A colloid, in chemistry, is a mixture in which one substance of microscopically dispersed insoluble particles is suspended throughout another substance. Sometimes the dispersed substance alone is called the colloid, the colloidal suspension refers unambiguously to the overall mixture. Unlike a solution, whose solute and solvent constitute only one phase, a colloid has a dispersed phase, to qualify as a colloid, the mixture must be one that does not settle or would take a very long time to settle appreciably. The dispersed-phase particles have a diameter between approximately 1 and 1000 nanometers, such particles are normally easily visible in an optical microscope, although at the smaller size range, an ultramicroscope or an electron microscope may be required. Homogeneous mixtures with a phase in this size range may be called colloidal aerosols, colloidal emulsions, colloidal foams, colloidal dispersions. The dispersed-phase particles or droplets are affected largely by the surface chemistry present in the colloid, some colloids are translucent because of the Tyndall effect, which is the scattering of light by particles in the colloid.
Other colloids may be opaque or have a slight color, Colloidal suspensions are the subject of interface and colloid science. This field of study was introduced in 1861 by Scottish scientist Thomas Graham, because of the size exclusion, the colloidal particles are unable to pass through the pores of an ultrafiltration membrane with a size smaller than their own dimension. The smaller the size of the pore of the ultrafiltration membrane, the measured value of the concentration of a truly dissolved species will thus depend on the experimental conditions applied to separate it from the colloidal particles dispersed in the liquid. This is particularly important for solubility studies of readily hydrolyzed species such as Al, Eu, Am, Cm, the colloid particles are attracted toward water. They are called reversible sols, hydrophobic colloids, These are opposite in nature to hydrophilic colloids. The colloid particles are repelled by water and they are called irreversible sols. In some cases, a colloid suspension can be considered a homogeneous mixture and this is because the distinction between dissolved and particulate matter can be sometimes a matter of approach, which affects whether or not it is homogeneous or heterogeneous.
The following forces play an important role in the interaction of particles, Excluded volume repulsion. Electrostatic interaction, Colloidal particles often carry a charge and therefore attract or repel each other. The charge of both the continuous and the phase, as well as the mobility of the phases are factors affecting this interaction. Van der Waals forces, This is due to interaction between two dipoles that are permanent or induced. Even if the particles do not have a permanent dipole, fluctuations of the electron density gives rise to a dipole in a particle
For example, when a solid vertical bar is supporting a weight, each particle in the bar pushes on the particles immediately below it. When a liquid is in a container under pressure, each particle gets pushed against by all the surrounding particles. The container walls and the pressure-inducing surface push against them in reaction and these macroscopic forces are actually the net result of a very large number of intermolecular forces and collisions between the particles in those molecules. Strain inside a material may arise by various mechanisms, such as stress as applied by external forces to the material or to its surface. Any strain of a material generates an internal elastic stress, analogous to the reaction force of a spring. In liquids and gases, only deformations that change the volume generate persistent elastic stress, however, if the deformation is gradually changing with time, even in fluids there will usually be some viscous stress, opposing that change. Elastic and viscous stresses are usually combined under the mechanical stress.
Significant stress may exist even when deformation is negligible or non-existent, stress may exist in the absence of external forces, such built-in stress is important, for example, in prestressed concrete and tempered glass. Stress may be imposed on a material without the application of net forces, for example by changes in temperature or chemical composition, stress that exceeds certain strength limits of the material will result in permanent deformation or even change its crystal structure and chemical composition. In some branches of engineering, the stress is occasionally used in a looser sense as a synonym of internal force. For example, in the analysis of trusses, it may refer to the total traction or compression force acting on a beam, since ancient times humans have been consciously aware of stress inside materials. Until the 17th century the understanding of stress was largely intuitive and empirical, with those tools, Augustin-Louis Cauchy was able to give the first rigorous and general mathematical model for stress in a homogeneous medium.
Cauchy observed that the force across a surface was a linear function of its normal vector, moreover. The understanding of stress in liquids started with Newton, who provided a formula for friction forces in parallel laminar flow. Stress is defined as the force across a small boundary per unit area of that boundary, following the basic premises of continuum mechanics, stress is a macroscopic concept. In a fluid at rest the force is perpendicular to the surface, in a solid, or in a flow of viscous liquid, the force F may not be perpendicular to S, hence the stress across a surface must be regarded a vector quantity, not a scalar. Moreover, the direction and magnitude depend on the orientation of S. Thus the stress state of the material must be described by a tensor, called the stress tensor, with respect to any chosen coordinate system, the Cauchy stress tensor can be represented as a symmetric matrix of 3×3 real numbers
Concrete is a composite material composed of coarse aggregate bonded together with a fluid cement that hardens over time. Most concretes used are lime-based concretes such as Portland cement concrete or concretes made with other hydraulic cements, when aggregate is mixed together with dry Portland cement and water, the mixture forms a fluid slurry that is easily poured and molded into shape. The cement reacts chemically with the water and other ingredients to form a matrix that binds the materials together into a durable stone-like material that has many uses. Often, additives are included in the mixture to improve the properties of the wet mix or the finished material. Most concrete is poured with reinforcing materials embedded to provide tensile strength, famous concrete structures include the Hoover Dam, the Panama Canal, and the Roman Pantheon. The earliest large-scale users of technology were the ancient Romans. The Colosseum in Rome was built largely of concrete, and the dome of the Pantheon is the worlds largest unreinforced concrete dome.
Today, large concrete structures are made with reinforced concrete. After the Roman Empire collapsed, use of concrete became rare until the technology was redeveloped in the mid-18th century, concrete is the most widely used man-made material. The word concrete comes from the Latin word concretus, the passive participle of concrescere, from con-. Perhaps the earliest known occurrence of cement was twelve years ago. A deposit of cement was formed after an occurrence of oil shale located adjacent to a bed of limestone burned due to natural causes and these ancient deposits were investigated in the 1960s and 1970s. On a human timescale, small usages of concrete go back for thousands of years and they discovered the advantages of hydraulic lime, with some self-cementing properties, by 700 BC. They built kilns to supply mortar for the construction of houses, concrete floors. The cisterns were kept secret and were one of the reasons the Nabataea were able to thrive in the desert, some of these structures survive to this day.
In the Ancient Egyptian and Roman eras, it was re-discovered that adding volcanic ash to the mix allowed it to set underwater, the Romans knew that adding horse hair made concrete less liable to crack while it hardened, and adding blood made it more frost-resistant. Crystallization of strätlingite and the introduction of pyroclastic clays creates further fracture resistance, german archaeologist Heinrich Schliemann found concrete floors, which were made of lime and pebbles, in the royal palace of Tiryns, which dates roughly to 1400–1200 BC. Lime mortars were used in Greece and Cyprus in 800 BC, the Assyrian Jerwan Aqueduct made use of waterproof concrete
Ettringite is a hydrous calcium aluminium sulfate mineral with formula, Ca6Al2312·26H2O. It is a colorless to yellow mineral crystallizing in the trigonal system, the prismatic crystals are typically colorless, turning white on partial dehydration. It is part of the ettringite-group which includes other sulfates such as thaumasite and bentorite, ettringite was first described in 1874 by J. Lehmann, for an occurrence near the Ettringer Bellerberg volcano, Rheinland-Pfalz, Germany. It occurs within metamorphically altered limestone adjacent to igneous rocks or within xenoliths. It occurs as weathering crusts on larnite in the Hatrurim Formation of Israel and it occurs associated with portlandite and hydrocalumite at Scawt Hill and with afwillite, hydrocalumite and gypsum in the Hatrurim Formation. In concrete chemistry ettringite is a hexacalcium aluminate trisulfate hydrate, of general formula, ettringite is formed in hydrated Portland cement system as a result of the reaction of calcium aluminate with calcium sulfate, both present in Portland cement.
Ettringite, the prominent representative of AFt phases or, can be synthesized in the laboratory by reacting stoichiometric amounts of calcium, aluminium. When the ratio is intermediate, only a portion of the converts to AFm. It represents a group of calcium sulfoaluminate hydrates, AFt has the general formula 2·X3·nH2O where X represents a doubly charged anion or, two singly charged anions. Ettringite is the most common and important member of the AFt group, AFm, abbreviation for alumina, ferric oxide, mono-sulfate or. It represents another group of calcium aluminate hydrates with general formula ·X·nH2O where X represents a singly charged anion or half a doubly charged anion, X may be one of many anions. The most important anions involved in Portland cement hydration are hydroxyl, the mineral ettringite has a structure that runs parallel to the c axis -the needle axis-, in the middle of these two lie the sulfate ions and H2O molecules, the space group is P31c. Ettringite crystal system is trigonal, crystals are elongated and in a needle like shape, occurrence of disorder or twining is common, which affects the intercolumn material.
The first X-ray study was done by Bannister, Hey & Bernal, from observations on dehydration and chemical formulas there were suggestions of the structure being composed of Ca2+ and Al63−, were between them lie SO42− ions and H2O molecules. Further X-ray studies ensued, namely Wellin which determined the structure of thaumasite. Further studies conducted on synthetic ettringite by use of x-ray and powder diffraction confirmed earlier assumptions, upon analyzing the structure of both ettringite and thaumasite, it was deduced that both minerals have hexagonal structures, but different space groups. Ettringite crystals have a P31c with a=11.224 Å, c=21,108 Å, while thaumasite crystals fall into space group P63 with a=11.04 Å, c=10.39 Å. While these two form a solid solution, the difference in space groups lead to discontinuities in unit cell parameters
In other words, compressive strength resists compression, whereas tensile strength resists tension. In the study of strength of materials, tensile strength, compressive strength, some materials fracture at their compressive strength limit, others deform irreversibly, so a given amount of deformation may be considered as the limit for compressive load. Compressive strength is a key value for design of structures, Compressive strength is often measured on a universal testing machine, these range from very small table-top systems to ones with over 53 MN capacity. Measurements of compressive strength are affected by the specific test method, Compressive strengths are usually reported in relationship to a specific technical standard. When a specimen of material is loaded in such a way that it extends it is said to be in tension, on the other hand, if the material compresses and shortens it is said to be in compression. On an atomic level, the molecules or atoms are forced apart when in tension whereas in compression they are forced together.
Since atoms in solids always try to find an equilibrium position, the phenomena prevailing on an atomic level are therefore similar. Tension tends to pull small sideways deflections back into alignment, while compression tends to amplify such deflection into buckling, Compressive strength is measured on materials and structures. By definition, the compressive strength of a material is that value of uniaxial compressive stress reached when the material fails completely. The compressive strength is usually obtained experimentally by means of a compressive test, the apparatus used for this experiment is the same as that used in a tensile test. However, rather than applying a uniaxial tensile load, a uniaxial compressive load is applied, as can be imagined, the specimen is shortened as well as spread laterally. In a compression test, there is a region where the material follows Hookes Law. Hence for this region σ = E ϵ where this time E refers to the Youngs Modulus for compression, in this region, the material deforms elastically and returns to its original length when the stress is removed.
This linear region terminates at what is known as the yield point, above this point the material behaves plastically and will not return to its original length once the load is removed. There is a difference between the stress and the true stress. By its basic definition the uniaxial stress is given by, σ = F A where, F = Load applied, A = Area As stated, in reality therefore the area is some function of the applied load i. e. Indeed, stress is defined as the force divided by the area at the start of the experiment, in engineering design practice, professionals mostly rely on the engineering stress. In reality, the stress is different from the engineering stress
The term is used to distinguish this process from the more conventional construction practice of transporting the basic materials to the construction site where all assembly is carried out. The term prefabrication applies to the manufacturing of other than structures at a fixed site. Prefabricated parts of the body of the machine may be called sub-assemblies to distinguish them from the other components, an example from house-building illustrates the process of prefabrication. The conventional method of building a house is to transport bricks, cement, sand and construction aggregate, etc. to the site, and to construct the house on site from these materials. The method finds application particularly where the structure is composed of repeating units or forms, prefabrication has been used since ancient times. Assembling sections in factories saved time on-site and the lightness of the reduced the cost of foundations. The Crystal Palace, erected in London in 1851, was a visible example of iron and glass prefabricated construction.
It can be difficult to construct the required to mould concrete components on site. Prefabricating steel sections reduces on-site cutting and welding costs as well as the associated hazards, prefabrication techniques are used in the construction of apartment blocks, and housing developments with repeated housing units. The quality of prefabricated housing units had increased to the point that they may not be distinguishable from traditionally built units to those live in them. The technique is used in office blocks and factory buildings. Prefabricated steel and glass sections are used for the exterior of large buildings. Detached houses, log cabin, etc. are sold with prefabricated elements, wood construction in particular benefits from the improved quality. However, tradition often favors building by hand in many countries, prefabrication saves engineering time on the construction site in civil engineering projects. This can be vital to the success of such as bridges and avalanche galleries. Prefabricated bridge elements and systems offer bridge designers and contractors significant advantages in terms of time, environmental impact, constructibility.
Prefabrication can minimize the impact on traffic from bridge building. Additionally, commonly used such as concrete pylons are in most cases prefabricated
Formwork is the term given to either temporary or permanent molds into which concrete or similar materials are poured. In the context of concrete construction, the falsework supports the shuttering molds, Formwork comes in several types, Traditional timber formwork. The formwork is built on site out of timber and plywood or moisture-resistant particleboard and it is easy to produce but time-consuming for larger structures, and the plywood facing has a relatively short lifespan. It is still used extensively where the costs are lower than the costs for procuring reusable formwork. It is the most flexible type of formwork, so even where other systems are in use and this formwork is built out of prefabricated modules with a metal frame and covered on the application side with material having the wanted surface structure. The two major advantages of formwork systems, compared to traditional timber formwork, are speed of construction and these interlocking and modular systems are used to build widely variable, but relatively simple, concrete structures.
The panels are lightweight and very robust and they are especially suited for similar structure projects and low-cost, mass housing schemes. This formwork is assembled on site, usually out of insulating concrete forms, ″Coffor″ is a structural stay-in-place formwork system to build constructions in concrete. It is composed of two filtering grids reinforced by vertical stiffeners and linked by articulated connectors that can be folded for transport. A standard panel 1.10 m x 2.70 m weighs 32.7 kg, after Coffor is placed, concrete is poured between the grids, excess water of concrete is eliminated by gravity and air is eliminated. Coffor remains in the construction after concrete is poured and acts as reinforcement, any type of construction can be built with Coffor, individual houses, multi-story buildings including high-rise buildings, commercial or administrative buildings. Several types of works can be done with Coffor. Coffor is delivered completely assembled from the factory, no assembly is necessary on the construction site.
This formwork is assembled on site, usually out of prefabricated fiber-reinforced plastic forms and these are in the shape of hollow tubes, and are usually used for columns and piers. Some of the earliest examples of concrete slabs were built by Roman engineers, because concrete is quite strong in resisting compressive loads, but has relatively poor tensile or torsional strength, these early structures consisted of arches and domes. The most notable concrete structure from this period is the Pantheon in Rome, to mould this structure, temporary scaffolding and formwork or falsework was built in the future shape of the structure. These building techniques were not isolated to pouring concrete, but were and are used in masonry. Similar to the method, but stringers and joist are replaced with engineered wood beams
Industrial autoclaves are pressure vessels used to process parts and materials which require exposure to elevated pressure and temperature. The manufacture of components from advanced composites often requires autoclave processing. An autoclave applies both heat and pressure to the workload placed inside of it, there are two classes of autoclave. Those pressurized with steam process workloads which can withstand exposure to water, while circulating heated gas provides greater flexibility, processing by autoclave is far more costly than oven heating and is therefore generally used only when isostatic pressure must be applied to a workload of comparatively complex shape. For smaller flat parts, heated presses offer much shorter cycle times, in other applications, the pressure is not required by the process but is integral with the use of steam, since steam temperature is directly related to steam pressure. Rubber vulcanizing exemplifies this category of autoclaving, the hydroclave is pressurized with water, the pressure keeps the water in liquid phase despite the high temperature.
The key component of the autoclave is the fast-opening door. On one hand, the operator must be able to open and close the door quickly and easily, on the other, such is the quality of autoclave door design that the US experiences as few as an estimated five or six autoclave failures annually. Autoclave design is driven by various safety standards, foremost among which is the ASME Pressure Vessel Code, while most nations use the ASME code, some have developed their own. The CE standard in Europe applies to vessels as well as to electrical controls, all codes specify conservative requirements intended to maximize safety. Local governments may impose licensing requirements related to autoclave operation, Pressure vessel design involves Barlows formula, used to calculate the required wall thickness. However, the design of a complex pressure containment system involves much more than the application of this formula, for almost all pressure vessels, the ASME code stipulates the requirements for design and testing.
Prior to delivery, the vessel is hydrostatically tested at 130% of its rated pressure under the supervision of an ASME code inspector. It is filled with water, and a small pump raises the pressure to the necessary test value, the inspector checks for leaks as well as evidence of flaws or inadequacies in the welding. The design of small autoclaves need not take into consideration the possibility of drawing a vacuum inside the pressure vessel, steam autoclaves, for example, can be exposed to an internal vacuum if the steam fully condenses while the vessel remains sealed. Although external pressure cannot exceed one atmosphere, that can suffice to collapse the vessel in some cases, in unusual situations, the autoclave itself might have to be square or rectangular instead of round, or it might be vertical instead of horizontal. If the autoclave is unusually large, it may have to be set into an excavation in the floor if there is to be floor level loading, the selection of the materials from which the autoclave is fabricated turns entirely upon the application.
For steam autoclaves, carbon steel is used, but an allowance is added to the calculated thickness
Fly ash, known as pulverised fuel ash in the United Kingdom, is one of the coal combustion products, composed of the fine particles that are driven out of the boiler with the flue gases. Ash that falls in the bottom of the boiler is called bottom ash, in modern coal-fired power plants, fly ash is generally captured by electrostatic precipitators or other particle filtration equipment before the flue gases reach the chimneys. Together with bottom ash removed from the bottom of the boiler, in the past, fly ash was generally released into the atmosphere, but air pollution control standards now require that it be captured prior to release by fitting pollution control equipment. In the US, fly ash is stored at coal power plants or placed in landfills. About 43% is recycled, often used as a pozzolan to produce hydraulic cement or hydraulic plaster, pozzolans ensure the setting of concrete and plaster and provide concrete with more protection from wet conditions and chemical attack. Coal Combustion Residuals are listed in the subtitle D, in that case the ash produced is often classified as hazardous waste.
Fly ash material solidifies while suspended in the exhaust gases and is collected by electrostatic precipitators or filter bags. Since the particles solidify rapidly while suspended in the exhaust gases, fly ash particles are spherical in shape. The major consequence of the cooling is that few minerals have time to crystallize. Nevertheless, some refractory phases in the coal do not melt. In consequence, fly ash is a heterogeneous material, siO2, Al2O3, Fe2O3 and occasionally CaO are the main chemical components present in fly ashes. The mineralogy of fly ashes is very diverse, the main phases encountered are a glass phase, together with quartz and the iron oxides hematite, magnetite and/or maghemite. Other phases often identified are cristobalite, free lime, calcite, halite, portlandite and anatase. The Ca-bearing minerals anorthite, gehlenite and various calcium silicates, the mercury content can reach 1 ppm, but is generally included in the range 0.01 -1 ppm for bituminous coal. The concentrations of trace elements vary as well according to the kind of coal combusted to form it.
In fact, in the case of coal, with the notable exception of boron. Two classes of fly ash are defined by ASTM C618, Class F fly ash, the chief difference between these classes is the amount of calcium, silica and iron content in the ash. The chemical properties of the fly ash are largely influenced by the content of the coal burned