Effect of Fine Aggregate Particle Characteristics on Mechanical Properties of Fly Ash-Based Geopolymer Mortar

: This research aimed to investigate the effect of ﬁne aggregate particles on mechanical properties of ﬂy ash-based geopolymer mortar. In this work, seven kinds of river sand particles were designed based on different ﬁne aggregate characteristics. The ﬁneness modulus was adopted to quantitatively describe the gradation of sands. The ﬂuidity, compressive, ﬂexural, and tensile strengths of geopolymer mortar with different sand gradations were analyzed by laboratory tests. Furthermore, the composition and morphology of ﬂy ash-based geopolymer mortar was analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The reasonable gradation range and ﬁlling effect of sand were obtained. The results show that ﬂuidity and compressive and ﬂexural strengths of geopolymer mortar both improve with the increase of the ﬁneness modulus, while speciﬁc surface area and voidage are opposite. The tensile strength of mortar largely lies on the interface properties between the geopolymer binder and ﬁne aggregates. When the pass rate of the key sieving size 1.18 mm is 75–95%, the pass rate of the key sieving size 0.15 mm is 15–25%, the ﬁneness modulus is 2.2–2.6 and the appropriate ﬁlling coefﬁcient of geopolymer paste is around 1.0–1.15, the comprehensive performance of geopolymer mortar is the best. This research paper could provide a basis for the design of geopolymer mortar based on ﬂy ash, and it is of great signiﬁcance for its popularization and application. available in respect to the effect of matrix internal composition structure on the mechanical performance of geopolymer mortar. In this research, a series of experiments are done to study the effect of sand content, gradation, speciﬁc surface area and voidage on mechanical properties of ﬂy ash-based geopolymer mortar. Compressive, ﬂexural and tensile strengths of geopolymer mortar with different sand gradations were analyzed by laboratory tests. The composition and morphology of ﬂy ash-based geopolymer mortar was analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). This research can provide some reference and theory basis for the reasonable design


Introduction
Cement is one of the most popular building materials in civil construction. In 2014, China's cement output reached 2.182 billion tons, accounting for 60% of the total world production [1]. Cement manufacturing releases a great amount of greenhouse gas into the environment. According to preliminary statistics, the cement industry contributes up to 5-10% by weight of the total anthropogenic emissions of carbon dioxide in the world [1][2][3]. As a result, the application of supplementary materials to partially displace cement and alternative cementing materials should be developed to decrease the usage of cement [4,5]. Geopolymer binder is one of such alternative cementing materials which are produced by mixing high alkali solutions and aluminosilicate material together [6][7][8]. Geopolymer has a rich source of preparation raw materials. The energy consumption and carbon emissions of geopolymer production are relatively low [9]. The CO 2 emission of producing one ton of geopolymer is only about 1/6 of cement production [10]. In addition, geopolymer has good mechanical properties and rapid hardening early strength Performance of sodium silicate was shown in Table 2. The alkali-activator used in the experiment was obtained by NaOH-modified sodium silicate modulus to 1.2 [14,41,42]. NaOH used in the test is in the solid granular state. Its purity is more than 96%. The water used in this research was conformed to ASTM C94 [43]. Two kinds of sands were adopted in this research. One is standard sand, which is conformed to the ISO 679 [44] method. The other was river sands with different gradations, which were shown in Table A1 in Appendix A. Seven types of river sands (K 1 -K 7 ) were designed with a different fineness modulus. Sand gradations of river sands and standard sand (K s ) were divided into three categories according to GB/T 14684-2011 [45], which were shown in Table 3. The fineness modulus was used to characterize the fineness and coarseness of the sand. The classification of sand is used to characterize the gradation of sand. The gradation curves of river sands (K 1 -K 7 ) and standard sand (K s ) were illustrated in Figure 1. The particle of river sands from coarse to fine are K 1 , K 2 , K 3 , K 4 , K 5 , K 6 , and K 7 . According to GB/T 14684-2011 [45], the silt content of the river sands is below 2.0%. The apparent density of river sands is between 2.65 g/cm 3 and 2.70 g/cm 3 . Table 3. Gradation of fine aggregate.

NO.
Fineness Modulus Sand Type Categories K 1 3.67 Coarse I K 2 3.19 Coarse II K 3 2.81 Medium I K 4 2.39 Medium III K 5 2.11 Fine II K 6 1.61 Fine III K 7 1

Preparation Procedure of Fly Ash-Based Geopolymer Mortar
The preparation procedure of fly ash-based geopolymer mortar was illustrated in Figure 2. The modulus of sodium silicate can be adjusted by adding different amounts of NaOH reagent [14,41,42]. In addition, some water was evaporated due to the heat from the mixing process of the NaOH reagent and sodium silicate, Therefore, we used an electronic balance to weigh the mass difference of the alkali-activator before and after cooling, and extra water was added to compensate the weight loss of the water.

Fluidity Property Tests
Fluidity property tests were performed by the flow table spread (FTS) test, according to ASTM C230 [46]. The consistency of geopolymer mortar was determined by flow value. An average flow value of minimum and maximum diameters of FTS was measured as flow spread in mm. The greater the flow value, the better fluidity of mortar it represents.

Preparation Procedure of Fly Ash-Based Geopolymer Mortar
The preparation procedure of fly ash-based geopolymer mortar was illustrated in Figure 2. The modulus of sodium silicate can be adjusted by adding different amounts of NaOH reagent [14,41,42]. In addition, some water was evaporated due to the heat from the mixing process of the NaOH reagent and sodium silicate. Therefore, we used an electronic balance to weigh the mass difference of the alkali-activator before and after cooling, and extra water was added to compensate the weight loss of the water.

Preparation Procedure of Fly Ash-Based Geopolymer Mortar
The preparation procedure of fly ash-based geopolymer mortar was illustrated in Figure 2. The modulus of sodium silicate can be adjusted by adding different amounts of NaOH reagent [14,41,42]. In addition, some water was evaporated due to the heat from the mixing process of the NaOH reagent and sodium silicate, Therefore, we used an electronic balance to weigh the mass difference of the alkali-activator before and after cooling, and extra water was added to compensate the weight loss of the water.

Fluidity Property Tests
Fluidity property tests were performed by the flow table spread (FTS) test, according to ASTM C230 [46]. The consistency of geopolymer mortar was determined by flow value. An average flow value of minimum and maximum diameters of FTS was measured as flow spread in mm. The greater the flow value, the better fluidity of mortar it represents.

Fluidity Property Tests
Fluidity property tests were performed by the flow table spread (FTS) test, according to ASTM C230 [46]. The consistency of geopolymer mortar was determined by flow value. An average flow value of minimum and maximum diameters of FTS was measured as flow spread in mm. The greater the flow value, the better fluidity of mortar it represents.

Mechanical Performance Tests
Compressive, flexural and tensile strength of geopolymer mortar with different sand gradations were analyzed by laboratory tests. The prism specimens were produced, adopting the mold size of 40 mm × 40 mm × 160 mm for flexural strength test and compressive strength according to ASTM C305 [47] and ASTM C348 [48] test methods. Specimens' size and test for tensile strength were illustrated in Figure 3. The tensile strength test was conducted according to ASTM C39 [49]. Three specimens were prepared for each individual mix ratio. After casting, the prism specimens were placed in their molds in a moisture room with a temperature of 23 ± 2 • C with 98% relative humidity (RH). After 24 ± 2 h, all specimens were removed from their molds, then cured in water at a temperature of 20 • C for up to 3, 7, and 28 days.  Figure 3. The tensile strength test was conducted according to ASTM C39 [49]. Three specimens were prepared for each individual mix ratio. After casting, the prism specimens were placed in their molds in a moisture room with a temperature of 23 ± 2 °C with 98% relative humidity (RH). After 24 ± 2 h, all specimens were removed from their molds, then cured in water at a temperature of 20 °C for up to 3, 7, and 28 days.

Specific Surface Area Tests
Specific surface area models of river sand with different characteristics were established. The specific surface area of sand is used to characterize the state of sand. Assuming that the density of the same river sand is uniform, the model is a uniform sphere, and each type of river sand has seven grades. The average value of each particle size range is the diameter of particle size. The derivation formula was as follows: where Vi is the volume fraction of each grade in river sand; Ni is the number of particles with grade I particle size per unit volume; S is the total specific surface area of sand per unit volume, m 2 /m 3 .

Voidage Tests
In order to study the effect of sand particle gradation on the self-compaction of mortar, sand bulk density of the above seven different gradations was measured, and then voidage was measured. Voidage is used to characterize the filling effect of sand, which can be obtained by the following formula: where p1 is bulk density kg/m 3 , and p2 is apparent density kg/m 3 .

Specific Surface Area Tests
Specific surface area models of river sand with different characteristics were established. The specific surface area of sand is used to characterize the state of sand. Assuming that the density of the same river sand is uniform, the model is a uniform sphere, and each type of river sand has seven grades. The average value of each particle size range is the diameter of particle size. The derivation formula was as follows: where V i is the volume fraction of each grade in river sand; N i is the number of particles with grade I particle size per unit volume; S is the total specific surface area of sand per unit volume, m 2 /m 3 .

Voidage Tests
In order to study the effect of sand particle gradation on the self-compaction of mortar, sand bulk density of the above seven different gradations was measured, and then voidage was measured. Voidage is used to characterize the filling effect of sand, which can be obtained by the following formula: where p 1 is bulk density kg/m 3 , and p 2 is apparent density kg/m 3 .

Scanning Electron Microscope Observation
SEM analysis was carried out to get a better understanding of the microstructure of geopolymer mortar specimens. At the specified age, specimens for SEM investigations were obtained near the specimen surface. Specimens were composed of clean solid blocks, and the surface was smoothed. After sampling, specimens were immersed in alcohol to terminate hydration. Images were obtained from specimens taken from prism-size samples using a saw. Specimens were vacuum-dried overnight prior to the SEM analysis. When starting the test, specimens were coated with a thin layer of platinum before observation, and the microstructure was investigated by a SU8010 scanning electron microscope.

X-ray Diffraction Tests
At the specified age, the specimens were immersed in alcohol to terminate hydration, then specimens were firstly dried and ground into power. The testing specimens were sealed in plastic bags. The mineralogical compositions of specimens were monitored by X-ray diffraction (XRD) with a D/Max-RB diffractometer using 40 KV and 50 mA current. MDI Jade (2009) software was applied to identify the crystalline phases.

Mix Ratio of Fly Ash-Based Geopolymer Mortar with Standard Sands
With respect to the mix ratio of the fly ash-based geopolymer binder, the water-binder ratio, alkali-activator dosage, GBFS/binder ratio and modulus of sodium silicate were considered as the four major mix design parameters. In this research, the water-binder ratio was adopted as 0.35, 0.40, and 0.45, respectively. The modulus of sodium silicate was adjusted to 1.2 by NaOH [14,41,42]. The solid mass of the alkali-activator is 14 wt.% to the weight of the geopolymer binder. The GBFS/binder ratio was set as 40%. The geopolymer binder is the sum of fly ash, GBFS, and the solid mass of the alkali-activator. The mix ratio of geopolymer mortar samples with standard sands was shown in Table 4. The property test results of geopolymer mortar samples with standard sands were shown in Table A2 in Appendix A. The fluidity and mechanical properties tests were performed to study the influence of sand gradation on the performance of geopolymer mortar. With respect to the optimal mix ratio of the geopolymer binder in research, the water-binder ratio and binder-sand ratio were adopted as 0.35 and 0.40, respectively. The modulus of sodium silicate was adjusted to 1.2 by NaOH [14,41,42]. The solid mass of the alkali-activator is 14 wt.% to the weight of the geopolymer binder. The GBFS/binder ratio was set as 40%. When the binder-sand ratio is determined, the type of sand is also one of the factors that affect the properties of the fly ash-based geopolymer mortar. The type of sand is mainly reflected by the thickness of sand, the gradation of sand, specific surface area of sand, and voidage of sand. The  Table 5. Property test results of geopolymer mortar samples with different grade sands were shown in Tables A3 and A4 in Appendix A.  Figure 4 showed the influence of the water-binder ratio and binder-sand ratio on the fluidity. On one hand, when the binder-sand ratio remains unchanged, the flow value of the mortar improves along with the increase of the water-binder ratio. On the other hand, when the water-binder ratio of the mixtures were retained at the same level, the flow value increased with the increase in the binder-sand ratio. Analyzing the flow value of mortar, the flow value is less than 130 mm when the water-cement ratio and binder-sand ratio are both below 0.40. The flow value of mortar decreases when more sand is added into the geopolymer paste, because the rough and angular surface area of sand traps free paste between sand particles. When the water-binder ratio is more than 0.4 and the binder-sand ratio is more than 0.45, the flow value of geopolymer mortar is more than 210 mm, which presents a good fluidity performance.   Figure 4 showed the influence of the water-binder ratio and binder-sand ratio on the fluidity. On one hand, when the binder-sand ratio remains unchanged, the flow value of the mortar improves along with the increase of the water-binder ratio. On the other hand, when the water-binder ratio of the mixtures were retained at the same level, the flow value increased with the increase in the binder-sand ratio. Analyzing the flow value of mortar, the flow value is less than 130 mm when the water-cement ratio and binder-sand ratio are both below 0.40. The flow value of mortar decreases when more sand is added into the geopolymer paste, because the rough and angular surface area of sand traps free paste between sand particles. When the water-binder ratio is more than 0.4 and the binder-sand ratio is more than 0.45, the flow value of geopolymer mortar is more than 210 mm, which presents a good fluidity performance.

Mechanical Performance
The influence of the water-binder ratio and binder-sand ratio on the compressive and flexural strength of specimens cured for 28 days was shown in Figures 5 and 6. When the water-binder ratio and binder-sand ratio are 0. 35  respectively. Moreover, when binder-sand ratio is more than 0.3, compressive and flexural strength both improve with the increase in the water-binder ratio of geopolymer mortar, and vice versa. However, when the binder-sand ratio is 0.3, compressive and flexural strength do not have the linear relationship with the water-binder ratio. Regarding mixtures with an increasing water-binder ratio, compressive and flexural strength first improved and then decreased. It can be seen from the test results that when the optimal value range of the water-binder ratio and binder-sand ratio are 0.35-0. 40   MPa, respectively. Moreover, when binder-sand ratio is more than 0.3, compressive and flexural strength both improve with the increase in the water-binder ratio of geopolymer mortar, and vice versa. However, when the binder-sand ratio is 0.3, compressive and flexural strength do not have the linear relationship with the water-binder ratio. Regarding mixtures with an increasing water-binder ratio, compressive and flexural strength first improved and then decreased. It can be seen from the test results that when the optimal value range of the water-binder ratio and binder-sand ratio are 0. 35 Figure 7 shows the specific surface area and voidage of different river sands. It can be seen that as the fineness modulus increased, specific surface area and voidage showed consistency, and both gradually decreased. At the same time, when the fineness modulus MPa, respectively. Moreover, when binder-sand ratio is more than 0.3, compressive and flexural strength both improve with the increase in the water-binder ratio of geopolymer mortar, and vice versa. However, when the binder-sand ratio is 0.3, compressive and flexural strength do not have the linear relationship with the water-binder ratio. Regarding mixtures with an increasing water-binder ratio, compressive and flexural strength first improved and then decreased. It can be seen from the test results that when the optimal value range of the water-binder ratio and binder-sand ratio are 0. 35 Figure 7 shows the specific surface area and voidage of different river sands. It can be seen that as the fineness modulus increased, specific surface area and voidage showed consistency, and both gradually decreased. At the same time, when the fineness modulus  Figure 7 shows the specific surface area and voidage of different river sands. It can be seen that as the fineness modulus increased, specific surface area and voidage showed consistency, and both gradually decreased. At the same time, when the fineness modulus Minerals 2021, 11, 897 9 of 22 is 3.67 (K 1 ), the specific surface area is the smallest, and the K 3 and K 4 groups have the smallest voidage.

Specific Surface Area and Voidage of River Sands
Minerals 2021, 11, x FOR PEER REVIEW 9 of 21 is 3.67 (K1), the specific surface area is the smallest, and the K3 and K4 groups have the smallest voidage.

Figure 7.
Relationship between the fineness modulus and specific surface area, voidage.

Fluidity
The influence of the fineness modulus on the fluidity of mortar was shown in Figure  8. The line graph observed in Figure 8 affirms the significant effect of the fineness modulus, specific surface area, and voidage on the fluidity of the geopolymer mortar. It could be seen that the flow value of mortar is approximately linearly proportional to the fineness modulus of sand. With the increase in the fineness modulus, the flow value increases gradually, while the change rule of specific surface area and voidage are completely opposite. According to the gradation curve of sand mentioned in Figure 1, it can be seen that the specimen showed better fluidity when the sand gradation belonged to Ⅰ and Ⅱ categories. The specific surface area is between 10,000 and 20,000 mm 2 /mm 3 , and voidage is about 45%; the flow value of K2 sample is the best, reaching 231.30 mm. When the gradation curve of the sand is close to the upper part of the curve map, the overall trend of fluidity is declining. This is because for samples with a low fineness modulus, there are more small particles in sands; the content of geopolymer paste is not enough to cover and bind these particles. Therefore, fluidity of the mortar decreases for the shortage of the paste. For the gradation curve near to the lower part of the curve map, the content of fine particles is less, and the content of coarse particles gradually increases. Therefore, the accumulation state of sands is obviously improved, and the fluidity of the mortar gradually increases. The change of the accumulation state results in the change of the specific surface area and voidage, which affects the fluidity of the sands. Thus, it can be seen that sand gradation, fineness modulus, specific surface area, and voidage have great influence on the fluidity of geopolymer mortar.

Fluidity
The influence of the fineness modulus on the fluidity of mortar was shown in Figure 8. The line graph observed in Figure 8 affirms the significant effect of the fineness modulus, specific surface area, and voidage on the fluidity of the geopolymer mortar. It could be seen that the flow value of mortar is approximately linearly proportional to the fineness modulus of sand. With the increase in the fineness modulus, the flow value increases gradually, while the change rule of specific surface area and voidage are completely opposite. According to the gradation curve of sand mentioned in Figure 1, it can be seen that the specimen showed better fluidity when the sand gradation belonged to I and II categories. The specific surface area is between 10,000 and 20,000 mm 2 /mm 3 , and voidage is about 45%; the flow value of K 2 sample is the best, reaching 231.30 mm. When the gradation curve of the sand is close to the upper part of the curve map, the overall trend of fluidity is declining. This is because for samples with a low fineness modulus, there are more small particles in sands; the content of geopolymer paste is not enough to cover and bind these particles. Therefore, fluidity of the mortar decreases for the shortage of the paste. For the gradation curve near to the lower part of the curve map, the content of fine particles is less, and the content of coarse particles gradually increases. Therefore, the accumulation state of sands is obviously improved, and the fluidity of the mortar gradually increases. The change of the accumulation state results in the change of the specific surface area and voidage, which affects the fluidity of the sands. Thus, it can be seen that sand gradation, fineness modulus, specific surface area, and voidage have great influence on the fluidity of geopolymer mortar.

Compressive and Flexural Strength
The effects of fineness modulus on the compressive and flexural strength of geopolymer mortar were illustrated in Figures 9 and 10. Typically, compressive and flexural strength both improve with the increase in the fineness modulus of sands, and vice versa. Compressive strength and flexural strength of K 1 , K 2 , K 3 , K 4 are relatively higher, and their gradation categories are I, II, I, III, respectively. The reason is that the characteristic of geopolymer binder is one of the key factors to the mechanical strength of mortar. The sands with a smaller fineness modulus contain more fine particles, especially for those less than 0.075 mm. This can be seen that specific surface area of sands improves with the increase in fine particles. The more specific surface area of sands is, the more the amount of geopolymer binder needs to cover and bind them. In each specimen of this experiment, the amount of geopolymer binder is constant. With respect to the sands with more fine particles, the amount of geopolymer binder is not enough to cover and binder these fine particles together, which will lead to the obvious decrease in compressive and flexural strength. The experimental data show that mechanical strength is not only related to the fineness modulus, but also to the technical type of sands. In general, mechanical strength of medium sand and coarse sands is higher than that of fine sands. The better the category (I, II), the higher the strength is. Compared with group K 7 , compressive strength and flexural strength of group K 1 increased by 19.3% and 21%, respectively. Strength of geopolymer mortar is closely related to the thickness of geopolymer binder around sands. However, sand gradation has little effect on the strength of the geopolymer mortar.
the specimen showed better fluidity when the sand gradation belonged to Ⅰ and Ⅱ categories. The specific surface area is between 10,000 and 20,000 mm 2 /mm 3 , and voidage is about 45%; the flow value of K2 sample is the best, reaching 231.30 mm. When the gradation curve of the sand is close to the upper part of the curve map, the overall trend of fluidity is declining. This is because for samples with a low fineness modulus, there are more small particles in sands; the content of geopolymer paste is not enough to cover and bind these particles. Therefore, fluidity of the mortar decreases for the shortage of the paste. For the gradation curve near to the lower part of the curve map, the content of fine particles is less, and the content of coarse particles gradually increases. Therefore, the accumulation state of sands is obviously improved, and the fluidity of the mortar gradually increases. The change of the accumulation state results in the change of the specific surface area and voidage, which affects the fluidity of the sands. Thus, it can be seen that sand gradation, fineness modulus, specific surface area, and voidage have great influence on the fluidity of geopolymer mortar.  Ⅰ Ⅱ Ⅰ Ⅲ, respectively. The reason is that the characteristic of geopolymer binder is one of the key factors to the mechanical strength of mortar. The sands with a smaller fineness modulus contain more fine particles, especially for those less than 0.075 mm. This can be seen that specific surface area of sands improves with the increase in fine particles. The more specific surface area of sands is, the more the amount of geopolymer binder needs to cover and bind them. In each specimen of this experiment, the amount of geopolymer binder is constant. With respect to the sands with more fine particles, the amount of geopolymer binder is not enough to cover and binder these fine particles together, which will lead to the obvious decrease in compressive and flexural strength. The experimental data show that mechanical strength is not only related to the fineness modulus, but also to the technical type of sands. In general, mechanical strength of medium sand and coarse sands is higher than that of fine sands. The better the category ( , Ⅰ Ⅱ), the higher the strength is. Compared with group K7, compressive strength and flexural strength of group K1 increased by 19.3% and 21%, respectively. Strength of geopolymer mortar is closely related to the thickness of geopolymer binder around sands. However, sand gradation has little effect on the strength of the geopolymer mortar.   The σ f /σ c ratio is one of the indexes used to evaluate the brittleness of cement-based materials [50,51]. The influence of the fineness modulus on the σ f /σ c ratio of geopolymer mortar was illustrated in Figure 11. The σ f /σ c ratio of geopolymer mortar does not represent the linear correlation to the fineness modulus of sands. This phenomenon is different from that of compressive and flexural strength. When the fineness modulus of sands is between 2.2 and 2.6, the specific surface area is between 35,000 and 45,000 mm 2 /mm 3 , and voidage is in the range of 45-47%; the σ f /σ c ratio of mortar specimens cured for 3, 7, and 28 days represents the peak value.   The σf/σc ratio is one of the indexes used to evaluate the brittleness of cement-based materials [50,51]. The influence of the fineness modulus on the σf/σc ratio of geopolymer mortar was illustrated in Figure 11. The σf/σc ratio of geopolymer mortar does not represent the linear correlation to the fineness modulus of sands. This phenomenon is different from that of compressive and flexural strength. When the fineness modulus of sands is between 2.2 and 2.6, the specific surface area is between 35,000 and 45,000 mm 2 /mm 3 , and voidage is in the range of 45-47%; the σf/σc ratio of mortar specimens cured for 3, 7, and 28 days represents the peak value. Figure 11. Influence of the fineness modulus on the σf/σc ratio of mortar. Figure 12 showed the influence of the fineness modulus on the tensile strength of geopolymer mortar cured for 28 days. From test results, the tensile strength of mortar specimens first increases and then decreases as the fineness modulus increases. The specimen with a fineness modulus between 2.2 and 2.6 has the highest tensile strength, which is more than 3.9 MPa. Compared with geopolymer mortar with a fineness modulus of 3.67, the tensile strength of mortar with a fineness modulus of 2.11 is increased by 25.6%.  Figure 11. Influence of the fineness modulus on the σ f / σ c ratio of mortar. Figure 12 showed the influence of the fineness modulus on the tensile strength of geopolymer mortar cured for 28 days. From test results, the tensile strength of mortar specimens first increases and then decreases as the fineness modulus increases. The specimen with a fineness modulus between 2.2 and 2.6 has the highest tensile strength, which is more than 3.9 MPa. Compared with geopolymer mortar with a fineness modulus of 3.67, the tensile strength of mortar with a fineness modulus of 2.11 is increased by 25.6%. The σf/σc ratio is one of the indexes used to evaluate the brittleness of cement-based materials [50,51]. The influence of the fineness modulus on the σf/σc ratio of geopolymer mortar was illustrated in Figure 11. The σf/σc ratio of geopolymer mortar does not represent the linear correlation to the fineness modulus of sands. This phenomenon is different from that of compressive and flexural strength. When the fineness modulus of sands is between 2.2 and 2.6, the specific surface area is between 35,000 and 45,000 mm 2 /mm 3 , and voidage is in the range of 45-47%; the σf/σc ratio of mortar specimens cured for 3, 7, and 28 days represents the peak value. Figure 11. Influence of the fineness modulus on the σf/σc ratio of mortar. Figure 12 showed the influence of the fineness modulus on the tensile strength of geopolymer mortar cured for 28 days. From test results, the tensile strength of mortar specimens first increases and then decreases as the fineness modulus increases. The specimen with a fineness modulus between 2.2 and 2.6 has the highest tensile strength, which is more than 3.9 MPa. Compared with geopolymer mortar with a fineness modulus of 3.67, the tensile strength of mortar with a fineness modulus of 2.11 is increased by 25.6%.

Classification and Analysis of Packed Sands Structure in Matrix
Particle characteristics of sand have a great effect on fluidity and mechanical properties of geopolymer mortar. Fluidity and compressive and flexural strength both improve with the increase in the fineness modulus of sand. This is because the specific surface area of sand improves with the increase in fine particles. The more fine particles in sand, the more specific the surface area of sand is. The content of geopolymer paste is not enough to cover and bond these particles. Therefore, the fluidity and compressive and flexural strength of the mortar decreases due to insufficient paste. When the fine modulus of sand is between 2.2 and 2.6, the σ f /σ c ratio and tensile strength of the mortar are the highest. The tensile strength of mortar largely lies on the interface properties between the geopolymer binder and fine aggregates. If the fineness modulus is too small or the gradation curve far exceeds K 4 , the content of fine particles is more. The geopolymer binder is not enough to form a strong interface between them. Moreover, the content of large-size sand particles exists if the fineness modulus is too high, or the gradation curve is far lower than K 3 . The more the content of large-size sand particles, the more the defects exist in the interface. Therefore, the fineness modulus and gradation of sands have an optimal range value to ensure good tensile strength of the geopolymer mortar. The gradation and particle size distribution of sands are important characteristics in the design procedure of geopolymer mortar. Geopolymer mortar is composed of tightly packed sand particles and geopolymer paste filled in voids. As shown in Figure 13, from the perspective of internal material composition, geopolymer mortar can be divided into two parts: sand part and paste part.

Classification and Analysis of Packed Sands Structure in Matrix
Particle characteristics of sand have a great effect on fluidity and mechanical properties of geopolymer mortar. Fluidity and compressive and flexural strength both improve with the increase in the fineness modulus of sand. This is because the specific surface area of sand improves with the increase in fine particles. The more fine particles in sand, the more specific the surface area of sand is. The content of geopolymer paste is not enough to cover and bond these particles. Therefore, the fluidity and compressive and flexural strength of the mortar decreases due to insufficient paste. When the fine modulus of sand is between 2.2 and 2.6, the σf/σc ratio and tensile strength of the mortar are the highest. The tensile strength of mortar largely lies on the interface properties between the geopolymer binder and fine aggregates. If the fineness modulus is too small or the gradation curve far exceeds K4, the content of fine particles is more. The geopolymer binder is not enough to form a strong interface between them. Moreover, the content of large-size sand particles exists if the fineness modulus is too high, or the gradation curve is far lower than K3. The more the content of large-size sand particles, the more the defects exist in the interface. Therefore, the fineness modulus and gradation of sands have an optimal range value to ensure good tensile strength of the geopolymer mortar. The gradation and particle size distribution of sands are important characteristics in the design procedure of geopolymer mortar. Geopolymer mortar is composed of tightly packed sand particles and geopolymer paste filled in voids. As shown in Figure 13, from the perspective of internal material composition, geopolymer mortar can be divided into two parts: sand part and paste part. The packing theory is the most commonly used method of aggregate particle gradation. It is a theory to obtain the maximum compactness through compact packing of particles. Both in terms of practicality and economy, this has great advantages. The most common such theories are the Feret packing theory, Fuller gradation curve model, and Andreasen and Andersen model. However, these three theoretical methods are more suitable for concrete with larger aggregate particles, and there are large errors for mortar with fine particles. However, the maximum compactness principle of packing theory is still applicable to mortar. In the state of maximum compactness, the mechanical properties of mortar are the best. In this paper, based on the maximum compactness principle of packing theory, referring to packing theory and according to the experimental results, the screening rate of related sieving size is proposed from the perspective of particle gradation. The voidage of sand particles can directly reflect the compactness of accumulation sand. The smaller the voidage is, the larger the bulk density of sand is. It can be seen from Figure 7 that K3 and K4 groups have the minimum voidage; therefore, K3 and K4 are the most compact and have the largest bulk density. Sand particle gradation with good continuity makes the peak values of σf/σc and the tensile strength of mortar appear in K3 and K4 groups. The gradation of sand particles of better continuity can effectively reduce the void ratio between sand particles, resulting in the maximum bulk density of aggregates and reducing the amount of geopolymer paste used in the mortar matrix. Therefore, when the The packing theory is the most commonly used method of aggregate particle gradation. It is a theory to obtain the maximum compactness through compact packing of particles. Both in terms of practicality and economy, this has great advantages. The most common such theories are the Feret packing theory, Fuller gradation curve model, and Andreasen and Andersen model. However, these three theoretical methods are more suitable for concrete with larger aggregate particles, and there are large errors for mortar with fine particles. However, the maximum compactness principle of packing theory is still applicable to mortar. In the state of maximum compactness, the mechanical properties of mortar are the best. In this paper, based on the maximum compactness principle of packing theory, referring to packing theory and according to the experimental results, the screening rate of related sieving size is proposed from the perspective of particle gradation. The voidage of sand particles can directly reflect the compactness of accumulation sand. The smaller the voidage is, the larger the bulk density of sand is. It can be seen from Figure 7 that K 3 and K 4 groups have the minimum voidage; therefore, K 3 and K 4 are the most compact and have the largest bulk density. Sand particle gradation with good continuity makes the peak values of σ f /σ c and the tensile strength of mortar appear in K 3 and K 4 groups. The gradation of sand particles of better continuity can effectively reduce the void ratio between sand particles, resulting in the maximum bulk density of aggregates and reducing the amount of geopolymer paste used in the mortar matrix. Therefore, when the sand gradation curve is within or near the range of K 3 and K 4 , the maximum compactness can be obtained. From the sand gradation curve in Figure 1, it can be seen that the Ks curve is closest to K 3 and K 4 . As shown in Figure 14, the Ks curve is wrapped by K 3 and K 4 curves. Therefore, when the pass rate of the key sieving size 1.18 mm is 75-95%, and the pass rate of the key sieving size 0.15 mm is 15-25%, the maximum aggregate density can appear.
Minerals 2021, 11, x FOR PEER REVIEW 13 of 21 sand gradation curve is within or near the range of K3 and K4, the maximum compactness can be obtained. From the sand gradation curve in Figure 1, it can be seen that the Ks curve is closest to K3 and K4. As shown in Figure 14, the Ks curve is wrapped by K3 and K4 curves. Therefore, when the pass rate of the key sieving size 1.18 mm is 75-95%, and the pass rate of the key sieving size 0.15 mm is 15-25%, the maximum aggregate density can appear. Furthermore, the filling effect and seeding effect of fine particles in sands are two possible mechanisms on the strength development of geopolymer mortar. The fine particles in mortar could fill voids in the matrix and make it denser. The fine particles could also act as nucleation centers for the crystallization of the geopolymer matrix. The strength increases as the number of crystals increases. However, when the number of fine particles continuously improved, it was difficult to obtain a uniform matrix due to the shortage of geopolymer binder and the poor workability. The demand for geopolymer paste is determined by voids between sand particles and specific surface area of sand particles. This paper defines the paste filling coefficient as the ratio of paste volume to sand void volume, which is used to describe the balance state of sand and paste volume in geopolymer mortar. The paste volume, sand void volume, and void ratio were shown in Table A5 in Appendix A. When the filling coefficient of paste is in a certain range, fluidity and mechanical properties of geopolymer mortar are better. As shown in Figure 15, it can be seen from  Furthermore, the filling effect and seeding effect of fine particles in sands are two possible mechanisms on the strength development of geopolymer mortar. The fine particles in mortar could fill voids in the matrix and make it denser. The fine particles could also act as nucleation centers for the crystallization of the geopolymer matrix. The strength increases as the number of crystals increases. However, when the number of fine particles continuously improved, it was difficult to obtain a uniform matrix due to the shortage of geopolymer binder and the poor workability. The demand for geopolymer paste is determined by voids between sand particles and specific surface area of sand particles. This paper defines the paste filling coefficient as the ratio of paste volume to sand void volume, which is used to describe the balance state of sand and paste volume in geopolymer mortar. The paste volume, sand void volume, and void ratio were shown in Table A5 in Appendix A. When the filling coefficient of paste is in a certain range, fluidity and mechanical properties of geopolymer mortar are better. As shown in Figure 15, it can be seen from Figures 8-12 that when the filling coefficient of geopolymer paste is around 1.0-1.15, the comprehensive properties of geopolymer mortar are the best. sand gradation curve is within or near the range of K3 and K4, the maximum compactness can be obtained. From the sand gradation curve in Figure 1, it can be seen that the Ks curve is closest to K3 and K4. As shown in Figure 14, the Ks curve is wrapped by K3 and K4 curves. Therefore, when the pass rate of the key sieving size 1.18 mm is 75-95%, and the pass rate of the key sieving size 0.15 mm is 15-25%, the maximum aggregate density can appear. Furthermore, the filling effect and seeding effect of fine particles in sands are two possible mechanisms on the strength development of geopolymer mortar. The fine particles in mortar could fill voids in the matrix and make it denser. The fine particles could also act as nucleation centers for the crystallization of the geopolymer matrix. The strength increases as the number of crystals increases. However, when the number of fine particles continuously improved, it was difficult to obtain a uniform matrix due to the shortage of geopolymer binder and the poor workability. The demand for geopolymer paste is determined by voids between sand particles and specific surface area of sand particles. This paper defines the paste filling coefficient as the ratio of paste volume to sand void volume, which is used to describe the balance state of sand and paste volume in geopolymer mortar. The paste volume, sand void volume, and void ratio were shown in Table A5 in Appendix A. When the filling coefficient of paste is in a certain range, fluidity and mechanical properties of geopolymer mortar are better. As shown in Figure 15, it can be seen from   Figure 16a is mainly composed of medium sand, coarser sand, and fine sand. The gradation of sand particles is a continuous gradation. After adding fine aggregate, the voidage of sand particles is the smallest, and the content of geopolymer paste can fill these voids sufficiently to bond sand particles tightly, and the geopolymer paste fill coefficient is the largest. Therefore, no large pores are found in the mortar structure. According to the maximum compactness principle of packing theory, this mortar structure is the most compact. It has compressive and tensile properties. According to the experimental results, the σ f /σ c ratio of K 3 and K 4 is the largest and the fluidity can reach more than 200 mm. The tensile, compressive, and flexural strengths are above 3.8 MPa, 60 MPa, and 6.0 Mpa, respectively.
content of geopolymer paste is not enough to fill these voids. The fill coefficient is also lower, as shown in Figure 15; therefore sand particles cannot be tightly bonded together, resulting in the structure of this kind of geopolymer mortar having compressive characteristics but not tensile characteristics. Therefore, the tensile strength of K1 and K2 is the lowest, but the aggregate mainly composed of coarse sand particles has a better framework support effect. Therefore, the compressive and flexural strength of K1 and K2 show good performance. However, according to packing theory, this type is not the most compact state. Figure 16c is mainly composed of finer sands. The fine sands surround a small number of coarser sands. The lack of medium sand and coarser sand filling is a discontinuous intermittent gradation; the experimental groups are specifically represented by the K6 and K7 groups. Due to the presence of a large number of fine particles, the content of geopolymer paste is not enough to cover and bond these particles. Therefore, the fluidity is the lowest. In addition, the "squeezing effect" of finer sands on coarser sands is obvious. The intermittent distribution of sands lacks necessary framework support, and sand particle structure presents a suspended state. Therefore, it can neither resist pressure nor resist pull. The physical and mechanical properties of geopolymer mortar are affected to a certain extent. Therefore, the mechanical properties of K6 and K7 geopolymer mortar are poor.

XRD Analysis
Some literature studies use FTIR and XRD to analyze microstructure of geopolymer to support XRD analysis results [52,53]. In future work, we will also use more methods to conduct more microscopic studies on geopolymer. In this paper, XRD is used to analyze the phase of GBFS, fly ash, and geopolymer paste, and results are shown in Figure 17. Fly  Figure 16b is mainly composed of medium sand and coarser sand. Although particle gradation is continuous, it lacks fine sand filling. It can be seen from Figure 7 that K 1 and K 2 groups are mainly composed of coarse sand particles with relatively large voids. The content of geopolymer paste is not enough to fill these voids. The fill coefficient is also lower, as shown in Figure 15; therefore sand particles cannot be tightly bonded together, resulting in the structure of this kind of geopolymer mortar having compressive characteristics but not tensile characteristics. Therefore, the tensile strength of K 1 and K 2 is the lowest, but the aggregate mainly composed of coarse sand particles has a better framework support effect. Therefore, the compressive and flexural strength of K 1 and K 2 show good performance. However, according to packing theory, this type is not the most compact state. Figure 16c is mainly composed of finer sands. The fine sands surround a small number of coarser sands. The lack of medium sand and coarser sand filling is a discontinuous intermittent gradation; the experimental groups are specifically represented by the K 6 and K 7 groups. Due to the presence of a large number of fine particles, the content of geopolymer paste is not enough to cover and bond these particles. Therefore, the fluidity is the lowest. In addition, the "squeezing effect" of finer sands on coarser sands is obvious. The intermittent distribution of sands lacks necessary framework support, and sand particle structure presents a suspended state. Therefore, it can neither resist pressure nor resist pull. The physical and mechanical properties of geopolymer mortar are affected to a certain extent. Therefore, the mechanical properties of K 6 and K 7 geopolymer mortar are poor.

XRD Analysis
Some literature studies use FTIR and XRD to analyze microstructure of geopolymer to support XRD analysis results [52,53]. In future work, we will also use more methods to conduct more microscopic studies on geopolymer. In this paper, XRD is used to analyze the phase of GBFS, fly ash, and geopolymer paste, and results are shown in Figure 17. Fly ash sample contains a small amount of mullite and quartz [52,53], mainly for its amorphous state. There is no obvious crystal phase in the XRD pattern of GBFS. The main mineral composition of GBFS slag is also the amorphous glassy state, which is the same as that of fly ash. With respect to the geopolymer paste sample, the main mineral components are quartz and mullite, which are mainly from fly ash. In geopolymer paste, GBFS and fly ash themselves contain a large amount of amorphous glassy state materials, as well as amorphous glassy state materials generated by geopolymerization and hydration in geopolymer paste; these amorphous glassy state materials are wrapped in quartz and mullite crystal, therefore the diffraction peaks of quartz and mullite in geopolymer paste are significantly lower than that in fly ash. The crystalline structure of hydrated calcium silicate is found in the geopolymer paste sample. This is mainly due to the slag containing large amounts of CaO and SiO 2 . SiO 2 generates sodium silicate in the process of hydration. Ca 2+ reacts with sodium silicate SiO 4 4− in the process of hydration to generate the hydrated calcium silicate. In 2 Theta between 15-40 • , the XRD of fly ash shows a broad hump, and the broad hump is not changed in the geopolymer paste; this is consistent with the findings of most scholars [54][55][56][57]. Liew et al. [58] believe that the broad hump corresponds to the amorphous aluminosilicate. Figure 17 XRD analysis results show that the broad hump is sillimanite and quartz. The main chemical components of sillimanite, quartz, and aluminosilicate are Al 2 O 3 and SiO 2 . Sillimanite is composed of double-stranded aluminosilicate; therefore, the broad hump is characteristic of the aluminosilicate diffraction peak, which is consistent with research results of Liew et al. [58]. It shows that broad hump is formed by geopolymerization.
ash sample contains a small amount of mullite and quartz [52,53], mainly for its amorphous state. There is no obvious crystal phase in the XRD pattern of GBFS. The main mineral composition of GBFS slag is also the amorphous glassy state, which is the same as that of fly ash. With respect to the geopolymer paste sample, the main mineral components are quartz and mullite, which are mainly from fly ash. In geopolymer paste, GBFS and fly ash themselves contain a large amount of amorphous glassy state materials, as well as amorphous glassy state materials generated by geopolymerization and hydration in geopolymer paste; these amorphous glassy state materials are wrapped in quartz and mullite crystal, therefore the diffraction peaks of quartz and mullite in geopolymer paste are significantly lower than that in fly ash. The crystalline structure of hydrated calcium silicate is found in the geopolymer paste sample. This is mainly due to the slag containing large amounts of CaO and SiO2. SiO2 generates sodium silicate in the process of hydration. Ca 2+ reacts with sodium silicate SiO4 4− in the process of hydration to generate the hydrated calcium silicate. In 2 Theta between 15°-40°, the XRD of fly ash shows a broad hump, and the broad hump is not changed in the geopolymer paste; this is consistent with the findings of most scholars [54][55][56][57]. Liew et al. [58] believe that the broad hump corresponds to the amorphous aluminosilicate. Figure 17 XRD analysis results show that the broad hump is sillimanite and quartz. The main chemical components of sillimanite, quartz, and aluminosilicate are Al2O3 and SiO2. Sillimanite is composed of double-stranded aluminosilicate; therefore, the broad hump is characteristic of the aluminosilicate diffraction peak, which is consistent with research results of Liew et al. [58]. It shows that broad hump is formed by geopolymerization.
In the hydration process of geopolymer paste, with the structure decomposition of fly ash and GBFS particles, the geological polymerization reaction is processed between active silica and active alumina. At the same time, C-S-H gel is generated by Ca 2+ in the paste. In this way, the geopolymer network structure is formed in the matrix. Therefore, the content of hydration products directly affects the geopolymer network structure.

SEM Analysis
The microstructure of the geopolymer paste sample cured for 28 days was shown in Figure 18. Under SEM at micro level, geopolymer matrix does not exhibit any noticeable cracks on the surface of the specimen. A large number of network structures of gel are observed in the micrograph of geopolymer paste sample. Gel network structures are porous and rough. It can be observed that some undissolved fly ash glass microspheres are packed by the gel. Figure 19 shows the hydration products completely filled gel pores and fully reacted fly ash particles. As the hydration process intensified, gel crystallized into a In the hydration process of geopolymer paste, with the structure decomposition of fly ash and GBFS particles, the geological polymerization reaction is processed between active silica and active alumina. At the same time, C-S-H gel is generated by Ca 2+ in the paste. In this way, the geopolymer network structure is formed in the matrix. Therefore, the content of hydration products directly affects the geopolymer network structure.

SEM Analysis
The microstructure of the geopolymer paste sample cured for 28 days was shown in Figure 18. Under SEM at micro level, geopolymer matrix does not exhibit any noticeable cracks on the surface of the specimen. A large number of network structures of gel are observed in the micrograph of geopolymer paste sample. Gel network structures are porous and rough. It can be observed that some undissolved fly ash glass microspheres are packed by the gel. Figure 19 shows the hydration products completely filled gel pores and fully reacted fly ash particles. As the hydration process intensified, gel crystallized into a large number of white crystals. Combined with XRD analysis, it can be seen that a large number of white crystals are calcium silicate hydrate [59]. In addition, there are a few small white crystals. According to the results of scanning electron spectroscopy, XRD, and Liew et al. [58], it can be judged that the main component of these small white crystals is aluminosilicate [21,59], and they are wrapped by hydration products and a large number of amorphous glassy state materials. This is also consistent with FTIR analysis results [53].  [59]. In addition, there are a few small white crystals. According to the results of scanning electron spectroscopy, XRD, and Liew et al. [58], it can be judged that the main component of these small white crystals is aluminosilicate [21,59], and they are wrapped by hydration products and a large number of amorphous glassy state materials. This is also consistent with FTIR analysis results [53].   Figure 20 showed the micrographs of fly ash and GBFS in geopolymer samples. It can be seen that there are glass microspheres of fly ash particles inside the mortar matrix. Microspheres have a smooth shell and fly ash particles have a porous interior [60]. The surfaces of GBFS micro particles are relatively rough, and large amount of amorphous glassy state materials exist. This is also proven by XRD analysis of GBFS. As the hydration process intensified, fly ash and GBFS micro particles could not be observed by SEM, which indicates that fly ash and GBFS micro particles have completely reacted to form a geopolymer matrix. The geopolymer matrix gradually contacted with sand and formed an obvious interfacial transition zone (ITZ), as shown in Figure 21. In this experiment, the amount of geopolymer binder is constant. If the content of fine aggregate is more, the content of geopolymer paste is not enough to cover and bind these particles. Therefore, the fluidity and compressive and flexural strength of K5, K6 and K7 are poor. However, if the fineness modulus is too high, there is a content of large-size sand particles. The more the content of large-size sand particles, the more the defects that exist in the interface. As shown in Figure 21a, ITZ is long, and has long and wide gaps. Therefore, mechanical properties of K1 and K2 are also poor. When the fineness modulus of sands is between 2.2 and 2.6, it is clear that there is an intimate bonding between the geopolymer matrix and sands. As shown in Figure 21b, sand is gradually wrapped by hydration products, forming a dense structure. Therefore, K3 and K4 have the best tensile strength. A close observation of the interfacial region uncovered that the bonding property of geopolymer matrix is good   [59]. In addition, there are a few small white crystals. According to the results of scanning electron spectroscopy, XRD, and Liew et al. [58], it can be judged that the main component of these small white crystals is aluminosilicate [21,59], and they are wrapped by hydration products and a large number of amorphous glassy state materials. This is also consistent with FTIR analysis results [53].   Figure 20 showed the micrographs of fly ash and GBFS in geopolymer samples. It can be seen that there are glass microspheres of fly ash particles inside the mortar matrix. Microspheres have a smooth shell and fly ash particles have a porous interior [60]. The surfaces of GBFS micro particles are relatively rough, and large amount of amorphous glassy state materials exist. This is also proven by XRD analysis of GBFS. As the hydration process intensified, fly ash and GBFS micro particles could not be observed by SEM, which indicates that fly ash and GBFS micro particles have completely reacted to form a geopolymer matrix. The geopolymer matrix gradually contacted with sand and formed an obvious interfacial transition zone (ITZ), as shown in Figure 21. In this experiment, the amount of geopolymer binder is constant. If the content of fine aggregate is more, the content of geopolymer paste is not enough to cover and bind these particles. Therefore, the fluidity and compressive and flexural strength of K5, K6 and K7 are poor. However, if the fineness modulus is too high, there is a content of large-size sand particles. The more the content of large-size sand particles, the more the defects that exist in the interface. As shown in Figure 21a, ITZ is long, and has long and wide gaps. Therefore, mechanical properties of K1 and K2 are also poor. When the fineness modulus of sands is between 2.2 and 2.6, it is clear that there is an intimate bonding between the geopolymer matrix and sands. As shown in Figure 21b, sand is gradually wrapped by hydration products, forming a dense structure. Therefore, K3 and K4 have the best tensile strength. A close observation of the interfacial region uncovered that the bonding property of geopolymer matrix is good  Figure 20 showed the micrographs of fly ash and GBFS in geopolymer samples. It can be seen that there are glass microspheres of fly ash particles inside the mortar matrix. Microspheres have a smooth shell and fly ash particles have a porous interior [60]. The surfaces of GBFS micro particles are relatively rough, and large amount of amorphous glassy state materials exist. This is also proven by XRD analysis of GBFS. As the hydration process intensified, fly ash and GBFS micro particles could not be observed by SEM, which indicates that fly ash and GBFS micro particles have completely reacted to form a geopolymer matrix. The geopolymer matrix gradually contacted with sand and formed an obvious interfacial transition zone (ITZ), as shown in Figure 21. In this experiment, the amount of geopolymer binder is constant. If the content of fine aggregate is more, the content of geopolymer paste is not enough to cover and bind these particles. Therefore, the fluidity and compressive and flexural strength of K 5 , K 6 and K 7 are poor. However, if the fineness modulus is too high, there is a content of large-size sand particles. The more the content of large-size sand particles, the more the defects that exist in the interface. As shown in Figure 21a, ITZ is long, and has long and wide gaps. Therefore, mechanical properties of K 1 and K 2 are also poor. When the fineness modulus of sands is between 2.2 and 2.6, it is clear that there is an intimate bonding between the geopolymer matrix and sands. As shown in Figure 21b, sand is gradually wrapped by hydration products, forming a dense structure. Therefore, K 3 and K 4 have the best tensile strength. A close observation of the interfacial region uncovered that the bonding property of geopolymer matrix is good enough to produce an intimate interface. In this way, the quantity of aluminosilicate and the interface between sand particles and gel in the geopolymer matrix has significant bearing on the overall development mechanical strength of the material. enough to produce an intimate interface. In this way, the quantity of aluminosilicate and the interface between sand particles and gel in the geopolymer matrix has significant bearing on the overall development mechanical strength of the material.

Conclusions
Based on the results obtained in this research, the following conclusions could be drawn: (1) When the water-cement ratio is more than 0.4 and the binder-sand ratio is more than 0.45, the fluidity of the geopolymer mortar with standard sand presents a good fluidity property. When the water-binder ratio and binder-sand ratio are in range of 0.35-0. 40   enough to produce an intimate interface. In this way, the quantity of aluminosilicate and the interface between sand particles and gel in the geopolymer matrix has significant bearing on the overall development mechanical strength of the material.

Conclusions
Based on the results obtained in this research, the following conclusions could be drawn: (1) When the water-cement ratio is more than 0.4 and the binder-sand ratio is more than 0.45, the fluidity of the geopolymer mortar with standard sand presents a good fluidity property. When the water-binder ratio and binder-sand ratio are in range of 0.35-0. 40

Conclusions
Based on the results obtained in this research, the following conclusions could be drawn: (1) When the water-cement ratio is more than 0.4 and the binder-sand ratio is more than 0.45, the fluidity of the geopolymer mortar with standard sand presents a good fluidity property. When the water-binder ratio and binder-sand ratio are in range of 0.35-0.40 and 0.35-0.50, respectively, geopolymer mortar samples with standard sand have good mechanical performance. The high value of compressive strength (50-70 MPa) and flexural strength (5-8 MPa) shows great promise for construction applications.
(2) For the fluidity of the geopolymer mortar, the gradation and fineness modulus of river sands both have great influence on it. The flow value of the mortar is approximately linearly proportional to the fineness modulus of sands. Geopolymer mortar has better fluidity when the sand gradation belongs to I and II categories.
(3) The compressive and flexural strength of geopolymer mortar both improve with the increase of the fineness modulus of sands, while specific surface area and voidage are opposite. Tensile strength first increases and then decreases as the fineness modulus increases; this is contrary to the law of specific surface area and voidage. Moreover, if sand gradation belongs to I and II categories, specimens of geopolymer mortar will show relatively higher strength.
(4) When the pass rate of the key sieving size 1.18 mm is 75-95%, the pass rate of the key sieving size 0.15 mm is 15-25%, fineness modulus is 2.2-2.6, and the appropriate filling coefficient of geopolymer paste is around 1.0-1.15, the maximum density of packed sands could appear. The σ f /σ c ratio of geopolymer mortar is the largest; the fluidity can reach more than 200 mm. The tensile, compressive, and flexural strengths are above 3.8 MPa, 60 MPa, and 6.0 Mpa, respectively. At this time, the comprehensive performance of geopolymer mortar is the best. Therefore, geopolymer mortar should be prepared with medium sand with good gradation.
(5) The geopolymer composites have crystal diffraction peak forms of quartz, mullite, hydrated calcium silicate, sillimanite, and zeolite in their crystalline phases. The results of microscopic tests indicate that a large number of network structures of gel are observed in the micrograph of geopolymer paste samples. Whether the interfacial transition zone (ITZ) between the geopolymer matrix and sand particles is dense determines the tensile strength performance of geopolymer mortar.