Method for adjusting pore distribution of fly ash geopolymer without additive

Abstract

The application provides a method for adjusting pore distribution of fly ash geopolymer without additive, and belongs to the technical field of fly ash geopolymer maintenance.The method comprises the following steps: (1) determining the amount of fly ash, obtaining the amounts of unknown NaOH, Na2SiO3 and water through matching design, and carrying out a polymerization reaction on the fly ash, NaOH, Na2SiO3 and water to obtain geopolymer A; (2) soaking the geopolymer A in lye, and carrying out a secondary polymerization reaction to obtain geopolymer B; (3) carrying out heating maintenance on the geopolymer B; and (4) after the heating maintenance is completed, the geopolymer B can be used at room temperature to the maintenance age period.The method adjusts the size of the overall porosity by soaking the fly ash in a suitable lye with a suitable concentration, so that the fly ash is subjected to a secondary polymerization reaction, thereby adjusting the size of the overall porosity, and the geopolymer is used according to the reaction process, so that the external addition is reduced, and the pore distribution, porosity and connected state of the fly ash-based porous material are controlled, which has a good application prospect.

Description

Technical Field

[0001] This invention relates to the field of fly ash geopolymer curing technology, and in particular to a method for adjusting the pore distribution of fly ash geopolymers without additives. Background Technology

[0002] Geopolymers are a type of low-carbon building material that has attracted considerable attention. Due to their excellent mechanical and durability properties, as well as various other advantages, they are considered a viable alternative to ordinary silicate cement (OPC). In particular, their low carbon emissions are significant; fly ash-based geopolymer composites produce approximately 40% to 60% less CO2 during their manufacturing process compared to OPC systems. Geopolymers belong to the category of alkali-activated materials (AAMs), and their gel phase primarily uses pure aluminosilicates or similar raw materials. This gel system is obtained by reacting alkali metal raw materials (solid or solution) with solid silicate powder. To form the gel of the main gel phase, the allowable calcium ion content in the reactants is typically very low, resulting in a zeolite-like network structure rather than chain-like hydrated calcium silicate. Low-calcium fly ash is the most common raw material in the artificial synthesis of geopolymers. For fly ash-based geopolymers, due to the low calcium oxide content of low-calcium fly ash, the degree of polymerization of [SiO4] tetrahedra is very high, making it almost insoluble in water. Therefore, the dissolution-depolymerization process of fly ash must be carried out under certain alkaline conditions. In other words, the synthesis reaction of fly ash-based polymers is a solid-liquid two-phase reaction between solid fly ash particles and an alkaline activator solution. Generally, the alkaline activator solution is prepared by mixing sodium silicate solution and sodium hydroxide solution in a certain proportion, resulting in a more prominent activating effect.

[0003] Porous materials are a new type of material, proposed in contrast to dense materials. They are materials with a network structure composed of interconnected or closed pores and a framework, possessing unique functions and structures. They are characterized by low density, low thermal conductivity, large specific surface area, high specific strength, high porosity, and light weight. Their porous nature leverages the unique characteristics of the porous structure on the basis of existing materials, significantly altering their mechanical properties, photoelectric properties, and chemical activity. They can play a significant role in separation, sound absorption, vibration damping, and structural applications, and are therefore widely used in building insulation materials, lightweight building materials, permeable concrete, adsorbents, membrane separation materials, porous ceramics, pH regulators, and more.

[0004] Many factors influence porous materials, including pore shape, pore size, pore surface properties, and framework structure. Among these, pore size is a crucial parameter of the pore structure, directly affecting the material's physical properties. Generally, porous materials are classified according to pore size into microporous (<2nm), mesoporous (2-50nm), and macroporous (>50nm) materials. Microporous materials with pore sizes less than 0.7nm are called ultramicroporous materials, while macroporous materials with pore sizes greater than 1m are called macroporous materials. Therefore, the distribution of different pore sizes and porosity directly impact the material.

[0005] Currently, there are many methods for preparing porous geopolymers, including particle packing, which creates free space by stacking; direct foaming, which introduces air; solvent evaporation, which creates pores by solvent evaporation; and porous filler, which fills the pores with external porous materials.

[0006] For fly ash, the main methods for preparing porous materials include solvent evaporation, direct foaming, and particle packing. Its applications include lightweight building insulation materials, adsorption of metal ions, and permeable concrete. However, most methods require multiple additives to initiate the reaction, and the type, method, and environment of these additives all have significant impacts, ultimately leading to large variations in porous materials that are difficult to quantitatively adjust. Previous studies on fly ash geopolymers have shown that increasing temperature and heating time increases porosity and alters pore distribution, primarily related to the combined effects of free water evaporation, internal rehydration, interlayer dehydration, and gel recombination. Porosity initially decreases and then increases with increasing temperature, while heating time also produces nonlinear changes. Furthermore, changes in alkali content also affect the generated product NASH, thus influencing the porosity and pore distribution.

[0007] Therefore, in order to better adjust the pore distribution of fly ash-based porous materials and ensure that the porosity can be adjusted more precisely, it is necessary to set up a control method that can adjust the pores based on its own reaction without external additives. Summary of the Invention

[0008] To overcome the aforementioned deficiencies in the existing technology, this invention provides a method for adjusting the pore distribution of fly ash geopolymers without additives. This method ensures the formation of geopolymers while reducing the amount of external additives, controlling the pore distribution, improving the accuracy of pore control in fly ash-based porous materials, and increasing the utilization value of geopolymers.

[0009] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0010] This invention provides a method for adjusting the porosity distribution of fly ash geopolymers without additives, comprising the following steps:

[0011] (1) Determine the amount of fly ash, and obtain the unknown amounts of NaOH, Na2SiO3 and water through the combination design. Then, carry out a one-time polymerization reaction of fly ash, NaOH, Na2SiO3 and water to obtain geopolymer A.

[0012] (2) Geopolymer A is soaked in alkaline solution to carry out a secondary polymer reaction to obtain geopolymer B;

[0013] (3) Heat-curing geopolymer B;

[0014] (4) After the heating curing is completed, it can be left at room temperature until the curing period is reached.

[0015] Optionally, the method for fit design in step (1) is as follows:

[0016] S1. Determine the liquid-to-solid ratio, activator modulus, and OH-. - The molar concentration, the liquid-to-solid ratio is the ratio of the sum of water and solid, the activator modulus is the SiO2 / Na2O ratio after preparing the composite activator from NaOH and Na2SiO3, and the OH... - Molar concentration refers to NaOH solutions with different molar concentrations;

[0017] S2. Determine the amount of fly ash used. Label the unknown quantities of Na2SiO3, NaOH, and water as A, B, and C, respectively. The specific calculation method is as follows:

[0018]

[0019]

[0020]

[0021]

[0022]

[0023] Among them, S DC S represents the concentration of the sodium silicate solution. N The modulus of sodium silicate solution; This represents the SiO2 content in the sodium silicate solution. This refers to the Na₂O content in the sodium silicate solution; This represents the SiO2 content in sodium hydroxide powder. This control method allows for the shift of the pore distribution peak towards larger pores by increasing the concentration of OH- ions, thereby altering the pore distribution.

[0024] Optionally, the concentration of the alkaline solution in step (2) is designed as follows:

[0025] The geopolymer A prepared in step (1) is immersed at room temperature in NaOH of different molar concentrations; the concentration can be designed according to the alkaline concentration of the mixing ratio during preparation.

[0026] Geopolymer A was immersed in a solution with a volume of 1000 cm³. 3 In the solution, a concentration test was conducted with a solid-liquid ratio of 0.064 to 0.25; after soaking, the concentration of the soaking solution of the relatively intact sample without cracks was selected as the concentration of the alkali solution.

[0027] Optionally, the concentration of NaOH can be two samples with the same concentration and one sample with a different concentration;

[0028] The soaking time is 24–168 hours;

[0029] If the local polymer did not crack at either of the two different concentrations, the higher concentration was chosen as the alkali concentration.

[0030] Optionally, the heating curing method in step (3) is to place the item in a hot air box for heating curing;

[0031] The temperature for heat curing is 50–100℃;

[0032] The heating and curing time is 24–48 hours. Temperature has a crucial impact on pore distribution. Within the limits defined in this application, as the temperature increases, the peak pore distribution shifts towards larger pores.

[0033] Optionally, the care period in step (4) is 7 days.

[0034] Compared with the prior art, the beneficial effects of this application include:

[0035] Based on existing mixing ratio designs, this invention enables precise control of OH. - The design dosage can be adjusted by adjusting OH - The method controls the pore formation area by adjusting the mix proportions; by soaking in an appropriate alkali concentration, the fly ash undergoes a secondary polymerization reaction, thereby adjusting the overall porosity. Through heat curing, under the specific conditions set in this application, the dehydration reaction is ensured, altering the pore connectivity. This method utilizes the geopolymer's own reaction process, reducing external additives, and controls the pore distribution, porosity, and connectivity of fly ash-based porous materials. Attached Figure Description

[0036] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0037] Figure 1 The adsorption-desorption curves and pore volume diagrams are shown at different temperatures, where (c) represents the adsorption-desorption curves at different temperatures; and (d) represents the pore volume diagrams at different temperatures.

[0038] Figure 2 The effect of alkaline immersion on adsorption-desorption curves and pore volume is shown, where (e) represents the adsorption-desorption curve under alkaline immersion and (f) represents the pore volume diagram under alkaline immersion.

[0039] Figure 3 To illustrate the effect of OH- concentration on adsorption-desorption curves and pore volume, (a) represents the adsorption-desorption curves for different OH- concentrations; (b) represents the pore volume plots for different OH- concentrations.

[0040] Figure 4 Pore ​​size distribution diagram of all samples

[0041] Figure 5 This is a SEM image of the pore distribution after the change in Example 1. Detailed Implementation

[0042] Example 1

[0043] Raw material composition:

[0044] The fly ash content is shown in Table 1. (SiO2+Al2O3+Fe2O3)=91.21%>70% is grade F fly ash, with a Si / Al molar ratio of 1.16, suitable for preparing cementitious materials. The calcium oxide content is 3.53%, which is low-calcium fly ash. Liquid sodium silicate, with a modulus of 2.31 and a content of 42%; white flaky sodium hydroxide.

[0045] Table 1. Composition and content of fly ash

[0046]

[0047]

[0048] NaOH solutions with molar concentrations of 6M, 8M, and 10M were prepared. For the alkaline activator solution, the SiO2 / Na2O ratio was 1.04. The optimal liquid-to-solid ratio was determined experimentally, and a design of 0.27 was used, as shown in Table 2 below.

[0049] Table 2. Preparation of Alkali Activator Solution

[0050]

[0051] By measuring the porosity of samples after 7 days with different NaOH preparation concentrations, curing temperatures, and soaking concentrations, the following conclusions can be drawn.

[0052] Figure 1 The effects of different curing temperatures on porosity are presented. For AT-6 cured at room temperature, a lower adsorption capacity is observed, indicating a lower actual porosity. Temperature has a significant effect on porosity, shifting the peak value towards larger pore sizes.

[0053] Figure 2 The results after soaking are shown. DW-6 (water immersion) exhibits a lower pore volume, but a peak point is located at 70 nm. As the NaOH concentration of the immersion solution increases, the peak point gradually shifts towards the mesopores, while a larger pore volume is generated. This also proves the effect of secondary hydration; the large capillaries are reduced due to secondary hydration, in other words, the specimen becomes more compact.

[0054] like Figure 3 It can be seen that the peak value of HT-6-60 is concentrated at 30 nm, while that of HT-8-60 and HT-10-60 is concentrated at 50 nm, which gives an approximate average porosity. However, due to the different distribution amounts, HT-8-60 has a wider pore distribution and a larger total adsorption volume than HT-10-60. This can be understood as the presence of more pores between 30-70 nm, ultimately resulting in a larger average pore volume.

[0055] Through such Figure 4 As shown, the pore size of all FA geopolymer samples was analyzed. It is evident that with increasing NaOH concentration during preparation, the proportion of medium-sized capillaries decreases, while the content of large capillaries increases. This is consistent with the shift in peak values ​​observed in Figure 3.

[0056] Depend on Figure 5 As can be seen, the fly ash geopolymer (b) prepared in this application has surface particle aggregation compared with conventional product (a), which proves that the fly ash in this application has a more complete reaction degree and a denser void distribution.

[0057] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for adjusting the porosity distribution of fly ash geopolymers without additives, characterized in that, Includes the following steps: (1) Determine the amount of fly ash, and obtain the unknown amounts of NaOH, Na2SiO3 and water through the combination design. Then, carry out a one-time polymerization reaction of fly ash, NaOH, Na2SiO3 and water to obtain geopolymer A. (2) Geopolymer A is soaked in alkaline solution to carry out a secondary polymer reaction to obtain geopolymer B; (3) Heat-curing geopolymer B; (4) After the heating curing is completed, the curing period can be reached at room temperature; The method for fit design in step (1) is as follows: S1. Determine the liquid-to-solid ratio, activator modulus, and OH-. - The molar concentration, the liquid-to-solid ratio is the ratio of the sum of water and solid, the activator modulus is the SiO2 / Na2O ratio after preparing the composite activator from NaOH and Na2SiO3, and the OH... - Molar concentration refers to NaOH solutions with different molar concentrations; S2. Determine the amount of fly ash used. Label the unknown quantities of Na2SiO3, NaOH, and water as A, B, and C, respectively. The specific calculation method is as follows: Among them, S DC S represents the concentration of the sodium silicate solution. N The modulus of sodium silicate solution; S SiO2 The content of SiO2 in sodium silicate solution; S Na2O The content of Na₂O in sodium silicate solution; N Na2O In sodium hydroxide powder Na2O The content; The concentration of the alkaline solution in step (2) is designed as follows: Geopolymer A prepared in step (1) is soaked in NaOH solution at room temperature. The alkaline concentration of the mixture is designed during preparation. Geopolymer A was immersed in a 1000 cm³ solution. 3 In the solution, a concentration test was conducted with a solid-liquid ratio of 0.064 to 0.25; after soaking, the concentration of the soaking solution of the relatively intact sample without cracks was selected as the concentration of the alkali solution.

2. The method for adjusting the porosity distribution of fly ash geopolymer without additives as described in claim 1, characterized in that, The concentration of NaOH can be two samples with the same concentration and one sample with a different concentration; The soaking time is 24–168 hours; If the local polymer does not crack at either of the two different concentrations, the higher concentration will be selected as the concentration of the alkali solution.

3. The method for adjusting the porosity distribution of fly ash geopolymer without additives as described in claim 1, characterized in that, The heating curing method in step (3) is to place the item in a hot air box for heating curing; The temperature for heat curing is 50–100℃; The heating and curing time is 24 to 48 hours.

4. The method for adjusting the porosity distribution of fly ash geopolymer without additives as described in claim 1, characterized in that, The nursing period in step (4) is 7 days.

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