One-step cement-free concrete and preparation method thereof

Abstract

The invention discloses one-step cement-free concrete and a preparation method thereof, and belongs to the technical field of concrete preparation, the one-step cement-free concrete takes fly ash and slag as cementing materials, machine-made sand as fine aggregate, fine stone as coarse aggregate, sodium tetraborate decahydrate as a retarder, and solid sodium silicate as an alkali activator; wherein the water-binder ratio (the mass ratio of water to the binding material) is 0.36-0.48; the mass ratio (slag-ash ratio) of the slag in the cementing material is 10%-30%; based on the mass of sodium oxide, the mass ratio (sodium-binder ratio) of the alkali activator to the binding material is 5-7%. According to the one-step cement-free concrete disclosed by the invention, by virtue of the raw material selection of all the components and the optimal design of the proportion, the prepared one-step cement-free concrete has relatively high mechanical properties, engineering benefits and environmental benefits.

Description

Technical Field

[0001] The present invention belongs to the technical field of concrete preparation, and particularly relates to a one-step cementless concrete and a preparation method thereof. Background Art

[0002] Cementless concrete is a new type of concrete material with excellent performance, which is prepared by activating industrial wastes such as fly ash, slag or other materials rich in silicon and aluminum with an alkaline solution and mixing them with aggregates. It has many excellent properties such as good self-healing performance, crack resistance and chemical erosion resistance. Compared with traditional Portland cement, cementless concrete reduces CO2 emissions and energy consumption by 60-80%, and at the same time provides a reasonable way for the resource utilization of industrial by-products such as fly ash and slag. Cementless concrete is not only applicable to traditional construction fields, but also can be extended to other fields such as marine engineering, roads and bridges, and underground engineering, providing more reliable and durable structural materials for various projects, with broad application prospects and important social and economic benefits, and is of great significance for improving project quality, protecting the environment and promoting sustainable development.

[0003] Cementless concrete can be widely used in various aspects. However, traditional two-step cementless concrete needs to prepare an alkaline activator slurry first and then mix it with aggregates. The pre-mixing step of the slurry will introduce additional energy consumption, resulting in a 25-30% extension of the construction period, and there are problems such as cumbersome process steps and low construction efficiency. In addition, if the storage time of the two-step alkaline activator is too long, its activity will decline, and the construction window period needs to be strictly controlled, which hinders the engineering application. While OPCFC (one-step cementless concrete) does not need to pre-mix the alkaline activator slurry, and can directly mix the solid alkaline activator with the precursor materials, which significantly simplifies the construction process and saves construction time and labor costs. OPCFC takes process streamlining and deep carbon reduction as its core advantages and has the potential to replace traditional OPC (ordinary Portland cement concrete).

[0004] However, the current research on one-step cementless concrete only focuses on single work performance or mechanical performance indicators, does not consider the cross-coupling effects of mixing variables on one-step cementless concrete, lacks a systematic mix ratio optimization method, and studies the mechanical properties of this mix ratio on this basis. Therefore, it is urgent to study the mix ratio design method of one-step cementless concrete under the coordinated control of multiple variables, and realize the global performance optimization of work performance-mechanical performance while ensuring the simplicity of the process. For this reason, the present invention proposes a one-step cementless concrete material and a preparation method thereof. Summary of the Invention

[0005] In view of the above technical problems, the present invention discloses a one-step cementless concrete and a preparation method thereof. Through the selection of raw materials and the design of mix ratios of each component, the prepared one-step cementless concrete has high mechanical properties.

[0006] The first aspect of the present invention discloses a one-step cementless concrete, which uses fly ash and slag as cementitious materials, manufactured sand as fine aggregate, fine gravel as coarse aggregate, sodium tetraborate decahydrate as a retarder, and solid sodium silicate as an alkali activator; wherein, the water-binder ratio (the mass ratio of water to cementitious materials) is 0.36 to 0.48; the mass proportion of the slag in the cementitious materials (slag-ash ratio) is 10% to 30%; calculated by the mass of sodium oxide, the mass ratio of the alkali activator to the cementitious materials (sodium-cement ratio) is 5% to 7%.

[0007] As a preference of the present invention, the modulus of the alkali activator is 1.6 to 1.8.

[0008] As a preference of the present invention, the manufactured sand is medium sand with a particle size of 0.25 mm to 0.5 mm; the particle size of the fine gravel is 10 mm to 20 mm.

[0009] As a preference of the present invention, the fly ash is Class F fly ash with a fineness of 10.8%, a loss on ignition of 4.6%, and a density of 2.3 g / cm 3 .

[0010] As a preference of the present invention, the slag is blast furnace S105 slag.

[0011] As a preference of the present invention, the dosage of the retarder is 3% to 6% of the mass of the cementitious materials.

[0012] As a preference of the present invention, the modulus of the alkali activator is 1.0 to 2.0.

[0013] As a preference of the present invention, the mass proportion of the slag in the cementitious materials is 10%; calculated by the mass of sodium oxide, the mass ratio of the alkali activator to the cementitious materials is 6%.

[0014] As a preference of the present invention, the modulus of the alkali activator is 1.6.

[0015] The purpose of the second aspect of the present invention is to provide a preparation method of the one-step cementless concrete. The steps include: weighing and adding the manufactured sand and the fine gravel into a mixer with the inner wall wetted and mixing dry for 2 min, then adding the cementitious materials, the alkali activator and the retarder to fully stir and mix the aggregate and the cementitious materials evenly; adding 50% of the total water amount under the stirring state, stirring for 2 min until the slurry is initially formed, then continuing to add the remaining water and stirring for another 2 min until the mixture reaches a uniform flow state, thus obtaining the one-step cementless concrete.

[0016] Compared with the prior art, the present invention has the following technical effects:

[0017] The one-step cement-free concrete prepared by the present invention uses solid waste fly ash and slag as cementitious materials, reducing carbon emissions; uses manufactured sand as fine aggregate, fine stone as coarse aggregate, sodium tetraborate decahydrate as a retarder, and solid sodium silicate as an alkali activator; by controlling the water-binder ratio to be 0.36 - 0.48, the slag-ash ratio to be 10% - 30%, and the sodium-binder ratio to be 5% - 7%, the prepared concrete has both good workability and mechanical strength. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The drawings forming a part of the present invention are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation to the present invention. In the drawings:

[0019] Figure 1 It is the compressive strength diagram of the OPCFC specimens prepared in Examples 1 - 9; wherein, A1 - A9 respectively correspond to Examples 1 - 9;

[0020] Figure 2 It is the workability diagram of the OPCFC specimens prepared in Examples 1 - 9; wherein, A1 - A9 respectively correspond to Examples 1 - 9;

[0021] Figure 3 It is the failure mode of the axial compression test of the five OPCFC specimens in Example 10;

[0022] Figure 4 It is the stress-strain curve of the axial compression test of the five OPCFC specimens in Example 10;

[0023] Figure 5 It is the peak strain curve of the axial compression test of the five OPCFC specimens in Example 10;

[0024] Figure 6 It is the failure mode of the splitting tensile test of the five OPCFC specimens in Example 10;

[0025] Figure 7 It is the load-strain curve of the splitting tensile test of the five OPCFC specimens in Example 10;

[0026] Figure 8 It is the tensile strength of the splitting tensile test of the five OPCFC specimens in Example 10. DETAILED DESCRIPTION OF THE INVENTION

[0027] Now, various exemplary embodiments of the present invention will be described in detail. This detailed description should not be considered as a limitation to the present invention, but should be understood as a more detailed description of certain aspects, characteristics, and implementation schemes of the present invention.

[0028] It should be understood that the terms used in the present invention are only for describing specific embodiments and are not intended to limit the present invention. Additionally, for the numerical ranges in the present invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Each intermediate value within any stated value or stated range, as well as each smaller range between any other stated value or intermediate value within the stated range, is also included in the present invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0029] Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention pertains. Although the present invention only describes preferred methods and materials, any methods and materials similar or equivalent to those described herein can also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials related to the said documents. In case of conflict with any incorporated document, the content of this specification shall prevail.

[0030] Without departing from the scope or spirit of the present invention, various improvements and changes can be made to the specific embodiments of the present invention specification, which are obvious to those skilled in the art. Other embodiments obtained from the present invention specification are obvious to those skilled in the art. The present invention specification and examples are merely exemplary.

[0031] Regarding the use of "comprising", "including", "having", "containing", etc. herein, they are all open-ended terms, meaning including but not limited to.

[0032] An embodiment of the present invention discloses a one-step cementless concrete, which uses fly ash and slag as cementitious materials, manufactured sand as fine aggregate, fine stones as coarse aggregate, sodium tetraborate decahydrate as a retarder, and solid sodium silicate as an alkali activator; wherein, the water-binder ratio is 0.36 - 0.48; the mass proportion of the slag in the cementitious materials is 10% - 30%; calculated by the mass of sodium oxide, the mass ratio of the alkali activator to the cementitious materials is 5% - 7%.

[0033] In some embodiments of the present invention, the modulus of the alkali activator is 1.6 - 1.8.

[0034] In some embodiments of the present invention, the manufactured sand is medium sand with a particle size of 0.25 mm - 0.5 mm; the particle size of the fine stones is 10 mm - 20 mm.

[0035] In some embodiments of the present invention, the fly ash is class F fly ash, and the chemical composition of the fly ash is shown in Table 1; the physical parameters of the fly ash are shown in Table 2.

[0036] Table 1 Chemical Composition Table of Fly Ash (unit: %)

[0037]

[0038] Table 2 Physical Parameter Table of Fly Ash

[0039]

[0040] In some embodiments of the present invention, the slag is blast furnace S105 slag. According to the provisions of GB / T 18046-2017 "Granulated Blast Furnace Slag Powder for Cement, Mortar and Concrete", the activity index of 28d≥105% is classified as S105 slag. The characteristics of the slag used in the embodiments of the present invention are shown in Table 3.

[0041] Table 3 Detection Results of S105 Slag

[0042]

[0043] In some embodiments of the present invention, the modulus of the alkali activator is 1.6-1.8. The alkali activator is solid sodium silicate. In the embodiments of the present invention, a sodium silicate reagent with a modulus of 1.0 and a sodium silicate reagent with a modulus of 2.0 are mixed and prepared to obtain the required modulus for the test.

[0044] In some embodiments of the present invention, the mass ratio of the slag in the cementitious material is 10%; calculated by the mass of sodium oxide, the mass ratio of the alkali activator to the cementitious material is 6%.

[0045] In some embodiments of the present invention, the modulus of the alkali activator is 1.6.

[0046] In some embodiments of the present invention, the dosage of the retarder is 3%-6% of the cementitious material dosage. The retarder in the embodiments is sodium tetraborate decahydrate; the test water is tap water, meeting the requirements of "Mixing Water for Concrete" (JGJ63-2006).

[0047] The embodiments of the present invention also provide a preparation method of one-step cementless concrete. The steps include: weighing and adding machine-made sand and fine stones according to the ratio into a mixer with a wetted inner wall, mixing and dry-stirring for 2 min, then adding the cementitious material, alkali activator and retarder, and fully stirring the aggregate and the cementitious material evenly; adding 50% of the total water amount under the stirring state, stirring for 2 min until the slurry is initially formed, then continuing to add the remaining tap water, and stirring for another 2 min until the mixture reaches a uniform flow state, thus obtaining the one-step cementless concrete.

[0048] All raw materials used in the embodiments of the present invention are obtained by purchasing from the market.

[0049] Examples 1-9

[0050] Preparation method of one-step cementless concrete, the steps include: weighing raw materials according to the raw material ratio in Table 4; weighing and adding manufactured sand and fine aggregate into the mixer with wetted inner wall according to the ratio and mixing dry for 2 minutes, then adding cementitious material, alkali activator and retarder to fully stir and mix the aggregate and cementitious material evenly; adding 50% of the total water amount under stirring state, stirring for 2 minutes until the slurry is initially formed, then continuing to add the remaining tap water, and stirring for another 2 minutes until the mixture reaches a uniform flow state, thus obtaining the one-step cementless concrete slurry. After injecting the freshly mixed slurry into the forming mold coated with release agent, place it on the vibrating table for mechanical compaction, and then complete the surface leveling treatment. After the specimens are sealed and cured with polyethylene film for 24 hours, demold them and transfer them to the standard curing water tank for water curing until the age of 28 days. During the whole curing process, the specimens are completely immersed below the curing liquid level to ensure the full progress of the hydration reaction. When the materials reach the predetermined age, perform performance experiments according to the test plan.

[0051] Table 4 Mix proportion of specimens in Examples 1 - 9 (kg / m 3 )

[0052]

[0053] Note: The sodium-cement ratio of Example 1, Example 6 and Example 8 is 5%, and the average modulus of the alkali activator is 1.8; the sodium-cement ratio of Example 2, Example 4 and Example 9 is 7%, and the average modulus of the alkali activator is 1.6; the sodium-cement ratio of Example 3, Example 5 and Example 7 is 6%, and the average modulus of the alkali activator is 1.7.

[0054] Detect the compressive strength and workability of the concrete specimens prepared in Examples 1 - 9, the compressive strength is shown in Figure 1 ; the workability is shown in Figure 2 . Among them, Figure 1-2 , A1 - A9 respectively correspond to Examples 1 - 9. From Figures 1 to 2It can be seen that the compressive strengths of the specimens prepared in Examples 1-9 are all greater than 60 MPa. There is a significant negative correlation between the water-binder ratio and the compressive strength of OPCFC. When the water-binder ratio is 0.36, the OPCFC specimens exhibit the most excellent compressive strength, among which the specimen prepared in Example 2 has the highest compressive strength, reaching 89.082 MPa. As the water-binder ratio gradually increases, the compressive strength shows a gradually decreasing trend. The possible reason is that during the hydration reaction process, the change of the water-binder ratio directly affects the consistency, fluidity of the OPCFC paste and the degree of hydration reaction. A lower water-binder ratio means relatively less water around the cementitious particles. During the hydration reaction process, the cementitious particles can be more closely packed, forming a denser microstructure, thereby effectively improving the compressive strength of the material. When the water-binder ratio is too high, excessive water will form pores inside the material after hardening. These pores become the weak links inside the material and are prone to stress concentration when bearing pressure, thereby reducing the compressive strength of the material.

[0055] In terms of the slag-ash ratio, the experimental results show that the slag-ash ratio plays a key role in the performance regulation of OPCFC. The change of the slag-ash ratio will affect the reaction degree inside the material and the interfacial bonding strength between the aggregate and the cementitious material. When the slag-ash ratio is at the lowest value, the performance of OPCFC is the best. Too high a slag-ash ratio may damage the structural balance inside the material, which is also not conducive to the improvement of the material performance. Therefore, the optimal slag-ash ratio is 10%.

[0056] For the sodium-binder ratio, when the sodium-binder ratio is 6%, the performance of the OPCFC specimens is the most stable and can better meet the requirements of compressive strength. When the sodium-binder ratio is too high or too low, it is difficult for OPCFC to obtain ideal compressive performance. When the sodium-binder ratio is too low, the concentration of the alkali activator is insufficient, and the activity of the cementitious material cannot be fully stimulated, resulting in incomplete hydration reaction and reduced material strength. When the sodium-binder ratio is too high, it may cause excessive chemical reactions, resulting in unstable internal structure of the material, which will also reduce the compressive strength of the material.

[0057] In terms of the activator modulus, when the activator modulus is too low, it will have an adverse effect on the working performance and mechanical properties of OPCFC. When the activator modulus is 1.6, the comprehensive performance of OPCFC reaches a better state. Further increasing the modulus can not only improve the compressive strength of OPCFC, but will instead lead to a decline in its working performance. The change of the activator modulus will affect the alkalinity of the alkali activator and the reaction kinetic process. A lower activator modulus means a weaker alkalinity of the alkali activator, which cannot effectively promote the hydration reaction of the cementitious material, thus affecting the material performance. While too high an activator modulus may make the reaction too intense, resulting in an increase in internal structural defects of the material, thereby reducing the working performance and compressive strength of the material.

[0058] Example 10

[0059] Through in-depth analysis of the key performance index of the compressive strength of OPCFC, based on the principle of maximizing the signal-to-noise ratio in Taguchi design method, the mix proportion was optimized, the influence law of each variable parameter on the performance of OPCFC was systematically evaluated, and the factor combination that maximizes the compressive strength of OPCFC under the combined action of multiple variable factors was successfully determined. And because the water-binder ratio variable shows a monotonic change, the change of the water-binder ratio can stably control the mechanical properties of OPCFC. Therefore, in this embodiment, the slag-ash ratio is fixed at 10%, the sodium-binder ratio is 6%, and the modulus of the alkali activator is 1.6. Five OPCFC specimens are prepared with the water-binder ratio as the variable (0.36, 0.39, 0.42, 0.45, and 0.48), and systematic research on their basic mechanical properties is carried out. The mix proportions of each specimen are shown in Table 5.

[0060] Table 5 Mix Proportions of OPCFC Specimens (kg / m 3 )

[0061]

[0062] The preparation method of each specimen is as follows:

[0063] For the one-step cementless concrete, a planetary mixer is used. The specific mixing sequence is as follows: Mechanically crushed sand and fine stones are weighed according to the ratio and added to the mixer with the inner wall wetted and mixed dry for 2 minutes, then the cementitious material, alkali activator, and retarder are added to fully stir the aggregate and the cementitious material evenly; 50% of the total water amount is added under the stirring state, and after stirring for 2 minutes until the slurry is initially formed, the remaining tap water is continuously added, and then stirred for 2 minutes until the mixture reaches a uniform fluid state, that is, the one-step cementless concrete slurry is obtained. After the freshly mixed slurry is injected into the forming mold coated with a release agent, it is placed on a vibrating table for mechanical compaction, and then the surface leveling treatment is completed. The specimen is demolded after being sealed and cured with a polyethylene film for 24 hours, and then transferred to a standard curing water tank for water curing until the age of 28 days. During the entire curing process, the specimen is completely immersed below the curing liquid level to ensure full hydration reaction. When the material reaches the predetermined age, performance experiments are carried out according to the test plan.

[0064] I. Static axial compression performance test on 5 specimens

[0065] Test method: Before the axial compression test, ensure that both ends of the specimen are flat and strain gauges are pasted. First, remove dust and other impurities from the surfaces at both ends of the specimen, apply high-strength gypsum for leveling in sequence, and use a level for calibration to ensure the flatness of the specimen. Finally, at the circumferential quarter position in the middle of the specimen, longitudinal and transverse strain gauges are alternately pasted to measure axial strain and circumferential strain respectively. According to ASTM C39M, a 500-ton MATEST press is used for the axial compression test, and the test rate is 0.18 mm / min. During the test, the axial deformation of the specimen is measured by two symmetric displacement transducers (LVDT), and the circumferential strain and axial strain are monitored by four strain gauges attached to the specimen. The test data is collected by a TDS-540 high-performance static data acquisition instrument at a sampling frequency of 1 Hz.

[0066] (1) The results of the failure modes in the axial compression test are shown in Figure 3 , and all specimens in the WB36 group presented a typical vertical crack splitting failure mode in the axial compression test. At the moment of failure, accompanied by a large area of concrete spalling, this not only led to obvious defects on the surface of the specimen but also severely weakened the bearing capacity of the specimen. Analyzed from the microscopic structure level, in the high-strength WB36 concrete, the bonding force between the cementitious material and the aggregate is relatively strong. Under the action of axial pressure, when the internal stress exceeds the ultimate strength of the material, cracks will rapidly develop along the weakest path, that is, the vertical direction, thus triggering a large area of concrete spalling.

[0067] With the gradual increase of the water-binder ratio, the failure modes of the OPCFC specimens changed significantly. The phenomenon of concrete spalling gradually decreased. At the same time, secondary cracks gradually increased, and the specimens could maintain integrity well during the failure process. For example, no large-scale concrete spalling occurred in the axial compression tests of the WB42 and WB45 groups of specimens. Their failure modes were mainly manifested as several clearly textured secondary cracks derived from the main crack after the main crack occurred. The scattered secondary cracks gradually expanded and connected in time and space to dissipate energy, rather than quickly forming a single through failure surface, thus inhibiting the rapid penetration and expansion of the cracks. This phenomenon indicates that the increase in the water-binder ratio makes the free water in the matrix increase. During the hardening process of concrete, the internal pore structure changes and the porosity relatively increases. These pores play a role in relieving stress concentration to a certain extent, making the cracks no longer concentrate in the vertical direction during development but extend in multiple directions, forming more secondary cracks. At the same time, due to the existence of pores, concrete has more deformation space during the stress process, thus reducing the brittleness of the material and improving the deformation capacity.

[0068] (2) Figure 4It is the axial compressive stress-strain curves of cylinders under different water-binder ratios. Comparing the curves of the two groups of specimens, WB39 and WB42, the peak stress of WB39 is slightly higher than that of WB42, but the descending section of the WB42 curve is relatively gentler. This is mainly because as the water content increases, the spreadability of the concrete is significantly improved. In this case, the cementitious materials can be more evenly distributed and filled between the aggregates, resulting in a denser microstructure during the forming and hardening processes of the specimens. This dense structure can be damaged relatively slowly after the specimens bear the load reaching the peak, leading to a gentler descending section of the curve.

[0069] Further comparing the specimens of group WB45 and group WB48, the specimens of WB48 show an increase in compressive strength and an increase in the curve slope. Exploring its internal mechanism in depth, when the water content is too high, the activator outside the concrete will react quickly with the water in a short time, forming a dense protective layer on the surface of the specimens. This protective layer acts like a barrier, isolating the further entry of water into the interior to react fully with the cementitious materials, making the chemical reaction inside the concrete unable to proceed completely. This incomplete reaction leads to an increase in the inhomogeneity of the internal structure of the concrete, thus macroscopically showing an increase in compressive strength and an increase in the slope of the stress-strain curve.

[0070] When the water-binder ratio gradually increases from 0.36 to 0.48, from the characteristics of the curve change, the peak stress shows a gradually decreasing trend, and at the same time, the slope of the ascending section of the curve is also continuously decreasing, while the descending section becomes gentler. These phenomena indicate that although the increase in the water-binder ratio will lead to a decrease in the compressive strength of the material, to a certain extent, it can effectively improve the brittleness of the material and significantly increase its compression deformation ability. Explained from the microscopic level, the increase in the water-binder ratio makes the free water in the OPCFC paste increase, forming more pore structures during the hardening process of the concrete. These pores relieve the stress concentration phenomenon in the specimens during the loading process to a certain extent, enabling the material to undergo greater deformation during the failure process, thus improving the brittleness of the material and increasing the compression deformation ability.

[0071] (3) Table 6 shows the results of this axial compression test (all values are averages without special instructions). Analyzing the test data comprehensively and in depth, it can be seen that there is a significant non-linear relationship between the compressive strength of OPCFC and the water-binder ratio. Initially, as the water-binder ratio gradually increases, the compressive strength shows a steady upward trend and reaches the peak when the water-binder ratio reaches 0.39. This phenomenon can be interpreted in depth from the microscopic reaction mechanism of the material. During the gradual increase of the water-binder ratio, the water content in the system increases correspondingly, providing more abundant free water for the chemical reaction between the solid activator and the alkali activator, making the reaction proceed more fully.

[0072] Taking WB36 and WB39 as examples, the difference in their compressive strengths mainly stems from the different degrees of reaction of the activator. Due to the higher water content in WB39, the alkali activator can come into full contact with the alkali activator in an environment rich in free water and undergo efficient chemical reactions, thereby generating a denser cementitious material. This dense cementitious material forms a more stable and higher-strength microstructure inside the concrete, greatly enhancing the concrete's ability to resist pressure and making the compressive strength of WB39 higher than that of WB36.

[0073] However, when the water-binder ratio continues to rise above 0.39, the compressive strength shows a gradually decreasing trend. This is because too much free water dilutes the activator, resulting in a weakened alkaline environment. In this case, the reaction environment between the alkali activator and the cementitious material changes, and the two cannot fully blend and complete the chemical reaction, thereby affecting the formation and development of the cementitious structure and ultimately leading to a decrease in compressive strength.

[0074] It is worth noting that although the average compressive strength of WB42 has decreased, it is still significantly higher than the standard of high-performance concrete (≥60 MPa). When the water-binder ratio continues to increase to 0.45, the average compressive strength of WB45 drops sharply from 71.02 MPa to 60.31 MPa, with a decrease amplitude of 15.1%. Analyzing the reasons in depth, although the increase in water content has, to a certain extent, improved the workability of the concrete, enabling the cementitious material and the activator to undergo chemical reactions more fully, which theoretically helps to form a denser structure, in fact, too much water destroys the stability of the internal microstructure of the concrete, forming a large number of pores during the hardening process of the concrete. These pores weaken the continuity and integrity of the internal structure of the concrete, making the internal stress distribution uneven when the concrete bears pressure and prone to stress concentration phenomena, thus reducing the compressive strength of the concrete.

[0075] Table 6 Axial compression test results

[0076]

[0077] (4) Figure 5 Shows the influence of the water-binder ratio on the peak strain of OPCFC. Through in-depth analysis of this figure, it can be seen that compared with the case of the water-binder ratio of WB36, when the water-binder ratios are WB39, WB42, WB45, and WB48, the peak strains of OPCFC increase by 14.23%, 36.30%, 20.28%, and 11.39% respectively.

[0078] From the perspective of the microstructure and mechanical properties of materials, the increase in the water-binder ratio enhances the compressive strength of concrete within a certain range. This is because the appropriately increased water helps the matrix to undergo sufficient hydration reactions, making the OPCFC structure more dense, enhancing the interfacial bonding strength between the aggregate and the cementitious material, and thus improving the overall ability of the concrete to resist external forces. At the same time, the increase in the water-binder ratio also increases the deformation ability of the concrete, enabling the internal structure of the concrete to undergo more sufficient adjustment and redistribution when under pressure, delaying the generation and propagation of cracks, and further delaying the failure process of the concrete, so the peak strain also increases accordingly.

[0079] When the water-binder ratio reaches 0.42, the enhancement effect of the deformation ability of OPCFC reaches its peak. This phenomenon can be explained as follows: under this water-binder ratio condition, the hydration reaction of OPCFC reaches a relatively ideal equilibrium state, and the strength of OPCFC and the bonding strength of the aggregate-matrix interface jointly reach a relatively optimal matching degree, making the overall plastic deformation ability of the concrete the best. However, when the water-binder ratio continues to increase, the dilution effect of the excessive water in the concrete becomes significantly enhanced. On the one hand, the excess water forms pores after the concrete hardens, weakening the structural strength of OPCFC and reducing the compressive strength of the concrete; on the other hand, the change in the pore structure caused by these excess waters intensifies the stress concentration phenomenon inside the concrete, inhibiting the development of plastic deformation and resulting in a decrease in the plastic deformation ability of the concrete.

[0080] II. Static splitting tensile properties

[0081] (1) According to the standard ASTM C496M-2011, a servo-hydraulic testing machine with a capacity of 5000 kN is used for the splitting tensile test, and the test rate is 0.05 mm / min. The standard test size is 300 mm in height and 150 mm in diameter. To explore the influence of the water-binder ratio on the static splitting tensile properties of OPCFC, the failure mode, load-displacement curve, and splitting tensile strength in the OPCFC splitting tensile test are analyzed respectively (each sample is tested 3 times repeatedly), and the test results are shown in Table 7.

[0082] Table 7 Summary of OPCFC splitting tensile test results

[0083]

[0084] (2) Figure 6The failure mode of OPCFC under static splitting tension is presented. After the OPCFC specimen fails, it exhibits typical characteristics, namely, a type-I crack penetrates the entire interface, splitting the specimen into two halves completely. This failure mode is due to the fact that under the action of splitting tensile stress, the tensile stress inside the specimen exceeds the tensile strength of the material, resulting in the crack rapidly expanding along the weakest path and finally forming a through crack. Taking the WB36 specimen as an example, its failure mode at the top of stiffness shows spalling. In the experimental environment with concentrated stress, due to the uneven internal stress distribution of the concrete with greater stiffness, it is easier to accumulate excessive energy in the stress concentration area. When the energy exceeds the bearing limit of the material, it will trigger local brittle spalling failure. This is because greater stiffness means stronger ability of the material to resist deformation. When subjected to external forces, deformation is difficult to be evenly distributed inside the material, leading to more prominent stress concentration phenomenon.

[0085] As the water-binder ratio gradually increases, the lateral deformation ability of the specimen significantly increases, and tiny secondary cracks begin to appear beside the main crack. These secondary cracks are initially relatively small, but with the continuous action of the load, they will gradually expand and connect with each other, finally converging at the upper and lower ends of the specimen, resulting in notch-type failure at the ends. However, when the water-binder ratio reaches 0.39, the failure of the WB39 specimen is the most serious. The main crack expands under high stress. Due to the strong stress concentration at the upper and lower ends, the stress is dispersed through the secondary cracks. Although the secondary cracks can disperse part of the stress, they also exacerbate the damage degree of the end structure, leading to large-notch failure at the ends. When the water-binder ratio increases to 0.42, the failure mode of the WB42 specimen changes significantly. The crack tip bifurcates during expansion, evolving into a multi-crack energy dissipation mechanism, which significantly improves the surface integrity of the specimen. At this time, the matrix porosity and gel toughness of OPCFC reach the best synergistic state, avoiding both the brittle spalling caused by stress concentration at low water-binder ratios and the strength degradation caused by the porous structure at high water-binder ratios. When the water-binder ratio continues to increase, the dilution effect of excessive water on the concrete becomes significantly stronger. From the microscopic level, the excess water forms pores after the concrete hardens, and the existence of these pores weakens the structural strength of OPCFC, further reducing the overall stiffness of the concrete. On the other hand, the change in the pore structure makes the internal stress distribution of the concrete more uneven, intensifying the stress concentration phenomenon. During the loading process, this stress concentration inhibits the development of plastic deformation, resulting in a decrease in the plastic deformation ability of the concrete. Due to the change in the internal stress distribution, the development of cracks is inhibited to a certain extent, enabling the specimen to maintain a relatively high integrity when it fails.

[0086] (3) Figure 7 is the load-strain curve of the OPCFC cylinder. From Figure 7It can be seen that the slope of the curve in this stage shows a typical linear change trend. As the water-binder ratio gradually increases from the initial value, the slope of the curve first rises and then falls. In the initial stage of the increase in the water-binder ratio, the increase in water in the system provides a more sufficient reaction environment for the hydration reaction of the matrix. Water molecules come into full contact with the matrix and participate in chemical reactions, promoting the OPCFC structure to gradually become denser, and the interfacial bonding strength between the aggregate and the cementitious material also increases accordingly. The optimization of this microstructure enables the material to more effectively resist the deformation caused by external loads in the elastic stage, macroscopically manifested as an increase in the slope of the load-strain curve. However, when the water-binder ratio exceeds a certain threshold, too much water is difficult to fully participate in the hydration reaction during the hardening process of the concrete, and a large number of pores are formed inside the material. The existence of these pores destroys the continuity and integrity of the material structure, weakens the stiffness of the material, and causes the slope of the load-strain curve to gradually decrease.

[0087] The appearance of the first crack is a key sign of the transition of the material from the elastic stage to the non-linear deformation stage. Experimental data show that the displacement value at the appearance of the first crack is between 0.03 mm (WB36) and 0.05 mm (WB39). This indicates that as the water-binder ratio increases, the deformation corresponding to the starting crack of the material increases. This is mainly because a higher water-binder ratio makes the internal microstructure of the material relatively looser, with more micro-defects and weak areas. Under the action of external loads, local stress concentration is more likely to occur in these weak areas. When the stress reaches the starting strength of the material, cracks begin to initiate and expand. Since the looseness of the internal structure of the material provides more space for the expansion of cracks, a larger deformation is required to initiate the appearance of cracks. At the same time, the starting load also changes significantly with the increase in the water-binder ratio. Compared with WB36, the starting loads of WB39, WB42, WB45, and WB48 are reduced by 40.12%, 57.91%, 66.36%, and 75.73% respectively. Among this series of data, the slope of the load-strain curve of WB39 is the largest among all experimental groups, which means that WB39 material has a strong ability to resist deformation in the elastic stage. However, this high stiffness also causes the elastic strain energy accumulated inside the material to be released instantaneously when the ultimate load is reached, resulting in a more violent failure process, and its load-strain curve shows a classic brittle failure trend. As the water-binder ratio further increases, the slope of the curve gradually decreases, and the slope gap between WB42 and WB45 is relatively small. This indicates that within a specific water-binder ratio change range, the additional water has a relatively small impact on the strength of the material. The reason may be that in this stage, the change in the internal microstructure of the material has entered a relatively stable transition state, and the impact of the newly added water on the material structure is not sufficient to cause a significant change in strength.

[0088] The duration of the hardening stage is extremely short. When the load borne by the material reaches the peak load, the specimen will fail instantaneously and the stress will drop rapidly. From the experimental data, as the water-binder ratio increases, the peak load of the specimen shows a gradually decreasing trend. However, when comparing WB39 and WB42, it is found that the peak load of WB39 is less than that of WB42. This is because the relative water content inside WB39 is insufficient, and during the hydration reaction of the matrix, a dense internal structure like that of WB42 cannot be formed. Due to the sufficient water content inside WB42, on the one hand, it can promote the full hydration of the cementitious material, form a denser microstructure, and enhance the overall strength of the material; on the other hand, the sufficient water dilutes the modulus of the solid alkali activator, changing the chemical reaction path inside the material and the microstructure of the material. It can be attributed that the increase in the water-binder ratio inhibits the generation of pores, thus significantly enhancing the bearing capacity of the specimen during the hardening stage.

[0089] The characteristics of the softening stage can be accurately reflected by the descending amplitude in the load-displacement curve. This amplitude intuitively reflects the rate at which the load drops as the displacement increases. The experimental results clearly show that increasing the water content has a certain effect on the softening behavior of the material, specifically manifested as a slightly longer descending section of the curve. When the water-binder ratio gradually increases from 0.36 to 0.45, the softening stage of the curve is significantly extended. This phenomenon fully indicates that within this range of water-binder ratio changes, increasing the water-binder ratio can effectively improve the performance of OPCFC materials. Analyzing in depth from the microscopic level, the change in the water-binder ratio will change the pore structure inside the material and the development mode of microcracks. The moderately increased water forms a more uniform pore distribution inside the material. When the material enters the softening stage, these pores can effectively disperse and absorb energy, slowing down the crack propagation speed, thus extending the softening stage of the curve and making the material exhibit better energy absorption capacity.

[0090] (4) Figure 8 Shows the effect of the water-binder ratio on the splitting tensile strength of OPCFC. When the water-binder ratio gradually increases from 0.36 to 0.48, the splitting tensile strength of OPCFC drops from 2.83 MPa to 2.22 MPa, showing an obvious negative correlation trend. Taking WB36 as a reference, the tensile strengths of WB39, WB for 42, WB45 and WB48 are reduced by 14.42%, 9.01%, 11.63% and 21.73% respectively. It is analyzed that the splitting tensile strength of OPCFC will decrease with the increase in water content, and this law is highly similar to the trend of the compressive strength of OPCFC changing with water content. Thus, it is concluded that the change in water content will significantly affect the hydration reaction process and the formation of the internal microstructure of the material, and thus have a similar impact on the compressive and tensile properties of the material.

[0091] However, when deeply analyzing the performance differences of OPCFC under different water-binder ratios, it was found that when in the water-binder ratio state of WB39, the splitting tensile strength of OPCFC decreased more significantly. In contrast, the splitting tensile strength of WB42 increased by about 6.28% compared to WB39. Delving into its internal mechanism, it is mainly due to the relatively insufficient water content inside WB39. During the matrix hydration reaction, water acts as a reactant and reaction medium, and its content directly affects the degree of hydration reaction and the structure of the products. The lack of water content in WB39 makes the cementitious material unable to hydrate fully and difficult to form a dense and uniform internal structure like WB42. While WB42 has sufficient water content inside, which provides favorable conditions for the matrix hydration. On the one hand, the abundant water can promote the full hydration of the cementitious material, generating more hydration products. These hydration products are intertwined to form a denser microstructure, thus effectively enhancing the overall strength of the material. On the other hand, with the increase of the water-binder ratio, the pore formation inside OPCFC is inhibited, reducing the stress concentration points and weak links inside the material, and significantly improving the ability of the specimen to withstand splitting tensile loads during the hardening stage.

[0092] Based on the above analysis, it can be concluded that in this invention, fly ash and slag are used as cementitious materials, manufactured sand is used as fine aggregate, fine stone is used as coarse aggregate, sodium tetraborate decahydrate is used as retarder, and solid sodium silicate is used as alkali activator; by controlling the water-binder ratio to be 0.36 - 0.48, the slag-ash ratio to be 10% - 30%, and the sodium-binder ratio to be 5% - 7%, the comprehensive mechanical properties and workability of OPCFC can meet the usage requirements. And compared with traditional high-performance concrete, OPCFC can maintain the mechanical properties of high-performance concrete at a higher water-binder ratio. When the water-binder ratio is 0.42, the slag-ash ratio is 10%, the sodium-binder ratio is 6%, and the modulus of the alkali activator is 1.6, the comprehensive performance is the best.

[0093] The above is only the preferred specific embodiment of this application, but the protection scope of this application is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed in this application should be covered within the protection scope of this application. Therefore, the protection scope of this application should be subject to the protection scope of the claims.

Claims

1. A one-step cementless concrete, characterized in that, Using fly ash and slag as cementitious materials, manufactured sand as fine aggregate, fine gravel as coarse aggregate, and sodium tetraborate decahydrate as a retarder, and solid sodium silicate as an alkali activator; wherein, in the preparation process of the one-step cement-free concrete, the water-binder ratio is controlled to be 0.36 to 0.48; the mass ratio of the slag in the cementitious materials is 10% to 30%; calculated by the mass of sodium oxide, the mass ratio of the alkali activator to the cementitious materials is 5% to 7%.

2. The one-step cement-free concrete according to claim 1, characterized in that The modulus of the alkali activator is 1.6 to 1.

8.

3. The one-step cementless concrete according to claim 1, characterized in that, The manufactured sand is medium sand with a particle size of 0.25 mm to 0.5 mm; the fine gravel has a particle size of 10 mm to 20 mm.

4. The one-step cementless concrete according to claim 1, wherein The fly ash is Class F fly ash, with a fineness of 10.8%, a loss on ignition of 4.6%, and a density of 2.3 g / cm 3 .

5. The one-step cementless concrete according to claim 1, characterized in that, The slag is blast furnace S105 slag.

6. The one-step cementless concrete according to claim 1, wherein The dosage of the retarder is 3% to 6% of the mass of the cementitious materials.

7. The one-step cementless concrete according to claim 1, wherein The mass ratio of the slag in the cementitious materials is 10%.

8. The one-step cementless concrete according to claim 1, wherein Calculated by the mass of sodium oxide, the mass ratio of the alkali activator to the cementitious materials is 6%.

9. The one-step cementless concrete according to claim 2, characterized in that, The modulus of the alkali activator is 1.

6.

10. A preparation method of the one-step cementless concrete according to any one of claims 1 to 9, characterized in that, Weigh the manufactured sand and fine gravel according to the ratio and add them to the mixer with the inner wall wetted and mix dry for 2 minutes, then add the cementitious materials, alkali activator and retarder to make the aggregate and the cementitious materials fully stirred and evenly mixed; add 50% of the total water amount under the stirring state, stir for 2 minutes until the slurry is initially formed, then continue to add the remaining water, and stir for another 2 minutes until the mixture reaches a uniform flow state, thus obtaining the one-step cement-free concrete material.

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