Fly Ash Dosage Research Articles

Effect of mineral admixtures on flow properties of fresh cemented paste backfill: Assessment of time dependency and thixotropy

Cemented paste backfill (CPB), an engineered mix consisting typically of tailings, cement and water, has become a regularly used structural material in most mines worldwide. The performance of the CPB structure is dependent both on the quality of each constituent and the rheological behavior of fresh CPB during placement into mined-out voids. A connection between the backfill's rheological behavior and the structural changes governing thixotropy and structural breakdown can provide a better understanding of the different yield stress-viscosity results. In this study, the mineral admixture effect on rheological properties of fresh CPB slurry is studied by considering a combined assessment of time dependency and thixotropy. A total of three mineral admixtures such as fly ash (FA), granulated blast furnace slag (Slag) and silica fume (SF) were used for the preparation of CPB material. The rheological parameters, mainly yield stress and plastic viscosity, were examined by the Bingham model. Results indicate that the thixotropy of fresh CPB decreases with FA dosage, while the addition of Slag and SF gives lower thixotropy values at early ages, and a higher rate of thixotropy acquisition at a later age. A partial replacement of cement with FA leads to an increase in the static deformability and significant improvement in fluidity of CPB, and a plot of thixotropy versus plastic viscosity results in a linear correlation between them. Moreover, the addition of slag particles reduces the yield stress and plastic viscosity of CPB, while SF-containing CPB exhibits improved yield stress and lower plastic viscosity. Plotting thixotropy versus plastic viscosity generates a poor linear relation for SF- and Slag-containing CPB slurries. The findings of this study will be useful in the lucrative design, efficient operation and safe production of early-age CPB material.

Characterisation of water stability of magnesium phosphate cement blended with steel slag and fly ash

The instability of magnesium phosphate cement (MPC) in moist environments limits its application. In the work reported in this paper, the residual strength and length evolution of MPCs under various curing conditions were investigated to assess the effect of steel slag and fly ash on the stability of MPC. The microstructure of hardened MPC was analysed using X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy. The results indicated that the water stability of MPC is highly dependent on the pre-curing regime. Although increasing the ratio of magnesia to phosphate is detrimental to the strength of MPC in air, it was able to improve the water stability. The strength development of MPCs with steel slag was found to similar to those of the fly ash blends, but showed poor water stability. The shrinkage of MPC paste was found to be more significant with an increase in curing age and the dosage of steel slag and fly ash for specimens cured in air, but an opposite trend was exhibited for specimens immersed in water. The cracks in hydrated MPC with steel slag could be the main reason for its poor water stability.

Removal of harmful algal blooms in freshwater by buoyant-bead flotation using chitosan-coated fly ash cenospheres.

Harmful algal blooms (HABs) are a growing problem worldwide, damaging human and ecosystem health. In this study, a novel buoyant-bead flotation (BBF) method using chitosan-coated fly ash cenospheres (CFACs) was developed to remove HABs in freshwater. To achieve a high removal efficiency of harmful algae (Chlorella vulgaris, Scenedesmus quadricauda, and Microcystis aeruginosa), this study investigated the effects of chitosan/fly ash ratios in CFAC composite, CFAC concentration, flotation time, and pH values on the microalgae removal. The optimized ratio of CFACs is 0.1:12, and the optimized CFAC concentration is 0.3-0.7gL-1. However, the lower or higher ratios (0.1:4, 0.1:8, 0.1:16) result in microalgae reaching a zero-point charge too late or early, which failed to effectively remove HABs with an appropriate coal fly ash dosage. An optimized removal efficiency of 98.50% for Microcystis aeruginosa was reached at pH of 6.0. The optimized efficiency of Scenedesmus quadricauda and Chlorella vulgaris was 99.37% and 91.63%, respectively, at pH of 8.0. At neutral pH conditions, the surface charge of microalgae cells and CFACs are different, promoting aggregate formation. When CFACs were used to remove microalgae, aggregate size significantly influenced removal efficiency. Meanwhile, at the optimized pH and concentration, the removal efficiency of all three algal species exceeded 90.00% in 5min. The study highlights an efficient and inexpensive method for removing HABs and obtains the optimized operational conditions.

Influence of Coal Fly-Ash on Soil Properties and Productivity of Chickpea Crop in Semi-Arid Region of Bundelkhand

Fly ash changes the soil properties which may cause disastrous influence on microbial activity and growth of the plant. However, the scientific studies on the influence of fly ash in various combination with an organic fertilizers on soil properties and microbial response at semi-arid region of Bundelkhand soil is scanty in India. The main objective of this study was to assess the impact of lower or higher doses of fly ash on the soil physico-chemical characteristic, microbial population and growth of leguminous plant chickpea (cicer arietinum L), an important crop of Bundelkhand. The field experiment was conducted during winter, different treatment were made such as control with no amendment of fly ash (T1), amendment of fly ash at the rate of 10tha-1(T2), 20tha-1(T3), 30tha-1 (T4), 40tha-1 (T5),50tha-1 (T6) in combination with vermi-compost (2tha-1 in soil) and Nitrogen, potassium and phosphorus (20kgN ha-1 +20kg k2O ha-1 + 50 kgP2O5 ha-1 in soil) with three replications. In the present study, it was seen that fly ash, increased water holding capacity (WHC), moisture content, pH, soil porosity, organic carbon and electrical conductivity values of the soil. An increasing trend was also seen in P, K, S, and Mn, concentration from 9.87 to 12.21kg ha-1, 121to 124 kg ha-1, 9.36 to 12.14mg/kg and 9.27 to 87 mg/kg, respectively whereas bulk density and total nitrogen decreased from 1.29 to 1.24 g/cm3 and 247 to 205 kg ha-1, respectively in the fly ash applied soil. The application of fly ash at 20tha-1 (T3) was found optimum for bacterial population though the fly ash level exceeding 20tha-1, resulted decline microbial population.

Mechanical properties and durability performance of fly ash based mortar containing nano- and micro-silica additives

The aim of this paper is to investigate the influence of nano-silica (NS) and micro-silica (MS) on the properties of fly ash based geopolymer mortar. The use of such additives has the potential to provide greener and cheaper concrete with enhanced properties. A screening study was conducted to compare the performance of mortar containing varying percentages of NS and MS (up to 15%). The performance of each specimen was determined through tests on compressive strength, splitting tensile strength, workability, and water absorption. Results indicated that mortar with a dosage of 5% NS shows optimum results in workability and mechanical properties. This is mainly due to the pozzolanic and filling effects of large surface area nanosized particles on the cementitious matrix. Moreover, the effect of 5% NS was investigated for mortars containing different amounts of fly ash (up to 30%) which replaced cement by weight. With the presence of 5% NS, variation in fly ash content has no significant influence on the flowability. Although replacement of cement with fly ash at 10% in the presence of 5% NS decreased the splitting tensile strength, no significant reduction was noticed when compared with control samples (without nanoparticle). Compressive strengths also show significant reduction as the percentage of fly ash increases, however the presence of 5% NS aids in the strength development over 28 days. Observations from this study show the effects of nano-silica and fly ash dosage in the performance of cementitious composites. Thus, the results of this study is beneficial for future applications of nanomaterials in cement-based composites for generation of multifunctional construction materials.

Flow Assessment of Self-Compacted Concrete incorporating Fly Ash

Flowability is one of the prime characteristics of good concrete and plays a key role in Self Compacting Concrete (SCC). This study assesses the flowability of SCC with reduced cement content which is replaced by fly ash. Several mixes were prepared by using 5% to 30% fly ash. From the experimental work it was realized that by adjusting a suitable percentage of Super Plasticizer (SP), fly ash dosage can be increased with satisfactory fresh properties of concrete in accordance with the guidelines of EFNARC. At T50cm time the optimum amount of SP was 2%. It was noted that with increase in dosage of fly ash the slump flow increased but T50cm and V-funnel time were reduced. J-Ring height value varied from 9 to 10 for all mixes.

Effect of Fly Ash and Un-crushed Coarse Aggregates on Characteristics of SCC

This research paper discusses the change in the workability and strength characteristics of Self Compacting Concrete (SCC) due to addition of fly-ash and use of un-crushed Coarse Aggregate (CA). Laboratory based experimental work was carried out by preparing 12 SCC mixtures among which six mixtures contained crushed aggregate and other six mixtures contained un-crushed coarse aggregate. A total of 550 kg/m3 binder content and fixed Water-Binder (W/B) ratio as 0.35 were used. Two mixtures were controlled by using Portland Cement (PC) and other ten mixtures contained PC and Fly Ash (FA). Slump flow time, slump flow diameter and J-ring height tests were conducted to study the fresh properties of SCC. Furthermore, compressive strength was calculated at 7, 14 and 28 days of curing. The outcomes indicated that the slump flow time, slump flow diameter and J-Ring height for all the mixes are within the limits specified by EFNARC guidelines. The compressive strength of SCCs depends upon dosage of fly ash. Compressive strength for SCCs with crushed CA was better than obtained in case of un-crushed CA. The maximum compressive-strengths were observed as 64.58 MPa and 58.05 MPa for SCC with crushed and un-crushed CA respectively.

Adsorption of Rhodamine B wastewater by sulphur acid modified fly ash

In this study, fly ash was modified using sulphur to remove Rhodamine B(RB)from aqueous solution. This paper discussed the influence of adsorption time, modified fly ash dosage, pH value, initial concentration on the removal efficiencies. The results showed that 50.0 mL RB wastewater whose concentration was 10.0 mg/L, the adsorbent dosage was 0.3000 g, the pH was 2.0, adsorption time was 60.0 min, the removal efficiencies(RE) and elimination capacity (EC) were 84.68% and 1.41 mg/L, respectivly. The modified fly ash adsorption of RB obeyed the Freundlich isotherm equation, belong to the multilayer adsorption. The adsorption process was accorded with second kinetics reaction of Lagergren. The results implied that fly ash can be effective used as adsorbent to RB wastewater.

Compressive strength development of steam cured cement paste with high volume wet-grinded coal fly ash

In this study, superfine fly ash slurry [WGFA] was prepared by wet grinding from the coal fly ash. Cement-fly ash binder with 30%, 50%, and 70% dosage of fly ash was designed, with a water/binder ratio of 0.5. The samples were firstly cured for 20 hours under steam curing at 60°C and then cooled to room temperature within 4 hours, followed by further curing to 28 days of age under standard curing condition. The compressive strength at 1 day and 28 days were examined, and the results were discussed in terms of hydration process and pore structure, which were evaluated by XRD, TG, SEM, NMR, and MIP. The results showed that 1 day compressive strength of the system with up to 50% WGFA was higher than that of the cement system. The reason was due to the high reaction activity of WGFA under steam curing and the excellent filling effect of fine particles. However, further curing to 28 days, the strength increase was surprisingly unacceptable, with an increase by 18% at 30% dosage, by 10% at 50% dosage, and by 11% at 70% dosage. These increases were much less than that in raw fly ash system. One reason was due to the denser structure which hindered the transportation of ions and water molecules for further hydration; the other reason was because of very little calcium hydroxide in the system, to activate the pozzolanic reaction of fly ash.

Application of High‐Strength Lightweight Concrete in Gob‐Side Entry Retaining in Inclined Coal Seam

Gob‐side entry retaining (GSER) is a popular no‐pillar mining technology that can increase coal recovery rate. We propose the application of high‐strength lightweight (HSLW) concrete to construct the gob‐side support body (GSSB) in NO. 411 inclined working face of Jingang Coal Mine. Firstly, the mechanical model of retained roadway was established, and the calculation for limit angle of GSSB stability and support resistance was mathematically derived. Using the performance test, the optimal proportion of LC50 concrete was determined as follows: the water‐binder ratio was 0.3; the silica fume dosage was not more than 10%; the fly ash dosage was 10–20%; and the sand ratio was 0.45–0.50. Based on theoretical deduction and laboratory analysis, the width of GSSB was obtained to be 0.75 m, and the optimal arrangement of concrete blocks with “two longitudinal and one horizontal, crisscross, and staggered joints” was determined. FLAC3D software was used to study the influence of different widths and material strengths on the surrounding rock deformation and verify the reasonable width and strength of the designed GSSB. Finally, field monitoring of retained roadway shows that the deformation is controlled in a small range, and the retained roadway effect is better, thus proving the feasibility of HSLW for constructing the support body for GSER. Our findings can serve as a theoretical guide for safety and effective implementation of HSLW as GSSB.

A Modified Chemical Index to Predict Fly Ash Dosage for Mitigating Alkali-Silica Reaction

Abstract It is well chronicled that coal fly ash can mitigate the risk of alkali-silica reaction (ASR) in concrete structures. However, the efficacy of individual fly ashes is highly dependent on their chemical composition. Prescriptive requirements for ASR mitigation have largely focused on whether the fly ash is classified as Class F or Class C per ASTM C618, Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete, whereas performance requirements may require extensive testing to satisfactorily demonstrate mitigation potential of a fly ash. Previous research established the need for an accurate model to predict required fly ash replacement rates for ASR mitigation based on the chemical constituents of cement and fly ash. Therefore, a new model is proposed to account for the availability of each chemical constituent from cement and fly ash separately, utilizing ASTM C1260, Standard Test Method for Potential Alkali Reactivity of Aggregates (Mortar-Bar Method), and ASTM C1567, Standard Test Method for Determining the Potential Alkali-Silica Reactivity of Combinations of Cementitious Materials and Aggregate (Accelerated Mortar Bar Method), test data. The successful development of models based on 14-day and 28-day expansion data from these tests is presented in this article.

Capillary suction induced water absorption and chloride transport in non-saturated concrete: The influence of humidity, mineral admixtures and sulfate ions

Concrete structures in marine environments are gravely jeopardized by chloride attack caused by capillary suction. To investigate the erosion of concrete induced by capillary suction under various factors, seawater and NaCl-Na2SO4 solution were used as corrosive solution to obtain the evolution law of water absorbed amount and chloride content in concrete with different water to binder ratio, fly ash and slag dosages and internal relative humidity. The results show that the decrease of internal relative humidity leads to the increase of seawater absorbed by capillary suction in a brief period, and boost rapid growth of chloride content in surface of concrete. For early age (28 d) concrete specimens, when the fly ash dosage is 0–30% or the slag dosage is 0–50%, the water absorption rate and chloride content of concrete are more likely to drop with the dosage increasing. Furthermore, under short-time capillary suction condition, SO42− has a minor impact on water absorption of concrete, while slightly increases the chloride content. However, the chloride content does not evolve coincidently with the change of SO42− concentration. Moreover, within a brief exposure period, the chloride content linearly develops with absorbed sweater amount, and it is inappropriate to use chloride diffusion coefficient to reflect the chloride transport rate of concrete under capillary absorption effect.

Preliminary Study on the Potential Use of Fly Ash as a Ventilated Improved Pit Latrine Additive

This study describes the results of a series of laboratory experiments on aimed at developing of a protocol for the use of fly ash as a pit additive in the ventilated improved pit latrines (VIPs) in South Africa. Model VIP vaults were prepared in the laboratory and filled with synthetic faecal sludge; and dosed with fly ash in amounts ranging from 5 to 20 % (w/w). The maximum leachable chemical oxygen demand (COD) accounted for 3.2 % of the total organic matter, while the maximum leachable ammonium accounted for 1.4 % of the leachable ammonium present in the synthetic faecal sludge. All phosphate in the synthetic faecal material was leachable. Mixing of the 20 % dose of fly ash into the synthetic faecal sludge resulted a minimum 98.9 % reduction (or about two log units) in the faecal coliform concentration after 28 days of incubation. As a result, addition of fly ash can decrease microbial threats to public and environmental health threats from the VIP faecal wastes, but can cause environmental contamination with phosphate. The fly ash dose and the duration of the faecal sludge treatment require further optimisation. Relationship between the chemical composition of the fly ash used as an additive to stabilise the VIP faecal sludge and the leaching of phosphorus from the sludge will also have to be investigated further.

Study on Test and Ratio Optimization of Waste Rock-Fly Ash Aggregate Filling Slurry

To obtain the optimum proportion of fly ash alternative for preparing filling slurry instead of waste rock, the effect of fly ash dosage on the filling body strength is predicted by utilizing BP neural network model on the basis of experimenting the particle size distribution, filling body strength and slurry working characteristics of waste rock-fly ash binary aggregate system under various fly ash dosages. As the results revealed, the waste rock-fly ash binary mixture system attains the densest accumulation when the fly ash dosage is 30%. At this point, the compressive strengths at all ages reach the maximum, according to the BP neural network model predictions. The minimum cost for preparing filling slurry is obtained to be 121 yuan/m3 based on the cost optimization model. At this time, the fly ash dosage is 26%, the cementitious material consumption is 286 kg/m3, the slurry concentration is 81%, the 3d strength is 1.26 MPa, the 7d strength is 1.52 MPa, the 28d strength is 5 MPa, the expansion degree is 51.4 cm, and the bleeding rate 5.7%. Both the filling body strength and slurry fluidity conform to the requirements, while the fly ash dosage is almost identical to that obtained from the particle size distribution and filling body strength tests.

Utilization of industrial and agricultural wastes for productions of sustainable roller compacted concrete pavement mixes containing reclaimed asphalt pavement aggregates

Asphalt pavement recycling has become a common practice across the globe and has been successfully employed in construction of new pavements. While several studies considered utilization of reclaimed asphalt pavement (RAP) aggregates for flexible and rigid pavements, very few attempted its possibility for roller compacted concrete pavements (RCCP). Additionally, studies on the possibility of enhancing the proportion of RAP for RCCP are very scanty. The present study is an attempt to increase the potential of RCCP mixes containing 50% RAP (dust contaminated & stiffened asphalt coated: 50RAP via including various industrial and agricultural wastes such as Silica Fume, Fly ash, and Sugarcane ash as partial replacement of conventional cement. It was observed that the inclusion of the stated admixtures had an insignificant effect on the density of the fresh RCCP mixes, however, increased the moisture demand considerably. In fact, the results firmly indicated the potential of silica fume for RAP-RCCP blends, as, it not only enhanced the physical and mechanical properties, but found to improve the durability of RCCP mixes considerably. Also, utilization of silica fume was found to be economical & environmentally friendly amongst all wastes: with reduced initial construction cost & CO2 emissions by up to 8.4% & 9.7%. As far as the other industrial wastes are concerned, 15% fly ash could also be utilized for producing sustainable RCCP mixes, whereas, higher dosage of fly ash (30%) and sugarcane ash (10 & 15%) may be employed as base layer material of conventional concrete pavements.

Workability and compressive strength properties of normal weight concrete using high dosage of fly ash as cement replacement

This study evaluates the workability and compressive strength properties of normal weight concrete (NWC) using high dosage of fly ash cement replacement. The goal is to find the suitable mix proportion of G40 NWC and to study the slump-loss at different interval time with fly ash replacement up to 60%. The strength properties of NWC were investigated using rebound hammer test as non-destructive and compressive strength test for verifying it. The acceptable mix proportion for fly ash replacement was then investigated using laboratory experiments. For strength properties of NWC, the samples were tested for 3, 7, 14 and 28 days. Results indicate that the inclusion of fly ash as partial cement replacement improves the workability of NWC concrete. The slump-loss of NWC with fly ash increases and it delays the setting time of fresh concrete. The strength of concrete reduces with an increment of fly ash. The strength measured using non-destructive test is lower when compared to compressive strength test. The results demonstrate that the replacement of cement with fly ash in concrete is beneficial and the targeted strength of concrete will achieve with time.

Mechanical Properties of High-Volume Fly Ash Strain Hardening Cementitious Composite (HVFA-SHCC) for Structural Application

Strain-hardening cementitious composite (SHCC) is a kind of construction material that exhibits multiple cracking and strain-hardening behaviors. The partial replacement of cement with fly ash is beneficial to the formation of the tensile strain-hardening property of SHCC, the increase of environmental greenness, and the decrease of hydration heat, as well as the material cost. This study aimed to develop a sustainable construction material using a high dosage of fly ash (no less than 70% of the binder material by weight). Based on the micromechanics analysis and particle size distribution (PSD) optimization, six mixes with different fly ash to cement ratios (2.4–4.4) were designed. The mechanical properties of the developed high-volume fly ash SHCCs (HVFA-SHCCs) were investigated through tensile tests, compressive tests, and flexural tests. Test results showed that all specimens exhibited multiple cracking and strain-hardening behaviors under tension or bending, and the compressive strength of the designed mixes exceeded 30MPa at 28 days, which is suitable for structural applications. Fly ash proved to be beneficial in the improvement of tensile and flexural ductility, but an extremely high volume of fly ash can provide only limited improvement. The HVFA-SHCC mix FA3.2 (with fly ash to binder ratio of about 76% by weight) designed in this study is suggested for structural applications.

Use of fly ash for conditioning the excess activated sludge during deliquefaction at chamber-membrane filter presses

Results of experimental studies of deliquefaction of excess activated sludge from municipal wastewater treatment facilities in chamber-membrane filter presses were presented. To condition the sludge, fly ash from thermal power plants was added as a mineral additive. Experimental studies have shown high efficiency of deliquefaction of the excess activated sludge with addition of fly ash. Increase in the dose of fly ash has led to an overall increase in filtration capacity and a decrease in moisture content in the filter cake. It was found that in the case of adding fly ash in an amount of 2 wt. % of the initial sludge, deliquefaction efficiency did not increase significantly. In this case, filtering time was reduced only by 15 % and specific capacity increased by 12 %. In the case of increasing the proportion of fly ash to 6 wt. % of initial sludge, a more intensive acceleration of the process was observed. Filtering duration was reduced from 100 to 10 min, specific capacity increased from 87 to 400 l/m2∙hr.Experimental studies have shown that addition of fly ash to sediments of municipal wastewater treatment facilities as a mineral component provides the possibility of deliquefaction of the suspensions obtained using a chamber-membrane filter press. With optimal doses of 4–5 wt. % of the mass of initial sludge, filter capacity is expected to be 350–400 l/m2∙hr. The resulting filter cake had moisture content of about 60 %, a dense, dry structure making it possible to transport it in bulk.

Effects of Fineness and Dosage of Fly Ash on the Fracture Properties and Strength of Concrete

This study focuses on evaluating the effects of the fineness of fly ash on the strength, fracture toughness, and fracture resistance of concrete. Three fineness levels of fly ash that respectively pass sieves—no. 175, no. 250, and no. 32—were used. In addition to the control concrete mixture without fly ash, two fly ash replacement levels of 10% and 20% by weight of the cementitious material were selected for the concrete mixture. The experimental results indicate that the compressive strength of the fly ash concrete decreases with the increase in the replacement ratio of fly ash but increases in conjunction with the fineness level of fly ash. The presence of finer fly ash can have beneficial effects on the fracture energy (GF) of concrete at an early age (14 days) and attain a higher increment of GF at a later age (56 days). The concrete containing finer fly ash was found to present larger critical stress intensity factors (KSIC) at various ages, and the KSIC also increases in conjunction with the fineness levels of fly ash.

Feasibility of coal combustion fly ash alone and in combination with gypsum and green manure for reclamation of degraded sodic soils of the Indo‐Gangetic Plains: A mechanism evaluation

AbstractThe disposal of coal combustion fly ash (FA) generated from thermal power plants is a worldwide problem. However, its low bulk density and presence of calcium and iron–aluminium sesquioxide make it a prospective ameliorant alternative to gypsum for amending degraded sodic soils. A field experiment consisting of eight treatments with variable doses of FA and gypsum along with green manure (GM) was conducted for evaluating the amelioration mechanism by measuring changes in pH, exchangeable sodium percentage, zeta (ζ) potential, and properties of meso‐aggregates (0.5–2.0 mm). The results showed that compared with control, FA significantly reduced ζ‐potential, bulk density, and pH. The GM with FA further decreased ζ‐potential and increased hydraulic conductivity. The hydrolysis of iron and aluminium sesquioxide, present in FA, functioned to lower soil pH, whereas the reduction in exchangeable sodium percentage is attributed to bioavailable Ca (27.4%). A lower FA application rate produced better crop yields. FA 2.5% (w/w) alone, compared to the FA 5% (w/w), resulted in higher yield of rice (1.96 tonnes ha−1), whereas 25% gypsum requirement (25GR) + FA 2.5% (w/w) and 25GR + FA 2.5% (w/w) + GM provided wheat yield on par with 50% gypsum requirement (50GR). The study identifies that where gypsum is scarce, the recommended amendment is 25GR + FA 2.5% or 25GR + FA 2.5% + GM when applied for sodic soil to achieve a similar yield response to standard 50GR. If gypsum is unavailable, a one‐time application of FA 2.5% (w/w) alone will improve the productivity of degraded sodic soils corresponding to more exchangeable calcium and organic carbon, which were found to be higher for meso‐aggregates (0.5–2.0 mm).