A one-step cementless concrete and its preparation method

Abstract

This invention discloses a one-step cementless concrete and its preparation method, belonging to the field of concrete preparation technology. The one-step cementless concrete uses fly ash and slag as cementitious materials, manufactured sand as fine aggregate, fine stone as coarse aggregate, sodium tetraborate decahydrate as a retarder, and solid sodium silicate as an alkali activator. The water-cement ratio (mass ratio of water to cementitious materials) is 0.36–0.48; the mass proportion of slag in the cementitious materials (slag-ash ratio) is 10%–30%; and the mass ratio of the alkali activator to the cementitious materials (sodium-cement ratio), based on the mass of sodium oxide, is 5%–7%. Through the optimized design of raw material selection and proportioning of each component, this invention enables the prepared one-step cementless concrete to possess high mechanical properties, engineering benefits, and environmental benefits.

Description

Technical Field

[0001] This invention belongs to the field of concrete preparation technology, specifically relating to a one-step cementless concrete preparation method and the same. Background Technology

[0002] Cement-free concrete is a novel type of concrete material with excellent properties, prepared by mixing industrial waste such as fly ash, slag, or other materials rich in silicon and aluminum with aggregates through alkaline activation. It possesses many superior properties, including excellent self-healing, crack resistance, and resistance to chemical attack. Compared to traditional silicate cement, cement-free concrete reduces CO2 emissions and energy consumption by 60-80%, while providing a rational resource utilization of industrial byproducts such as fly ash and slag. Cement-free concrete is not only suitable for traditional construction but can also be applied to other fields, such as marine engineering, roads and bridges, and underground engineering, providing more reliable and durable structural materials for various projects. It has broad application prospects and significant socio-economic benefits, playing a vital role in improving engineering quality, protecting the environment, and promoting sustainable development.

[0003] Cement-free concrete has wide applications, but traditional two-step cement-free concrete requires the preparation of an alkali activator slurry before mixing it with aggregates. This slurry premixing step introduces additional energy consumption, extending the construction cycle by 25-30%. It also suffers from cumbersome processes and low construction efficiency. Furthermore, prolonged storage of the alkali activator in the two-step process leads to a decrease in activity, necessitating strict control of the construction window and hindering engineering applications. In contrast, OPCFC (one-step cement-free concrete) eliminates the need for premixed alkali activator slurry; the solid alkali activator is directly mixed with precursor materials, significantly simplifying the construction process and saving construction time and labor costs. With its streamlined process and deep carbon reduction as core advantages, OPCFC has the potential to replace traditional OPC (ordinary silicate cement concrete).

[0004] However, current research on one-step cementless concrete focuses only on single workability or mechanical performance indicators, neglecting the cross-coupling effects of mixed variables on one-step cementless concrete, and lacking a systematic mix proportion optimization method, and further researching the mechanical properties of this mix proportion. Therefore, there is an urgent need to study a mix design method for one-step cementless concrete with multi-variable synergistic regulation, to achieve comprehensive performance optimization of workability and mechanical properties while ensuring process simplicity. To this end, this invention proposes a one-step cementless concrete material and its preparation method. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention discloses a one-step cementless concrete and its preparation method. Through the selection of raw materials and the design of the proportions of each component, the prepared one-step cementless concrete exhibits high mechanical properties.

[0006] The first aspect of this invention discloses a one-step cementless concrete, using fly ash and slag as cementing materials, manufactured 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-cement ratio (mass ratio of water to cementing materials) is 0.36 to 0.48; the mass percentage of slag in the cementing materials (slag-ash ratio) is 10% to 30%; and the mass ratio of the alkali activator to the cementing materials (sodium-cement ratio) is 5% to 7% based on the mass of sodium oxide.

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

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

[0009] As a preferred embodiment of the present invention, the fly ash is grade F fly ash with a fineness of 10.8%, a loss on ignition of 4.6%, and a density of 2.3 g / cm³. 3 .

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

[0011] As a preferred embodiment of the present invention, the amount of the retarder is 3% to 6% of the mass of the cementitious material.

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

[0013] As a preferred embodiment of the present invention, the slag accounts for 10% of the mass of the cementitious material; and the mass ratio of the alkali activator to the cementitious material is 6% based on the mass of sodium oxide.

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

[0015] The second aspect of this invention aims to provide a one-step cementless concrete preparation method, comprising the following steps: weighing manufactured sand and fine aggregate according to the specified proportions and adding them to a mixer after wetting the inner wall, mixing and dry-mixing for 2 minutes; then adding cementitious materials, alkali activators, and retarders to ensure that the aggregates and cementitious materials are fully and evenly mixed; adding 50% of the total water while stirring, stirring for 2 minutes until the slurry is initially formed, then adding the remaining water, and stirring for another 2 minutes 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 cementless concrete prepared by this invention uses solid waste fly ash and slag as cementing materials, reducing carbon emissions; it uses manufactured sand as fine aggregate, fine stone as coarse aggregate, sodium tetraborate decahydrate as retarder, and solid sodium silicate as alkali activator; by controlling the water-cement ratio to 0.36-0.48, the slag-ash ratio to 10%-30%, and the sodium-cement ratio to 5%-7%, the prepared concrete has both good workability and mechanical strength. Attached Figure Description

[0018] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0019] Figure 1 The diagram shows the compressive strength of the OPCFC samples prepared in Examples 1-9; where A1-A9 correspond to Examples 1-9 respectively.

[0020] Figure 2 The diagram shows the performance of the OPCFC samples prepared in Examples 1-9; where A1-A9 correspond to Examples 1-9 respectively.

[0021] Figure 3 The failure modes of the five OPCFC specimens in Example 10 are shown in the axial compression test.

[0022] Figure 4 The stress-strain curves of the axial compression test of the five OPCFC specimens in Example 10 are shown.

[0023] Figure 5 The peak strain curves of the axial compression test of the five OPCFC specimens in Example 10 are shown.

[0024] Figure 6 The failure modes of the five OPCFC specimens in Example 10 are shown in the splitting tensile test.

[0025] Figure 7 Load-strain curves of splitting tensile tests on five OPCFC specimens in Example 10;

[0026] Figure 8 The tensile strength of the five OPCFC specimens in Example 10 is the tensile strength obtained from the splitting tensile test. Detailed Implementation

[0027] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0028] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Every smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0029] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0030] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be readily apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0031] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0032] This invention discloses a one-step cementless concrete method, using fly ash and slag as cementing materials, manufactured 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-cement ratio is 0.36-0.48; the slag accounts for 10%-30% of the mass of the cementing materials; and the alkali activator to cementing materials, based on the mass of sodium oxide, has a mass ratio of 5%-7%.

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

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

[0035] In some embodiments of the present invention, the fly ash is grade 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 of Fly Ash (Unit: %)

[0037]

[0038] Table 2 Physical Parameters of Fly Ash

[0039]

[0040] In some embodiments of the present invention, the slag is blast furnace S105 slag. According to GB / T 18046-2017 "Granulated Blast Furnace Slag Powder for Cement, Mortar and Concrete", slag with an 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 S105 Slag Test Results

[0042]

[0043] In some embodiments of the present invention, the modulus of the alkali activator is 1.6 to 1.8. The alkali activator is solid sodium silicate. In 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 to prepare the required modulus for the experiment.

[0044] In some embodiments of the present invention, the slag accounts for 10% of the mass of the cementitious material; and the mass ratio of the alkali activator to the cementitious material is 6% based on the mass of sodium oxide.

[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% to 6% of the amount of cementitious material. In the embodiments, the retarder is sodium tetraborate decahydrate; the test water is tap water, which meets the requirements of "Mixing Water for Concrete" (JGJ63-2006).

[0047] This invention also provides a one-step cementless concrete preparation method, comprising the following steps: weighing manufactured sand and fine aggregate according to the proportion and adding them to a mixer after wetting the inner wall, mixing and dry-mixing for 2 minutes; then adding cementitious materials, alkali activator and retarder to fully mix the aggregate and cementitious materials evenly; adding 50% of the total water while stirring, stirring for 2 minutes until the slurry is initially formed, then adding 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.

[0048] All raw materials used in the embodiments of this invention were obtained through commercial purchase.

[0049] Examples 1 to 9

[0050] The preparation method of one-step cementless concrete includes the following steps: weighing raw materials according to the raw material ratio in Table 4; adding manufactured sand and fine aggregate according to the ratio to a mixer after wetting the inner wall, mixing dry for 2 minutes, then adding cementitious materials, alkali activator, and retarder to ensure the aggregate and cementitious materials are fully mixed; adding 50% of the total water while mixing, mixing for 2 minutes until the slurry is initially formed, then adding the remaining tap water, and mixing for another 2 minutes until the mixture reaches a uniform flow state, thus obtaining the one-step cementless concrete slurry. The freshly mixed slurry is poured into a molding mold coated with a release agent, placed on a vibrating table for mechanical compaction, and then the surface is leveled. After the specimens are sealed and cured with polyethylene film for 24 hours, they are demolded and transferred to a standard curing water tank for water curing to 28 days. Throughout the curing process, the specimens are completely submerged below the curing liquid surface to ensure sufficient hydration reaction. Once the material reaches the predetermined age, performance tests are conducted according to the test plan.

[0051] Table 4. Mix proportions of specimens 1-9 in Examples (kg / m³) 3 )

[0052]

[0053] Note: The sodium colloid ratio in Examples 1, 6, and 8 was 5%, and the average modulus of the alkali activator was 1.8; the sodium colloid ratio in Examples 2, 4, and 9 was 7%, and the average modulus of the alkali activator was 1.6; the sodium colloid ratio in Examples 3, 5, and 7 was 6%, and the average modulus of the alkali activator was 1.7.

[0054] The compressive strength and workability of the concrete specimens prepared in Examples 1-9 were tested. The compressive strength is shown in the figure. Figure 1 For working performance, please refer to Figure 2 .in, Figure 1-2 In the diagram, A1 to A9 correspond to Examples 1 to 9, respectively. From... Figures 1-2It can be seen that the compressive strength of the specimens prepared in Examples 1-9 is 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 specimen exhibits the best compressive strength, with the specimen prepared in Example 2 showing the highest compressive strength at 89.082 MPa. As the water-binder ratio gradually increases, the compressive strength shows a gradual decreasing trend. This may be because, during the hydration reaction, the change in the water-binder ratio directly affects the consistency, fluidity, and degree of hydration of the OPCFC slurry. A lower water-binder ratio means relatively less water around the cementitious particles, allowing them to pack more tightly during the hydration reaction, forming a denser microstructure and effectively improving the material's compressive strength. Conversely, when the water-binder ratio is too high, excessive water forms pores inside the material after hardening. These pores become weak points within the material, easily causing stress concentration under pressure, thus reducing the material's compressive strength.

[0055] Regarding the slag-to-ash ratio, experimental results show that it plays a crucial role in the performance regulation of OPCFCs. Changes in the slag-to-ash ratio affect the degree of internal reactivity of the material and the interfacial bond strength between the aggregate and the cementitious material. OPCFCs exhibit optimal performance when the slag-to-ash ratio is at its lowest value. An excessively high slag-to-ash ratio may disrupt the internal structural balance of the material, which is also detrimental to improving material performance. Therefore, the optimal slag-to-ash ratio is 10%.

[0056] Regarding the sodium-to-cement ratio, OPCFC specimens exhibited the most stable performance and best met the compressive strength requirements when the sodium-to-cement ratio was 6%. OPCFCs struggled to achieve ideal compressive strength when the sodium-to-cement ratio was too high or too low. When the sodium-to-cement ratio was too low, the concentration of the alkali activator was insufficient, failing to fully activate the cementitious material, leading to incomplete hydration and reduced material strength. Conversely, when the sodium-to-cement ratio was too high, it might trigger excessive chemical reactions, resulting in unstable internal material structure and similarly reducing compressive strength.

[0057] Regarding the activator modulus, a low activator modulus negatively impacts the working and mechanical properties of OPCFCs. OPCFCs achieve optimal overall performance with an activator modulus of 1.6. Further increasing the modulus not only fails to improve the compressive strength of OPCFCs but also leads to a decrease in their working performance. Changes in the activator modulus affect the alkalinity of the alkali activator and the reaction kinetics. A lower activator modulus indicates weaker alkalinity, which cannot effectively promote the hydration reaction of the cementitious material, thus affecting its properties. Conversely, an excessively high activator modulus may cause the reaction to be too vigorous, leading to increased internal structural defects and consequently reducing the material's working performance and compressive strength.

[0058] Example 10

[0059] Through in-depth analysis of the key performance indicator of OPCFC compressive strength, and based on the principle of maximizing the signal-to-noise ratio in the Taguchi design method, the mix proportion was optimized. The influence of various variable parameters on OPCFC performance was systematically evaluated, and the combination of factors that maximizes the compressive strength of OPCFC under the combined effect of multiple variables was successfully determined. Furthermore, since the water-cement ratio exhibits monotonic variation, changing the water-cement ratio can stably regulate the mechanical properties of OPCFC. Therefore, in this embodiment, with a fixed slag-to-ash ratio of 10%, a sodium-cement ratio of 6%, and an alkali activator modulus of 1.6, five OPCFC specimens were prepared with the water-cement ratio as a variable (0.36, 0.39, 0.42, 0.45, and 0.48), and their basic mechanical properties were systematically studied. The mix proportions of each specimen are shown in Table 5.

[0060] Table 5. OPCFC specimen mix proportions (kg / m³) 3 )

[0061]

[0062] The preparation methods for each specimen are as follows:

[0063] One-step cementless concrete is produced using a planetary mixer. The specific mixing sequence is as follows: Weigh the manufactured sand and fine aggregate according to the specified ratio and add them to the mixer after wetting the inner wall; mix dry for 2 minutes. Then add the cementitious materials, alkali activator, and retarder, ensuring the aggregate and cementitious materials are thoroughly mixed. While mixing, add 50% of the total water and mix for 2 minutes until a preliminary slurry is formed. Add the remaining tap water and mix for another 2 minutes until the mixture reaches a uniform flow state, thus obtaining the one-step cementless concrete slurry. The freshly mixed slurry is poured into a mold coated with a release agent and mechanically compacted on a vibrating table, followed by surface leveling. After curing in a polyethylene film for 24 hours, the specimens are demolded and transferred to a standard curing water tank for 28 days of water curing. Throughout the curing process, the specimens are completely submerged below the curing liquid surface to ensure sufficient hydration. Once the material reaches the predetermined age, performance tests are conducted according to the testing protocol.

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

[0065] Test Method: Before the axial compression test, ensure both ends of the specimen are flat and strain gauges are attached. First, remove dust and other impurities from the surfaces of both ends of the specimen, apply high-strength plaster sequentially for leveling, and calibrate using a level to ensure the flatness of the specimen. Finally, alternately attach longitudinal and transverse strain gauges at the mid-circumferential quarter-position of the specimen to measure axial and circumferential strain respectively. According to ASTM C39M, the axial compression test was conducted using a 500-ton MATEST pressure testing machine at a test rate of 0.18 mm / min. During the test, the axial deformation of the specimen was measured using two symmetrical displacement gauges (LVDT), while the circumferential and axial strains were monitored by four strain gauges attached to the specimen. Test data were acquired using a TDS-540 high-performance static data acquisition instrument at a frequency of 1 Hz.

[0066] (1) The failure mode results of the axial compression test are shown in Figure 3 In the axial compression test, all WB36 specimens exhibited a typical vertical crack splitting failure mode. At the moment of failure, large-scale concrete spalling occurred, resulting not only in obvious defects on the specimen surface but also severely weakening its load-bearing capacity. Microstructural analysis reveals that the high-strength WB36 concrete exhibits strong bonding between the cementitious material and aggregate. Under axial pressure, when the internal stress exceeds the material's ultimate strength, cracks rapidly propagate along the weakest path, i.e., the vertical direction, leading to large-scale concrete spalling.

[0067] With the gradual increase of the water-cement ratio, the failure modes of OPCFC specimens changed significantly. Concrete spalling gradually decreased, while secondary cracks gradually increased, and the specimens were able to maintain their integrity better during failure. For example, neither WB42 nor WB45 specimens showed large-scale concrete spalling in the axial compression test. Their failure mode was mainly characterized by the emergence of several well-defined secondary cracks after the main crack appeared. These dispersed secondary cracks gradually expanded and connected in time and space, dissipating energy, rather than rapidly forming a single through-type failure surface, thus inhibiting the rapid penetration and expansion of cracks. This phenomenon indicates that the increase in water-cement ratio increases the amount of free water in the matrix, and the internal pore structure changes during concrete hardening, with a relative increase in porosity. These pores, to some extent, alleviate stress concentration, causing cracks to extend in multiple directions instead of being concentrated vertically, forming more secondary cracks. At the same time, due to the presence of pores, the concrete has more deformation space during stress, thereby reducing the brittleness of the material and improving its deformation capacity.

[0068] (2) Figure 4The axial compressive stress-strain curves of cylinders under different water-cement ratios are shown. Comparing the curves of specimens WB39 and WB42, the peak stress of WB39 is slightly higher than that of WB42, but the descent of the WB42 curve is relatively gentler. This is mainly because the spreadability of concrete is significantly improved with increasing water content. In this case, the cementitious material can be more evenly distributed and filled between the aggregates, resulting in a denser microstructure during the molding and hardening process. This dense structure can fail relatively slowly after the specimen reaches the peak load, thus causing the descent of the curve to be gentler.

[0069] Further comparison of specimens from groups WB45 and WB48 revealed that specimen WB48 exhibited increased compressive strength and a steeper stress-strain curve. In-depth investigation into the underlying mechanism revealed that when the water content is too high, the activator on the exterior of the concrete reacts rapidly with the water within a short time, forming a dense protective layer on the specimen surface. This protective layer acts as a barrier, preventing water from further penetrating the interior and reacting fully with the cementitious materials, thus hindering the complete chemical reaction within the concrete. This incomplete reaction leads to increased heterogeneity in the internal structure of the concrete, which macroscopically manifests as increased compressive strength and a steeper stress-strain curve slope.

[0070] As the water-cement ratio gradually increases from 0.36 to 0.48, the peak stress shows a gradual decreasing trend in the curve characteristics. Simultaneously, the slope of the rising segment of the curve continuously decreases, while the falling segment becomes more gradual. This series of phenomena indicates that although increasing the water-cement ratio leads to a slight decrease in the material's compressive strength, it can effectively improve the material's brittleness to a certain extent, significantly enhancing its compressive deformation capacity. From a microscopic perspective, the increased water-cement ratio leads to an increase in free water in the OPCFC paste, forming more pore structures during the concrete hardening process. These pores alleviate stress concentration in the specimen to some extent during the stress process, allowing the material to undergo greater deformation during failure, thereby improving the material's brittleness and enhancing its compressive deformation capacity.

[0071] (3) Table 6 shows the results of this axial compression test (unless otherwise specified, all values ​​are averages). A comprehensive and in-depth analysis of the test data reveals a significant nonlinear relationship between the compressive strength of OPCFC and the water-cement ratio. Initially, as the water-cement ratio gradually increases, the compressive strength shows a steady upward trend, reaching a peak when the water-cement ratio reaches 0.39. This phenomenon can be further explained from the perspective of the material's microscopic reaction mechanism. As the water-cement ratio gradually increases, the water content in the system increases accordingly, providing more sufficient free water for the chemical reaction between the solid activator and the alkali activator, allowing the reaction to proceed more fully.

[0072] Taking WB36 and WB39 as examples, the difference in their compressive strength mainly stems from the different degrees of activator reaction. Because WB39 has a higher water content, the alkali activator can fully contact and undergo a highly efficient chemical reaction with the alkali activator in an environment rich in free water, thus generating a denser cementitious material. This dense cementitious material forms a more stable and stronger microstructure inside the concrete, greatly enhancing the concrete's resistance to pressure, resulting in a higher compressive strength for WB39 compared to WB36.

[0073] However, when the water-cement ratio continues to rise above 0.39, the compressive strength shows a gradual decreasing trend. This is because excessive free water dilutes the activator, weakening its alkaline environment. Under these circumstances, the reaction environment between the alkaline activator and the cementitious material changes, preventing them from fully integrating and completing the chemical reaction. This, in turn, affects the formation and development of the cementitious structure, ultimately leading to a decrease in compressive strength.

[0074] It is noteworthy that although the average compressive strength of WB42 decreased, it was still significantly higher than the standard for high-performance concrete (≥60MPa). When the water-cement ratio continued to increase to 0.45, the average compressive strength of WB45 dropped sharply from 71.02MPa to 60.31MPa, a decrease of 15.1%. A deeper analysis reveals that while the increased water content improved the spreadability of concrete to some extent, allowing the cementitious materials and activators to react more fully and theoretically contributing to a denser structure, excessive moisture actually disrupted the stability of the concrete's internal microstructure. This resulted in numerous pores during the hardening process, weakening the continuity and integrity of the internal structure. Consequently, under pressure, the internal stress distribution was uneven, leading to stress concentration and ultimately reducing the concrete's compressive strength.

[0075] Table 6 Results of Axial Compression Test

[0076]

[0077] (4) Figure 5 The effect of water-binder ratio on the peak strain of OPCFC is shown. In-depth analysis of the figure reveals that, compared to a water-binder ratio of WB36, the peak strain of OPCFC increases by 14.23%, 36.30%, 20.28%, and 11.39% for water-binder ratios of WB39, WB42, WB45, and WB48, respectively.

[0078] From the perspective of material microstructure and mechanical properties, increasing the water-cement ratio enhances the compressive strength of concrete within a certain range. This is because a moderate increase in water facilitates the full hydration reaction of the matrix, making the OPCFC structure denser and strengthening the interfacial bond strength between aggregates and cementitious materials, thereby improving the overall resistance of concrete to external forces. Simultaneously, the increased water-cement ratio also increases the deformation capacity of concrete, allowing for more thorough adjustment and redistribution of the internal structure under pressure, delaying the initiation and propagation of cracks, and thus prolonging the concrete failure process. Consequently, the peak strain also increases.

[0079] When the water-cement ratio reaches 0.42, the deformation capacity enhancement effect of OPCFC reaches its peak. This phenomenon can be explained by the fact that, under this water-cement ratio condition, the hydration reaction of OPCFC reaches a relatively ideal equilibrium state, and the strength of OPCFC and the bond strength at the aggregate-matrix interface reach a relatively optimal match, resulting in the optimal overall plastic deformation capacity of concrete. However, as the water-cement ratio continues to increase, excessive water leads to a significant enhancement of the dilution effect of concrete. On the one hand, excess water forms pores after the concrete hardens, weakening the structural strength of OPCFC and reducing the compressive strength of concrete; on the other hand, the changes in pore structure caused by this excess water exacerbate the stress concentration phenomenon inside the concrete, inhibiting the development of plastic deformation and leading to a decrease in the plastic deformation capacity of concrete.

[0080] II. Static splitting tensile properties

[0081] (1) According to the standard ASTM C496M-2011, the splitting tensile test was conducted using a 5000 kN servo hydraulic testing machine at a test rate of 0.05 mm / min. The standard test dimensions were 300 mm in height and 150 mm in diameter. To investigate the effect of the water-cement ratio on the static splitting tensile properties of OPCFC, the failure mode, load-displacement curve, and splitting tensile strength of OPCFC in the splitting tensile test were analyzed (each sample was tested three times). The test results are shown in Table 7.

[0082] Table 7 Summary of OPCFC Splitting Tensile Test Results

[0083]

[0084] (2) Figure 6The failure modes of OPCFC under static splitting tension are shown. After failure, the OPCFC specimen exhibits a typical characteristic: a type I crack runs through the entire interface, completely splitting the specimen in two. This failure mode occurs because, under splitting tensile stress, the tensile stress inside the specimen exceeds the material's tensile strength, causing the crack to propagate rapidly along the weakest path, eventually forming a through crack. Taking the WB36 specimen as an example, its failure mode at the peak stiffness is spalling. In stress-concentrated experimental environments, concrete with high stiffness, due to its uneven internal stress distribution, is more prone to accumulating excessive energy in stress concentration areas. When the energy exceeds the material's bearing capacity, it triggers localized brittle spalling failure. This is because high stiffness means a strong resistance to deformation; under external forces, deformation is difficult to distribute evenly within the material, leading to a more pronounced stress concentration phenomenon.

[0085] As the water-cement ratio gradually increases, the lateral deformation capacity of the specimens increases significantly, and tiny secondary cracks begin to appear beside the main crack. These secondary cracks are initially small, but with the continuous application of load, they gradually expand and connect with each other, eventually converging at the upper and lower ends of the specimen, resulting in notch-type failure at the ends. However, when the water-cement ratio reaches 0.39, the failure of specimen WB39 is the most severe. The main crack expands under high stress, and 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 some stress, they also exacerbate the damage to the end structure, leading to large notch failure at the ends. When the water-cement ratio increases to 0.42, the failure mode of specimen WB42 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 point, the matrix porosity and gel toughness of OPCFC reach an optimal synergistic state, avoiding brittle fracture caused by stress concentration at low water-cement ratios and suppressing strength degradation caused by porous structures at high water-cement ratios. As the water-cement ratio continues to increase, the excess water significantly enhances the dilution effect of concrete. From a microscopic perspective, excess water forms pores after the concrete hardens. These pores weaken the structural strength of OPCFC, further reducing the overall stiffness of the concrete. On the other hand, the altered pore structure leads to a more uneven stress distribution within the concrete, exacerbating stress concentration. During stress loading, this stress concentration inhibits the development of plastic deformation, resulting in a decrease in the concrete's plastic deformation capacity. Due to the altered internal stress distribution, crack development is inhibited to some extent, allowing the specimen to maintain a relatively high degree of integrity upon failure.

[0086] (3) Figure 7 For the load-strain curve of the OPCFC cylinder, from Figure 7As can be seen, the slope of the curve in this stage exhibits a typical linear trend. As the water-cement ratio gradually increases from its initial value, the slope of the curve shows a pattern of first rising and then falling. In the initial stage of increasing the water-cement ratio, the increase in water content within the system provides a more sufficient reaction environment for the hydration reaction of the matrix. Water molecules fully contact the matrix and participate in the chemical reaction, causing the OPCFC structure to gradually become denser, and the interfacial bond strength between the aggregate and the cementitious material also increases. This optimization of the microstructure allows the material to more effectively resist 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-cement ratio exceeds a certain threshold, excessive water cannot fully participate in the hydration reaction during the concrete hardening process, thus forming a large number of pores within the material. The presence of these pores disrupts the continuity and integrity of the material structure, weakens the material's stiffness, and causes the slope of the load-strain curve to gradually decrease.

[0087] The appearance of the first crack is a key indicator of the material's transition from the elastic stage to the nonlinear deformation stage. Experimental data shows that the displacement value at the appearance of the first crack ranges from 0.03 mm (WB36) to 0.05 mm (WB39). This indicates that the deformation corresponding to the initiation crack increases with the increase of the water-cement ratio. This is mainly because a higher water-cement ratio makes the internal microstructure of the material relatively looser, with more microscopic defects and weak areas. Under external loads, these weak areas are more prone to local stress concentration, and when the stress reaches the material's initiation strength, the crack begins to initiate and propagate. Because the looseness of the material's internal structure provides more space for crack propagation, a larger deformation is required to trigger crack appearance. Meanwhile, the initiation load also changes significantly with the increase of the water-cement ratio. Compared with WB36, the initiation loads of WB39, WB42, WB45, and WB48 are reduced by 40.12%, 57.91%, 66.36%, and 75.73%, respectively. In this series of data, WB39 exhibits the steepest load-strain curve slope among all experimental groups, indicating that WB39 possesses strong resistance to deformation during the elastic stage. However, this high stiffness also leads to the instantaneous release of accumulated elastic strain energy upon reaching the ultimate load, resulting in a more severe failure process, and its load-strain curve displays a classic brittle failure trend. As the water-cement ratio further increases, the curve slope gradually decreases, with a smaller difference in slope between WB42 and WB45. This suggests that within a specific water-cement ratio variation range, the additional water has a relatively small impact on the material's strength. This may be because, at this stage, the microstructural changes within the material have entered a relatively stable transition state, and the impact of added water on the material structure is insufficient to cause a significant change in strength.

[0088] The hardening stage is extremely short; the specimen fails instantly when the load reaches its peak, and the stress drops rapidly. Experimental data shows that the peak load gradually decreases with increasing water-to-binder ratio. However, comparing WB39 and WB42, the peak load of WB39 is lower than that of WB42. This is because WB39 has relatively insufficient water content, preventing the formation of a dense internal structure like that of WB42 during the matrix hydration reaction. WB42, with its ample water content, promotes complete hydration of the cementitious material, forming a denser microstructure and enhancing the overall strength of the material. Furthermore, the sufficient water dilutes the modulus of the solid alkali activator, altering the internal chemical reaction pathways and microstructure of the material. This can be attributed to the increased water-to-binder ratio inhibiting pore formation, thus significantly enhancing the load-bearing capacity of the specimen during the hardening stage.

[0089] The characteristics of the softening stage can be precisely reflected by the magnitude of the decrease in the load-displacement curve. This magnitude intuitively reflects the rate of load decrease as displacement increases. Experimental results clearly show that increasing the water content has a certain impact on the softening behavior of the material, specifically manifested as a slight increase in the downward segment of the curve. When the water-cement ratio gradually increases from 0.36 to 0.45, the softening stage of the curve is significantly prolonged. This phenomenon fully demonstrates that within this range of water-cement ratio variation, increasing the water-cement ratio can effectively improve the performance of OPCFC materials. A deeper analysis at the microscopic level reveals that changes in the water-cement ratio alter the internal pore structure and microcrack propagation patterns of the material. A moderate increase in water content creates a more uniform pore distribution within the material. When the material enters the softening stage, these pores can effectively disperse and absorb energy, slowing crack propagation and thus prolonging the softening stage of the curve, resulting in better energy absorption capacity of the material.

[0090] (4) Figure 8 The effect of water-cement ratio on the splitting tensile strength of OPCFC was shown. As the water-cement ratio gradually increased from 0.36 to 0.48, the splitting tensile strength of OPCFC decreased from 2.83 MPa to 2.22 MPa, exhibiting a clear negative correlation. Using WB36 as a reference, the tensile strengths of WB39, WB42, WB45, and WB48 decreased by 14.42%, 9.01%, 11.63%, and 21.73%, respectively. Analysis revealed that the splitting tensile strength of OPCFC decreases with increasing water content, a trend highly similar to that of OPCFC compressive strength with varying water content. This indicates that changes in water content significantly affect the hydration reaction process and the formation of the material's internal microstructure, thus having a similar impact on the material's compressive and tensile properties.

[0091] However, a deeper analysis of the performance differences of OPCFC under different water-cement ratios revealed a more significant decrease in splitting tensile strength when the water-cement ratio was WB39. In contrast, the splitting tensile strength of WB42 increased by approximately 6.28% compared to WB39. The underlying mechanism is primarily due to the relatively insufficient water content in WB39. During the matrix hydration reaction, water acts as both a reactant and a reaction medium, and its content directly affects the degree of hydration and the structure of the products. The insufficient water content in WB39 prevents the cementitious material from fully hydrating, making it difficult to form the dense and uniform internal structure of WB42. WB42, on the other hand, possesses a sufficient water content, providing favorable conditions for matrix hydration. On the one hand, abundant moisture can promote the full hydration of cementitious materials, generating more hydration products. These hydration products intertwine to form a denser microstructure, thereby effectively enhancing the overall strength of the material. On the other hand, with the increase of the water-cement ratio, the formation of pores inside OPCFC is suppressed, reducing stress concentration points and weak links inside the material, and significantly improving the specimen's ability to withstand splitting tensile loads during the hardening stage.

[0092] Based on the above analysis, it can be concluded that this invention uses fly ash and slag as cementing materials, 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-cement ratio to be 0.36–0.48, the slag-ash ratio to be 10%–30%, and the sodium-cement ratio to be 5%–7%, the comprehensive mechanical and workability of OPCFC can meet the application requirements. Furthermore, compared with traditional high-performance concrete, OPCFC can maintain the mechanical properties of high-performance concrete at higher water-cement ratios. The optimal comprehensive performance is achieved when the water-cement ratio is 0.42, the slag-ash ratio is 10%, the sodium-cement ratio is 6%, and the alkali activator modulus is 1.6.

[0093] The above are merely preferred embodiments of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A one-step cement-free concrete, characterized in that, The fly ash and the slag are used as cementitious materials, the machine-made sand is used as fine aggregate, the fine stone is used as coarse aggregate, the sodium tetraborate decahydrate is used as a retarder, and the solid sodium silicate is used as an alkali activator; wherein, the water-binder ratio is controlled to be 0.42 in the preparation process of the one-step cement-free concrete; the mass ratio of the slag in the cementitious materials is 10%; the mass ratio of the alkali activator to the cementitious materials is 6% in terms of the mass of sodium oxide; and the modulus of the alkali activator is 1.

6.

2. The one-step cementless concrete according to claim 1, characterized in that, The machine-made sand is medium sand, and the particle size is 0.25mm-0.5mm; and the particle size of the fine stone is 10mm-20mm.

3. The one-step cement-free concrete according to claim 1, characterized in that, The fly ash is F-grade fly ash, fineness is 10.8%, loss on ignition is 4.6%, density is 2.3 g / cm 3 .

4. The one-step cement-free concrete according to claim 1, characterized in that, The slag is a blast furnace S105 slag.

5. The one-step cement-free concrete according to claim 1, characterized in that, The dosage of the retarder is 3%-6% of the mass of the cementitious materials.

6. A method for the production of cement-free concrete according to any one of claims 1 to 5, characterized in that The machine-made sand and the fine stone are weighed according to the proportion, added into a mixer with a wet inner wall, and dry-mixed for 2min, then the cementitious materials, the alkali activator and the retarder are added, so that the aggregate and the cementitious materials are fully and uniformly stirred; 50% of the total water is added under the stirring state, the stirring time is 2min, then the remaining water is continuously added, and the mixture is stirred for 2min until the mixture reaches a uniform flow state, and the one-step cement-free concrete is obtained.

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