Microstructure and Chloride Diffusion Properties of Hardened Fly Ash Cement Paste with Three-dimensional Graphene

In order to characterize the influence of 3DG content on the workability of fly ash cement paste, the fluidity of cement paste was tested according to the test method for homogeneity of concrete admixtures (GB/T 8077-2012). The mixtures were poured into a truncated cone mold immediately after mixing. The dimensions of the cone model were top diameter 36 mm, bottom diameter 60 mm, and height 60 mm. The cement paste was smoothed with a spatula. Then the mold was removed vertically. The maximum diameter of the fresh fly ash cement paste in two directions perpendicular to each other was measured. The average value of the two vertical directions was recorded as the final fluidity.

According to the Chinese standard GB/T 17671-2020, the compressive strength of samples was tested at 28days by the MTS810 hydraulic servo testing machine. The size of specimens for measurements was 40 × 40 × 40 mm. The loading rate was set to 2.4 kN/s. Three specimens were repeated for compressive strengths. The average of three values was taken as representative compressive strength.

The Ion Selective Electrode method (ISE) was applied to evaluate the effect of 3DG on chloride diffusion of hardened fly ash cement paste in this study. The content of water-soluble chloride in fly ash cement-based composites was determined by the NJCL-B chloride ion rapid determination instrument (Beijing, China). The size of the specimens was 10 × 20 × 30 mm. The samples cured for 28 days were conducted with the chloride diffusion tests. In order to bond the epoxy resin tightly to the sample, the five surfaces of the dry test block were coated with epoxy resin. The uncoated epoxy surface was 10 × 20 mm in size and served as a chloride diffusion surface. After the epoxy resin was dried, the sample was immersed in a NaCl solution (5% by mass). After the immersion of 7 days, the specimens were removed and split along the direction of chloride diffusion. Then, a 0.1 mol/L silver nitrate solution was sprayed onto the surface of the split samples. The color change border was identified as the chloride diffusion depth. After that, half of the sample was used to investigate the chloride content at different depths. Layers of 4 mm depths (0–20 mm) were milled, and the powders were collected. After being dried at 60 ℃ for 6 h in an oven, the powders were put in the bottle with deionized water. The bottle was shaken 3–5 times before the test.

The porosity and pore structure of the hardened fly ash cement paste was tested by an Autopore IV9500 automatic mercury porosimeter (Micromeritics Co., USA) with a pressure ranges up to 228 MPa. The pore diameter was performed from 0.003 μm to 1000 μm. Cubes of size 10 × 10 × 10 mm were used for MIP measurements. After 28 days of curing, the samples were soaked in acetone to stop the hydration process. Then, the samples were dried at 60 ℃ for 6 hours before MIP measurements.

The fly ash cement paste of 28 days was carried out by X-ray diffraction (Mini Flex 600) using Cu Kα radiation (λ  = 1.54056 Å) and operating at 40 kV and 15 mA with a step size of 0.02. The qualitative analysis of cement hydrates was examined with a scanning rate of 4°/min in the 2θ range of 2–70 °. Powder samples of hardened fly ash cement paste were produced by grinding.

To better understand the microstructure evolution of 3DG-fly ash cement paste, the field emission scanning electron microscope (Carl Zeiss, Germany and Hitachi-SU8220, Japan) was used. The fracture surfaces of the composites after the compressive strength test were selected to characterize the morphology. Samples were dried under vacuum conditions for 6 hours before the test. The working voltage of the scanning electron microscope was 5.0 kV.

The pore characteristics of the 3DG fly ash cement system of 28d were characterized by MIP. The cumulative and log differential intrusion curves are shown in Fig. 2. The detailed results of the mercury intrusion test are shown in Table 4. With the addition of 20% fly ash, the cumulative intrusion curve shifts down significantly in Fig. 2a. With the addition of 3DG, the cumulative intrusion curves shift downward and then upward. The sample with 0.1% content 3DG has the smallest cumulative intrusion volume. Compared with the FG0 group, the cumulative intrusion volume drops by 8.1%. The addition of fly ash and 3DG plays the synergetic role in reducing the pore volume of cement-based materials. The peak value of six specimens is mainly distributed between 10 and 100 nm in Fig. 2b. With the addition of 3DG, it is evident that the peaks of the differential distribution curves shift toward smaller pore sizes. The most probable pore size, which corresponds to the peak in the differential distribution curves, represents pore size with the highest probability. The specific data measured in the test are listed in Table 4. Compared with the reference group FG0, the most probable pore size is reduced by 19.96% with 0.02% 3DG. When the 3DG content is 0.1%, the reduction quantity reaches a maximum and the decrease is 35.67%. When the content of 3DG further increases, the most probable pore size begins to rise.

The incorporation of 3DG not only affects the porosity and the most probable pore size of cement paste but also affects its pore size distribution. According to the classification standard proposed by Wu (1979), the pore sizes can be divided into four categories, namely harmless pores (< 20 nm), less-harmful pores (20–50 nm), harmful pores (50–200 nm), and more-harmful pores (> 200 nm). The pore size distribution measured by MIP is shown in Fig. 3. Results indicate that the introduction of 3DG into the fly ash cement paste has a minor effect on both harmless pores (< 20 nm) and more-harmful pores (> 200 nm). The less-harmful pores (20–50 nm) and the harmful pores (50–200 nm) changed obviously. With the increase of 3DG content, less-harmful pores gradually become the dominant pores. The fraction of the less-harmful pores and the harmful pores are shown in Table 4. The fraction of less-harmful pores is calculated by dividing the porosity of less-harmful pores (20–50 nm) by the corresponding total porosity. The fraction of less-harmful pores in FG02, FG05, FG10, and FG20 samples are 56.18%, 52.85%, 61.67%, and 51.95%, respectively. Therefore, the addition of 3DG can refine the pore structure of the fly ash cement system and promote the transformation of the harmful pores (50–200 nm) to less-harmful pores (20–50 nm).

XRD was used to determine the qualitative interpretation of various phase compositions and evaluate hydration reactions in the cement paste. The XRD results of ordinary cement paste (OPC), cement paste with 20% fly ash (FG0), and 0.1% 3DG-fly ash cement paste sample (FG10) at 28 days of curing are shown in Fig. 4. It shows that the XRD diffraction peaks of FG0 and FG10 samples are similar to ordinary cement paste. It indicates that no new hydration products generate by adding fly ash and 0.1% 3DG compared with ordinary cement paste. However, the peak intensity of the CH crystals in FG10 is stronger than in the FG0 sample. This shows that CH has better crystallinity. Thus, it is indicated that the addition of 3DG may promote hydration reactions in the cement matrix and create conditions for the secondary hydration of fly ash.

In this research, SEM was employed to observe the microstructure of the cement composites. Ordinary cement paste (OPC), reference group FG0 (20% FA), and FG10 (20% FA-0.1% 3DG) samples at 28 days of curing were tested to study the effect of 3DG on the microstructure and hydration product morphology of fly ash cement paste. SEM images are shown in Figs. 5,  6,  7, respectively.

Fig. 5a, b illustrates SEM images of the ordinary cement paste (OPC) cured at 28 days. There are still big and sparse holes randomly distributed in Fig. 5a, which decrease the density of the specimens. Fig. 5b presents a large number of needle-like hydration products randomly distributed throughout the matrix. And the cracks distribute in the slack hydration products in a straight manner. Thus, the structure of hydration products in ordinary cement paste is loose.

Fig. 6a, b depicts the morphology of 20% fly ash cement paste. Many spherical FA particles distributed in hydration products are shown in Fig. 6a. Some fine particles of hydration products have been adhered to the surface of FA, indicating the hydration of FA is just at the beginning stage in fly ash cement composites. At this stage, FA particles mainly play the role of filling and micro-aggregate in cement. In addition, it is also can be observed that the continuous fine cracks pass through the cement matrix. As shown in Fig. 6b, the loose structure is C–S–H gel is distributed in a disorder condition. Compared with the OPC group, the morphology of C–S–H changes from fibrous to loose network structure.

Fig. 7a–i shows the microscopic morphology of the fly ash cement sample with 0.1% 3DG. It can be observed from Fig. 7a that the surface of some FA particles is uneven. Fig. 7b, c depicts the morphology of hydration products attached to the surface of fly ash particles. A small amount of CH and a large amount of particle hydration products are attached to the surface of FA particles. Compared with the FA particle in Fig. 6a, the degree of secondary hydration of FA is higher. It is indicated that the addition of 3DG has accelerated the pozzolanic reactions of FA particles. This can be attributed to the fact that three-dimensional honeycomb graphene provides nucleation sites to facilitate the precipitation of primary cement hydration products, which further triggers the secondary hydration of the FA particles (Kaur & Kothiyal, 2020). Meanwhile, the accumulated layered C–S–H can be observed around the fly ash. It shows that the combined effect of 3DG and fly ash is more beneficial to the formation of cement hydration products.

Fig. 7d–f shows the microstructure of 3DG in cement and its combination with hydration products. As shown in Fig. 7d, the three-dimensional honeycomb structure of graphene can be observed in the fly ash cement matrix. At the same time, the 3DG is closely connected with the cement matrix by providing the attachment point of hydration products. Fig. 7e, f shows the details of the connection between graphene and cement products. It can be observed that many small particles of C–S–H gel adhere to the cavity and thin wall of 3DG. 3DG is used as the nucleation site of C–S–H gel and used as a carrier to accelerate the growth of C–S–H. The continuously generated C–S–H forms a three-dimensional connecting network along the single-layer graphene wall of 3DG. The thin graphene wall acts as a bridge between the cement matrix and graphene to connect them into an integral structure. Fig. 7g shows that the dense graphene sheet is closely bonded to the cement. The dense graphene sheet can effectively block the development of cracks to prevent the diffusion of chloride ions. This can be attributed to the barrier formed by the graphene sheet, which makes the entry path of chloride ions more tortuous (Du et al., 2016). It can be observed that a stacked connecting wall is formed at the connection between graphene and cement hydration products in Fig. 7h. Fig. 7i shows the details of the three-dimensional graphene. Graphene is used as the substrate for crystal formation and deposition because of its huge specific surface area, which provides a platform for crystal formation and growth. At the same time, it can not only enhance the adhesion between cement and graphene but also prevent crack propagation to enhance the overall mechanical properties of cement-based materials (Li et al., 2018). Fig. 7j–l shows the morphology of C–S–H. Compared with C–S–H in Fig. 6b, C–S–H in Fig. 7i is denser and more regular. It is indicated that the addition of graphene and fly ash changes the trend of C–S–H and makes its arrangement more orderly. It is shown in Fig. 7l that the C–S–H are staggered, which is conducive to anchoring and bonding. The above results show that graphene can not only improve the morphology of cement hydration products but also enhance the combination of them. It helps prevent the diffusion of chloride ions and improve its mechanical properties.

In this research, a mini-slump cone test was conducted to find out the influence of 3DG on the fluidity of fly ash cement paste. At the same time, the increased content of dispersant will also affect the fluidity of cement-based materials. Therefore, fly ash cement containing different doses of dispersant was added in this experiment. And the results are given in Fig. 8. When 3DG and PC were added into fly ash cement paste at the same time, it was found that the fluidity of FG02, FG05, FG10 and FG20 groups was lower than that of fly ash cement paste containing only PC. Due to the large specific area of 3DG, flocculation shape and accumulation of cement particles are formed. The flocculated structure entraps a large amount of water which leads to lower fluidity (Indukuri et al., 2019). The fluidity diameters are shown in Table 3. At the 20% replacement of fly ash in cement paste, the fluidity is obtained as 91 mm. Compared with the FG0 group, the fluidity of samples with 0.2 wt% 3DG is the largest, which is increased by 34.07%. It can be attributed to the following two factors. Firstly, due to the less water demand and containing a lot of smooth spherical particles, fly ash particles take action as a ball between the cement particles to reduce the resistance of cement particles (Wang et al., 2017). As a result, the homogeneity and fluidity of the fly ash cement paste are improved. Secondly, because of the addition of PC with the same mass fraction as 3DG, PC can promote the dispersion of 3DG and enhance the fluidity of the cement-based system. Therefore, fly ash and high-efficiency polycarboxylate superplasticizer can help offset the obstruction of 3DG to the fluidity of cement paste.

The effect of 3DG on the compressive strength of fly ash cement-based materials was studied after 28 days of curing. The results are illustrated in Fig. 9. Compared with the OPC sample, the compressive strength of cement paste with a content of 20% fly ash (FG0) has been found to show a slight decrease. Due to the substitution of cement with 20% FA in the FG0 group, the proportion of cement clinker minerals in the system decreases. Therefore, hydrated calcium silicate gel also declines (Shi & Fang, 2004). The compressive strength is reduced accordingly.

Moreover, with the increase of 3DG content, the compressive strength of the paste presents a changing law that first increases and then drops. The compressive strength is improved progressively by increasing the 3DG range from 0.02 to 0.1%. The percentage improvement in compressive strength for FG02, FG05, and FG10 has been observed to be 2.21%, 21.20%, and 31.33% with respect to the FG0 group without 3DG. After that, the compressive strength shows a reduction with the increasing content of 3DG. A similar trend in these results was also found in previous studies (Birenboim et al., 2019). The results demonstrate that the compressive strength can be improved when the content of 3DG does not exceed 0.1%. An appropriate amount of 3DG dispersed in the cement matrix can refine the pore size distribution and adjust the degree of compaction of hydration products. So, the compressive strength of fly ash cement is improved.

After 7 days of diffusion in sodium chloride solution, the cement samples were axially split and sprayed with AgNO₃. The chloride diffusion depth of fly ash cement paste with different 3DG contents is shown in Fig. 10. The chloride diffusion depth of the OPC group is 16 mm. Compared with the OPC sample, the chloride diffusion depth of the FG0 sample decreases slightly. With the increase of 3DG content, the chloride diffusion depth of fly ash cement drops gradually until 3DG content reaches 0.1%. Comparing to the FG0 samples, the chloride diffusion depth of FG02, FG05, FG10, and FG20 are reduced by 20%, 40%, 53.3%, and 26.67%, respectively.

The diffusion of chloride ions into cement paste can be described by Crank's solution to Fick’s second law:where \(C\left( {x{,}\,\;t} \right)\) is the chloride content at depth x (mm) and time t (s), \({ }C_{0}\)(%) is the initial chloride ion content in cement, \({ }C_{S}\) (%) is the chloride content at the surface, the erf is the error function, and \(D_{{{\text{app}}}} \left( {10^{ - 12} {\text{m}}^{2} /{\text{s}}} \right)\) is the chloride diffusion coefficient.

The best-fitted curves of different 3DG contents are displayed in Fig. 11a. It is observed in the figure that the chloride content decreased with the increase of depth. And every sample has the same trend. The chloride diffusion coefficient and surface chloride concentration are shown in Fig. 11b. The chloride diffusion coefficient and chloride content at the cement surface drop gradually until 0.1% and then increase. The chloride diffusion coefficient and chloride content at the cement surface of the FG0 sample are 34.95 × 10^–12 m²/s and 2.722%, respectively. The chloride diffusion coefficient and surface chloride concentration reduce by 13.22% and 19.36% with the addition of 0.02% 3DG. The minimum chloride diffusion coefficient and surface chloride concentration value are achieved when the 3DG content is 0.1%. Additionally, the chloride diffusion coefficient and surface chloride concentration displayed a maximum reduction of approximately 49.44% and 73.29% compared to the FG0 mix.

In this experiment, the chloride diffusion depth and chloride diffusion coefficient gradually decrease when the content of 3DG is less than 0.1%. It indicates that the appropriate addition of 3DG can effectively improve the barrier performance of fly ash cement composites. This enhancement effect can be attributed to the following three aspects. Firstly, the addition of fly ash and polycarboxylate superplasticizer reduces the viscosity and improves the fluidity of the 3DG-fly ash cement system. It contributes to enhancing the compactness of composite cementitious materials and reducing the generation of pores (Wang et al., 2019a). Secondly, MIP results show that the addition of 3DG significantly enhances the pore structure of fly ash cement-based materials. The pore volume, most probable pore diameter, and the fraction of harmful pores (50–200 nm) are decreased by the addition of 3DG. The decrease in the proportion of macropores restricts the movement of chloride ion molecules into cement-based materials (Du et al., 2016). Finally, 3DG becomes the nucleation point of hydration products with its large surface area, and the shape of hydration products gradually presents a regular and compact appearance. The improvement of hydration product morphology enhances the ability to resist chloride ion diffusion. However, when the dosage of 3DG exceeds 0.1%, the chloride diffusion depth and chloride diffusion coefficient start to increase, which can be attributed to the limitation of the current dispersion technique to disperse high concentration of 3DG uniformly. And the aggregated graphene finally increases the porosity of the system, which reduces the resistance to chloride diffusion.