Preparation method of SAP-loaded composite aggregate and application of SAP-loaded composite aggregate in internal curing of cement-based materials

Abstract

The invention provides a preparation method of SAP-loaded composite aggregate and application of the SAP-loaded composite aggregate in internal curing of cement-based materials, and relates to the technical field of building materials. According to the method, the pores of the aggregate (porous lightweight aggregate such as coral sand and the like) are filled with the SAP pre-polymerization liquid, and then the polymerization curing reaction is performed in situ, so that the SAP-loaded composite aggregate is prepared. The composite aggregate prepared by the method can fully utilize the SAP to enhance the internal curing performance and interface performance of the aggregate in concrete, so that the high water absorption and water retention performance of the aggregate in the concrete in a high-alkaline environment is enhanced, and compared with a traditional internal curing agent SAP, the composite aggregate also enhances the skeleton filling effect of water release holes of an internal curing material; the interface bonding performance between the aggregate and the concrete is promoted, and the method has the advantages of being simple in technological process, capable of achieving industrial production, easy to popularize and low in cost.

Description

Technical Field

[0001] The present invention relates to the technical field of building materials, and in particular to a method for preparing a SAP-loaded composite aggregate and application of the composite aggregate in the internal curing of cement-based materials. Background Art

[0002] Concrete is currently one of the most important civil engineering materials, and high performance is the main direction of concrete material development. In addition, traditional building materials such as sand and gravel are in short supply and expensive, and cannot meet the engineering needs of modern social development. Ultra-high performance concrete (UHPC) is a cement-based composite material with an optimal gradation of constituent material particles, a water-cement ratio of less than 0.25, and an internal non-connected pore structure. It has a high resistance to gas and liquid infiltration, and its durability can be greatly improved compared to traditional concrete and high-performance concrete (HPC). However, due to the low water-to-binder ratio of UHPC, after the cementitious material is hydrated to a certain extent, it further consumes water, resulting in high internal drying and causing autogenous shrinkage. At the same time, UHPC uses the principle of closest packing to achieve high strength, and its internal structure is dense. When autogenous shrinkage occurs, it is difficult for external curing moisture to enter the interior, and therefore materials with a certain water absorption and storage capacity need to be added to UHPC to alleviate and suppress its autogenous shrinkage from the inside. Internal curing involves introducing absorbent materials as curing media, which release water to low-water-binder ratio cement-based materials as cement hydrates, thereby maintaining relative humidity within the cement-based materials and inhibiting shrinkage. Existing internal curing media primarily fall into two categories: polymer-based superabsorbent polymers (SAP) and porous inorganic materials (coral, expanded clay, etc.). Their effectiveness in internal curing within UHPC has been widely recognized. Porous inorganic materials absorb water through the capillary action of their internal pores by rationally replacing fine aggregate within UHPC, but their water absorption ratio and rate are low, limiting their internal curing effect. SAP, on the other hand, primarily stores water through physical adsorption within its polymer network, offering the advantages of high water absorption and high water retention. However, the residual pores after water release present a new defect.

[0003] In view of this, it is necessary to design an improved preparation method of SAP-loaded composite aggregate and its application in the internal curing of cement-based materials to solve the above problems. Summary of the Invention

[0004] The object of the present invention is to provide a method for preparing a SAP-loaded composite aggregate and application thereof in internal curing of cement-based materials.

[0005] To achieve the above-mentioned object of the invention, in a first aspect, the present invention provides a method for preparing a composite aggregate loaded with SAP, comprising the following steps:

[0006] The aggregate is immersed in the SAP prepolymer liquid, and the pores inside the aggregate are filled with the SAP prepolymer liquid; then, a polymerization and curing reaction is carried out to obtain a composite aggregate loaded with SAP;

[0007] The SAP prepolymer solution comprises monomer raw materials, a cross-linking agent and an initiator, and the mass ratio of the three is 1: (0.05-0.3%): (0.3-3%).

[0008] Preferably, the polymerization curing reaction temperature is 70-80° C. and the time is 1-2 hours.

[0009] Preferably, the SAP is at least one of polyacrylates, polyacrylamides, and acrylic acid-acrylamide.

[0010] Preferably, the crosslinking agent is at least one of N,N-methylenebisacrylamide, divinylbenzene, trimethylolpropane triacrylate, pentaerythritol triacrylate, epichlorohydrin, and glutaraldehyde; and the initiator is ammonium persulfate or potassium persulfate.

[0011] Preferably, the temperature of the aggregate is 55-65° C. before the aggregate is immersed in the SAP prepolymer solution.

[0012] Preferably, the aggregate is a porous lightweight aggregate of natural, artificially sintered, and industrial waste types. The natural aggregate includes coral aggregate and volcanic slag, the artificially sintered aggregate includes clay aggregate, fly ash aggregate, and sintered silt aggregate, and the industrial waste aggregate includes expanded slag and biomass ash aggregate.

[0013] In a second aspect, the present invention provides a SAP-loaded composite aggregate.

[0014] In a third aspect, the present invention provides an application of a SAP-loaded composite aggregate in the internal curing of cement-based materials.

[0015] In a fourth aspect, the present invention provides a cement-based material comprising the SAP-loaded composite aggregate provided by the present invention, wherein the preparation method comprises the following steps:

[0016] After cement, fly ash and silica fume are mixed, river sand, SAP-loaded composite aggregate, water reducer and water are added to prepare mortar, and the mortar is cured at 19-21° C. and relative humidity ≥95% for 3-28 days to prepare the cement-based material.

[0017] Preferably, the mass ratio of cement, fly ash and silica fume is 65:25:15, the particle size range of river sand is 0-1.18 mm, and the amount added is 1.1 times the total mass of cement, fly ash and silica fume, wherein the part of the river sand with a particle size range of 0.6-1.18 mm is replaced by an equal amount of SAP-loaded composite aggregate by volume, and the replacement rate is 5-25%; the amount of water reducer added is 1.5% of the total mass of cement, fly ash and silica fume.

[0018] The beneficial effects of the present invention are:

[0019] 1. The present invention provides a method for preparing a SAP-loaded composite aggregate. This method involves in-situ polymerization within the pores of aggregate (porous lightweight aggregate such as coral sand) to form a super absorbent resin (SAP). This allows the coral sand to be loaded with SAP, resulting in a porous lightweight aggregate with both water absorption and water storage properties. The porous lightweight aggregate produced by this method can be used to replace river sand in cement-based materials, improving the internal curing and interfacial adhesion of cement-based materials, thereby enhancing the internal curing effect of cement-based materials and resolving the durability issues of existing concrete caused by shrinkage.

[0020] 2. The composite aggregate loaded with SAP provided by the present invention can utilize this unique structure to enhance the water absorption and water storage performance of the composite aggregate by loading SAP with a three-dimensional network structure in the aggregate. Secondly, the SAP load is first filled inside the aggregate and then polymerized. Since the filling process is achieved by means of the liquid fluidity of the SAP prepolymer, it can ensure that the aggregate is fully filled, which is conducive to improving the loading efficiency of SAP. Furthermore, the SAP with high water absorption and water storage performance forms multiple independent or interconnected water reservoirs in the pores inside the coral aggregate. When used as a curing material in concrete, it improves the water absorption of the coral aggregate. When the relative humidity of the surrounding matrix decreases, it can release water in time, extend the curing time, and inhibit the development of internal microcracks and self-shrinkage. The composite aggregate provided by the present invention achieves a comprehensive improvement in the water absorption performance, water storage performance and interface bonding performance of the material, and provides a solution with reference value for the development of coral sand in concrete structure engineering.

[0021] 3. The SAP-loaded composite aggregate provided by the present invention combines SAP with high water absorption and water storage properties with coral aggregate. The coral aggregate serves as a physical shell for the SAP inside. As the SAP slowly releases water, the matrix around the coral aggregate fully hydrates, generating more CSH gel. The CSH gel can fill the interface and micropores within the coral, reducing the porosity at the interface while improving the mechanical bite and chemical adhesion between the interfaces. Secondly, the porous structure of the coral aggregate can be used to slow the water release rate of SAP without affecting the formation of SAP water-release pores, providing dual effects for internal curing (internal curing and interfacial adhesion), meeting the requirements of internal curing of concrete to inhibit autogenous shrinkage while improving mechanical properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is a SEM image of the porous lightweight aggregate prepared in Example 1 of the present invention;

[0023] Figure 2 TG curve of the porous lightweight aggregate prepared in Example 1 of the present invention;

[0024] Figure 3 This is the liquid absorption rate curve of the porous lightweight aggregate prepared in Example 1 of the present invention;

[0025] Figure 4 The flexural and compressive strength results of concrete mortar specimens with different compositions in Example 1 of the present invention are shown;

[0026] Figure 5 The autogenous shrinkage test results of concrete mortar specimens with different compositions in Example 1 of the present invention are shown in FIG.

[0027] Figure 6 The effect of different compositions of concrete mortar specimens on fluidity in Example 1 of the present invention;

[0028] Figure 7 This is the effect of concrete mortar specimens with different compositions on the setting time in Example 1 of the present invention. DETAILED DESCRIPTION

[0029] In order to make the objectives, technical solutions and advantages of the present invention more clear, the present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0030] It should also be noted here that, in order to avoid obscuring the present invention due to unnecessary details, only structures and / or processing steps closely related to the solutions of the present invention are shown in the drawings, while other details that are not closely related to the present invention are omitted.

[0031] In addition, it should be noted that the terms "comprises", "includes" or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method, article or apparatus that includes a series of elements includes not only those elements, but also includes other elements not explicitly listed, or also includes elements inherent to such process, method, article or apparatus.

[0032] The present invention provides a method for preparing a SAP-loaded composite aggregate, comprising the following steps:

[0033] The aggregate is completely immersed in a SAP prepolymer solution until the aggregate is completely absorbed by the SAP prepolymer solution. Then, a polymerization and curing reaction is carried out to load the aggregate with SAP, producing a SAP-loaded composite aggregate with a particle size of 0.6-1.18 mm. The SAP prepolymer solution includes a monomer raw material, a crosslinking agent, and an initiator in a mass ratio of 1: (0.05-0.3%): (0.3-3%).

[0034] In the above technical solution, by compounding SAP and aggregate (coral sand), a composite aggregate with both water absorption and water release properties can be produced, and its water absorption and release characteristics are as follows: in the early stage, SAP can significantly improve the water absorption and water storage performance of the porous lightweight aggregate; in the later stage, the process of SAP releasing water to the surrounding substrate promotes the hydration reaction and enhances the interfacial adhesion between the porous lightweight aggregate and the cement-based material, which is of great significance for inhibiting the self-shrinkage of concrete. At the same time, the porous lightweight aggregate can serve as a skeleton support for the SAP release holes, which has a significant improvement effect on the mechanical properties and durability of the concrete. On the one hand, the coexistence of porous lightweight aggregate and SAP can significantly improve the compressive strength, flexural strength and durability of concrete. SAP releases water as the humidity inside the concrete decreases, and promotes the strength growth of the cement-based material around the porous lightweight aggregate, avoiding cracks at the interface between the aggregate and the cement-based material under stress during the hydration reaction, thereby improving the compressive and flexural strength of the material. On the other hand, the synergistic effect of porous lightweight aggregate and SAP also slows down the water release rate of SAP inside the concrete: the humidity inside the concrete decreases, and the SAP releases water first through the porous lightweight aggregate and then transfers it to the cement-based material, avoiding the disadvantage of the local water-cement ratio increasing and the strength being reduced due to the excessive release of SAP. At the same time, the porous lightweight aggregate acts as a channel for the slow release and diffusion of water, guiding the uniform diffusion of water and avoiding the phenomenon of uneven water dispersion under the action of gravity.

[0035] In some embodiments, the SAP comprises at least one of acrylic acid, acrylamide, and acrylic acid-acrylamide, and its monomer raw materials are substances that can form the aforementioned categories. For example, when the SAP is an acrylic acid-based superabsorbent resin, its monomer raw materials include a neutralized acrylic acid solution, and the acrylamide is specifically acrylamide. When the SAP is an acrylic acid-acrylamide copolymer, the molar ratio of acrylic acid to acrylamide is (0-1):(1-0). Preparation of the SAP prepolymer solution includes: neutralizing the acrylic acid with a NaOH solution to a neutralization degree of 55-80%. The concentration of the NaOH solution can be selected as desired, as long as the neutralization purpose is achieved, and is not limited to this. The neutralized acrylic acid solution is mixed with acrylamide, and the mixture is prepared with deionized water to obtain a 30% mixed monomer solution.

[0036] In some embodiments, the crosslinking agent is a bifunctional crosslinking agent, a multifunctional crosslinking agent, and one of other types of crosslinking agents. The bifunctional crosslinking agent includes N,N-methylenebisacrylamide and divinylbenzene. The multifunctional crosslinking agent includes trimethylolpropane triacrylate and pentaerythritol triacrylate. Other types of crosslinking agents include epichlorohydrin and glutaraldehyde.

[0037] In some embodiments, the initiator includes at least one of ammonium persulfate and potassium persulfate.

[0038] In some embodiments, the aggregate includes lightweight aggregates such as 0.3-0.6 mm coral sand, 0.6-1.18 mm coral sand, and coral coarse aggregate. The temperature of the aggregate before immersing the aggregate in the SAP prepolymer liquid is 55-65°C to promote the reaction of the monomer raw material with the cross-linking agent and the initiator. It should be noted that the coral sand here refers to fine aggregate made by crushing and screening coral aggregate, which can be used to replace traditional fine aggregate in concrete and as an internal curing medium. In other embodiments, the aggregate can also be a porous lightweight aggregate of natural, artificial sintered, and industrial waste. Natural aggregates include coral aggregate and volcanic slag, artificial sintered aggregates include clay ceramsite, fly ash ceramsite, and sintered silt aggregate, and industrial waste includes expanded slag and biomass ash aggregate.

[0039] In some embodiments, the polymerization curing reaction temperature is 70-80° C. and the time is 1-2 hours.

[0040] The preparation method of the SAP-loaded composite aggregate proposed by the present invention and its application in the internal curing of cement-based materials are further described below with reference to specific embodiments:

[0041] In the embodiment part, coral sand is used as the matrix aggregate, and its particle size is 0.6-1.18 mm, which is purchased from Beihai Fuqida Trading Co., Ltd.; SAP is acrylic acid-acrylamide type SAP, and the monomer ratio is 1:1. The acrylic acid-acrylamide type SAP includes the following components by mass fraction: 30% by mass fraction of NaOH solution and acrylic acid solution (purity>99%) are neutralized to obtain a neutralization degree of 65%, and the concentration of the mixed solution of acrylic acid neutralization solution and acrylamide (purity>99.9%) is 30%.

[0042] Example 1

[0043] This embodiment prepares a SAP-loaded composite aggregate, and the preparation method thereof comprises the following steps:

[0044] S1. Mix 30% by mass of sodium hydroxide and acrylic acid solution (purity>99%) in an ice-water bath to obtain a neutralized solution with a neutralization degree of 65%; mix the neutralized solution with acrylamide (purity>99.9%) to obtain a monomer solution with a mass fraction of 30%, stir well, heat in a 25°C water bath, and stir for 10 minutes; raise the water bath temperature to 45°C, add N,N-methylenebisacrylamide (cross-linker), stir for 10 minutes, then raise the water bath temperature to 65°C, add initiator ammonium persulfate (initiator), and continue stirring for 5 minutes to obtain a SAP prepolymer solution; wherein, the mass ratio of the monomer solution to N,N-methylenebisacrylamide is 1:0.001, and the mass ratio of the monomer solution to ammonium persulfate is 1:0.036.

[0045] S2. Put coral sand with a particle size of 0.6-1.18 mm into a gauze bag and completely immerse it in the SAP prepolymer liquid. As the surface and interior of the coral sand are completely filled and wrapped by the SAP prepolymer liquid, take out the gauze bag containing the coral sand. After the excess solution is filtered out under the action of gravity (usually 5-10 minutes), the excess SAP prepolymer liquid adhering to and wrapped on the surface of the coral sand is removed by stirring and filtering (filtering until no water droplets fall). After sealing, polymerize and cure at 80°C for 2 hours. The SAP prepolymer liquid inside the coral sand is polymerized to form SAP, thereby obtaining a composite aggregate with loaded SAP.

[0046] SEM images of coral sand before and after loading SAP Figure 1 As shown, Figure 1 Figure (a) is the SEM image of coral sand before loading SAP (the left image is the original image, and the right image is a partial enlarged image). Figure 1 Figure (b) is the SEM image of the coral sand after loading with SAP (the left image is the original image, and the right image is a partial enlarged image). Comparative analysis shows that the pore structure of the coral sand after loading with SAP is filled with SAP.

[0047] The TG curve of porous lightweight aggregate is as follows: Figure 2 The results show that the thermal decomposition temperature of SAP is between 400-480℃, while the main component of coral sand is CaCO3, which decomposes at a temperature range of 625-800℃. Based on the percentage of the ignition residue from the thermogravimetric reaction curve, the coral-loaded SAP content is calculated to be 1.5%, which is consistent with the water absorption rate calculation data of SAP-loaded coral sand and unloaded coral sand. The liquid absorption rate curve of porous lightweight aggregate is shown in Figure 2. Figure 3 As shown, Figure 3 Figure (a) shows the liquid absorption rate of 100g of porous lightweight aggregate with a particle size of 0.6-1.18mm in deionized water. Figure 3 Figure (b) shows the liquid absorption rate of 100g of porous lightweight aggregate with a particle size of 0.6-1.18mm in cement filtrate, where the cement filtrate is prepared by mixing cementitious material (cement, fly ash, and silica fume in a mass ratio of 65:25:15): water in a ratio of 1:5, stirring for 24h (stirring for 5min per hour), and then filtering the supernatant to obtain it. During the stirring process, the whole is sealed with plastic wrap to prevent carbonization.

[0048] Liquid absorption rate test steps:

[0049] a) Weigh 100g of sample to the nearest 0.1g and record the mass as m. Add all the samples into a beaker;

[0050] b) adding deionized water and cement slurry filtrate to a beaker in sequence for soaking for 10 min, 1 h, 4 h, 8 h, and 24 h;

[0051] c) Quickly transfer the soaked sample from the beaker to a ceramic container on a filtration flask connected to a circulating water vacuum pump. Place filter paper at the bottom of the ceramic container and filter for 3-5 minutes. When no solution drops, turn off the instrument, wait for 1 minute, and then turn it on again one or two times to ensure the filtration effect.

[0052] d) Weigh the mass of the sample after filtration as m1;

[0053] e) Place the filtered sample in a 60°C electric blast drying oven and dry it for 24 hours, and weigh the dried sample again (m2);

[0054] f) Repeat the above experiment and weigh the dry filter paper m3; the mass of the filter paper after filtration m4. The liquid absorption rate is calculated as (1):

[0055]

[0056] In the formula: ω is the liquid absorption rate; m1 is the mass of water absorbed by the sample after filtration at a certain moment, in g; m2 is the mass of the sample after drying in the drying oven, in g; m3 is the mass of the dried filter paper, in g; m4 is the mass of the filter paper after filtration, in g; m is the mass of the weighed sample, in g.

[0057] The sample is tested three times and the arithmetic mean is taken as the test result. The result is accurate to one decimal place. If one of the measured values ​​exceeds 10% of the median value, the median value should be taken as the sample's absorbency test result. If two measured values ​​exceed 10% of the median value, the test should be repeated.

[0058] The results showed that after 8 hours, the water absorption rate of the SAP-loaded coral sand reached 45%, while the water absorption rate of the unloaded coral sand was only 11%. More significantly, the liquid absorption rate of the SAP-loaded coral sand was 30%, which was three times higher than that of the unloaded coral sand. The physical properties of the sand used in concrete were then tested, as shown in Table 1.

[0059] Table 1 Physical properties of fine aggregate

[0060]

[0061] Note: RS represents river sand with a particle size of 0-1.18 mm; CS represents coral sand with a particle size of 0.6-1.18 mm; CS+SAP represents SAP-loaded coral sand with a particle size of 0.6-1.18 mm.

[0062] Referring to the national standard JGJ / T70-2009, the porous lightweight aggregate prepared in this embodiment was further configured into mortar, and the compressive strength and autogenous shrinkage tests were performed on the specimens. The specimen preparation method was as follows: cement, fly ash, and silica fume were mixed in a mass ratio of 65:25:15, and then river sand, coral sand or SAP-loaded coral sand, polycarboxylate water reducer (the addition amount was 1.5% of the sum of the mass of cement, fly ash, and silica fume), and water were added to prepare mortar. The water-cement ratio of the mortar was w / c = 0.18. The obtained concrete mortar specimens were placed in a standard curing box for curing (20±1°C (according to GB / T 17671-2021), relative humidity must be ≥95%).

[0063] The specific composition of the concrete mortar specimens is shown in Table 2, where the meanings of the symbols in the table are as follows: Cement represents cement, FA represents fly ash, SF represents silica fume, Sand represents the total amount of coral sand, w / b represents the designed water-binder ratio (i.e., water: (cement + fly ash + silica fume)), (w / b) e Indicates the additional water-binder ratio released by SAP, (w / b) Trepresents the actual total water-binder ratio (i.e., the designed water-binder ratio + the additional water-binder ratio released by SAP), Coral+SAP / Coral Sand / v% (0.6-1.18 mm) represents the proportion of SAP-loaded coral sand in the particle size range of 0.6-1.18 mm, SP / % represents the dosage of polycarboxylate superplasticizer (polycarboxylate-type high-efficiency water-reducing agent with a solid content of 44% and a water-reducing efficiency greater than 45%), CS0 represents the mortar control group obtained by using ordinary coral sand without SAP loading, 5CS / 10CS / 15CS / 20CS / 25CS+AW represent mortars prepared under the volume replacement of SAP-loaded coral sand in the particle size range of 0.6-1.18 mm, for example, 5CS+AW means that the volume replacement rate of river sand in the mortar with a particle size range of 0.6-1.18 mm by SAP-loaded coral sand is 5%, and additional water corresponding to the content of SAP-loaded coral sand is included; 15CS+AW represents the volume replacement of river sand in the mortar under this condition. The river sand with a particle size of 0.6-1.18 mm is replaced by SAP-loaded coral sand at a volume rate of 15%, and additional water corresponding to the content of SAP-loaded coral sand is included; 15CS-AW represents the river sand with a particle size of 0.6-1.18 mm in the mortar with a volume rate of 15%, and no additional water corresponding to the content of SAP-loaded coral sand is included; 15C+AW represents the river sand with a particle size of 0.6-1.18 mm in the mortar with a volume rate of 15%, and additional water corresponding to the content of pure coral sand is included, where the proportion of additional water is calculated by the ratio of the composite material to the stable water absorption rate and the relevant composite material, where the stable liquid absorption rate of CS is 0.3 g / g and the stable liquid absorption rate of pure coral sand is 0.1 g / g. The calculation of the equal volume amount of coral sand can be carried out according to the following formula, and the calculation method is shown in formula (2):

[0064]

[0065] Where v is the volume of coral sand replaced; m5 is the mass of river sand in g; ρ1 is the apparent density of river sand, which is 2630 kg / m 3 m6 is the mass of SAP-loaded coral sand, in g; ρ2 is the apparent density of SAP-loaded coral sand, which is 2280 kg / m 3 .

[0066] Table 2 Specific composition of concrete mortar specimens

[0067]

[0068]

[0069] According to the national standard JGJ / T70-2009, the flexural and compressive strength of the concrete mortar specimens prepared under the formula in Table 2 were tested. The test results are as follows: Figure 4 As shown, Figure 4 Figure (a) shows the compressive strength results. Figure 4 Figure (b) shows the flexural strength results. The results show that the replacement rate of SAP-loaded coral sand is negatively correlated with the early strength: the strength of the coral aggregate is slightly reduced. The main reason is that the dynamic water absorption and release of SAP is regulated according to the internal moisture demand. After SAP absorbs water, the coral aggregate is lubricated, resulting in poor ITZ density. After water release, the local water-cement ratio increases, which is not conducive to early strength. After 7 days, the strength rebounded slightly, indicating that the later water release promoted hydration and increased density. After 28 days, it was basically the same as the control group. The flexural strength is more sensitive to the porosity and interface integrity of the coral. This shows that the defects formed by the coral aggregate and the cement matrix after sufficient additional water in the early stage are limited. Comprehensive analysis shows that the compressive and flexural strengths of 15CS+AW are as high as 126MPa and 19MPa. Compared with the river sand control group (0CS) of 128MPa and 19MPa, the compressive strength is only reduced by 1.5%, and the flexural strength remains almost unchanged. The autogenous shrinkage test results of concrete mortar specimens are shown in the figure below. Figure 5 As shown in the results, the autogenous shrinkage of 15CS+AW was reduced by 44% compared with the control group, and this group had the best performance. Table 2 Effect of the concrete mortar specimens prepared under the formula on the fluidity Figure 6 As shown in Table 2, the effect of the concrete mortar specimens prepared under the formula on the setting time is as follows Figure 7The results show that the overall fluidity of the mortar remains essentially unchanged or slightly increases after the addition of coral sand. This suggests that the early water absorption mechanism of coral sand loaded with SAP and the additional water replenishment mechanism significantly buffers early water loss and improves particle lubricity during SAP absorption. An increase in the replacement rate did not lead to a decrease in fluidity; sufficient additional water and an increased water absorption gradient after the coral sand was loaded with SAP had minimal impact on fluidity. The lowest fluidity was observed when the coral sand was loaded with SAP at a 15% SAP content but without additional water. SAP absorption caused early water depletion in the mortar, resulting in dry particles and difficulty in mixing. At a 15% coral sand content alone, the lack of SAP loading likely reduced the additional water content, leading to poor particle lubricity. Furthermore, compared to the control (0CS), the setting time of the 5CS+AW to 25CS+AW content increased slightly with increasing replacement rate. This is because SAP dynamically regulates moisture, delaying local drying, which overall facilitates internal curing but delays setting. The initial and final setting times of coral sand loaded with 15% SAP but without added water were slightly shorter than those of the sample with added water. This was primarily due to a lack of water in the slurry, which resulted in a "false setting" phenomenon, resulting in early crusting but limited hydration later on, which was detrimental to the uniform development of structural strength. The setting time of the 15% coral sand unloaded with SAP was similar to that of the control, indicating that the coral sand maintained its hydration rhythm after water addition. The overall setting time of the slurry without additional water was faster than that of the coral sand loaded with 15% SAP and with added water.

[0070] The above results show that the SAP-loaded composite aggregate not only effectively overcomes the problems of SAP water-release pores reducing strength and insufficient water absorption capacity of single coral sand limiting internal curing effect, but also significantly improves the mechanical properties of concrete after replacing river sand with marine material coral sand, fully demonstrating its superiority in durability and internal curing, and is a highly promising shrinkage-reducing lightweight aggregate.

[0071] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, it should be understood by those skilled in the art that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for preparing a SAP-loaded composite aggregate, characterized in that: The steps include: The aggregate is immersed in the SAP prepolymer liquid, and the pores inside the aggregate are filled with the SAP prepolymer liquid; then, a polymerization and curing reaction is carried out to obtain a composite aggregate loaded with SAP; The SAP prepolymer solution comprises monomer raw materials, a cross-linking agent and an initiator, and the mass ratio of the three is 1: (0.05-0.3%): (0.3-3%).

2. The preparation method according to claim 1, characterized in that The polymerization curing reaction temperature is 70-80° C. and the time is 1-2 hours.

3. The preparation method according to claim 1, characterized in that SAP is polyacrylic acid salt, polyacrylamide, and acrylic acid-acrylamide.

4. The preparation method according to claim 1, characterized in that The crosslinking agent is at least one of N,N-methylenebisacrylamide, divinylbenzene, trimethylolpropane triacrylate, pentaerythritol triacrylate, epichlorohydrin, and glutaraldehyde; and the initiator is ammonium persulfate or potassium persulfate.

5. The preparation method according to claim 1, characterized in that The temperature of the aggregate before being immersed in the SAP prepolymer solution is 55-65°C.

6. The preparation method according to claim 1, characterized in that The aggregates are porous lightweight aggregates of natural, artificially sintered, and industrial waste types. Natural aggregates include coral aggregates and volcanic slag, artificially sintered aggregates include clay aggregate, fly ash aggregate, and sintered silt aggregate, and industrial waste aggregates include expanded slag and biomass ash aggregate.

7. A SAP-loaded composite aggregate prepared by the preparation method according to any one of claims 1 to 6.

8. Use of the SAP-loaded composite aggregate prepared by the preparation method according to any one of claims 1 to 6 or the SAP-loaded composite aggregate according to claim 7 in internal curing of cement-based materials.

9. A cement-based material, characterized in that: The SAP-loaded composite aggregate prepared by the preparation method according to any one of claims 1 to 6 comprises the following steps: After cement, fly ash and silica fume are mixed, river sand, SAP-loaded composite aggregate, water reducer and water are added to prepare mortar, and the mortar is cured at 19-21° C. and relative humidity ≥95% for 3-28 days to prepare the cement-based material.

10. The cement-based material according to claim 9, characterized in that The mass ratio of cement, fly ash and silica fume is 65:25:

15. The particle size of river sand ranges from 0 to 1.18 mm, and the amount added is 1.1 times the total mass of the three ingredients. Among them, the part of the river sand with a particle size range of 0.6 to 1.18 mm is replaced by an equal amount of SAP-loaded composite aggregate, with a replacement rate of 5-25%. The amount of water reducer added is 1.5% of the total mass of the three ingredients.

Images