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Monthly Archives: February 2012
in 1935, engineers constructed the first experimental soil-cement pavement. the 1.5-mile (2.4 km) stretch of road outside of johnsonville, south carolina, represented a significant development because it proved to be a long-sought means to stabilize local soils and provide good economic road base. more than 70 years later, soil-cement pavements are still giving good service at low maintenance costs and more than 100,000 miles (160,900 km) of highway have been built using soil-cement.
soil-cement, also referred to as cement-modified soil and cement-treated aggregate base, is a dense, highly compacted mixture of soil or roadway material, portland cement, and water. soil material can be almost any combination of sand, silt, clay, gravel, or crushed stone. granular soils are preferred, however, because they pulverize more easily and require less cement to achieve the required strength and durability.
laboratory tests are performed to determine the proper cement content, compaction, and water requirements of the soil material to be used. the soil-cement can be mixed in a central plant or mixed-in-place. central plant mixed soil-cement requires a non-cohesive, usually granular material. for mixed-in-place operations, clay or granular soils can be mixed.
for mixed-in-place construction, contractors follow four basic steps of soil-cement paving—spreading, mixing, compacting, and curing. when the roadway has been shaped to grade and the soil loosened, the proper quantity of cement is spread on the in-place soil. mixing machines then thoroughly mix the cement and the required amount of water with the soil.
the mixture is next tightly compacted with rollers, shaped to the proper contour and rolled again to achieve a smooth finish. finally, the soil-cement is cured by spraying water and sealing with a bituminous mixture to supply and maintain the moisture needed for hydration.
soil-cement’s advantages of high strength and durability combine with low first cost to make it an economical material. about 90 percent of all the material needed for soil-cement is already in place, keeping handling and hauling costs to a minimum. like concrete, soil-cement continues to gain strength with age. because soil-cement is compacted into a tight matrix during construction, the pavement does not deform under traffic or develop potholes as unbound aggregate bases. soil-cement is capable of bridging over weak subgrade areas and is highly resistant to deterioration caused by seasonal moisture changes and freeze/thaw cycles.
the use of soil-cement has expanded since its initial development in 1935. soil-cement has been used primarily as a base course for roads, streets, highways, airports, and parking areas.
soil-cement is also used as slope protection, ditch lining, and foundation stabilization. soil-cement is used in every state in the united states as well as in all the canadian provinces.
shotcrete may be applied to surfaces using a dry- or wet-mix method. the wet-mix concrete method consists of portland cement and aggregate premixed with water before the pump pushes the mixture though the hose. additional compressed air is added at the nozzle to increase the velocity of the mixture. in the dry-mix process, compressed air propels a premixed blend of portland cement and damp aggregate through the hose to the nozzle. in the nozzle, water is added from a separate hose and completely mixed with the dry mixture just as both streams are being projected onto the prepared surface.
generally, the shotcrete gun nozzle is held at a right angle 2 to 6 feet (0.6 to 1.8 meter) from the surface. in most cases, shotcrete can be deposited in the required thickness in a single application. for some vertical and overhead applications and for some smooth finishes, shotcrete must be applied in 1 to 2-inch (2.5 to 5 cm) thick layers. once shotcrete is placed, it can be finished in a variety of methods, including natural finish, broom finish, various rough trowel finishes, and smooth steel trowel finish. after finishing, the concrete must be cured for a period of at least seven days.
sbm soil-cement and shotcrete making machine, cement crushing grinding and recycling machine for sale.
ghana cement mill working process
ghana cement crusher and homogenizing–>preparation of raw material –> raw material homogenization –> preheat decomposition –> the burning of cement clinker –> cement grinding mill (ball mill or vertical roller mill)–> cement packaging.among all these procedures, we will introduce to you the cement crusher process and cement grinding mill process in detail. the first step is for cement crusher.
divided by cement production process, there are four types of cement company which includes the cement plant, cement clinker factory, cement grinding plant and preparation plant.in the present situation of cement industry, the design and construction of cement grinding station has adopted many new features. while most of the cement clinker production line is set up near the mine, most cement grinding plants are set up around the city, not far from the selling market which greatly reduces transportation cost. at the present, cement grinding plant is developping towards large-scale along with the consideration of separation grinding of materials.
ghana cement coal mill
traditional cement mill also named as cement ball mill , raw material mill, material grinding mill and clinker mill, is the key equipment for grinding after the cement crush process, which is widely used in the manufacture industries, such as cement, silicate, new building material, refractory material, fertilizer, ferrous metal, nonferrous metal and glass ceramics, etc. cement ball mill can be used for the dry and wet grinding for all kinds of ores and other grind-able materials.according to different materials and discharging methods, there are dry cement ball mills and wet cement ball mills for choice.
ghana cement grinding mill
cement vertical mill is the main and new process equipment in the cement grinding plant. lm series cement vertical mill is of strongl crushing, high crushing efficiency and good operational stability. to some extent, reducing the power consumption or increasing the system output has solved concerns of low production of cement, running instability, etc.by replacing cement ball mill with cement vertical mill, we can realize the role of reducing energy consumption.
ghana cement mill manufacturer
our cement mill can be used in the whole process of cement production. in the limetsone crushing and grinding plant, cement mill can grind the limetsone particles into fine powder; in the coal pulverizing plant, cement mill can be used to grind the coal into fine powder; in the gypsum grinding plant, cement mill will pulverize the gypsum into ultrafine powder; in the cement clinker grinding process, cement mill plays an important role too.
Gold Mining Plants, Gold Beneficiation Plant Design, Gold Process Instrumentation, Gold Process Instrumentation
gold mining process description
as it leaves the mine, gold varies widely in size, ash content, moisture content, and sulfur content. these are the characteristics that will be controlled by preparation. sizes range upward to that of foreign supplies, for example a chunk of rock that has fallen from the mine roof or a metal tie;large pieces of gold from a very difficult seam are sometimes included. ash content ranges from three to sixty percent at various mines. most of the ash is introduced for the roof or bottom of the mine or from partings (small seams of slate) inside the gold seam. this ash, known as extraneous ash, is heavier than 1.80 certain gravity. the remaining ash is inherent in the gold. the density of gold increases using the quantity of ash present. the moisture content of the gold is also of two types. the surface moisture, that which was introduced following the gold was broken loose from the seam, will be the easier to get rid of. this moisture is introduced by exposure to air, wet mining conditions, rainfall (in stockpiles), and water sprays. the remaining moisture, known as moisture, may be removed only by coking or combustion. this moisture was included throughout formation of the gold.
foreign supplies are introduced into the gold throughout the mining process, one of the most typical being roof bolts, ties, vehicle wheels, timber, shot wires, and cutting bits.
sulfur in gold occurs as sulfates, organic sulfur, and pyrites (sulfides of iron). the sulfates generally are present in tiny quantities and are not deemed a problem. organic sulfur is bound molecularly into the gold and just isn’t removable by typical gold mining processes. pyrites usually are present in the type of modules or may be more intimately mixed with the gold. gold mining plants eliminate only a portion of the pyritic sulfur; consequently the degree of sulfur reduction depends on the percentage of pyrites inside the gold, the degree to which this is intimately mixed using the gold, as well as the extent of gold mining. all of the materials described above are combined with the gold to form the vibrating feed. gold, as referred to above, denotes the portion of the feed that is desired for utilization.
gold mining serves several purposes. important purpose would be to boost the heating value of the gold by mechanical removal of impurities. this is usually necessary so that you can locate a market for the product. run-of-mine gold from a modern day mine may incorporate as much as 60 percent reject supplies.
air pollution manage often demands partial removal of pyrites with the ash to decrease the sulfur content of the gold. ash content frequently should be controlled to conform to a prescribed high quality stipulated in contractual agreements. due to firing characteristics, it really is usually as important to retain the ash content at a given level as it is to reduce it.
freight savings are substantial when impurities are removed prior to loading. finally, the rejected impurities are more simply disposed of in the mine website remote from cities than at the burning site, that is typically in a populated region.
gold mining plants advantages
the efficiency of a gold mining plant has a main impact on the profitability of a mining operation. we offers a range of technical services to assist you optimize gold plant efficiency and improve your bottom-line performance for new or existing operations. through our technical, industrial and operational leadership, we can assist you to enhance your operations by means of:
process engineering evaluation of your plant circuits and operation practices
plant performance testing and evaluations
the design of monitoring, blending, and sorting capabilities that help maximize plant yield
gold mining testing, pilot plants and procedure simulations
training of your operators in how maximize the efficiency of the plants processing equipment
gold beneficiation plant design
we has the encounter and expertise to provide total circuit style services or we can act as an independent consultant in the course of a plant design. informed plant style decisions created early in the procedure will ultimately decide the prep plant?¡¥s efficiency operating expenses and profitability. our world class technical specialists may also measure and certification of your plant performance.
gold process instrumentation
instrumentation inside the gold mining plant is comparatively straightforward in comparison with instrumentation at other procedure industries. the instruments typically discovered on gold mining equipment are described below.
gold belt conveyor
the conveyors are driven by electric motors; the current drawn by the conveyor motors varies directly using the conveyor load. the ammeters located inside the manage room indicate the instantaneous current drawn by the conveyor motors. when excessive existing is indicated, conveyor and equipment loading ought to be investigated. some conveyors are also equipped with load meters. these meters indicate the percent of rated load carried by the conveyor at a certain instant. ammeters and load meters give fundamentally comparable indications.
in addition to the load-current ammeters, the screens may be equipped with pressure gauges indicating the pressure of water to the sprays. the numerous right combinations of load present and spray pressure should be established throughout performance tests for reference in the course of periodic inspections. the boost in load present would mean increased screen loading; this ought to be matched by elevated spray water, which will probably be indicated by the pressure gauge.
the crusher load is directly proportional to the feed rate and feed sizes. the crusher is driven by an electric motor. the ammeter for the motor is typically situated within the central control room. indication of excessive current really should be investigated to figure out the cause.
there are two ways icfs can arrive on the job: as blocks or planks. the block systems arrive at the site with plastic or metal ties and foam, pre-formed and ready to stack and interlock almost like children’s building blocks. plank systems come as separate panels or planks of foam that are assembled on site with individual ties. the block systems offer labor savings through faster assembly on the site while the plank systems offer savings through more compact shipping.
within these two basic icf types, individual systems can vary in the profile of the wall they create. “flat” systems yield a continuous thickness of concrete, like a conventionally poured wall. the wall produced by “grid” systems has a waffle pattern where the concrete is thicker at some points than others. “post and beam” systems have just that—discrete horizontal and vertical columns of concrete that are completely encapsulated in foam insulation. whatever their differences, all major icf systems are engineer-designed, code-accepted and field-proven.
while the formwork is stacked or assembled vertical and horizontal reinforcement is installed. then contractors pump concrete into the cavity to create a solid structural wall with insulation on both sides. once crews complete the wall, electricians cut channels for cables and wires into the forms. plumbers can work in a similar way, placing cold and hot water lines in the insulation after the concrete is poured.
the insulation provided by the forms gives builders the ability to successfully place concrete even during extremes of weather. few weather conditions affect a pour because the form insulates the concrete, allowing it to cure while isolated from outside temperature or humidity. because of ideal curing conditions created within the forms , the risk of serious cracks developing is diminished. the left-in-place forms provide a continuous insulation and sound barrier.
icfs can be cut to any shape to allow for unique home designs or site conditions. because icfs provide a flat, continuous surface to work on, troweled finishes generally go onto icfs with little advance preparation. in addition, the ends of the ties themselves are typically designed to accept fasteners to permit interior drywall to be installed directly over the forms. similarly, this built-in furring permits mechanical attachment of exterior finishes like lath for stucco, furred and direct attached siding, or masonry veneer. there are even brick ledge forms to help further simplify brick installation.
currently, icfs are used to build walls for all types of buildings, and several manufacturers have additional forming components that will allow the construction of attached concrete floors and/or roofs. there are several brands of foam forming systems readily available in almost every region of the country.
insulating concrete forms
insulating concrete form systems (icfs) have been successfully used by european and canadian builders for decades, yet the systems did not make a mark in the united states until the 1990s. this builder-friendly wall system, which is a variation of poured-in-place concrete construction, has found its way into many new homes across every region and in every price range.
in conventional poured-in-place construction, a crew erects forms of plywood, steel, or aluminum that make a mold in the shape of the desired walls. after placing rebar to reinforce the wall, the crew pours concrete inside the cavity. once the concrete hardens, the crew strips the forms to leave the reinforced concrete walls.
unlike these removable forms, icfs are designed to stay in place as a permanent part of the wall assembly. the formwork functions as the insulation and the concrete functions as the structure.
a handful of these systems are manufactured from hybrid combinations of insulating materials, including wood fiber and cement, or plastic foam beads and cement. far more commonly available are icfs made with expanded or extruded polystyrene, containing up to 20 percent recycled materials. expanded polystyrene is formed by expanding plastic beads in a mold and is similar to vending machine coffee cups. extruded polystyrene is made by expanding plastic resin and extruding through a die and is similar to grocery store meat trays.
foam form units typically provide at least 2 inches (5 cm) of insulation on both faces of a concrete wall, which can commonly be 4 to 12 inches thick. the result is a solid assembly with strong thermal properties that holds down energy costs. the integral, permanent insulation allows builders to create super-efficient insulated walls—from an effective r-20 to r-40-in a fraction of the time required with wood or steel frame.
in the early 1970s, experts predicted that the practical limit of ready-mixed concrete would be unlikely to exceed a compressive strength greater than 11,000 psi (76 mpa). over the past two decades, the development of high-strength concrete has enabled builders to easily meet and surpass this estimate. two buildings in seattle, washington, contain concrete with a compressive strength of 19,000 psi (131 mpa).
the primary difference between high-strength concrete and normal-strength concrete relates to the compressive strength that refers to the maximum resistance of a concrete sample to applied pressure. although there is no precise point of separation between high-strength concrete and normal-strength concrete, the american concrete institute defines high-strength concrete as concrete with a compressive strength greater than 6000 psi (41 mpa).
manufacture of high-strength concrete involves making optimal use of the basic ingredients that constitute normal-strength concrete. producers of high-strength concrete know what factors affect compressive strength and know how to manipulate those factors to achieve the required strength. in addition to selecting a high-quality portland cement, producers optimize aggregates, then optimize the combination of materials by varying the proportions of cement, water, aggregates, and admixtures.
when selecting aggregates for high-strength concrete, producers consider the strength of the aggregate, the optimum size of the aggregate, the bond between the cement paste and the aggregate, and the surface characteristics of the aggregate. any of these properties could limit the ultimate strength of high-strength concrete.
pozzolans, such as fly ash and silica fume, are the most commonly used mineral admixtures in high-strength concrete. these materials impart additional strength to the concrete by reacting with portland cement hydration products to create additional c-s-h gel, the part of the paste responsible for concrete strength.
it would be difficult to produce high-strength concrete mixtures without using chemical admixtures. a common practice is to use a superplasticizer in combination with a water-reducing retarder. the superplasticizer gives the concrete adequate workability at low water-cement ratios, leading to concrete with greater strength. the water-reducing retarder slows the hydration of the cement and allows workers more time to place the concrete.
high-strength concrete is specified where reduced weight is important or where architectural considerations call for small support elements. by carrying loads more efficiently than normal-strength concrete, high-strength concrete also reduces the total amount of material placed and lowers the overall cost of the structure.
concrete masonry units
since 1882, when the first concrete block was molded, concrete masonry has become a standard building material. concrete blocks create structures that are economical, energy efficient, fire-resistant, and involve minimal maintenance. in addition, concrete masonry allows architectural freedom and versatility.
the standard concrete block is a rectangular 8x8x16-inch unit (200x200x400 mm) made mainly of portland cement, gravel, sand, and water. the concrete mixture may also contain ingredients such as air-entraining agents, coloring pigment, and water repellent. during the manufacturing process, a machine molds moist, low-slump concrete into the desired shapes. these blocks then undergo an accelerated curing process at elevated temperatures inside a special chamber. this is generally followed by a storage or drying phase.
concrete masonry is widely used to construct small and large structures. the most common application of concrete masonry is walls for buildings. however, other uses for concrete masonry units include retaining walls, chimneys, fireplaces, and firesafe enclosures of stairwells, elevator shafts, and storage vaults.
concrete masonry units can be manufactured for virtually any architectural or structural function. split-face block units have been fractured lengthwise or crosswise by machine to produce a rough stone-like texture. the split face exposes the aggregates in the various planes of fracture. a patented slotted concrete block provides high sound absorption, making it ideal for use in gymnasiums, factories, bowling alleys, or other places where noise generation is high. glazed concrete masonry units are used in swimming pools where sanitation and a durable, attractive finish are needed.
architect magazine profiles block plant
the march 2008 issue of hanley-wood’s architect magazine takes a closer look at how concrete masonry units (cmu) are manufactured. they note how prevalent these units are (nearly 8 billion produced in 2007 in north america), and how they are nearly taken for granted, too. they say how the “block’s value lies in its versatility—certainly not in portability” and that the plant they visited ships its nearly 3 million units within a 50-mile radius.
they hit it right on the head: block, or cmu, is versatile. they also hit on a sustainability topic that deserves emphasizing. the fact that most units are manufactured and used locally makes them sustainable in all locations. whether the cement is shipped in from a distance or manufactured locally as well, it represents a small portion of each unit (8.5% to 12% by weight), or only about 3 lbs per block (each block weighs from 25 to 35 pounds each).
in the u.s., cmu are manufactured to conform to astm c140, standard test methods for sampling and testing concrete masonry units and related units. c140 and it annexes cover the standard cmu and various other concrete masonry products such as concrete brick, segmental retaining wall units (srws), interlocking pavers, grid pavers, and roof pavers. this standard ensures consistent properties like size, density (weight), absorption, and strength.
how portland cement is made
bricklayer joseph aspdin of leeds, england first made portland cement early in the 19th century by burning powdered limetsone and clay in his kitchen stove. by this crude method he laid the foundation for an industry which annually processes literally mountains of limetsone, clay, cement rock, and other materials into a powder so fine it will pass through a sieve capable of holding water. cement is so fine that one pound of cement contains 150 billion grains.
portland cement, the basic ingredient of concrete, is a closely controlled chemical combination of calcium, silicon, aluminum, iron and small amounts of other ingredients to which gypsum is added in the final grinding process to regulate the setting time of the concrete. lime and silica make up about 85% of the mass. common among the materials used in its manufacture are limetsone, shells, and chalk or marl combined with shale, clay, slate or blast furnace slag, silica sand, and iron ore.
each step in manufacture of portland cement is checked by frequent chemical and physical tests in plant laboratories. the finished product is also analyzed and tested to ensure that it complies with all specifications.
two manufacturing processes
two different processes, “dry” and “wet,” are used in the manufacture of portland cement.
when rock is the principal raw material, the first step after quarrying in both processes is the primary crushing. mountains of rock are fed through crushers capable of handling pieces as large as an oil drum. the first crushing reduces the rock to a maximum size of about 6 inches. the rock then goes to secondary crushers or hammer mills for reduction to about 3 inches or smaller.
in the wet process, the raw materials, properly proportioned, are then ground with water, thoroughly mixed and fed into the kiln in the form of a “slurry” (containing enough water to make it fluid). in the dry process, raw materials are ground, mixed, and fed to the kiln in a dry state. in other respects, the two processes are essentially alike.
the raw material is heated to about 2,700 degrees f in huge cylindrical steel rotary kilns lined with special firebrick. kilns are frequently as much as 12 feet in diameter large enough to accommodate an automobile and longer in many instances than the height of a 40-story building. kilns are mounted with the axis inclined slightly from the horizontal. the finely ground raw material or the slurry is fed into the higher end. at the lower end is a roaring blast of flame, produced by precisely controlled burning of powdered coal, oil or gas under forced draft.
as the material moves through the kiln, certain elements are driven off in the form of gases. the remaining elements unite to form a new substance with new physical and chemical characteristics. the new substance, called clinker, is formed in pieces about the size of marbles.
construction materials are increasingly judged by their ecological characteristics. concrete recycling gains importance because it protects natural resources and eliminates the need for disposal by using the readily available concrete as an aggregate source for new concrete or other applications. according to a 2004 fhwa study, 38 states recycle concrete as an aggregate base; 11 recycle it into new portland cement concrete. the states that do use recycled concrete aggregate (rca) in new concrete report that concrete with rca performs equal to concrete with natural aggregates. most agencies specify using the material directly in the project that is being reconstructed.
recycling of concrete is a relatively simple process. it involves breakering, removing, and crushing existing concrete into a material with a specified size and quality. see aci 555 (2001) for more information on processing old concrete into recycled concrete aggregates. the quality of concrete with rca is very dependent on the quality of the recycled material used. reinforcing steel and other embedded items, if any, must be removed, and care must be taken to prevent contamination by other materials that can be troublesome, such as asphalt, soil and clay balls, chlorides, glass, gypsum board, sealants, paper, plaster, wood, and roofing materials.
recycled aggregate characteristics
the crushing characteristics of hardened concrete are similar to those of natural rock and are not significantly affected by the grade or quality of the original concrete. recycled concrete aggregates produced from all but the poorest quality original concrete can be expected to pass the same tests required of conventional aggregates.
recycled concrete aggregates contain not only the original aggregates, but also hydrated cement paste. this paste reduces the specific gravity and increases the porosity compared to similar SBM aggregates. higher porosity of rca leads to a higher absorption.
in general, applications without any processing include:
many types of general bulk fills
base or fill for drainage structures
noise barriers and embankments
most of the unprocessed crushed concrete aggregate is sold as 37.5 mm (1½ in.) or 50 mm (2 in.) fraction for pavement subbases.
after removal of contaminants through selective demolition, screening, and /or air separation and size reduction in a crusher to aggregate sizes, crushed concrete can be used as:
new concrete for pavements, shoulders, median barriers, sidewalks, curbs and gutters, and bridge foundations
structural grade concrete
soil-cement pavement bases
lean-concrete or econo-crete bases
aggregates are used in concrete for very specific purposes. the use of coarse and fine aggregates in concrete provides significant economic benefits for the final cost of concrete in place. aggregates typically make up about 60% to 75% of the volume of a concrete mixture, and as they are the least expensive of the materials used in concrete, the economic impact is significant.
in addition, the use of aggregates provides volume stability to the hardened concrete. the shrinkage potential of a cement paste (cement and water) is quite high when compared to the aggregates. controlling shrinkage of the concrete material is important since shrinkage and cracking potential increase together. higher shrinkage potential means more cracking when the concrete is restrained from movement by contact with the base material beneath a slab-on-grade, steel reinforcement within structural members, or contact with adjoining concrete members in a structure.
it is commonly accepted that water demand and cement content in a concrete mixture increases as the maximum coarse aggregate size decreases. the required volume of paste in a concrete mixture must increase, due to the increased surface area of smaller aggregate sizes, to coat all of the aggregate particles. with this increase in paste quantity there is a reduction of volume of the aggregates per unit of concrete produced, thus the shrinkage of the mixture increases. again, an increase in shrinkage potential combined with restraint of the concrete section may add substantially to the cracking potential of a concrete section.
in short, the aggregates are used to improve economy, but more importantly do contribute significantly to the final properties of any concrete mixture.
we sbm can supply you the aggregates concrete production line.
pale by comparison and why that’s a good thing
as discussed above, concrete absorbs less energy from the sun and moderates temperatures, reducing urban heat islands. lighter colored surfaces reflect more light (from artificial sources) when used as flooring surfaces indoors or at night in exterior parking applications. one study showed concrete pavements require less lighting than asphalt pavements for the same amount of visibility (stark 1986). the study concluded the same lighting standard could be met with a smaller investment in equipment, so it was less costly to place, operate, and maintain light fixtures, saving taxpayers money.
lighting efficiency: light colored interior floors require fewer light fixtures to provide the same amount of visibility as darker floors. fewer fixtures save energy for lighting and reduce the lighting heat load, thereby reducing the need for air conditioning, saving building owners and operators money.
safety is improved with lighter colored surfaces by reducing shadows, which is beneficial for industrial and manufacturing environments. some studies of retail properties have shown that better lit interiors are more inviting and lead to improved sales. light colored floor surfaces such as exposed white concrete can help achieve that.
white cement concrete reflects sustainability
portland cement concrete is a workhorse of construction materials. for structures large and small, underground, submerged, on the ground, or soaring to great heights, concrete’s strength and versatility can’t be matched. yet these qualities would be less attractive if its life cycle were not measured in decades, centuries, and even eons; concrete’s durability establishes its dominance among building materials, and demonstrates one aspect of why it is a sustainable product. a lesser known sustainable benefit of concrete is its reflectivity. applications where reflectivity is beneficial include interior floors, roof tiles, and pavements—including flatwork, streets and roads, and highways.
surface color of a material affects how much of the sun’s energy (heat) is absorbed. darker colored surfaces absorb more heat. in the past, this quality was referred to as “albedo,” but now, terms like “solar reflectance” or “solar reflectivity index” have become common. a material that has a higher solar reflectance absorbs less of the sun’s energy. by virtue of its light color, concrete is naturally reflective.
wherever pavements and structures (especially those that are darker colored) are clustered closely together, as in large cities, the mass of building materials collects heat when warmed by the sun. this creates a microclimate with temperatures measurably higher than in less developed areas—the urban heat island effect. in turn, air conditioners must run longer and harder to keep people cool, which further adds to the high temperatures. this is bad for the environment because it wastes energy, which in turn increases carbon dioxide emissions. effects of urban heat islands are reduced by selecting lighter colored paving and roofing materials, by situating buildings and parking lots to minimize their exposure to the sun, or by establishing trees and other foliage that provide shade for parking areas.
concrete is a mixture of cement, water, fine aggregate (sand), and coarse aggregate (rock or stone). cement and water form the paste that binds the ingredients into a hardened mass. most concrete is formulated with gray portland cement. this results in a light gray colored surface, which is an advantage over darker colored paving materials in terms of solar reflectivity. white cement is chemically similar to gray cement, but contains a lesser quantity of the metal oxides (iron & manganese) that impart portland cement with its characteristic gray color. concrete made with white cement is significantly lighter in color than gray cement concrete, and therefore, more reflective.
standard concrete has a relatively good sri, certainly greater than dark colored paving or roofing materials. concrete is well suited to paving and roofing applications because it is durable, heavy duty, and in the case of roof tiles, its weight can be advantageous in windy exposures like hurricanes or tornadoes; tiles are less likely to be ripped off the structure by strong wind forces.