Portland cement

History

Portland cement was developed from cements (or correctly hydraulic limes) made in Britain in the early part of the nineteenth century, and its name is derived from its similarity to Portland stone, a type of building stone that was quarried on the Isle of Portland in Dorset, England. Joseph Aspdin, a British bricklayer, in 1824 was granted a patent for a process of making a cement which he called Portland cement. His cement was an artificial hydraulic lime similar in properties to the material known as "Roman Cement" (patented in 1796 by James Parker) and his process was similar to that patented in 1822 and used since 1811 by James Frost who called his cement "British Cement". The name "Portland cement" is also recorded in a directory published in 1823 being associated with a William Lockwood and possibly others. Aspdin's son William in 1843 made an improved version of this cement and he initially called it "Patent Portland cement" although he had no patent. In 1848 William Aspdin further improved his cement and in 1853 moved to Germany where he was involved in cement making.[1] Many people have claimed to have made the first Portland cement in the modern sense, but it is generally accepted that it was first manufactured by William Aspdin at Northfleet, England in about 1842[2]. The German Government issued a standard on Portland cement in 1878.

Types of Portland cement

Genera

lThere are different standards for classification of Portland cement. The two major standards are the ASTM C150 used primarily in the U.S. and European EN-197. EN 197 cement types CEM I, II, III, IV, and V do not correspond to the similarly-named cement types in ASTM C 150.

 

Production

Schematic explanation of Portland cement production

There are three fundamental stages in the production of Portland cement:

1.                Preparation of the raw mixture

2.                Production of the clinker

3.                Preparation of the cement

The chemistry of cement is very complex, so cement chemist notation was invented to simplify the formula of common oxides found in cement. This reflects the fact that most of the elements are present in their highest oxidation state, and chemical analyses of cement are expressed as mass percent of these notional oxides.

 

Rawmix preparation

 

A limestone prehomogenization pile being built by a boom stacker

 

A completed limestone prehomogenization pile

The raw materials for Portland cement production are a mixture (as fine powder in the 'Dry process' or in the form of a slurry in the 'Wet process') of minerals containing calcium oxide, silicon oxide, aluminium oxide, ferric oxide, and magnesium oxide. The raw materials are usually quarried from local rock, which in some places is already practically the desired composition and in other places requires the addition of clay and limestone, as well as iron ore, bauxite or recycled materials. The individual raw materials are first crushed, typically to below 50 mm. In many plants, some or all of the raw materials are then roughly blended in a "prehomogenization pile". The raw materials are next ground together in a rawmill. Silos of individual raw materials are arranged over the feed conveyor belt. Accurately controlled proportions of each material are delivered onto the belt by weigh-feeders. Passing into the rawmill, the mixture is ground to rawmix. The fineness of rawmix is specified in terms of the size of the largest particles, and is usually controlled so that there are less than 5-15% by mass of particles exceeding 90 μm in diameter. It is important that the rawmix contains no large particles in order to complete the chemical reactions in the kiln, and to ensure the mix is chemically homogenous. In the case of a dry process, the rawmill also dries the raw materials, usually by passing hot exhaust gases from the kiln through the mill, so that the rawmix emerges as a fine powder. This is conveyed to the blending system by conveyor belt or by a powder pump. In the case of wet process, water is added to the rawmill feed, and the mill product is a slurry with moisture content usually in the range 25-45% by mass. This slurry is conveyed to the blending system by conventional liquid pumps.

Formation of clinker

The raw mixture is heated in a cement kiln, a slowly rotating and sloped cylinder, with temperatures increasing over the length of the cylinder up to a peak temperature of 1400-1450 °C. A complex succession of chemical reactions take place (see cement kiln) as the temperature rises. The peak temperature is regulated so that the product contains sintered but not fused lumps. Sintering consists of the melting of 25-30% of the mass of the material. The resulting liquid draws the remaining solid particles together by surface tension, and acts as a solvent for the final chemical reaction in which alite is formed. Too low a temperature causes insufficient sintering and incomplete reaction, but too high a temperature results in a molten mass or glass, destruction of the kiln lining, and waste of fuel. The resulting material is clinker. On cooling, it is conveyed to storage. Some effort is usually made to blend the clinker, because although the chemistry of the rawmix may have been tightly controlled, the kiln process potentially introduces new sources of chemical variability. The clinker can be stored for a number of years before use. Prolonged exposure to water decreases the reactivity of cement produced from weathered clinker.

The enthalpy of formation of clinker from calcium carbonate and clay minerals is ~1700 kJ/kg. However, because of heat loss during production, actual values can be much higher. The high energy requirements and the release of significant amounts of carbon dioxide makes cement production a concern for global warming. See "Environmental effects" below.

 

Cement grinding

A 10 MW cement mill, producing 270 tph

In order to achieve the desired setting qualities in the finished product, a quantity (2-8%, but typically 5%) of calcium sulfate (usually gypsum or anhydrite) is added to the clinker and the mixture is finely ground to form the finished cement powder. This is achieved in a cement mill. The grinding process is controlled to obtain a powder with a broad particle size range, in which typically 15% by mass consists of particles below 5 μm diameter, and 5% of particles above 45 μm. The measure of fineness usually used is the "specific surface", which is the total particle surface area of a unit mass of cement. The rate of initial reaction (up to 24 hours) of the cement on addition of water is directly proportional to the specific surface. Typical values are 320-380 m².kg-1 for general purpose cements, and 450-650 m².kg-1 for "rapid hardening" cements. The cement is conveyed by belt or powder pump to a silo for storage. Cement plants normally have sufficient silo space for 1-20 weeks production, depending upon local demand cycles. The cement is delivered to end-users either in bags or as bulk powder blown from a pressure vehicle into the customer's silo. In developed countries, 80% or more of cement is delivered in bulk, and many cement plants have no bag-packing facility. In developing countries, bags are the normal mode of delivery.

Typical constituents of Portland clinker and Portland cement. Cement industry style notation in italics:

Clinker

Mass%

Cement

Mass%

Tricalcium silicate (CaO)3.SiO2, C3S

45-75%

Calcium oxide, CaO, C

61-67%

Dicalcium silicate (CaO)2.SiO2, C2S

7-32%

Silicon oxide, SiO2, S

19-23%

Tricalcium aluminate (CaO)3.Al2O3, C3A

0-13%

Aluminium oxide, Al2O3, A

2.5-6%

Tetracalcium aluminoferrite (CaO)4.Al2O3.Fe2O3, C4AF

0-18%

Ferric oxide, Fe2O3, F

0-6%

Gypsum CaSO4

2-10%

Sulfate

 

 

Use

 Decorative use of Portland cement panels on London’s Grosvenor estate[3]

The most common use for Portland cement is in the production of concrete. Concrete is a composite material consisting of aggregate (gravel and sand), cement, and water. As a construction material, concrete can be cast in almost any shape desired, and once hardened, can become a structural (load bearing) element. Users may be involved in the factory production of pre-cast units, such as panels, beams, road furniture, or may make cast-in-situ concrete such as building superstructures, roads, dams. These may be supplied with concrete mixed on site, or may be provided with "ready-mixed" concrete made at permanent mixing sites. Portland cement is also used in mortars (with sand and water only) for plasters and screeds, and in grouts (cement/water mixes squeezed into gaps to consolidate foundations, road-beds, etc).

Setting and hardening

When water is mixed with Portland cement, the product sets in a few hours and hardens over a period of weeks. These processes can vary widely depending upon the mix used and the conditions of curing of the product, but a typical concrete sets (i.e. becomes rigid) in about 6 hours, and develops a compressive strength of 8 MPa in 24 hours. The strength rises to 15 MPa at 3 days, 23 MPa at one week, 35 MPa at 4 weeks, and 41 MPa at three months. In principle, the strength continues to rise slowly as long as water is available for continued hydration, but concrete is usually allowed to dry out after a few weeks, and this causes strength growth to stop.

Setting and hardening of Portland cement is caused by the formation of water-containing compounds, forming as a result of reactions between cement components and water. Usually, cement reacts in a plastic mixture only at water/cement ratios between 0.25 and 0.75. The reaction and the reaction products are referred to as hydration and hydrates or hydrate phases, respectively. As a result of the reactions (which start immediately), a stiffening can be observed which is very small in the beginning, but which increases with time. The point in time at which it reaches a certain level is called the start of setting. The consecutive further consolidation is called setting, after which the phase of hardening begins.

Stiffening, setting and hardening are caused by the formation of a microstructure of hydration products of varying rigidity which fills the water-filled interstitial spaces between the solid particles of the cement paste, mortar or concrete. The behaviour with time of the stiffening, setting and hardening therefore depends to a very great extent on the size of the interstitial spaces, i. e. on the water/cement ratio. The hydration products primarily affecting the strength are calcium silicate hydrates ("C-S-H phases"). Further hydration products are calcium hydroxide, sulfatic hydrates (AFm and AFt phases), and related compounds, hydrogarnet, and gehlenite hydrate. Calcium silicates or silicate constituents make up over 70 % by mass of silicate-based cements. The hydration of these compounds and the properties of the calcium silicate hydrates produced are therefore particularly important. Calcium silicate hydrates contain less CaO than the calcium silicates in cement clinker, so calcium hydroxide is formed during the hydration of Portland cement. This is available for reaction with supplementary cementitious materials such as ground granulated blast furnace slag and pozzolans. The simplified reaction of alite with water may be expressed as:

2Ca3OSiO4 + 6H2O → 3CaO.2SiO2.3H2O + 3Ca(OH)2

This is a relatively fast reaction, causing setting and strength development in the first few weeks. The reaction of belite is:

2Ca2SiO4 + 4H2O → 3CaO.2SiO2.3H2O + Ca(OH)2

This reaction is relatively slow, and is mainly responsible for strength growth after one week. Tricalcium aluminate hydration is controlled by the added calcium sulfate, which immediately goes into solution when water is added. Firstly, ettringite is rapidly formed, causing a slowing of the hydration (see tricalcium aluminate):

Ca3(AlO3)2 + 3CaSO4 + 32H2O → Ca6(AlO3)2(SO4)3.32H2O

The ettringite subsequently reacts slowly with further tricalcium aluminate to form "monosulfate" - an "AFm phase":

Ca6(AlO3)2(SO4)3.32H2O + Ca3(AlO3)2 + 4H2O → 3Ca4(AlO3)2(SO4).12H2O

This reaction is complete after 1-2 days. The calcium aluminoferrite reacts slowly due to precipitation of hydrated iron oxide:

2Ca2AlFeO5 + CaSO4 + 16H2O → Ca4(AlO3)2(SO4).12H2O + Ca(OH)2 + 2Fe(OH)3

The pH-value of the pore solution reaches comparably high values and is of importance for most of the hydration reactions.

Soon after Portland cement is mixed with water, a brief and intense hydration starts (pre-induction period). Calcium sulfates dissolve completely and alkali sulfates almost completely. Short, hexagonal needle-like ettringite crystals form at the surface of the clinker particles as a result of the reactions between calcium- and sulphate ions with tricalcium aluminate. Further, originating from tricalcium silicate, first calcium silicate hydrates (C-S-H) in colloidal shape can be observed. Caused by the formation of a thin layer of hydration products on the clinker surface, this first hydration period ceases and the induction period starts during which almost no reaction takes place. The first hydration products are too small to bridge the gap between the clinker particles and do not form a consolidated microstructure. Consequently the mobility of the cement particles in relation to one another is only slightly affected, i. e. the consistency of the cement paste turns only slightly thicker. Setting starts after approximately one to three hours, when first calcium silicate hydrates form on the surface of the clinker particles, which are very fine-grained in the beginning. After completion of the induction period, a further intense hydration of clinker phases takes place. This third period (accelerated period) starts after approximately four hours and ends after 12 to 24 hours. During this period a basic microstructure forms, consisting of C-S-H needles and C-S-H leafs, platy calcium hydroxide and ettringite crystals growing in longitudinal shape. Due to growing crystals, the gap between the cement particles is increasingly bridged. During further hydration, the hardening steadily increases, but with decreasing speed. The density of the microstructure rises and the pores fill: the filling of pores causes strength gain.

ASTM C150

There are five types of Portland cements with variations of the first three according to ASTM C150.

Type I Portland cement is known as common or general purpose cement. It is generally assumed unless another type is specified. It is commonly used for general construction especially when making precast and precast-prestressed concrete that is not to be in contact with soils or ground water. The typical compound compositions of this type are:

55% (C3S), 19% (C2S), 10% (C3A), 7% (C4AF), 2.8% MgO, 2.9% (SO3), 1.0% Ignition loss, and 1.0% free CaO.

A limitation on the composition is that the (C3A) shall not exceed fifteen percent.

Type II is intended to have moderate sulfate resistance with or without moderate heat of hydration. This type of cement costs about the same as Type I. Its typical compound composition is:

51% (C3S), 24% (C2S), 6% (C3A), 11% (C4AF), 2.9% MgO, 2.5% (SO3), 0.8% Ignition loss, and 1.0% free CaO.

A limitation on the composition is that the (C3A) shall not exceed eight percent which reduces its vulnerability to sulfates. This type is for general construction that is exposed to moderate sulfate attack and is meant for use when concrete is in contact with soils and ground water especially in the western United States due to the high sulfur content of the soil. Because of similar price to that of Type I, Type II is much used as a general purpose cement, and the majority of Portland cement sold in North America meets this specification.

Note: Cement meeting (among others) the specifications for Type I and II has become commonly available on the world market.

Type III is has relatively high early strength. Its typical compound composition is:

57% (C3S), 19% (C2S), 10% (C3A), 7% (C4AF), 3.0% MgO, 3.1% (SO3), 0.9% Ignition loss, and 1.3% free CaO.

This cement is similar to Type I, but ground finer. Some manufacturers make a separate clinker with higher C3S and/or C3A content, but this is increasingly rare, and the general purpose clinker is usually used, ground to a specific surface typically 50-80% higher. The gypsum level may also be increased a small amount. This gives the concrete using this type of cement a three day compressive strength equal to the seven day compressive strength of types I and II. Its seven day compressive strength is almost equal to types I and II 28 day compressive strengths. The only downside is that the six month strength of type III is the same or slightly less than that of types I and II. Therefore the long-term strength is sacrificed a little. It is usually used for precast concrete manufacture, where high 1-day strength allows fast turnover of molds. It may also be used in emergency construction and repairs and construction of machine bases and gate installations.

Type IV Portland cement is generally known for its low heat of hydration. Its typical compound composition is:

28% (C3S), 49% (C2S), 4% (C3A), 12% (C4AF), 1.8% MgO, 1.9% (SO3), 0.9% Ignition loss, and 0.8% free CaO.

The percentages of (C2S) and (C4AF) are relatively high and (C3S) and (C3A) ar

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