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Monday, March 27, 2023

# Affect of the ultra-fine fly ash dosages on the mechanical properties of the sulfoaluminate cement-based excessive water backfilling materials

### The setting time of the HWBMs blended with UFA

In response to the coal industrial normal (MT/T420-1995) of the HWBMs in China, the setting time of the HWBMs must be lower than 20 min. On this work, the setting time of the HWBMs was 7 min, which is as much as normal as proven in Fig. 4. The setting time of the HWBMs elevated with the rising of the UFA dosages. And notably, the setting time of the HWBMs blended with the UFA elevated quickly because the UFA dosages elevated from 20 to 30%. Nonetheless, the setting time of the HWBMs blended with the UFA of 30% nonetheless didn’t exceed 20 min. It signifies that the addition of the UFA delayed the hydration means of the HWBMs, and the 20% is a inflection level, which the setting time of the samples rising quickly because the UFA dosages exceeded 20%.

### Mechanical properties of the HWBMs blended with UFA

#### Mechanical properties of the HWBM cured at the usual curing situation

At the usual curing situation, the stress–pressure curves of the HWBM blended with UFA of various dosages had been examined proven, and the change within the UCS, elastic modulus and deformation modulus of the HWBM with completely different UFA dosages had been examined and calculated for analyzed the affect of the UFA on the mechanical properties of the HWBM.

Underneath the usual curing situation, the power, elastic modulus, and deformation modulus of the HWBM blended with the UFA of various dosages had been calculated in Fig. 5. Firstly, as proven in Fig. 5a, it may be noticed that the power of the HWBM decreased with the rising of the UFA dosage at completely different curing ages (1, 3, 7, 14 and 28 days), and the power of the HWBM present a pointy decreased when the dosage of UFA was 30%, this result’s comparable with earlier research5. Nonetheless, with the rising of the curing time, the later power (on the age of 28 days) of the preliminary HWBM decreased by 8.15%, however the power of HWBM blended with UFA maintained rising on the later age. It signifies that the addition of UFA has a constructive affect on the rising of power on the later age.

In the meantime. Elastic modulus (E) is a vital efficiency parameter to guage the capability of resistance to elastic deformation of backfill physique in mine backfill area, which might mirror the elastic deformation capability of HWBM underneath the roof stress. As proven in Fig. 5b, there are similar change pattern in E and UCS of preliminary HWBM, which signifies there are a number of limitations on elastic deformation capability of pure HWBM. Nonetheless, after blended with UFA, the elastic modulus of the HWBM have barely elevated when the dosages had been 5% and 15%, however with the elastic modulus decreased sharply with the dosage rising to 30%.

Moreover, the deformation modulus of HWBM additionally is a vital parameter to guage the deformation capability and compressibility of backfill physique. As proven in Fig. 5c, the deformation modulus of the HWBM first elevated after which decreased, whereas the height occurred on the age of 14 days. After addition of UFA, the deformation modulus elevated with curing time rising. It signifies that the addition of UFA has a constructive impact on deformation modulus of the HWBM on the later ages.

In abstract, there are a number of limitations on later mechanical properties of pure HWBM, however the addition of UFA has a constructive impact on these limitations. Nonetheless, with the UFA dosages rising, the mechanical properties of the HWBM blended with UFA nonetheless decreased evaluating with pure HWBM, particularly the mechanical properties decreased sharply when the UFA dosage was 30%. Thus, we advise that the UFA dosage must be lower than 30% in mining backfill engineering.

Furthermore, for additional investigating the change charge of the HWBM blended with UFA evaluating with the preliminary HWBM, the variation charge of the UCS, elastic modulus and deformation modulus of the HWBM with completely different UFA contents in contrast with the management group at the usual situation had been calculated in Fig. 6.

Determine 6 shows the variation charge of the UCS, elastic modulus and deformation modulus of the HWBM with completely different UFA contents in contrast with the management group at the usual situation for additional analyzing the variation charge of the mechanical properties of the HWBM blended with UFA of various dosages. It may be noticed that the hole of the mechanical properties between the pure HWBM and the HWBM blended with UFA decreased with the curing time rising underneath the usual curing situation, whereas the power reducing by lower than 10% because the UFA dosages lower than 10%. Furthermore, the E and D have elevated to some extent when the UFA dosages are 5% and 10%, which signifies that the a number of additions of UFA have a constructive affect on the event of mechanical properties of the HWBM on the later ages.

For analyze the deformation attribute of the HWBM with the UFA of various dosages, the stress–pressure curves of the samples after curing for 28 days at the usual curing situation had been proven in Fig. 7. It may be noticed that the pressure of compaction stage of the preliminary HWBM is shorter than the samples blended with UFA, and shortly enter elastic stage, then the pressure is lesser when attained peak stress, whereas the pressure on the peak stress elevated with UFA dosages rising. It signifies that the preliminary HWBM can notice help of the roof because the stain is lesser, Nonetheless, after combined with UFA, the HWBMs nonetheless have increased power after attained peak power, whereas the stress of preliminary HWBM decreased shortly. It signifies that the UFA can improve the residual power of the HWBM, which makes the HWBM has increased stress to help the roof deformation and floor floor settlement.

#### Mechanical properties of the HWBM cured on the laboratory air situation

Evaluating with the moist and closed atmosphere in coal aim, the dry air and high-temperature atmosphere in roadway has a clearly adverse have an effect on on the mechanical properties of the HWBM. The earlier studies3 discover that the power of superhigh water materials (water content material attains 95–97%) decreased with the rising of curing ages in weathering situation and misplaced its stability after cured for 28 days solely. Subsequently, it’s vital to analyze the impact of the UFA on mechanical properties of the HWBM on the weathering atmosphere. On this work, the laboratory air situation was used to analyze the change within the mechanical properties and deformation traits of the HWBM because the simulation of the coal roadway.

Determine 8a exhibits the UCS of the HWBM blended with completely different dosages of UFA on the ages of 1, 3, 7, 14 and 28 days cured on the laboratory air situation. It exhibits that the power of the preliminary HWBM first elevated after which decreased with the rising of curing time, whereas the height power happens on the age of seven days. Nonetheless, after mixing with UFA, the 28 days power was increased than the preliminary HWBM when the dosages of UFA had been lower than 15%. Nonetheless, the power of the HWBM scale back sharply after ages of the 14 days when the dosages had been 20% and 30%, this end result counsel that the UFA dosage must be lower than 20% when the HWBM is utilized within the gob-side entry retaining of mining engineering.

In the meantime, elastic modulus of the HWBM mix with completely different dosages of UFA at completely different curing ages had been calculated in Fig. 8b. It may be discovered that the E of preliminary pattern decreased with curing time rising, whereas the E of the HWBM confirmed the pattern of first reducing, then rising and closing reducing with UFA dosages rising. It’s noteworthy that the E of the HWBM is increased the preliminary pattern by 17.05%, 70.81% and 89.39% respectively on the ages of seven, 14 and 28 days when the dosage of UFA was 15%, which signifies the capability of resistance to elastic deformation elevated considerably within the HWBM blended with UFA of 15% on the later ages. In one other side, when the UFA dosages exceeded 20%, the elastic modulus of the HWBM decreased sharply on the later ages.

Deformation modulus of the HWBM is proven in Fig. 8c. The Deformation modulus of HWBM first elevated after which decreased with the curing time rising, whereas the deformation modulus of the samples blended with UFA decreased on the early ages (1 and three days). It signifies that the brittle deformation of HWBM elevated, which suggests compressibility of the HWBM decreased after blended with UFA. Notably, when the UFA dosages had been 20% and 30%, the deformation modulus of the HWBM decreased sharply on the later ages. Nonetheless, when the UFA dosage was 15%, the change in deformation modulus of the HWBM is comparable with preliminary HBWM. It signifies that the impact of UFA of 15% on compressibility of the HWBM is minimal on the laboratory air situation.

In the meantime, Fig. 9 is displayed for analyzing the impact of UFA dosage on variation charge of mechanical properties. It may be inferred that UFA dosages of 20% and 30% have a major weakening impact on the event of mechanical properties of the HWBM UCS of the HWBM with curing time rising underneath the laboratory air situation. Nonetheless, when the dosage of UFA was 15%, the UCS and elastic modulus of the HWBM elevated considerably on the later ages, whereas the deformation modulus decreased with the rising of curing ages. This end result means that the 15% is an optimum UFA dosage of HWBM with the water stable charge of two.5:1 for decreasing backfill price and rising mechanical properties of the HWBM.

As well as, for additional exploring the affect of the UFA dosage on the mechanical properties and deformation traits of the HWBM, the stress–pressure curves of the HWBM with completely different UFA dosages on the laboratory air situation for 28 days are analyzed. It may be noticed that the power decreased, and deformation elevated considerably when the UFA dosages had been 20% and 30%, which signifies that UFA dosage of exceeding 20% have an apparent adverse impact on the mechanical properties of the HWBM underneath open dry atmosphere. Nonetheless, the mechanical properties of the HWBM elevated in a deep when the UFA dosage was lower than 15%, and the HWBM nonetheless has increased power evaluating with the preliminary HWBM with the pressure rising as proven in Fig. 10. It signifies that the addition of small quantity of UFA can scale back the weathering depth of the HWBM successfully underneath the laboratory air situation.

### Impact of UFA on the hydration merchandise of the HWBM

To analyze the impact of UFA on the hydration course of and merchandise of HWBM, the XRD assessments had been carried out to investigate the variation of mineralogical composition within the HWBM with completely different dosages of UFA.

Determine 11 exhibits the XRD sample of the HWBM blended with completely different contents of UFA cured for 7 days at completely different curing circumstances, it may be noticed that the principle hydration merchandise are ettringite, and some C–S–H and AH3 gel as proven in Eqs. (1)– (3). Moreover, there are small quantity of CH2 and gypsum. It’s noteworthy that the UFA particles haven’t any vital diffraction peaks, which can be the results of too small particle measurement of UFA and overlaying by different hydration merchandise construction.

$${textual content{C}}_{{4}} {textual content{A}}{}_{{3}}overline{{textual content{S}}} + 2{textual content{CaSO}}_{4}^{{}} + 38{textual content{H}}_{{2}} {textual content{O}} to {textual content{C}}_{{2}} {textual content{A}} cdot 3{textual content{CaSO}}{}_{4} cdot 32{textual content{H}}_{{2}} {textual content{O}} + 2{textual content{AH}}_{3} (gel)$$

(1)

$${textual content{C}}_{{2}} {textual content{S}} + 2{textual content{H}}_{{2}} {textual content{O}} to {textual content{C – S – H}} + {textual content{Ca}}({textual content{OH}})_{2}$$

(2)

$$3{textual content{Ca}}({textual content{OH}})_{2} + 3{textual content{CaSO}}_{4} + {textual content{AH}}_{3}^{{}} + 26{textual content{H}}_{2} {textual content{O}} to {textual content{C}}_{2} {textual content{A}} cdot 3{textual content{CaSO}}_{4} cdot 32{textual content{H}}_{2} {textual content{O}}$$

(3)

As proven in Fig. 11a, it may be noticed that the kinds of hydration merchandise within the HWBM haven’t any clearly adjustments after blended with completely different dosages of UFA when the pattern had been cured at the usual curing situation, which signifies that there are different reactions occurred within the HWBM on the early ages after addition of UFA. This result’s in step with earlier research17. Therefore, it may be inferred that the principle results of UFA addition are filler and dilution on the early hydration course of, which brought about the mechanical properties of the HWBM reducing on the early ages, and the power of the HWBM decreased with UFA dosages rising. Notably, the mechanical properties of the HWBM decreased sharply when the UFA dosages exceed 20%.

Nonetheless, when the HWBMs had been cured on the laboratory air situation, the hydration merchandise have apparent adjustments as proven in Fig. 11b. It may be noticed that the diffraction peak of ettringite decreased with the UFA dosages reducing whereas there was calcite fashioned as a result of carbonation of ettringite underneath the open dry atmosphere. Nonetheless, it’s noteworthy that the ettringite content material elevated when the UFA dosage was 30%, which can be the results of sufficient UFA crammed within the pore of ettringite and delayed its carbonation. Whereas overmuch UFA additionally destroyed the skeleton construction of ettringite within the HWBM, brought about the mechanical properties of the HWBM decreased.

Furthermore, for additional investigating the change in hydration merchandise of the HWBM with curing time rising, the XRD patterns had been displayed in Fig. 12.

In contrast with the XRD patterns of samples on the age of seven days in Fig. 11a, the kinds of hydration merchandise haven’t any clearly adjustments on the ages of 28 days as proven in Fig. 12a. Whereas the diffraction peak of ettringite decreased considerably when the UFA dosages exceeded 20%, it signifies that the overmuch UFA addition have a adverse affect on the hydration means of the HWBM, which brought about the mechanical properties of the HWBM decreased sharply as proven in Fig. 5a.

Alternatively, when the HWBMs had been cured on the laboratory air situation, there are apparent variations within the hydration merchandise of the HWBM with the rising of the curing ages and UFA dosages. As proven in Fig. 12b, after cured for 28 days, the diffraction peak of the ettringite and C–S–H additional decreased whereas diffraction peak of the calcite additional elevated in contrast with the pattern cured for 7 days, it signifies that the evaporation of free water accelerated carbonation of ettringite as a result of rising of porosity and get in touch with space between ettringite and CO2 within the air. Whereas, after blended with UFA, UFA particles crammed into the pore of the HWBM as fillers, brought about the reducing of contact space between hydration merchandise and air, which restrained the carbonation of ettringite. Thus, the power of the HWBM elevated on the ages of 28 days when the UFA dosages had been lower than 15% as proven in Fig. 8a, however when UFA dosages had been 20% and 30%, quantities of UFA particles destroyed the integrity of ettringite construction, it brought about the power of the HWBM decreased sharply. Subsequently, on this work, the UFA dosages of 15% is cheap in view of using of fly ash and mechanical properties of the HWBM.

### Diploma of crystallinity of the HWBM

Because of the diploma of crystallinity have a major impact on mechanical properties of supplies, we calculated the diploma of crystallinity of samples in keeping with the XRD sample for additional analyzing the impact mechanism of fly ash on mechanical properties of the HWBM as proven in Fig. 13. Firstly, the principle crystal part is ettringite within the HWBM, so the content material of ettringite will be quantified in a level by the diploma of crystallinity. Determine 13a shows the diploma of crystallinity of samples cured at the usual curing situation, it may be noticed that the diploma of crystallinity of HWBM elevated with curing time rising, which signifies that the relative content material of ettringite elevated with the curing time rising. After blended with UFA, the diploma of crystallinity elevated when the UFA dosages had been lower than 15%, it signifies {that a} small variety of UFA can speed up the hydration means of the HWBM on the early ages. Whereas the addition of UFA restricted the hydration means of later ages.

Alternatively, when the samples had been cured on the laboratory air situation, the diploma of crystallinity decreased in contrast with the samples cured at the usual curing situation, which signifies that the formation of ettringite had been delayed as a result of lack of free water and carbonation of ettringite. Whereas, after including UFA, the crystallinity of the HWBM first elevated after which decreased with the UFA dosages rising, it signifies that the UFA can enhance the weathering resistance of the HWBM when the UFA dosages didn’t exceed 15%. This result’s in conformity with the height power as proven in Fig. 8a.

### Impact of UFA on the microstructure of HWBM

#### Curing at the usual curing situation

For observing the hydration merchandise and its morphology of the HWBM with completely different UFA dosages cured at the usual curing situation, the SEM photographs of the HWBM samples are given after curing 28 days.

Determine 14 exhibits the microstructure of the HWBM with completely different dosages of UFA on the ages of 28 days underneath the usual curing situation, it may be noticed that the principle hydration merchandise of preliminary HWBM are ettringite and C–S–H as proven in Fig. 14a. The formation of community skeleton construction which consisted of needle-like ettringite is the principle supply of mechanical properties of the HBWM. However, with the curing time rising underneath the usual curing situation, there are a number of micro-pore and cracks within the pure HWBM, which can be the results of ettringite formation and growth in restrict house, it brought about the failure of an intact construction and the reducing of the mechanical properties on the 28 days.

In contrast with the pure HWBM, the morphology of the HWBM is important completely different after including UFA. It may be discovered that the content material of sheet and cotton-shaped C–S–H gel elevated with the UFA dosages rising as proven in Fig. 14b–f), these gels improved the cohesive drive of the ettringite construction, which is useful for the later mechanical properties on the ages of 28 days. Moreover, the merchandise of pozzolanic response between fly ash and Ca(OH)2 additionally enhanced the residual power and compressive capacity of the HWBM as proven in Fig. 7.

Moreover, the construction of the HWBM grew to become denser when the UFA dosage was 5%, and there are a number of globular vitreous particles adhere to the floor of community skeleton construction as proven in Fig. 14b, which signifies that the UFA particles additionally performed a filler position within the HWBM. Thus, when the UFA dosages had been lower than 20%, the merchandise of pozzolanic response improved the cohesion of the samples, and the UFA particles crammed within the pores and cracks of hydration merchandise of the HWBM, which brought about the mechanical properties parameters elevated. Nonetheless, a quantity of UFA particles will lead the dilution of the hydration merchandise and failure of total construction as proven in Fig. 14f, which brought about the power of the HWBM clearly decreased.

#### Curing on the laboratory air situation

Underneath the laboratory air situation, there could be completely different hydration reactions occurring within the inside of HWBM compared to the usual curing situation, akin to evaporation of inner free water and carbonation of ettringite, which impacts the hydration course of and microstructure of the HWBM. Thus, the SEM photographs of HWBM with completely different dosages of UFA had been displayed in Fig. 15.

As proven in Fig. 15a, it may be noticed that the content material of ettringite within the HWBM decreased considerably underneath the laboratory air situation in contrast with the usual curing situation, whereas the content material of calcite elevated. It signifies that the hydration merchandise of the HWBM occurred the weathering carbonation response with the CO2 within the air, and the formation of the calcite weakened the adhesive property of the HWBM, which make its power decreased quickly on the later curing ages. As well as, the evaporation of free water accelerated the weathering carbonation of the hydration merchandise, lead the power of samples decreased on the 28 days.

Nonetheless, the including of UFA crammed within the cracks of ettringite and C–S–H delayed the carbonation response of the hydration merchandise as a result of adhering of UFA particles lead the reducing of the contact space between hydration merchandise and the CO2 within the air, whereas there isn’t a response between the glass-globular fly ash particles and the CO2 as proven in Fig. 15b–f. Nonetheless, as a result of UFA particles haven’t any adhesive property, the including of UFA simply play a filler position within the HWBM. On the one hand, the filling of UFA particles brought about the reducing of the weathering carbonation within the HWBM, additional delayed the reducing of its power on the 28 days when the UFA dosages had been lower than 15%. Alternatively, as proven in Fig. 15e and f, quantities of fly ash particles adhered to the hydration merchandise of the HWBM, which there was a dilution impact on the its microstructure, additional destroyed the entire construction of the HWBM, lastly lead the apparent reducing of the power of the HWBM as proven in Fig. 8a.

In abstract, in keeping with the evaluation in regards to the results of UFA on the microstructure of the HWBM, we advise that the UFA dosages must be not more than 15%, this result’s agreed with the XRD analyze as talked about within the sections “Mechanical properties of the HWBMs blended with UFA” and “Impact of UFA on the hydration merchandise of the HWBM”.