A fly ash ultralightweight aerated concrete based on silica fume slurry and its preparation method

Abstract

This invention discloses an ultra-lightweight aerated concrete based on silica fume slurry and its preparation method, belonging to the field of building materials technology. The ultra-lightweight aerated concrete of this invention comprises the following components by weight percentage: CBCSA cementitious material: 30-40%; secondary fly ash: 36-57%; silica fume slurry: 10-20%; natural gypsum: 3-4%; and additives comprising the following percentages by weight of the above components: aluminum powder paste: 3-5‰; sodium dodecylbenzenesulfonate: 4.0‰. The product of this invention has high production efficiency and features low shrinkage, high strength, and low thermal conductivity.

Description

Technical Field

[0001] This invention relates to the field of building materials technology, specifically to an ultra-lightweight aerated concrete based on silica fume slurry made from fly ash and its preparation method. Background Technology

[0002] Autoclaved aerated concrete (AAC) blocks are porous silicate concrete made from cement, lime, sand (fly ash, stone powder, etc.), aluminum paste, admixtures, etc., through processes such as ball milling, metering, mixing and pouring, gas expansion, static curing, cutting, and autoclaving.

[0003] However, the thermal insulation effect of aerated concrete is not yet satisfactory. Under conventional production processes, the oven-dry density of aerated concrete usually needs to be controlled at 500–600 kg / m³. 3 (i.e., B05 and B06 grades), with a thermal conductivity of approximately 0.12–0.16 W / (m·K). To further reduce the thermal conductivity and improve the insulation and energy-saving effect, the dry density must be significantly reduced to 300 kg / m³. 3 However, at this point, the product is very prone to problems such as increased shrinkage, deterioration of pore structure, and even casting collapse, making it difficult to meet the requirements of ultra-lightweight, high strength, and low thermal conductivity.

[0004] In traditional aerated concrete, the quartz sand or fly ash used as siliceous materials have limited reactivity. Under low-density conditions, the formation of hydration products is insufficient, and the development of major strength-contributing crystalline phases such as tobermorite is incomplete, resulting in a loose skeleton and excessively low strength in the finished product. Simultaneously, the bubble stability of the low-density slurry deteriorates, making it prone to bubble coalescence, escape, or slurry sedimentation, leading to uneven green body structure and low yield. Although some studies have attempted to improve performance by adding ultrafine slag, metakaolin, or adjusting the autoclaving regime, none of these methods have fundamentally solved the problem of ultra-lightweight aerated concrete (dry density ≤300 kg / m³). 3 The challenges of large-scale production and engineering application of ( ).

[0005] Silica fume is a byproduct of smelting ferrosilicon alloys or industrial silicon. Its main component is amorphous silica, with extremely fine particles (average particle size 0.1–0.3 μm) and a huge specific surface area, exhibiting extremely high pozzolanic activity. Currently, silica fume is widely used in high-strength concrete and self-compacting concrete. However, in the field of aerated concrete, especially in the preparation of ultra-lightweight aerated concrete as a primary siliceous material, a mature technical solution has yet to be found. In the few existing reports, silica fume is mostly used in small amounts as an external admixture to improve the later-stage strength or impermeability of aerated concrete. Furthermore, silica fume is usually used in a denser or thicker dry powder form, which presents problems such as difficulty in dispersion, high water demand, and a sharp increase in slurry viscosity. This can easily lead to decreased slurry fluidity and hindered gas generation, limiting its dosage and effectiveness in ultra-lightweight products. In addition, in ultra-lightweight aerated concrete slurry, the volume ratio of air bubbles exceeds 85%, requiring the slurry to thicken and harden rapidly after gas generation to stabilize the bubbles and prevent their coalescence. Therefore, ultra-lightweight aerated concrete requires high-performance cementitious materials with good setting and hardening properties.

[0006] Based on the above background, this invention proposes an ultra-lightweight aerated concrete and its preparation method, which uses silica fume slurry as a secondary siliceous material and CaO-C2S-C4A3 (CBCSA) cementitious material as a calcareous material. The aim is to fully utilize the advantages of silica fume and special cementitious materials through the slurry treatment and mix ratio optimization of silica fume, and to construct a uniform and dense microporous structure, thereby significantly reducing the dry density of the product while maintaining high compressive strength and excellent thermal insulation performance. Summary of the Invention

[0007] The purpose of this invention is to provide an ultra-lightweight aerated concrete based on silica fume slurry and its preparation method. This aerated concrete uses calcined CaO-C2S-C4A3 (CBCSA) cementitious material as the main calcareous material, fly ash as the main siliceous material, dispersed silica fume slurry as the secondary siliceous material, natural gypsum as a setting regulator, aluminum powder paste as an air-entraining agent, and sodium dodecylbenzenesulfonate as a foam stabilizer. It is prepared using autoclaving. The product has high production efficiency and features low shrinkage, high strength, and low thermal conductivity.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows: In a first aspect, the present invention provides a fly ash ultralight aerated concrete based on silica fume slurry, comprising the following components by weight percentage: CBCSA cementitious material: 30~40%; Secondary fly ash: 36~57%; Silica slurry: 10~20%; Natural gypsum: 3-4%; And additives comprising the following percentages by weight of the above components: Aluminum powder paste: 3~5‰; Sodium dodecylbenzenesulfonate: 4.0‰.

[0009] Furthermore, the CBCSA cementitious material comprises the following chemical components by weight percentage: CaO 67%~76%, SiO2 8%~16%, Al2O3 5%~9%, Fe2O3 1.0~1.5%, MgO 1.5~1.6%, SO3 2.9~3.8%, Na2O and K2O 0.4~0.6%, and TiO2 0.2~0.5%.

[0010] Furthermore, the CBCSA cementitious material comprises the following mineral components by weight percentage: 25%~41% active calcium oxide, 24%~45% C2S, 5.5%~11.9% C4A3, 1.8%~3.3% C4AF, 0.8%~1.2% f-CaSO4, and 10.5%~12.6% amorphous components.

[0011] The amorphous state is a vitreous substance with no definite chemical or mineral composition.

[0012] Furthermore, the secondary fly ash has a water requirement ratio of <95%, an activity index of >65%, a SiO2 content of >55%, and an Al2O3 content of >29%.

[0013] Furthermore, the silica paste is prepared according to the following method: Step 1. Weigh 2 kg of silica fume, add 3 kg of water, and mix evenly using a high-speed mixer to obtain the first slurry; Step 2. Add 60-120g of fly ash glass microspheres, 12-16g of polycarboxylate superplasticizer, 1-3g of starch ether, and 1-3g of suspending agent to the first slurry, stir evenly, and obtain the second slurry. Step 3. Place the second slurry in a wet horizontal ball mill and grind for 10-20 minutes to obtain a silica slurry with a flowability of 190-210 mm.

[0014] Furthermore, gypsum dihydrate comprises the following mineral components by weight percentage: CaSO4·2H2O 79~80%, CaMg(CO3)2 20~21%.

[0015] In a second aspect, this invention provides a method for preparing the above-mentioned fly ash ultralight aerated concrete, comprising the following steps: S1. Mix the weighed CBCSA cementitious material, secondary fly ash, gypsum dihydrate, and sodium dodecylbenzene sulfonate evenly to obtain dry material; S2. Heat the water to 30~40℃ and weigh it at a ratio of 0.56~0.6 of the dry material mass, and set aside; S3. Dissolve the weighed aluminum powder paste in 20% water and stir until well mixed; S4. Add 80% water to the dry material and stir quickly for 2 minutes. Then add silica fume slurry and stir for another 1 minute to obtain the slurry. S5. Pour the solution containing aluminum powder paste into the slurry, stir quickly for 30 seconds, then pour it into the mold. After static curing and demolding, autoclave the mixture to obtain fly ash ultralight aerated concrete.

[0016] Furthermore, the specifics of autoclaving are as follows: After standing in a steam environment at 50℃ for 4 hours, the material is demolded to obtain a blank. The blank is placed in an autoclave and heated to 180°C and 1.0 MPa at a uniform rate within 2 hours. It is then kept at this temperature for 6 hours, and the pressure is released. After opening the autoclave, fly ash ultralight aerated concrete is obtained.

[0017] The high-performance, low-density aerated concrete prepared by this invention has the following mechanism: 1. CBCSA contains C4A3$ with high hydration activity. During the static resting stage after slurry pouring, C4A3$ and gypsum rapidly hydrate under alkaline conditions to produce AFt, which prevents bubbles from disproportionating and merging, thereby forming a uniform pore structure and preventing the green body from collapsing.

[0018] 2. Compared to the traditional cement-lime system, CBCSA (Super-Lightweight Aerated Concrete) slowly releases Ca(OH)₂ during autoclaving due to the lower hydration activity of C₂S compared to C₃S. This forms low-alkalinity CSH gel, which bonds together with increasing insulation time to form a dense whole, thus improving the strength of the super-lightweight aerated concrete. In contrast, the traditional cement-lime system, with its high alkalinity, forms high-alkalinity Type II CSH gel in the initial stage of autoclaving. As insulation time increases, this Type II CSH gel absorbs SiO₂, forming Type I CSH gel and tobermorite, resulting in higher porosity and lower strength compared to the CBCSA system.

[0019] 3. Adding highly active silica slurry allows it to rapidly dissolve SiO2 during the autoclaving stage, reducing the calcium-to-silicon ratio in the slurry solution and forming more low-calcium-to-silicon CSH gel, thereby improving strength.

[0020] Compared with the prior art, the present invention has the following beneficial effects: 1. The method for preparing silica paste according to the present invention is simple to operate and can be mass-produced.

[0021] 2. This invention combines silica fume slurry and CBCSA material, resulting in the formation of low-alkalinity CSH in the slurry during autoclaving, with no tobermorite formation, resulting in low porosity and high strength of the system.

[0022] 3. This invention uses CBCSA to replace traditional cement and quicklime. Due to the presence of the fast-setting and fast-hardening C4A3, it is possible to prepare 300kg / m³ cement. 3 When making ultra-lightweight aerated concrete, the collapse of the green body can be prevented. Attached Figure Description

[0023] Figure 1 This is a flowability test diagram of the silica slurry of the present invention.

[0024] Figure 2 This is a density test diagram of the fly ash ultralight aerated concrete of the present invention.

[0025] Figure 3 This is a CT scan image of the fly ash ultralight aerated concrete of the present invention. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments and accompanying drawings. Obviously, the described embodiments are merely some embodiments of this invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0027] In this embodiment of the invention, the CBCSA gelling material comprises the following chemical components by weight percentage: CaO 70.4%, SiO2 16%, Al2O3 7%, Fe2O3 1.5%, MgO 1.5%, SO3 3.0%, Na2O and K2O 0.4%, and TiO2 0.2%.

[0028] In this embodiment of the invention, the CBCSA cementitious material comprises the following mineral components by weight percentage: 25% active calcium oxide, 45% C2S, 9.0% C4A3, 2.0% C4AF, 1.0% f-CaSO4, 11.0% amorphous form, 4.2% C3A, and 2.8% C3S, where $ represents sulfur.

[0029] In the embodiments and comparative examples of this invention, the water requirement ratio of secondary fly ash is 93%, the activity index is 70%, the SiO2 content is 57%, and the Al2O3 content is 31%.

[0030] In this embodiment of the invention, the silica paste is prepared according to the following method: Step 1. Weigh 2 kg of silica fume, add 3 kg of water, and mix evenly using a high-speed mixer to obtain the first slurry; Step 2. Add 90g of fly ash glass microspheres, 14g of polycarboxylate superplasticizer, 2g of starch ether, and 2g of suspending agent to the first slurry, stir evenly, and prepare the second slurry. Step 3. Place the second slurry in a wet horizontal ball mill and grind for 15 minutes to obtain silica slurry with a flowability of 195 mm.

[0031] In this embodiment of the invention, the preparation method of fly ash ultralight aerated concrete includes the following steps: S1. Mix the weighed CBCSA cementitious material, secondary fly ash, gypsum dihydrate, and sodium dodecylbenzene sulfonate evenly to obtain dry material; S2. Heat the water to 30~40℃ and weigh it at a ratio of 0.58 of the dry material mass, and set aside; S3. Dissolve the weighed aluminum powder paste in 20% water and stir until well mixed; S4. Add 80% water to the dry material and stir quickly for 2 minutes. Then add silica fume slurry and stir for another 1 minute to obtain the slurry. S5. Pour the solution containing aluminum powder paste into the slurry, stir quickly for 30 seconds, and then pour it into the mold; place the mold in a steam environment at 50℃, let it stand for 4 hours, and then demold it; place the green body in an autoclave, raise it to 180℃ at a constant speed within 2 hours, pressurize it to 1.0MPa, keep it at that temperature for 6 hours, then depressurize it, and after opening the autoclave, you will get fly ash ultra-lightweight aerated concrete.

[0032] Example 1, as a preferred embodiment of the present invention, the fly ash ultralight aerated concrete of this embodiment comprises the following components by weight percentage: CBCSA cementitious material: 35%; Secondary fly ash: 46.5%; Silica mortar: 15%; Natural gypsum: 3.5%; And additives comprising the following percentages by weight of the above components: Aluminum powder paste: 5‰; Sodium dodecylbenzenesulfonate: 4.0‰.

[0033] Example 2, as a preferred embodiment of the present invention, the fly ash ultralight aerated concrete of this embodiment comprises the following components by weight percentage: CBCSA cementitious material: 40%; Secondary fly ash: 41.5%; Silica mortar: 15%; Natural gypsum: 3.5%; And additives comprising the following percentages by weight of the above components: Aluminum powder paste: 5‰; Sodium dodecylbenzenesulfonate: 4.0‰.

[0034] Example 3, as a preferred embodiment of the present invention, the fly ash ultralight aerated concrete of this embodiment comprises the following components by weight percentage: CBCSA cementitious material: 30%; Secondary fly ash: 51.5%; Silica mortar: 15%; Natural gypsum: 3.5%; And additives comprising the following percentages by weight of the above components: Aluminum powder paste: 5‰; Sodium dodecylbenzenesulfonate: 4.0‰.

[0035] Comparative Example 1: The fly ash ultralight aerated concrete of this comparative example comprises the following components by weight percentage: Quicklime: 8.8% P.O42.5R: 26.2% Secondary fly ash: 61.5%; Natural gypsum: 3.5%; And additives comprising the following percentages by weight of the above components: Aluminum powder paste: 5‰; Sodium dodecylbenzenesulfonate: 4.0‰.

[0036] Comparative Example 2, the fly ash ultra-lightweight aerated concrete of this comparative example, comprises the following components by weight percentage: Quicklime: 8.8% P.O42.5R: 26.2% Secondary fly ash: 46.5%; Silica mortar: 15%; Natural gypsum: 3.5%; And additives comprising the following percentages by weight of the above components: Aluminum powder paste: 5‰; Sodium dodecylbenzenesulfonate: 4.0‰.

[0037] The physical and mechanical properties and drying shrinkage of the aerated concrete in the examples and comparative examples were tested, and the results are shown in the table below.

[0038] Table 1 Mechanical properties and drying shrinkage properties of aerated concrete

[0039] As shown in Table 1, the bulk density of the comparative examples increased due to collapse. Examples 1-3 employing the technical solution of this invention have a bulk density ranging from 190 to 214 kg / m³. 3Within the specified range, the density was significantly lower than that of the comparative example. Although the comparative example had a higher density, its strength and drying shrinkage were essentially equivalent to those of the example due to uneven pore distribution caused by collapse. This indicates that CBCSA, in synergy with silica fume slurry, can produce high-performance ultra-lightweight aerated concrete.

[0040] The flowability of the silica slurry prepared according to this invention was tested, such as... Figure 1 As shown, its fluidity is 200 mm, which meets the fluidity requirements of aerated concrete slurry. Furthermore, observation from the image reveals that the silica fume slurry is uniform, without segregation or bleeding.

[0041] The bulk density of the aerated concrete prepared in Example 1 of this invention is as follows: Figure 2 As shown, it is 199.6 kg / m 3 Its density is far lower than that of ordinary aerated concrete.

[0042] CT scan of the aerated concrete prepared in Example 1 of this invention is as follows: Figure 3 As shown, from Figure 3 As can be seen, most of the pores are circular, and some pores have diverged and merged, with pore sizes ranging from 1 to 2 mm.

[0043] Finally, it should be noted that the above embodiments are merely preferred embodiments of the present invention used to illustrate the technical solutions of the present invention, and are not intended to limit the invention, nor are they intended to limit the patent scope of the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. These modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention. That is to say, any changes or refinements made to the main design concept and spirit of the present invention that are not of substantial significance, but whose technical problems are still consistent with the present invention, should be included within the protection scope of the present invention. In addition, the direct or indirect application of the technical solutions of the present invention to other related technical fields are similarly included within the patent protection scope of the present invention.

Claims

1. A fly ash ultralightweight aerated concrete based on silica fume slurry, characterized in that, It includes the following components by weight percentage: CBCSA cementitious material: 30~40%; Secondary fly ash: 36~57%; Silica slurry: 10~20%; Natural gypsum: 3-4%; And additives comprising the following percentages by weight of the above components: Aluminum powder paste: 3~5‰; Sodium dodecylbenzenesulfonate: 4.0‰.

2. The fly ash ultralight aerated concrete according to claim 1, characterized in that, CBCSA cementitious material comprises the following chemical components by weight percentage: CaO 67%~76%, SiO2 8%~16%, Al2O3 5%~9%, Fe2O3 1.0%~1.5%, MgO 1.5%~1.6%, SO3 2.9%~3.8%, Na2O and K2O 0.4%~0.6%, and TiO2 0.2%~0.5%.

3. The fly ash ultralight aerated concrete according to claim 1, characterized in that, CBCSA cementitious materials contain the following mineral components by weight percentage: active calcium oxide 25%~41%, C2S 24%~45%, C4A3S 5.5%~11.9%, C4AF 1.8%~3.3%, f-CaSO4 0.8%~1.2%, and amorphous 10.5%~12.6%.

4. The fly ash ultralight aerated concrete according to claim 1, characterized in that, Secondary fly ash has a water requirement ratio of <95%, an activity index of >65%, a SiO2 content of >55%, and an Al2O3 content of >29%.

5. The fly ash ultralight aerated concrete according to claim 1, characterized in that, Silica slurry is prepared according to the following method: Step 1. Weigh 2 kg of silica fume, add 3 kg of water, and mix evenly using a high-speed mixer to obtain the first slurry; Step 2. Add 60-120g of fly ash glass microspheres, 12-16g of polycarboxylate superplasticizer, 1-3g of starch ether, and 1-3g of suspending agent to the first slurry, stir evenly, and obtain the second slurry. Step 3. Place the second slurry in a wet horizontal ball mill and grind for 10-20 minutes to obtain a silica slurry with a flowability of 190-210 mm.

6. The fly ash ultralight aerated concrete according to claim 1, characterized in that, Gypsum dihydrate comprises the following mineral components by weight percentage: CaSO4·2H2O 79~80%, CaMg(CO3)2 20~21%.

7. A method for preparing fly ash ultralight aerated concrete as described in any one of claims 1-6, characterized in that, Includes the following steps: S1. Mix the weighed CBCSA cementitious material, secondary fly ash, gypsum dihydrate, and sodium dodecylbenzene sulfonate evenly to obtain dry material; S2. Heat the water to 30~40℃ and weigh it at a ratio of 0.56~0.6 of the dry material mass, and set aside; S3. Dissolve the weighed aluminum powder paste in 20% water and stir until well mixed; S4. Add 80% water to the dry material and stir quickly for 2 minutes. Then add silica fume slurry and stir for another 1 minute to obtain the slurry. S5. Pour the solution containing aluminum powder paste into the slurry, stir quickly for 30 seconds, then pour it into the mold. After static curing and demolding, autoclave the mixture to obtain fly ash ultralight aerated concrete.

8. The preparation method according to claim 7, characterized in that, The specific steps for autoclaving are as follows: After standing in a steam environment at 50℃ for 4 hours, the material is demolded to obtain a blank. The blank is placed in an autoclave and heated to 180°C and 1.0 MPa at a uniform rate within 2 hours. It is then kept at this temperature for 6 hours, and the pressure is released. After opening the autoclave, fly ash ultralight aerated concrete is obtained.

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