Cemented Paste Backfill Specimens Research Articles

Characterisation of strength and deformation characteristics of alkali-activated rice husk ash filler-assemblage perimeter rock

In this study, the strength characteristics, deformation characteristics and damage characteristics of three kinds of specimens, namely, surrounding rock, cemented paste backfill (CPB) and a surrounding rock-CPB combination, were studied by uniaxial compression testing using rice husk ash and slag as cementing materials, and the mechanical properties of the combination specimens with different height ratios were also analyzed. The results showed that the surrounding rock specimens were the strongest, followed by the composite body, and the CPB was the weakest. The relationship between different height ratios of the assemblage and the cut line modulus was found according to the fitted curves. The CPB specimens and the surrounding rock specimens showed ductile damage, while the assemblage specimens showed brittle damage.

Reactivity of cemented paste backfill containing polycarboxylate-based superplasticizer

Superplasticizers are used in the preparation of cemented paste backfill (CPB) to improve its rheological properties. CPB produced from sulphide-bearing tailings can generate acid mine drainage (AMD), which has negative environmental impact. The AMD potential can be assessed based on the reactivity of the CPB when exposed to the atmosphere. However, the effect of superplasticizers on the reactivity of CPB is unknown. This paper presents findings from experimental investigations on the effects of polycarboxylate superplasticizers (PCSP) on the reactivity of CPB containing pyrite. The reactivity of cured CPB specimens was determined by monitoring the rate of oxygen consumption in an enclosed chamber. Microstructural tests, namely mercury intrusion porosimetry, X-ray diffraction, and thermogravimetric analyses were also carried out to have a better understanding of the reactive mechanisms. The results obtained show that the addition of PCSP leads to a proportionate reduction of reactivity. The admixture causes the refinement of the pore structure of the CPB by mainly improving binder hydration and self-consolidation. It was also observed that less pyrite crystals were present in the CPB containing the superplasticizer. High curing temperature was also found to improve binder hydration, leading to a decrease in reactivity of CPB with PCSP. Due to the micro-filer effects on the CPB pore structure, partial replacement of cement with fly ash and blast furnace slag also decreases the reactivity of CPB. These research findings will be valuable for the design of cost-effective, more environmentally friendly and sustainable CPB structures.

The Role of Rheological Additives on Fresh and Hardened Properties of Cemented Paste Backfill.

Cemented paste backfill (CPB) has become a significant structural material in most mines across the world. In this study, the effects of chemical rheological additives including viscosity modifying agent (i.e., polyacrylamide) and polycarboxylate superplasticizer (PCE) on fresh and hardened properties of CPB with different water-to-solid (W/S) ratios and water-to-cement (W/C) ratios were investigated. The microstructure of CPB specimens was also characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and backscattered electron image (SEM-BSE). The obtained results indicate that PAM (polyacrylamide) dosage and W/S are the most significant parameters influencing the workability of fresh CPB mixtures. For the hardened CPB specimens, the decreasing W/S ratio leads to higher flexural and compressive strength values and lower dry shrinkage strains. The interfacial transition zone (ITZ) between the cement matrix and the tailings sand was also observed to be narrower, with fewer micro cracks and capillary pores. Meanwhile, the existence of PAM decreased the number of hydration products and retarded the hydration reaction. Overall, the CPBs with high W/C ratios (i.e., 1.0 and 1.2), low W/S ratios (i.e., 0.3), and moderate amounts of rheological additives (i.e., 0.05% PAM and 1.0% PCE) have excellent fresh and hardened properties. The findings of this study contribute to better optimization of CPB mixtures in backfill construction, bringing benefits of low costs and low environmental impacts.

Fresh and hardened properties of cemented paste backfill: Links to mixing time

Current technical knowledge is insufficient regarding the influence of mixing time on fresh and hardened properties of cemented paste backfill (CPB). This work focused on experimental studies to gain a deeper understanding of the influence of mixing time on the rheological properties (yield stress, viscosity), self-desiccation, permeability and mechanical properties or behavior (strength, deformation behavior) of CPB. A series of CPB specimens were prepared with various mixing times (1, 2, 4, 7, 10, and 15 min) and cured at different times (1, 2, 4, 7, 10 and 15 min) for rheological property testing. Unconfined compressive strength (UCS), hydraulic conductivity and microstructural tests were performed on CPBs that were made with various mixing times and cured for different times (up to 90 days). Monitoring experiments were also carried out on CPBs prepared with various mixing times. The results obtained indicate that the mixing time significantly affects the rheological, mechanical, hydraulic and microstructural properties of CPB. Moreover, the self-desiccation of CPB is a function of the mixing time. A longer mixing time increased the yield stress and decreased the viscosity. The longer mixing time promoted binder dissolution and hydration in the CPB, which increased the amount of hydration products and decreased the volumetric water. Longer mixing time intensified the CPB self-desiccation, which is beneficial for the early age mechanical strength of CPB. CPB specimens made with longer mixing time experienced a higher strength and elastic modulus due to the generation of more cement hydration products and the reduction of the volume of pores in CPB. A longer mixing time reduced the CPB hydraulic conductivity, due to the refinement of the CPB pore structure. This study provides relevant knowledge and technical information that will contribute to the design of more cost-effective and sustainable CPB structures.

Utilization of Low-Alkalinity Cementitious Materials in Cemented Paste Backfill of Gold Mine Tailings

The purpose of this paper was to explore the possibility of using low-alkalinity cementitious materials as binders, in which ground blast furnace slag and fly ash acted as a partial replacement of ordinary Portland cement, and CaSO4, Na2SO4, and CaO were used as a sulfate activator and alkali-activated additives, to solidify gold mine tailings for preparation of a green, inexpensive cemented paste backfill (CPB). For this target, the effects of cement/ tailings ratio, superplasticizer dosage, solid content, tailings fineness on the mechanical properties of the CPB were investigated. Additionally, the hydration mechanism of the CPB was analyzed based on X-ray diffraction and scanning electron microscopy results. The results showed that the fluidity of the CPB slurry could be improved by adding polycarboxylic acid superplasticizer. The unconfined compressive strength (UCS) of the CPB specimens was increased with the increase of cement/tailings ratio and solid content. Under the same experimental conditions, the 28 d UCS of the CPB specimens was 3.8~4.9 times higher than that of ordinary Portland cement. The softening coefficient of the CPB specimens was increased with the increasing cement/tailings ratio, ranging from 0.83 to 0.92. The shrinkage rate of the CPB specimens was decreased from 0.70% to 0.54% with the increase of cement/tailings ratio from 1:12 to 1:4. The UCS of the full tailings CPB was the highest, followed by the fine tailings CPB specimens, and the UCS of the coarse tailings CPB specimens was the lowest. The low-alkalinity binder was proved to be a promising material to improve the engineering performances of the CPB. The optimal mixing ratio is 1:6 cement/tailings ratio, 0.15 wt% superplasticizer dosage, and 70 wt% solid content. Prepared by this mixing ratio, the UCS values of the CPB after 3, 7, and 28 d curing ages reached 1.85, 5.87, and 9.16 MPa, respectively, which were suitable as CPB for the Zhaoyuan gold mine in terms of strength requirements.

Flow ability of cemented paste backfill with chloride-free antifreeze additives in sub-zero environments

Currently, the addition of antifreeze additives or admixtures to cemented paste backfill (CPB) during its preparation is an inexpensive technology to address the risk of CPB freezing during its transportation in sub-zero environments. However, the effects of chloride-free antifreeze additives on the flow ability of CPB under sub-zero conditions are not well understood. No studies have been performed on the rheological propertiefs of CPB with chloride-free antifreeze additives in sub-zero environments. Hence, the objective of this study was to assess and gain a deeper insight into the effects of chloride-free antifreeze additives on the rheological properties of CPB with different binder types (Portland cement, ground blast furnace slag) and contents in sub-zero conditions. CPB specimens with different types of chloride-free antifreeze (calcium nitrite (Ca(NO2)2), calcium nitrate (Ca(NO3)2), and urea (CO(NH2)2)) and concentrations (5, 15, and 35 g/L) were prepared and cured under sub-zero conditions (−6, −12, and −20 °C). The rheological properties (yield stress and viscosity) of CPB were determined at curing times of 0 min, 15 min, 1 h, and 2 h. In addition, microstructural analyses, investigation of electrochemical properties, and a series of monitoring experiments were conducted. It was found that the addition of chloride-free antifreeze reduced yield stress and increased viscosity. Meanwhile, the significant effect on the rheological properties of CPB also depends on the curing time, antifreeze concentration, cement content, sub-zero temperature, and cement to slag ratio. The findings of this study can advance the development of chloride-free antifreeze CPB technology and provide important guidance for successful pipeline transportation of CPB in sub-zero environments.

Experimental Investigation on Strength and Failure Characteristics of Cemented Paste Backfill

The strength characteristics of cemented paste backfill (CPB) in mining areas are key control factors for the safety assessment of overlying strata. A series of experiments about uniaxial compression and triaxial compression were carried out to study the influence of cement content of filling slurry, curing age, and curing temperature on strength behavior of CPB specimens. The failure mechanism and damage feature of CPB have been investigated. The results show the following: (1) The uniaxial compressive strength of CPB specimens exhibits an upward trend with the increase of cement content and curing age. When the cement content is high, the uniaxial compressive strength increases sharply with increasing curing age. (2) The cohesion of CPB specimens increases with the increase of cement content of filling slurry, curing age, and curing temperature. The cohesion of CPB specimens with curing age 7 days and 14 days increases linearly with increasing cement content. At the later stage of curing age, the strength growth of high cement content backfill is significantly greater than that of low cement content. The internal friction angle of the filling increases slightly with increasing filling cement content, curing age, and curing temperature. (3) The shear strength of CPB specimens at curing age 7 days exhibits an upward trend with the increase of confining pressure, while the shear strength at 14 days and 28 days curing age decreases slightly as the confining pressure increases. (4) With the increase of cement content in backfill, the brittleness increases significantly when the backfill is damaged. The failure mode of CPB specimens changes from monoclinic section shear failure to X-type failure with increasing curing age, and the failure process is divided into four stages: pore compaction, linear elastic deformation, plastic deformation, and post-peak failure.

Influences of mine water on the properties of construction and demolition waste-based cemented paste backfill

Construction and demolition waste (CDW) is a solid waste generated by urbanisation development around the world, and its treatment and utilization are burgeoning universally. The CDW are generally prepared into a slurry of cemented paste backfill (CPB), which is pumped to the goaf to control surface subsidence and utilize CDW. The underground mine water affects the mechanical properties of CPB. To simulate influences of mine water environment, the CPB specimens sampled onsite were subjected to soaking and drying-wetting cycles (soaking for 12 h and then drying for 12 h constituted one cycle) tests, and corroded at different durations (0, 2, 4, 6, 8, 10, 12, 14, 28, and 56d). Then, unconfined compressive strength (UCS) tests were conducted to investigate the effect of mine water corrosion on mechanical behavior of CPB, a gray model was established to predict long-term strength based on the measured compressive strength, and microstructural analysis such as X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) were performed to reveal the microstructure changes associated with mine water erosion. The results demonstrate that the erosion environments of soaking and drying-wetting cycles can both degrade the UCS of CPB, which first shows a rapidly decreasing trend (slowing later). The maximum degradation of UCS of CPB reached 6.856% when soaking duration reached 8d, and 15.311% when drying-wetting cycles reached 10d. And that the evolution of the modulus of elasticity of CPB is like that of the UCS and an exponential function was proposed to characterise the correlation between them, with a goodness of fit of 0.969. Similarly, both SEM and XRD of specimens at erosion durations of 8d and 10d indicate that, by decomposing hydrated gels in CPB, HCO3−, and SO42− ions in the mine water generate bulk calcite and columnar gypsum to expand crack structures in CPB, which reduces the strength of CPB. Furthermore, through drying-wetting cycles, the cracks blocked by corrosives are coalesced many times, facilitating the repeated migration of corrosive ions, so that the corrosion under drying-wetting cycles is stronger than that under soaking. In addition, according to the grey model, the changes in the UCS of CPB within 150 d under soaking and drying-wetting cycles were predicted and it was determined that UCS of CPB first reached the critical value (at failure) in drying-wetting cycles, and its erosion durations was 136d. This guides the environmental management of underground CPB and protection of the surface ecological environment.

Substitution of Cement with Granulated Blast Furnace Slag in Cemented Paste Backfill: Evaluation of Technical and Chemical Properties

Cemented paste backfill (CPB) offers an environmentally sustainable way to utilize mine tailings, one of the largest waste streams in the world. CPB is a support and filler material used in underground mine cavities, which consists of mine tailings, water, and binder material that usually is cement. Replacing cement with secondary raw materials like granulated blast-furnace slag reduces the total CO2 emissions and strengthens the internal microstructure of the CPB. This study characterizes the total- and soluble contents of CPB starting materials and five CPB specimens containing different levels of slag substitution. In addition, phase composition (mineral liberation analysis, MLA) and internal structure (X-ray tomography) of five CPB specimens is documented, and measurements of compressive strength are used to evaluate their suitability as backfill material. Mine tailings and CPB specimens used in this study are rich in sulphates and arsenic, but low in sulphides. Stronger As leaching of ground CPB specimens compared with ground mine tailings is related to the elevating pore water pH during the cement hydration. The hydration product ettringite is found in all CPB specimens and its content is the lowest in the slag containing specimens. X-ray tomography revealed vertically differentiated density structures in the CPB specimens. The lower parts of all specimens are denser in comparison with the upper parts, which is probably due to the compaction of the solid particles at the base. The compressive strength test results indicate that partial substitution of cement with slag improves the strength of the CPB. The total replacement of cement with slag reduces the early strength but gives excellent strength and lower porosity over longer time intervals. The results of the study can be utilized in developing more durable and environmentally responsible CPB recipes for gold mines of similar mineral composition and gold extraction method.

Non-isothermal evolution of mechanical properties, pore structure and self-desiccation of cemented paste backfill

In the field, cemented paste backfill (CPB) structures are subjected to non-isothermal curing conditions. In this study, the non-isothermal strength, pore structure development, and self-desiccation of CPB were investigated. CPB specimens with different types of binders and tailings were prepared and cured under isothermal (room temperature: 20 °C) and field non-isothermal conditions. Unconfined compressive strength (UCS) tests, microstructural tests and analyses as well as monitoring experiments, were conducted on CPB specimens cured over various ages. The obtained results show that the non-isothermal curing condition significantly increases the strength as well as the strength increase rate of the CPB regardless of the type of binder and tailings used. Moreover, the non-isothermal condition leads to more intense self-desiccation of CPB as well as the refinement of the pore structure of CPB due to the generation of more hydration products, such as calcium–silicate–hydrate (C–S–H), in the non-isothermal CPB. It is also found that a mine cavity (stope) backfilled with CPB material made with Portland cement type I (PCI) has a significant advantage over stope backfilled with CPB made with slag/PCI (50%/50%) in terms of strength formation and enhancement of self-desiccation due the difference in the non-isothermal curing conditions. The findings of this study provide insight into the development of the mechanical and microstructure properties as well as self-desiccation of CPB structures when exposed to field thermal curing condtions, thereby contributing to the improvement of mechanical stability and economic performance of future CPB designs. The impact of non-isothermal curing conditions on CPB properties need to be considered in mine backfill design and operations.

Damage Strengthening Constitutive Model of Cemented Paste Backfill

In order to consider the influence of mesoscopic characteristics of materials on the constitutive model of cemented paste backfill (CPB), the uniaxial compression variables and the damage constitutive model, considering the influence of porosity and pore size of filling materials, were derived based on the strain equivalence principle and Weibull probability distribution function. The nuclear magnetic resonance (NMR) tests and unconfined compression strength (UCS) tests were carried out on 8 groups of CPB specimens with different slurry concentrations and cement-tailings ratios. Then, the expression of damage strengthening coefficient is determined, and the stress-strain curves measured by the theoretical model were compared with the experimental ones. The results show that the uniaxial compression constitutive model proposed is in good agreement with UCS test results and can effectively describe the damage evolution law and the development process of stress-strain curve of CPB under uniaxial compression. The 28-day compressive strength of CPB can reach 8 MPa, the residual strength is about 1∼2 MPa, the elastic modulus is about 200∼2000 MPa, and the porosity is about 3∼5%. The CPB with slurry concentration of 74% and 76% and cement-tailings ratio of 1 : 4 and 1 : 6 is more reasonable, and the relevant mechanical parameters are more stable.

Drained Triaxial Compressive Shear Response of Cemented Paste Backfill (CPB)

Cemented paste backfill (CPB) has been widely used as local and regional underground support to reduce host rock wall closure in mined-out areas and also to reduce rockfall and rockburst incidents. However, analyzing the rock mass—CPB interactions in the first month after backfill placement must account for the CPB’s time-dependent strength, stiffness, and volume change characteristics during binder hydration. This article presents the first such comprehensive study made for CPB from the Williams mine in Ontario, Canada. Monotonic isotopically consolidated drained compressive triaxial tests were conducted on cured CPB specimens using the lubricated-ends test technique. The specimens had 3.0–7.5% Cement Content (CC); Curing Times (CTs) were from 3 to 28 days; and the confining stress ranged from 25 to 350 kPa. During shearing, all tests exhibited an initial contractive phase leading to a Characteristic State (CHS or point of volumetric strain reversal) followed by dilation with a maximum dilation rate corresponding to peak stress at the Failure State (FS). Both CHS and FS are adequately described by the Mohr–Coulomb criterion and a framework formulation was proposed to predict the evolution of CHS and FS based on CC and CT. Furthermore, the volumetric strains at CHS and FS can be defined as linear functions of the respective axial strains at the CHS and FS. Quantification of the observed behaviors through these functional relationships can help develop future constitutive models that better represent CPB’s transient response to triaxial stress loading while curing, which is essential to understanding as-placed backfill properties and its interaction with surrounding rock mass.

Experimental investigation on some performance of rubber fiber modified cemented paste backfill

Incorporating scrap tire rubber fibers into cemented paste backfill (CPB) may be a good solution to dispose of scrap tires. Thus the performance of the rubber fiber modified CPB was investigated. The results reveal that the modified CPB has acceptable workability, lower density, and higher ductility. Although the increase in the rubber fibers content leads to the decrease in the unconfined compressive strength, the highest toughness which represents the optimal balance of strength and ductility is noted for the modified CPB with 4% rubber fibers content. Besides, the modified CPB specimens could maintain their integrity during the compression test.

Effect of external sulfate attack on the mechanical behavior of cemented paste backfill

The sulfate-rich mine water in underground metal mines causes external sulfate attack on cemented paste backfill (CPB) and affects its mechanical stability. To simulate the effect of mine water environment in metal mines, CPB specimens were immersed into the sulfate solutions with different concentration (0, 0.1, 1 and 10 g/L) and cured at different times (30, 90, 160, and 240 days). Then, unconfined compressive strength (UCS) tests were conducted to investigate the effect of external sulfate attack on mechanical behavior and properties of CPB. Moreover, bulk volume change was measured at the macroscale and a series of microstructural analysis was performed to reveal the microstructure change associated with external sulfate attack. In addition, a BP neural network model was established to predict the UCS of CPB under external sulfate attack. The results show that sulfate concentration has a greater impact on the formation and crystallization of hydration products, which directly affect the pre- and post-failure behavior and bulk volume of CPB at the macroscale. Therefore, the obtained results indicate the effect of mine water induced external sulfate attack on the CPB should be fully considered for the safe design of CPB.

Effects of curing time, cement content, and saturation state on mode-I fracture toughness of cemented paste backfill

Due to the wide existence of tensile stresses in cemented paste backfill (CPB) mode-I fracture toughness (KIC) plays a crucial role in the mechanical stability of CPB under complex field loading conditions. However, the studies on KIC of CPB materials and associated influential factors are very limited. Hence, the main objective of this study is to experimentally investigate the effects of curing time (3, 7, 28, and 90 days), cement content (2%, 4.5%, and 7%) and saturation state (unsaturated and saturated states) on the evolution of KIC through single-edge-notch bending tests on CPB specimens. The obtained results show that KIC displays a strongly time-dependent and nonlinear evolution with curing time, especially during the early age. Moreover, the development of KIC is sensitive to changes in cement content, which can be attributed to the precipitation of cement hydration products as well as the self-densification process in CPB. In addition, through the vacuum-based rapid re-saturation approach proposed in this study, it has been concluded that the re-saturated CPB has a lower KIC compared with the CPB without re-saturation treatment and thus confirm the dependence of KIC on the saturation state.

Flow ability of cemented pastefill material that contains nano-silica particles

No investigations have been performed on the rheological characteristics of cemented paste backfill (CPB) that contains nano-silica (nS) particles, although the addition of nS to CPB has been regarded as a promising approach to enhance the strength and strength gain rate of CPB at an early age. The present papers present the results of an experimental study on the impact of nano-silica (nS) on the rheological characteristics of CPB. It is found that the nanoparticles considerably impact the rheological properties of CPB, and thus alter its flow ability. The yield stress and viscosity of the CPB specimens incorporating nS are higher than that of the samples without nS. However, the magnitude of this effect is dependent on the binder type, curing time and water content. The findings described and discussed in this manuscript will support the development of the nano-CPB technology and its successful application in underground mine backfill operations.

Mechanical properties and microstructure of layered cemented paste backfill under triaxial cyclic loading and unloading

In this paper, an experimental investigation was conducted to study mechanical properties and microstructural characteristic of layered cemented paste backfill (CPB) under triaxial cyclic loading and unloading condition. A number of CPB specimens were prepared with different number of layers (1, 2, 3, and 4). Four confining pressures (0, 0.2, 0.5, and 0.8 MPa) and three cycles were designed. The results indicated that cyclic loading and unloading had obvious influence on the mechanical properties of the CPB, which was closely related to the number of layers and confining pressure. Compared with the conventional triaxial compression test results, when the confining pressure is 0 and 0.2 MPa, the cyclic loading and unloading can increase the uniaxial compressive strength (UCS) of CPB, and when the confining pressure is 0.8 MPa, it decreased the UCS. When the confining pressure is 0.5 MPa, it increased the strength of the CPB 1 and 2 layers, but decreased the strength of CPB with 3 and 4 layers. The change of UCS had a positive linear correlation with the change of secant modulus of unloading curve caused by the cycle loading and unloading. The results of SEM and CT scanning showed that there were more pores in the post filling layers due to the difference of curing time which were compacted easily under small confining pressure. Under the action of cyclic loading and unloading, the CPB was repeatedly compacted, resulting in the increase of UCS. With the increase of confining pressure, the difficulty of pore compaction increases gradually and stress concentration was easy to form around the pores, which resulting in local fatigue damage and reduce the strength under cyclic loading and unloading.

Effect of Moisture Content on the Ductility of Cemented Paste Backfill

The ductility of cemented paste backfill at different moisture content were researched in this paper. The cemented paste backfill specimens at three different moisture content were prepared and the uniaxial compressive tests were conducted. The results show that the compressive strength of specimen decreases with the increase of moisture content. However, the ductility of specimen increases with the increase of moisture content. The elastic modulus and the rate of post peak stress reduction decreases with the increase of moisture content. The obtained results are helpful to better understand the behavior of the cemented paste backfill under different moisture condition, which could provide guidance for safe mining.

Mechanical behaviour and failure characteristics of cemented paste backfill under lateral unloading condition

A key issue associated with mine backfill design is the determination of mechanical behaviour and failure characteristics of the cemented paste backfill (CPB), especially for CBP under lateral unloading conditions caused by excavation of adjacent stopes. In this research, a series of lateral unloading tests with RFPA3D were conducted to investigate the relationships among the cement-tailings ratio (1:4 and 1:8), lateral unloading stress (0 MPa, 0.2 MPa, 0.4 MPa, 0.6 MPa, 0.8 MPa and 1.0 MPa), mechanical behaviour, failure modes and acoustic emissions (AE) characteristics. Similar mechanical behaviour of all the CPB specimens was obtained. With different cement-tailings ratios and preloading stress values, the ultimate failure modes of CPB specimens show different forms. Additionally, increasing the preloading stress value caused a rising of AE counts. The results of this study can provide a significant reference for the backfill design and study of underground mining.

Quantitative investigation on micro-parameters of cemented paste backfill and its sensitivity analysis

The mechanical properties of cemented paste backfill (CPB) depend heavily on its pore structural characteristics and micro-structural changes. In order to explore the variation mechanisms of macro-mechanical characteristics and micro-structure of CPB. CPB specimens with different mass concentrations prepared from the full tailings of Xianglushan Tungsten Ore were micro-tests. Moreover, acquired pore digital images were processed by using the pores (particles) and cracks analysis system (PCAS), and a sensitivity analysis was performed. The results show that as the mass concentration of CPB increases from 70% to 78%, the porosity, the average pore area and the number of pores drop overall, leading to a decline in the pores opening degree and enhancing the mechanical characteristics. As the mass concentration of CPB increases, the trend of fractal dimension, probability entropy and roundness is reduced, constant and increased, which can result in an enhancement of the uniformity, an unchanged directionality and more round pores. According to the definition of sensitivity, the sensitivities of various micro-parameters were calculated and can be ranked as porosity > average pore area > number of pores > roundness > fractal dimension > probability entropy.