## Consistency

As shown in Fig. 6, the consistency of the geopolymer is closely related to the dosage of all four factors (alkali equivalent, slag substitution rate, modulus and water-cement ratio). The consistency of specimen is decreased with increasing alkali equivalent and increased with increasing slag substitution rate, increasing modulus and increasing water-cement ratio. As can be seen in Fig. 6, the consistency of thirteen groups of specimens, including N10, N12, N14, S15, S30, S45, S60, M1.2, M1.4, W4.0, W4.5, W5.0, W5.5, is in the range of 7 cm ~ 11 cm, which meets the basic requirements of the consistency of mortar in Chinese standard (GB/T 25181 − 2019) and conditions of actual construction in China31.

## Compressive strength

Figure 7 describes the compressive strength for geopolymers with different factor (alkali equivalent, slag substitution rate, modulus and water-cement ratio). It is found that the compressive strength of the specimens is increased and then decreased with the increase of alkali equivalent (Fig. 7a), and the compressive strengths at 7 days and 28 days reached the maximum value when the alkali equivalent was taken as 10%, which increased by 53.62% and 60.99% compared with that when the alkali equivalent is 16%. OH− ions provide an alkaline environment for the hydration reaction and accelerate the dissolution of active units such as [AlO4]5− ions and [SiO4]4− ions in the precursors32. Thus, the hydration reaction is facilitated, thus the amount of gelling products generated is increased and the internal structure is filled, resulting in higher compressive strength. However, when OH− ions are in excess, the rate at which the structure of the precursor is destroyed and the rate at which the hydration products are formed are too fast33. Therefore, the products do not have time to diffuse and adhere to the surface of precursor to form a protective film, so that the later reaction process is blocked. Thus, the compressive strength is reduced by too high alkali equivalent.

The compressive strength of the geopolymer is significantly increased with increasing slag substitution rate (Fig. 7b). When the slag substitution rate is reached 60%, the compressive strength of the specimens under the age of 7 days and 28 days is as high as 66.07 MPa and 81.33 MPa, respectively, which is 374.38% and 226.72% higher than that of the all-gangue based geopolymer, indicating that the compressive strength of the specimens is greatly affected by the slag substitution rate. This is due to the fact that the content of Ca2+ in the system is continuously increased with the increase of slag substitution rate, and more [AlO4]5− and [SiO4]4−units in the pores are combined to produce C-S-H and C-A-S-H gel crystals. Meanwhile, the high charge density of Ca2+ ions drives gel precipitation to from the formation of the three-dimensional disordered mesh that fills the internal structure of the geopolymer12, and ultimately the compressive strength of the geopolymer is improved.

The compressive strength of the geopolymer is increased and then decreased with increasing modulus (Fig. 7c). The maximum compressive strength of the geopolymer is obtained when the modulus is 1.4. As can be seen in Fig. 8, the specific surface area of the micelles in the alkali exciters is increased with increasing modulus, resulting in an enhanced ability of the micelles to adsorb silica-oxygen anion groups, metal ions, and polymerization products. Thus, OH− ions penetrate into the precursor material more easily, so that the later hydration process is promoted. Eventually, a dense and stable internal structure is formed, leading to an increase in compressive strength. However, an increase in modulus also means a relative decrease in the content of OH− ions in the pores, and when its content is not enough to drive the active units to dissolve, the compressive strength of the geopolymer is decreased34. Therefore, the compressive strength is significantly decreased when the modulus is taken to 1.6.

The compressive strength of the geopolymer is reduced with the increasing water-cement ratio (Fig. 7d). The free water content within the geopolymer is increased with the increase of the water-cement ratio, resulting in the increase of pore size and the number of pores, and the internal densification and stability of the geopolymer is reduced, so that the compressive strength of the geopolymer is decreased. In addition, an increase in the water-cement ratio causes the concentration of alkali exciter to be reduced, which may also induce lower compressive strength. It is worth noting that when the water-cement ratio is 4.0, the compressive strength of geopolymer at 28 days is reached 80.10 MPa, which is similar to that of geopolymer when the slag substitution rate is 60%. This shows that the appropriate reduction of the water-cement ratio, not only to obtain a higher compressive strength, but also to increase the utilization rate of coal gangue, maximizing the realization of its secondary high-value utilization.

## Resistivity

## Influence of ratios of geopolymer

The effect of alkali equivalent on resistivity is shown in Fig. 9a. It can be seen that the resistivity is decreased and then increased with increasing alkali equivalent. As can be seen in Eq. 4, the alkali exciter undergoes hydrolysis in contact with water to produce NaOH as well as Si (OH)4, and NaOH exists in the pore solution in free states such as Na+ and OH− ions.

2Na2O·*n*SiO2 + 2(*n*-1) H2O→4NaOH + *n*Si(OH)4 (4)

However, Na+ ions act as charge balancers, and a small amount of Na+ ions bind to the active units to produce N-A-S-H gels, while a large amount of Na+ ions remain in the free state in the pore solution35. Therefore, the amount of ionic liquid in the pores is enlarged with the increase of equivalent, which leads to the increase of carrier concentration, resulting in the enhancement of ionic conductivity. However, in combination with the test results of alkali equivalent on compressive strength, it is concluded that the hydration process was seriously inhibited when the alkali equivalent was increased to 16%, and the internal structural densification is reduced (Fig. 9a), which is unfavorable for migration of ions. Carrier concentration and ion mobility are the main factors affecting ionic conductivity24, thus only increasing the equivalent in the appropriate interval could reduce the resistivity.

The effect of slag substitution ratio on resistivity is shown in Fig. 9b. It can be seen that the resistivity is decreased and then increased with increasing slag substitution ratio, and the minimum of resistivity was obtained at slag substitution rate of 15%. As can be seen in Fig. 9b, When the slag substitution rate is 0, there are penetrating cracks in the specimen, and the internal structure is seriously degraded. When the slag substitution rate reaches 15%, the number of cracks inside the specimen is significantly reduced, and it shows a short and fine morphology, and the density and integrity of its microstructure are significantly improved. The microstructure of the geopolymer has a significant effect on its resistivity, and a compact and dense structure could generate more electronic circuits, resulting in a lower resistivity. Therefore, when the substitution rate is less than 15%, the density of the structure is increased with increasing slag substitution rate, and thus the resistivity is shown to be significantly reduced with the increase of substitution rate. After that, the phenomenon of resistivity being increased with increasing substitution rate may be attributed to two reasons. Firstly, the reaction rate of alkali exciters with slag is much faster than that with coal gangue, resulting in a sharp decrease in the concentration of conductive ions36,37. Therefore, although the addition of slag makes the microstructure denser, it reduces the content of conductive ions in the system. Secondly, the presence of active groups on the surface of coal gangue could improve the conductivity of the geopolymer, but the number of active groups is reduced with the increase of slag substitution rate, resulting in an increase in resistivity.

The effect of modulus on resistivity is shown in Fig. 9c. It can be seen that the resistivity is decreased and then increased with increasing modulus. The resistivity of modulus taken 0.8 and 1.6 increased by 71.42% and 90.24% respectively as compared to modulus taken 1.0. As can be seen in Fig. 9c, the compressive strength at 28 days does not reach 42.5 MPa when the modulus is taken as 0.8 and 1.6, indicating that the internal structure is severely degraded, and thus the migration of ionic is suppressed, so the resistivity of specimen is larger. When the modulus is taken between 1.0 and 1.4, the resistivity is increased with the increase of modulus, which may be due to the increase of modulus promoting the hydration reaction, thus the consumption of ions is increased and the concentration of internal carriers is reduced, resulting in the increase of resistivity.

The effect of water-cement ratio on resistivity is shown in Fig. 9d. It can be seen that the resistivity is decreased with increasing water-cement ratio. This phenomenon is attributed to pore connectivity and water saturation. The connectivity porosity inside the matrix is a key factor affecting diffusion of ions, and more connectivity porosity means easier diffusion of ions, thus more favorable for ions to migrate and diffuse in the capillary channels filled with free water38,39. Therefore, the increase in the water-cement ratio causes the connectivity porosity of matrix (Fig. 9d, Fig. 10) and free water content to be increased, resulting in lower resistivity.

## Influence of Curing Ages

The effect of curing ages on resistivity is shown in Fig. 11. It can be seen that the resistivity is increased with increasing age. The above phenomenon could be attributed to two reasons. Firstly, the hydration reaction of the geopolymer takes a period of time. At early age, the free ions such as Ca2+, K+ and SO42− ions are high inside the pores, and the ions are gradually consumed with the extension of curing ages. Secondly, when the age is prolonged, the free water in the pores is continuously absorbed by the hydration reaction and forced to be in the form of crystal-water. Therefore, the content of ionic liquid is decreased with age, resulting in a weakening of ionic conductivity and thus an increase in resistivity.

The SEM micrograph of raw materials and specimen (N12) at 7 days and 28 days of age are shown in Fig. 12. Comparing Fig. 12c and Fig. 12d, it can be seen that at 7 days of age, the amount of internal gelation and flocculation hydration products is low, but at 28 days of age, there is a significant increase in internal agglomerates and flocculation gel crystal products as well as spongy hydrated calcium-aluminum particles, and the densification of the internal structure is improved. Further comparing the SEM images of the specimens of N12 at 7 and 28 days for the paste (Fig. 11e and Fig. 11f), it can also be clearly found that the amount of hydration products and the densification of the internal structure were significantly enhanced with increasing age. That is to say, the hydration reaction inside the matrix is still going on after 7 days of age, and the consumption of ionic and the transformation of free water to crystal-water also exist, so the content of ionic liquid is still reduced. The decrease in ionic liquid leads to a deterioration of ionic conductivity, which is consistent with the results of the change in resistivity. In addition, the density and integrity of internal structural is enhanced leading to higher the development of strength, which is also consistent with the results of compressive strength with age.

The XRD diffractograms and EDS spectra of specimen (N12) are shown in Fig. 13. As shown in Fig. 13a, a small sharp peak is found near 29.5°, which could correspond to the C-S-H and C-A-S-H gel products generated by the reaction, as shown against the standard diffraction pattern. Then in the EDS spectra, it is found that the product is mainly composed of elements such as C, O, Si, Al, and Ca, which is consistent with the results of the XRD spectra. It is inferred that the hydration products are between C-S-H and C-A-S-H or mixtures thereof, which is similar to the experimental results of Ma et al12. It can be further found by EDS spectroscopy that at the age of 7d (Fig. 12a), the ratios of Ca/Si and Al/Si in the products are 0.67 and 0.38, respectively. However, at 28 d of age (Fig. 12b), the ratios of Ca/Si and Al/Si are 0.58 and 0.40, respectively. Decreased the ratio of Ca/Si and increased the ratio of Al/Si indicate a gradual transition from C-S-H gels to C-A-S-H gels with increasing age40. It is confirmed that Ca2+ ions produced by the dissolution of slag under the action of alkali are consumed during the reaction, while Al3+ ions produced by dissolution are further involved in the reaction. This also indicates to some extent that the extension of age increases the consumption of ionic, resulting in an increase in specimen resistivity.

In addition, in order to explore the changing law of the degree of influence of four factors on resistivity with age, such as alkali equivalent (N), slag substitution rate (S), modulus (M) and water-cement ratio (W), the specimens under the difference of ages are subjected to range analysis, and the results are shown in Fig. 14.

R' is range of a particular factor and the expression is shown in Eq. 5:

$$\:{R}^{’}={max}({p}_{i})-{min}({p}_{i})$$

5

Where: *p**i* is the actual test indicator corresponding to the *i* level of a factor.

As can be seen in Fig. 14, the relationship between the effects of the four factors on the resistivity of the specimens for 7 days is N > M > S > W; the relationship for both 14 days and 28 days is M > S > N > W. At different ages, the influence of W on resistivity was lower than that of other factors. And with the increase of age, the influence of N on resistivity is decreased, and the influence of M and S on resistivity is increased. This is mainly due to the fact that there are two ways of “external introduction” and “internal consumption” of N to affect the content of ionic liquid of the matrix compared to M and S. Increasing N means the content of Na+ ions in the pores increased. What's more, the content of Na+ ions in the pores is nearly 103 mmol/L higher than that of other ions35. Therefore, compared with the difference in resistivity caused by the consumption of hydration reaction, the difference in resistivity brought about by the difference in the content of externally introduced ions is greater. And the difference in the content of externally introduced ions is created when the alkali exciter was added, and therefore, the effect of N on resistivity is more significant at the early age.

## Influence of Water Content

The effect of water content on resistivity is shown in Fig. 15. It can be seen that the resistivity is increased with increasing ages. When the specimen is changed from the surface dry state to the dry state, the resistivity of different ratios is increased by nearly 5 ~ 6 orders of magnitude, and the value of FCR is reached 104, indicating that the conductivity of the specimen is significantly affected by the water content, and the ambient humidity can affect the resistivity of specimen by influencing the water content. Reduced water content restricts the diffusion of ions, resulting in increased resistivity41. The free water in the pores is the carrier of the migration and diffusion of ions, and the decrease in water content results in the internal channels, which were filled with free water, becoming anhydrous.