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Scientific Reports volume 14, Article number: 19212 (2024 ) Cite this article Polycarboxylate Superplasticizer For Dry-Mixed Mortar
The application environment for concrete is becoming increasingly complex, accompanied by an intensification of its functional requirements. This paper presents a method for developing self-compacting concrete with conductive properties using limonite and graphite as the concrete conductive phases. In the process of concrete preparation, the limonite is initially treated by a pre-wetting method to prevent the surface depression caused by the addition of limonite during the concrete curing process. The second stage of the process involved optimising different proportions of limonite and graphite and different dosages of water-reducing agent, defoamer and dispersant to prepare concrete. The influence of different dosages of limonite and graphite and different dosages of water-reducing agent on the mechanics and electrical conductivity of concrete was studied in order to obtain self-compacting conductive concrete with performance indicators meeting the requirements of self-compacting and electrical conductivity. The results demonstrate that the mechanical and electrical properties of self-compacting conductive concrete prepared with polycarboxylic acid superplasticizer and retarding superplasticizer combined with superplasticizer are satisfactory, and the composite superplasticizer can function in conjunction with dispersant. The self-compaction index, slump expansion, expansion time T50 and J-ring expansion of fluid concrete meet the requisite standards. Once the concrete has reached the designated curing age, its compressive strength and flexural strength align with the anticipated design expectations, while its resistivity meets the stipulated conductivity index requirements.
Concrete is the most widely used material in civil engineering, accounting for approximately 10–20% of the total volume of materials used in this field1,2. In 2019, global production of concrete is projected to reach 4.5 billion cubic metres, with an anticipated increase to 7 billion cubic meters by 2027. It is estimated that 80% of the carbon emissions associated with concrete are attributable to the production of cement. The concrete industry is confronted with significant challenges and opportunities pertaining to the concept of "carbon peak and carbon neutrality".
The investigation of self-compacting concrete in China commenced in the early 1990s, originating from the concept of fluid concrete proposed by Professor Feng Naiqian of Tsinghua University in 19873. Over the following decades, self-compacting concrete has been subjected to continuous research and development. In 2015, the internal structure of the giant column of the outer frame of the Tianjin Gaoyin 117 Building was complex and could not be vibrated for construction purposes. Consequently, it was necessary to pour C70 high-strength vibration-free self-compacting concrete4. Gao Xiaojian, Sun Bochao and colleagues5 conducted a study to investigate the influence of various mineral admixtures on the variable flow properties of freshly mixed self-compacting concrete. In order to achieve this objective, they employed a novel concrete rheometer. In a study conducted by Ning Xiliang and colleagues6, the effects of various fibres on the bursting resistance, residual compressive strength, residual bending and tensile strength, and mass loss of self-compacting concrete subjected to high temperatures were investigated. In a study conducted by Li Jianwei and Qin Qirong et al.7, self-compacting concrete samples containing stone chips and active fibers were prepared and their mechanical properties were studied. In a study published by Liu Yu, Huang Lei and colleagues8, the influence of sodium citrate on the compressive strength and paste micro-structure evolution of self-compacting concrete was investigated. The strength development mechanism was explored through hydration degree and rheology tests. In essence, China has pursued innovation and development in self-compacting concrete from a multitude of vantage points, with a relentless pursuit of optimising the performance of self-compacting concrete.
In 1986, Okamura, a foreign scholar, proposed the concept of self-compacting concrete9. In contrast to conventional concrete, self-compacting concrete does not necessitate additional manual vibration during mixing. It is capable of filling all corners of the formwork with concrete solely through the force of its own weight. Since the 1990s, the application of self-dense concrete in actual engineering structures in Japan has gradually increased, reflecting the advantages of its construction rationalisation10. Following subsequent development, the use of self-compacting concrete in Japan, Germany, the United Kingdom, the United States and other countries has become relatively common. Concrete accounted for 30–40% of the total output. By the end of 1994, 28 Japanese construction companies had acquired the requisite expertise to utilize self-compacting concrete in practical projects. Concurrently, self-compacting concrete has also been extensively utilized in Europe and North America. In 2005, the proportion of self-compacting concrete in Sweden, Norway and the Netherlands in northern Europe reached 10–15% of the total amount of concrete in the corresponding countries11,12,13,14. In the course of their research, scholars from a number of countries identified the need to optimise the preparation of self-compacted concrete. Consequently, Danar Altalabani et al.15 investigated the mechanical properties of self-compacted lightweight concrete reinforced with limestone powder and polypropylene fibre. Victor Revilla-Cuesta et al.16 conducted a study on the preparation of self-compacting concrete with varying proportions of recycled concrete fine aggregate and recycled concrete coarse aggregate. The performance of these concretes was then evaluated, and the influence of recycled concrete fine aggregate on self-compacting concrete was analysed when the amount of recycled concrete coarse aggregate was substantial.
As scientific and technological developments progressed, numerous scholars initiated efforts to create functional concrete, incorporating minute quantities of steel or carbon fibres into the concrete matrix with the intention of imparting a conductive effect. Verma Abhishek and colleagues17 conducted research into the electromagnetic characteristics of conductive concrete, enhancing the contact between steel fibres and graphite, and developing conductive concrete. Faneca et al.18 conducted an experimental study on conductive concrete made of recycled carbon fibre for self-heating and deicing applications in urban homes. The results demonstrated that the technology has the potential to be applied in deicing applications under climate conditions where the temperature is maintained at 3–5 °C or − 5 °C. Heydar Dehghanpour and colleagues19 investigated the effects of conductive additives derived from waste materials on the mechanics and conductive behaviour of concrete. Furthermore, research in this field is also developing rapidly in the domestic context. The utilization of conductive concrete represents an efficacious approach to addressing the issue of snow and ice accumulation on roadways in regions characterised by low temperatures. Furthermore, the potential of graphene composite conductive concrete for snow melting in cold environments has been explored through simulation20. Some scholars have identified potential applications for conductive concrete in grounding transmission line poles and towers21. For instance, in the context of domestic research on the grounding electrode of transmission poles and towers22, carbon fiber and carbon black composite conductive concrete is frequently employed as a novel grounding material, exhibiting commendable grounding performance.
From what has been discussed above, the combined effect of particle, filling and superposition is enhanced when mineral admixtures are mixed into concrete in the presence of a water reducing agent. During the process of cement hydration, the chemical reaction between various mineral admixtures results in the production of a series of effects, including induction activation, surface microcrystallisation and interface coupling. These effects contribute to the improvement of the mechanical properties and durability of concrete23. Furthermore, the particle size of mineral admixtures exerts a significant influence on the fillability, shape and economic benefits of self-compacting concrete24,25. The addition of graphite to the cementing material of self-compacting concrete can enhance its conductive function. At present, there is a paucity of research on self-compacting conductive concrete. In order to expand the application conditions of concrete and facilitate the development of functional concrete from a single-function to a multi-function material, this paper presents a novel type of self-compacting green functional concrete. The significance of developing limonite self-compacting conductive concrete is as follows: first, self-compacting property makes concrete widely used in complex components and key projects; Secondly, its electrical conductivity can make concrete have piezoresistive characteristics, electromagnetic shielding effect, snow melting, deicing and health monitoring of engineering structures. Third, improve the mechanical properties and durability of concrete, enhance the quality and use efficiency of concrete structural materials; Fourth, the use of limonite can make solid waste resource utilization and reduce carbon emissions.
In this paper, limonite sand and graphite are employed as conductive fillers for concrete. In addition, dispersant and water-reducing agents are utilized to enhance the working performance of concrete. The principal components of limonite are goethite (Fe2O3·H2O), hydrogoethite (2Fe2O3·H2O) and iron oxide containing different crystalline water, which has a high iron content, is relatively loose, is easily smelted and has a high yield. It can therefore be used as an ideal auxiliary filler in carbon-based conductive composite materials26,27. In this paper, limonite sand is partially replaced by river sand as fine aggregate. The rough outer surface of limonite sand can mitigate the loss of concrete strength that may result from the incorporation of excessive graphite into concrete28. Consequently, the particle size of the limonite was reduced to 50–75 μm by grinding technology as an admixture. In addition, HPMA (Hydrolyzed Polymaleic Anhydride), a dispersant, polycarboxylic acid retarding compound, a water reducing agent and sodium sulfate were combined in prehumidification to promote the water absorption of the limonite sand through the prehumidification method. This process effectively reduces the phenomenon of surface depression that can occur following the condensation of concrete. The addition of graphite to the mixed limonite self-compacting concrete results in the formation of a conductive concrete.
The P·O42.5 ordinary Portland cement provided by a company in Weifang, Shandong Province, exhibits an initial setting time of more than 150 min and a final setting time of less than 240 min.
Fly ash is a high-quality, first-class fly ash (particle size ≤ 12 μm) provided by a company in Yuncheng, Shanxi Province. Fly ash represents the optimal cement admixture, exhibiting high potential activity, a stable mineral chemical composition, fine particles and a low concentration of harmful substances.
The coarse aggregate is the coarse stone produced by a company in Shijiazhuang. The particle size is between 3 and 6 mm, the apparent density is 2700 kg/m3, the water content is 0.48%, and the flatness index is 21%.
The fine aggregate is the quartz sand between 80 and 120 mesh (120–180 μm) provided by the Anhui Shengli Quartz Sand Factory. Its main chemical composition is 99.34% silicon dioxide (SiO2), 0.3% aluminium oxide (Al2O3), and 0.0225% iron oxide (Fe2O3). The water content is 2.51%, the water absorption is 1.39%, and the particle density is 2.67 t/m3. The incorporation of quartz sand into concrete as a fine aggregate can enhance the compressive strength, tensile strength, and compactness of concrete, as well as improve the bearing capacity and stability of the structure.
The water-reducing agent employed is a polycarboxylic acid concrete water-reducing agent and a retarding concrete water-reducing agent provided by the manufacturer. The combination of polycarboxylic acid superplasticiser and retarding superplasticiser has been demonstrated to effectively reduce the incomplete phenomenon of subsequent concrete surface and improve the fluidity of concrete. The composition of the polycarboxylic acid concrete water reducing agent is presented in Table 1, while the composition of the retarded concrete water reducing agent is shown in Table 2. Polycarboxylic acid superplasticiser is a mixture of polycarboxylic acid superplasticiser and retarding superplasticiser (the principal component is citric acid). The water absorption of limonite sand can be enhanced by the combination of polycarboxylic acid superplasticiser and other admixtures when utilised in the pre-humidification process. The incorporation of limonite sand into concrete mixtures ensures the uniform dispersion of nano-graphite particles, thereby enhancing the fluidity and workability of the concrete29.
The nano-graphite is produced by the Liugong Graphite Company with a carbon content of 98.50%, and its apparent morphology is illustrated in Fig. 1a,b. Limonite is composed of goethite (Fe2O3·H2O), hydrogoethite (2Fe2O3·H2O), iron oxide containing different crystal water, and a mixture of pelitic substances. The iron content of limonite is 37–55%, as shown in Fig. 1c,d.
Apparent form of conductive filler.
The dispersant employed in this experiment is hydrolyzed polymaleic anhydride (HPMA), and its characteristics are presented in Table 3. The defoamer utilized is a polyether defoamer, and its characteristics are presented in Table 4. HPMA is a low molecular weight polyelectrolyte with a relative molecular weight of 400–800. It is non-toxic, soluble in water, chemically stable and thermally stable, with a decomposition temperature above 330 °C.
The copper mesh utilised in the aforementioned experiment was produced by the esteemed colleagues at Hardware Co., Ltd. through the process of standardisation. The mesh number of the copper mesh is 40, comprising 65% copper and 35% zinc. In this experiment, the brass mesh is employed as the electrode in the conductive test.
The sodium sulfate utilized in this experiment was procured from the Tianjin Dengfeng Chemical Reagent Factory. The substance is a white crystalline powder that is soluble in water. The composition of the substance is presented in Table 5.
It is assumed that the water-cement ratio, sand rate and other component parameters remain constant. Limonite sand is added to replace a portion of quartz sand as the fine aggregate of concrete in order to investigate the impact of limonite content on concrete performance and subsequently optimise the composition of the material. Furthermore, the adjustment of the fluidity of concrete through the use of a water reducing agent represents a crucial aspect of this test. In order to gain further insight into the comprehensive effect of these factors, three distinct types of graphite powder with varying mesh numbers were employed in the mixing of concrete samples, with the objective of elucidating the influence of graphite powder mesh number on concrete properties.
In order to ascertain the mesh number of graphite powder, the existing research results30 were consulted. This revealed that three graphite powders with different mesh numbers (200 mesh, 4000 mesh and 15,000 mesh) were required to explore their effects on the mechanical and conductive properties of concrete.
As a conductive material, limonite exhibits certain conductive properties, although its conductive effect is inferior to that of metals, which are considered excellent conductive materials. Consequently, the proportion of limonite sand incorporated into the concrete was set at 20%, 40% and 60%, respectively, in order to assess the impact of varying addition rates on the concrete's performance.
The utilisation of a water-reducing agent is also a central focus of this examination, which concerns the treatment of limonite sand by the pre-wetting method and the fluidity of concrete. In the experiment, polycarboxylic acid superplasticiser and polycarboxylic acid retarding compound superplasticiser were employed to assess the impact of distinct superplasticisers on the operational efficacy of concrete.
The target design strength grade of ordinary concrete used for comparison is C30, and the matrix concrete mix ratio is selected through a large number of concrete adapts. That is to say, the basic mix ratio of concrete is designed as W (water):C (cement):S (sand):G (stone) = 0.38:1:0.88:2.13. A total of nine control groups were designed. In this study, orthogonal test factors were employed to investigate the effects of A (mesh number of graphite powder), B (content of limonite) and C (type of water reducing agent) on the properties of limonite self-compacting concrete. A total of 18 groups were designed in the experimental group, and their combinations are presented in Tables 6, 7, 8 and 9. In these tables, G and M represent graphite and limonite sand, respectively, while P and R represent polycarboxylic acid high-efficiency water reducing agent and polycarboxylic acid slow-setting compound water reducing agent, respectively. The numbers 2, 4 and 15 after G indicate that the mesh number of graphite powder added is 200 mesh, 4000 mesh and 15,000 mesh, respectively. Similarly, the numbers 20, 40 and 60 after M signify that the content of limonite sand is 20%, 40% and 60% of the total quality of fine aggregate in concrete, respectively. The control group consisted of the following combinations: G0M0, G0M2, G0M4, G0M6, G2M0, G4M0, G15M0, G0M0R and G0M0P.
In the preparation process of self-compacting conductive concrete, two key processes are employed in order to ensure that the performance of the concrete meets the expected standards. Firstly, the limonite is pre-humidified, which is of critical importance in order to ensure the smooth hydration reaction within the concrete. By wetting the limonite to a sufficient degree, its capacity to absorb water within the concrete is diminished. This process serves to reduce the occurrence of clumping, thereby enhancing the compactness and overall strength of concrete. The second is the rational combination of a water-reducing agent. This additive can effectively reduce the water consumption of concrete while improving its fluidity through the precise calculation and verification of tests, thus determining the formula of the water reducer. This ensures that the concrete has the requisite fluidity and cohesion when it reaches the predetermined strength, in the preparation of specimens, in strict accordance with the standards set out in GB/T 50107-2010.
In accordance with the stipulations of the "Concrete Strength Inspection and Evaluation Standards"31, the dimensions of the specimen are calibrated in accordance with the dimensions of the test instrument employed, the dimensions of the mould, and the prevailing actual production conditions. For instance, the dimensions of the concrete cube test block are reduced by threefold in comparison to the original dimensions of the concrete cube compressive test block, in order to accommodate the smaller dimensions of the test instrument and mould space. Additionally, cuboid test blocks are scaled down by a factor of 2.5 in order to more accurately reflect the conditions that may be encountered in actual use.
In accordance with the specifications of the test, the concrete test block is prepared through the following steps:
Initially, the requisite quantity of HPMA dispersant is combined with the water reducer, in addition to the total weight of 5% of the weight of the limonite sand water. This mixture is then poured into the prepared container. The solution should be stirred until the components are sufficiently integrated to form a uniform solution. Subsequently, the mixture is poured into the prepared limonite sand and continues to be stirred in order to ensure that the limonite is dispersed effectively and to avoid clumping. In order to ascertain the water content of the limonite following the pre-wetting treatment, the following steps are required: Firstly, the limonite material is completely soaked in the solution until it has absorbed an adequate quantity of water. Once this point has been reached, the material is removed from the solution and wiped with a paper towel, thus removing any excess water from the surface. Finally, the wet weight of the limonite material is measured and compared with its initial dry mass. The water content of the limonite was found to be 4.6% following the pre-wetting treatment.
The pre-wet limonite sand and graphite are added to the mixed material composed of ordinary Portland cement, first-class fly ash and quartz sand. The mixing process should be continued until the mixed material becomes fine and uniform. At this point, the coarse aggregate is added to the mixture. At this time, the remaining water and the mixture of defoamer should be slowly added, and the mixing process should be repeated until the mixture is uniform. The mixed concrete slurry should then be poured into the mould.
Following the completion of the pouring process, the concrete sample should be promptly transferred to a curing box with an appropriate relative humidity (maintained above 95% RH) and temperature control (20 ± 2 °C) for curing. This is essential to ensure the humid environment required by the concrete sample, which is conducive to its curing and molding, and to facilitate the demolding of the concrete after solidification and molding. Following demoulding, the sample was returned to the curing box and the curing period was 7, 14, 21 and 28 days, respectively. The preparation process of the concrete specimen is illustrated in Fig. 2.
Preparation process of self-compacting conductive concrete.
Finally, 324 cube samples of 50 mm × 50 mm × 50 mm and 324 cuboid samples of 40 mm × 40 mm × 160 mm were prepared according to the standard method (81 cuboid samples were modified with copper mesh during the preparation process for conductive property testing), and a total of 27 groups of concrete samples were prepared. A total of 648 were prepared. The specimens that were poured for testing are shown in Fig. 3.
As illustrated in Table 10, the collapse-expansion degree of self-compacting conductive concrete of newly mixed limonite falls within the range of 630 to 720 mm, which aligns with the specifications for filling grade SF1 or SF2. The flow time T500 is 3.7–5.2 s, which conforms to VS1 level. Furthermore, the difference between slump expansion and expansion with ring is 0 mm ≤ PA ≤ 25 mm, which meets the requirements of PA2 index. Therefore, the preliminary conclusions are as follows:
The fluidity of the test group treated with the pre-humidification method was found to be superior to that of the test group not treated with the pre-humidification method. Furthermore, the test group without the pre-humidification method exhibited a greater degree of dilute slurry precipitation from the bottom after lifting the slump cylinder, which indicated that the concrete mix of the latter test group exhibited a lack of water retention.
The fluidity and compactness of the self-compacting conductive concrete with polycarboxylic acid retarding and water-reducing agent as admixture are superior to those with polycarboxylic acid water-reducing agent as admixture.
The results of the seven groups of tests indicate that the G15M60R test group has the best fluidity and compactness.
As illustrated in Fig. 7, the PA values are all within 30 mm, which fulfills the requirements of the specification33. The concrete in the J-ring of the unmixed superplasticiser exhibits severe accumulation, and its fluidity is inferior to that of the self-compacting conductive concrete with polycarboxylic acid retarding superplasticiser.
The mechanical properties tests included in this paper are the compressive strength test and the flexural strength test of self-compacted conductive concrete of limonite.
In this study, cubes of a size of 50 mm × 50 mm × 50 mm were prepared and cured for 7, 14 and 28 days, respectively, in order to perform compressive strength tests. The objective of this study was to determine the influence of different water-reducing agents, graphite parameters, limonite content, and the use of pre-humidification on the compressive properties of the test block.
A servo hydraulic universal testing machine was employed for the tests. The loading speed of the compressive strength test load was set at 0.6 MPa/s. The displacement curve of each load is read by the built-in software of the testing machine until the specimen fails34.
The compressive test and flexion test are in accordance with the "Standard for Test Method of Mechanical Properties of Ordinary Concrete" (GB/T50081-2019)35, as follows:
where Fcc: Compressive strength of concrete test block, MPa; F: Specimen failure load, N; A: Specimen bearing area, mm2.
In this study, the 40 × 40 × 160 mm test blocks were subjected to a curing period of 7 days, 14 days and 28 days, respectively. The objective of this study was to determine the influence of different water-reducing agents, graphite parameters, limonite content and the use of a pre-humidification method on the bending resistance of the test block. The load rate employed in the flexural strength test was 0.03 MPa/s. The spacing between loading points in the flexural strength test was 140 mm. The software of the testing machine is employed to read each load displacement curve until the test piece fails.
where ft indicates flexural strength of concrete test block(MPa), the calculation result should be accurate to 0.01 MPa; F specimen failure load (N); l span between supports (mm); b specimen section width (mm); h specimen section height (mm).
Table 11 and Fig. 8 present the data and graphs of the test control group. Table 11 and Fig. 8 demonstrate that the compressive and bending strengths of LSCCC without graphite and water reducing agent are significantly lower than those of LSCCC with graphite and water reducing agent. It can be demonstrated that the incorporation of graphite and a water-reducing agent can enhance the mechanical properties of LSCCC.
The compressive and flexural strength of LSCCC in control group.
Tables 12, 13 and Figs. 9 and 10 demonstrate that the addition of polycarboxylic acid superplasticiser to test blocks of 7 days, 14 days and 28 days results in a graphite mesh number of 200 mesh and 4000 mesh. Furthermore, the compressive strength of the test blocks increases with the increase of limonite content. When the mesh number of graphite is 15,000, the compressive strength of the LSCCC mixed with 40% limonite is the highest, while the compressive strength of the LSCCC mixed with 60% limonite shows a downward trend. This is due to the uneven distribution of limonite in the LSCCC, resulting in a decrease in its compressive strength. The compressive strength of LSCCC with different graphite mesh numbers increases with the addition of limonite when polycarboxylic acid retarding compound water reducer is added. On the whole, however, preliminary tests show that the compressive strength of LSCCC supplemented with polycarboxylic acid retarding compound superplasticizer is generally lower than that of LSCCC supplemented with polycarboxylic acid superplasticizer. This indicates that the fluidity and self-compaction of LSCCC with polycarboxylic acid retarding compound superplasticiser can only be enhanced, which is convenient for construction. However, from the perspective of late intensity, it has not been observed to undergo synchronised growth, and further investigation is required to increase the sample size.
Compressive strength of LSCCC mixed with different water reducing agents.
Flexural strength of LSCCC mixed with different water reducing agents.
As illustrated in Fig. 10 and Table 13, the bending strength of test blocks increased with the addition of limonite content when graphite mesh number was 200 mesh, 4000 mesh and 15,000 mesh. However, the bending strength of LSCCC supplemented with a polycarboxylic acid retarding and water-reducing agent was found to be significantly lower than that of LSCCC supplemented with a polycarboxylic acid water-reducing agent. The rationale is analogous to that previously outlined in the compressive strength analysis.
In order to gain further insight into the internal microstructure of LSCCC, a scanning electron microscope (SEM) was employed to observe and analyse the internal composition of the material. As illustrated in Fig. 11, limonite sand that has not undergone pretreatment exhibits a pronounced tendency to agglomerate when in contact with water. In contrast, limonite sand that has been subjected to the pre-wetting method displays a more porous and uniform morphology than its untreated counterpart. This observation suggests that the pre-wetting method enhances the water absorption capacity of limonite sand, facilitating its uniform dispersion within the concrete mixture. Consequently, upon hardening, the surface of the concrete is less susceptible to the formation of dents or other defects, thereby ensuring the maintenance of structural integrity. Furthermore, the good electrical conductivity of the limonite sand and its uniform dispersion facilitate the enhancement of the electrical conductivity of LSCCC through the pre-wet treatment.
SEM micrographs of (a) untreated limonite, (b) Limonite after prewetting treatment.
Figure 12 illustrates the SEM diagram and binarization diagram of LSCCC under the condition of polycarboxylic acid retarding and water reducing agent. This allows the internal pore distribution to be obtained and the inner density of LSCCC to be observed in greater detail. When the particle size of the graphite remains unchanged, the internal structure of LSCCC becomes more compact with an increase in the content of limonite sand. Further observation of Figs. 12 and 13 reveals that in the LSCCC test block containing a polycarboxylic acid retarded compound water reducing agent and graphite particle size of 15,000 mesh, when the limonite content is 60%, the internal distribution of LSCCC is uniform and the concrete density is optimal. The above translation results were generated by the Universal Neural Network Translation (UNMT) system, which was designed for general use. When the concrete admixture is a polycarboxylic acid water reducing agent, the graphite particle size is 15,000 mesh and the limonite content is 40%, the internal structure of LSCCC is compact, and the density of LSCCC is the best. The type of water-reducing agent, graphite particle size, and limonite content have been demonstrated to influence the compactness of LSCCC. The addition of limonite can fill the void between aggregates and improve the compactness of concrete. The chemical action of the water-reducing agent in the pre-humidification method promotes better water absorption of limonite sand, resulting in a favourable combination effect between limonite sand and cement slurry, thus improving the compactness of LSCCC. Furthermore, a comparison of Figs. 12 and 13 reveals that the use of polycarboxylic acid retarding and water reducing agents can markedly enhance the dispersion of limonite sand within the material. This degree of dispersion directly impacts the overall performance of LSCCC, ensuring a favourable combination between nano-graphite and other components of concrete, and thus optimising the conductive characteristics of LSCCC. In comparison to LSCCC utilising solely polycarboxylic acid superplasticisers, it can be observed that a superior degree of compaction is achieved.
SEM micrographs and binary picture of Polycarboxylic acid retarding compound water reducer LSCCC.
SEM micrographs of polycarboxylic acid water reducer LSCCC sample.
A four-point method was employed to conduct a conductivity test on specimens at 7d, 14d, 21d, and 28d, with the objective of measuring their resistivity. The dimensions of the specimen were 40 mm × 40 mm × 160 mm, and 30 mm × 80 mm metal mesh electrodes were embedded parallel to the specimen during the placement of the concrete. The resistance of the conductive concrete was recorded using a digital multimeter. Figure 14 illustrates the schematic diagram of the electrical conductivity test for concrete specimens.
Schematic diagram of the four-electrode method.
In accordance with the “Standard Test Method for Resistivity of Conductive Materials” (ASTM B193-2002)36, the formula for calculating the resistivity of concrete is as follows:
where ρ is resistivity (Ω·cm); U is voltage (V); I is current (A); A is cross-sectional area of the specimen (cm2); L is distance between electrodes (cm).
Figure 15 illustrates that when the graphite particle size remains unchanged and the type of water reducer is consistent, an increase in limonite content results in a gradual decline in resistivity and a corresponding increase in conductivity within 28 days. When the water-reducing agent remains constant and the limonite content is maintained, the 28-day resistivity of LSCCC decreases with an increase in graphite particle size. In the case of differing water-reducing agents, the 28-day resistivity of LSCCC with a polycarboxylic acid retarding compound water-reducing agent is found to be lower than that with a polycarboxylic acid water-reducing agent.
As illustrated in Table 14, when the graphite particle size in LSCCC remains constant and the limonite content remains unaltered, the 28-day resistivity of LSCCC mixed with polycarboxylic acid and a water reducing agent is observed to be lower than that of LSCCC mixed with a polycarboxylic acid and a water reducing agent. This is due to two reasons:
The action of a polycarboxylic acid retarding and water reducing agent results in concrete exhibiting enhanced fluidity and a more compact internal structure. Limonite and graphite are uniformly dispersed in LSCCC as conductive fillers, thereby enhancing its conductive properties.
The polycarboxylic acid retarding superplasticiser is composed of two distinct components: the polycarboxylic acid retarding superplasticiser and the retarding superplasticiser. The primary component of the retarding superplasticiser is citric acid, which inhibits the dissolution of cement particles, thereby enabling the polycarboxylic acid superplasticiser to promote the water absorption of limonite sand in the early stages and disperse in cement and other materials. This improves the electrical conductivity of LSCCC.
In this paper, the objective of this study was to investigate the effects of varying limonite content, graphite particle size and water reducing agent on the mechanics, conductivity and fluidity of concrete. The aim was to determine the optimal concrete mix, which informed the preparation of a limonite self-dense conductive concrete with the best performance. The following conclusions can be drawn from the results of the study:
The incorporation of graphite and limonite into concrete enhances the electrical conductivity of the concrete. When the admixture of concrete is a polycarboxylic acid retarding and water reducing agent, the concrete with a 15,000 mesh graphite and 60% limonite sand content exhibits the highest electrical conductivity electrical conductivity. When the concrete admixture is a polycarboxylic acid superplasticiser, the concrete exhibits the highest electrical conductivity when the graphite particle size is 15,000 mesh and the limonite sand content is 40%.
The substitution of limonite sand for quartz sand as a fine aggregate results in a more compact condensation process of the concrete mix. When the concrete admixture is a polycarboxylic acid retarding and water reducing agent, and the limonite sand is mixed with 60%, the particle size of graphite added is 15,000 mesh, and the compressive and bending properties of concrete are optimised. However, if the concrete admixture is changed to a polycarboxylic acid superplasticiser, with a limonite sand content of 40%, and a graphite particle size of 15,000 mesh, the concrete will exhibit the best compressive and bending resistance.
The water reducing agent has the dual effect of reducing the water consumption of concrete mixing and promoting the water absorption of limonite sand in the pre-humidification method. The limonite sand has been pre-humidified at an early stage, which results in it being full of water. This subsequently reduces its ability to absorb water in the concrete, reduces the occurrence of agglomeration, and ensures that the pre-wet slurry is evenly dispersed throughout the concrete mix. The phenomenon of sagging after concrete condensation is effectively reduced.
The microstructure of LSCCC was analysed by electron microscope scanning, and the results were consistent with the experimental conclusions, thereby further corroborating the veracity of this study.
The datasets generated during and analyzed during the current study are available from the corresponding author (Xiantao Zeng) on reasonable request.
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The project is supported by Hunan Key Laboratory of Intelligent Disaster Prevention and Mitigation and Ecological Restoration in Civil Engineering; Catastrophe and Reinforcement of Dangerous Engineering Structures of Hunan Provincial Engineering Research Center; Natural Science Foundation of Hunan Province (Grant No. 2022JJ30193); Hunan Provincial Science and Technology Promotion Talent Project (Grant No. 2022TJ-Q17); The science and technology innovation Program of Hunan Province (Grant No. 2022RC4032).
Hunan Provincial Key Laboratory of Intelligent Disaster Prevention-Mitigation and Ecological Restoration in Civil Engineering, Hunan Institute of Engineering, Xiangtan, 411104, China
Zhenhua Ren, Jia Guo, Wei Chen & Xiantao Zeng
School of Civil Engineering and Architecture, East China Jiao Tong University, Nanchang, 330013, China
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Conceptualization, Z.H.R. and X.T.Z.; methodology, Z.H.R. and X.T.Z.; software, J.G.; validation, J.G. and W.C.; formal analysis, Z.H.R. and X.T.Z; investigation, Z.H.R.; resources, Z.H.R. and X.T.Z.; data curation, J.G.; writing—original draft preparation, J.G.; writing—review and editing, Z.H.R. and X.T.Z.; visualization, J.G. and W.C.; supervision, Z.H.R. and X.Y.W.; project administration, Z.H.R. and X.T.Z.; funding acquisition, Z.H.R. All authors have read and agreed to the published version of the manuscript.
Correspondence to Zhenhua Ren, Xiantao Zeng or Xiang Yu Wang.
The authors declare no competing interests.
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Ren, Z., Guo, J., Chen, W. et al. Impact of different water-reducing agents on the properties of limonite self-compacting conductive concrete. Sci Rep 14, 19212 (2024). https://doi.org/10.1038/s41598-024-69671-2
DOI: https://doi.org/10.1038/s41598-024-69671-2
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