in Example 1 was lower than that in Comparative Example 1. This is because diatomaceous earth has strong water absorption, which absorbs moisture from the cement matrix, hindering the normal precipitation and crystallization of cement particles, affecting the rate of cement hydration reaction, and thus impacting the development of early strength. With increasing age, up to 56 days, its strength reached 85.57 MPa, an increase of 1.6% compared to Comparative Example 1. This is because diatomaceous earth has potential pozzolanic activity; during cement hydration, it can undergo a secondary reaction with Ca(OH)₂ to form hydrated calcium silicate gel, which consumes Ca(OH)₂, lowers the pH of the pore solution, and increases the density of the cement matrix. The micro-aggregate effect of diatomaceous earth can also fill the pores of the cement matrix, improving the strength and stability of the material. Diatomaceous earth reduces the pore alkalinity of cement-based materials and improves their compressive strength through adsorption, potential pozzolanic effect, and micro-aggregate effect.

[0066] The strength values ​​of the cement paste in Example 2 from 3 to 28 days were lower than those in Comparative Example 2. This is because oxalic acid reacts with Ca(OH)₂ produced during cement hydration to form calcium oxalate and water, increasing porosity and thus reducing strength. When the Ca(OH)₂ concentration decreases, the alkalinity of the cement matrix decreases. This change in alkaline environment affects the hydration process, leading to a reduction in hydration products that contribute to strength, resulting in lower strength. Overall, the compressive strength of the cement matrix in Example 2 decreased by 5.4%, 3.1%, and 9.6% at 3, 28, and 58 days, respectively, all below 10%, meeting the strength requirements. The pH value at 56 days could be reduced to 8.49, demonstrating excellent alkalinity reduction effects at all ages.

[0067] In Example 3, the compressive strength of the cement matrix from 3 to 56 days was increased by 9.2%, 7.5%, 9.1%, and 3.6% respectively compared to Comparative Example 2, demonstrating a significant strengthening effect. This is because ferric sulfate promotes the hydration reaction of cement, accelerating the formation of hydration products such as Ca(OH)2, hydrated calcium silicate gel, and hydrated calcium aluminate gel, thereby increasing strength. This also explains why the pH value of the cement paste aqueous solution was higher and decreased more slowly in the early stages. When ferric sulfate dissolves in water, it also releases Fe. 3+ and SO4 2- Fe 3+ With OH - The reaction produces Fe(OH)3 colloids, which have strong adhesive properties, increasing the nucleation sites for cement hydration, enhancing the interfacial structure, filling the pores in the cement matrix, increasing its density and strength, and also consuming alkaline substances, thus reducing the alkalinity of the pores.

[0068] In Example 4, the strength of the cement matrix increased by 8.4%, 3.2%, and 6.5% at 3d, 7d, and 56d, respectively, compared to Comparative Example 1. Compared to Example 1, the strength of Example 4 at 3d and 56d was higher. This is because oxalic acid and ferric sulfate consumed Ca(OH)2 in the cement through acid-base neutralization. The resulting calcium oxalate and ferric hydroxide not only bonded a large number of particles in the cement, providing more nucleation sites, but also filled the pores of the cement matrix. Together with diatomaceous earth, they generated hydration products with a low Ca / Si ratio, which not only reduced the alkalinity of the cement matrix, providing environmental conditions for normal plant growth, but also improved the strength of the cement matrix and increased its stability.

[0069] Finally, the above are merely preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A low-alkali, high-strength cementitious material for ecological porous concrete, characterized in that: It is composed of the following raw materials in parts by weight: 2-10 parts diatomaceous earth, 68-76 parts cement, 20-30 parts water, 0.1-0.2 parts water-reducing agent, 1-5 parts oxalic acid, 1-5 parts ferric sulfate, and a water-cement ratio of 0.25-0.

3. The cement is P·O 42.5 ordinary Portland cement, with a specific surface area of ​​327 m². 2 ·kg -1 The compressive and flexural strengths after 3 days were 28.1 MPa and 5.3 MPa, respectively. The water-reducing agent is a high-performance polycarboxylate water-reducing agent with a water reduction rate of 30%. Diatomaceous earth, oxalic acid, and ferric sulfate interact and participate in the entire process of cement hydration. The combined use of these three can reduce the alkalinity of the cement matrix and improve its strength. The diatomaceous earth contains more than 80% silica, and is calcined diatomaceous earth of 200-400 mesh, with a pH value of less than 10 and a loss on ignition of less than 1.0%.

2. The low-alkali, high-strength cementitious material for ecological porous concrete according to claim 1, characterized in that: The water in question is tap water.

3. The low-alkali, high-strength cementitious material for ecological porous concrete according to claim 1, characterized in that: The oxalic acid is oxalic acid dihydrate with a molecular weight of 126.

07.

4. The low-alkali, high-strength cementitious material for ecological porous concrete according to claim 1, characterized in that: The ferric sulfate mentioned is hydrated ferric sulfate with a molecular weight of 399.

88.

5. The method for preparing low-alkali, high-strength cementitious material for ecological porous concrete as described in any one of claims 1 to 4, characterized in that, The process includes the following steps: dissolving oxalic acid and ferric sulfate in water and stirring until there are no particles to obtain an aqueous solution; mixing diatomaceous earth, cement, and water-reducing agent until there are no obvious white particles to obtain a mixture; adding the aqueous solution to the mixture and stirring at a speed of 200 r / min until uniform; stirring for 120 s to obtain a low-alkali, high-strength cementitious material for ecological porous concrete.

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