PORTLAND CEMENT INTRODUCTION
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binder is a hydrophilic, resulting from the calcination of limestone, sandstone and clay, in order to obtain a fine powder that hardens when water is present gaining strength and adhesion properties)
As mentioned in Chapter 1, the name comes from the similarity in appearance and advertising intended to give effect in 1824 Joseph Apsdin an English builder, for patenting a process of calcination of limestone clay producing a hydrated cement acquired according to him, the same resistance that stone island near the port of Portland in Dorset.
is in 1845 when developing the industrial process of modern Portland cement with some variation persists to this day and consists of ground limestone with some clay shale composition and subjecting the powder to temperatures above 1300 ° C producing what is called the clinker, consisting of hardened balls of different diameters, Lesnar finally ground gypsum as a final product having an extremely fine powder.
PORTLAND CEMENT MANUFACTURING
The starting point of the manufacturing process is the selection and exploitation of raw materials for processing accordingly.
Components main chemical raw materials for cement manufacturing and general proportions involved are
Chemical Component
Hometown Usual calcium oxide (CaO)
limestone
95% Silica Oxide (SiO2)
Sandstone
Aluminum Oxide (Al2O3) Clays
Iron Oxide (Fe2O3)
Clays, Iron Ore, Pyrite
magnesium oxide, sodium, 5%
Potassium, Titanium, Sulphur,
Several
Match Minerals and manganese
typical percentages involved in the Portland cement mentioned oxides are:
Component Percent Oxide Typical Abbreviation
CaO 61% - 67% SiO2
C 20% - 27% Al2O3
S
4% - 7% Fe2O3
A 2% - 4% F
SO3 1% - 3% MgO
1% - 5% K2O and Na2O
0.25% - 1.5%
can see an overview of modern manufacturing in the system called for dry ", which is cheaper because it needs less power, and is the highest employment in our area, however we must bear in mind that each manufacturer has a particular equipment available depending on your needs.
It begins with the quarrying of raw materials and subjected to a primary crushing process is reduced in size to around 5 stones "and then processes this material into a secondary crusher which reduces them to a size of about 3 / 4 ", which are capable of being subjected to grinding. The materials are ground individually in a ball mill to be converted into a fine powder, impalpable, being dosed and then intimately mixed in suitable proportions for the type of cement that is desired.
The mixture is then introduced into a rotary kiln consisting of a large metal cylinder of refractory material coated with diameters ranging between 2 and 5 m. and lengths between 18 and 150 meters. The oven has a slight inclination with respect to the horizontal of around 4% and a rotation speed of 30 to 90 revolutions per hour. Depending on the size of the oven, can be produced daily from 30 to 700 tons. The heat source is at the opposite end of the income and material can be obtained by injection of pulverized coal, oil or gas ignition, with temperatures between 1.250 and 1.900 ° C. Developed
temperatures along the furnace produced first open water evaporation, then the release of CO2 and finally in the area of \u200b\u200bhigher temperature melting occurs around 20% 30% of the load and when the lime, silica and alumina are recombined swarming in nodules of various sizes, usually 1 / 4 "to 1" diameter characteristic black color, bright and hard when cooled, called " Portland cement clinker. "
In the final stage, the clinker is cooled and ground in a ball mill together with plaster in small amounts (3 to 6%) to control the hardening violent. The grinding produces a fine powder containing up to 1.1 x 10 12 particles per kg and that passes completely through a sieve No. 200 (0.0737 mm., 200 openings per square inch). Finally, the cement passes be stored in bulk, being then supplied in this form or heavy bagging for distribution.
In the wet process the raw material is ground and mixed with water to form a slurry which is introduced into the rotary kiln using a similar process as above but with higher consumption of energy to remove the water added. The process used depends on the characteristics of raw materials, economy, and in many cases by ecological considerations as the wet process is that the dry cleaner.
During all processes running detailed checks the manufacturer to ensure both quality and proportions of ingredients such as temperature and final product properties, for which there are a series of physical tests and standardized chemical and laboratory equipment developed specifically for this work.
can see the sources of raw materials of which you can get the components to make cement, which appreciates the variety of possibilities in nature in order to produce this material.
COMPOSITION OF PORTLAND CEMENT.
After the training process and final grinding of clinker, the following compounds are obtained first established by Le Chatelier in 1852, and which are those that define the behavior of hydrated cement and detail its chemical formula and name abbreviation
current
a) Tricalcium Silicate (3CaO.SiO2 -> C3S -> Alita-
Define the initial resistance (in the first week) and is very important in the heat of hydration.
b) dicalcium silicate (2CaO.SiO2- -> C2S -> Belita) .-
Define long-term resistance and has minor effect on the heat of hydration.
c) Tricalcium Aluminate (3CaO.Al2O3) -> C3A) .- Singly
has no bearing on the resistance, but with silicate setting determines the violent act as a catalyst, making it necessary to add gypsum in the process (3% - 6%) to control it. Responsible
cement resistance to sulfates and to react with these results sulfoaluminate expansive properties, so you have to limit its content.
d) Aluminium-Ferrito tetracalcium (4CaO.Al2O3.Fe2O3--> C4AF - Celita) .- .-
Table 3.1 Sources of raw materials used in manufacturing portland cement
. (Ref. 3.3)
Cal Silica Alumina
SaO
SiO2 Al2O3
Aragonite
Clay calcareous clay (Marga) Calcite
Seashells
Waste Slag alkaline
Marble Slate Limestone
Residual dust from clinker
Rock Chalk
calcareous clay
calcareous clay (Marga)
Arena Sandstone Basalt
fly ash
ash rice husk. Slag
Quartzite Limestone limestone
calcium silicate clay
calcareous clay (Marga)
Bauxite Waste fly ash
aluminum ore.
copper slag slag
Staurolite
Granodiorite
Slate Limestone
washing waste aluminum ore limestone
Gypsum Iron Magnesia
CaSO4.2H2O
Fe2O3 MgO
Clay
ash and blast furnace slag from iron pyrite laminations
Boards
iron ore washing
waste iron ore
Calcium Sulfate Anhydrite Gypsum
Slag Limestone limestone
has significance in the rate of hydration and secondarily in the heat of hydration.
e) Magnesium oxide (MgO) .-
Despite being a minor component, is important because for contents greater than 5% expansion brings problems hydrated and hardened paste.
f) Potassium and Sodium Oxide (K2O, Na2O -> Alkalis) .-
cases have special importance for chemical reactions with certain aggregates, and soluble in water contribute to producing efflorescence with limestone aggregates.
g) manganese oxide and titanium (Mn2O3, TiO2) .-
The former has no special significance in the properties of cement, except in color, which tends to be brown if they contained more than 3%. It has been observed that in cases where the content exceeds 5% decrease in resistance is obtained over time. (Ref.3.2)
The second influence on the resistance, reducing it to more than 5% contained. For lower contents is of no importance. Of the compounds
mentioned, silicates and aluminates are the major components, but not necessarily the most significant ones, because as we shall see some of the minor components are very important for certain conditions of use of cement.
BOGUE
THE FORMULA FOR THE CALCULATION OF THE POTENTIAL FOR CEMENT COMPOSITION.
in 1929 following a series of experimental investigations, the chemical formulas RHBogue states that allow the calculation of the components of cement on the basis of knowing the percentage of oxides containing, having been taken as a standard by ASTM C-allowing practical approach to any potential behavior normal Portland cement is not mixed.
then establish Bogue formulas should be clear that based on the following assumptions: ¨
compounds have the exact composition. (It is not entirely true because in practice have impurities). ¨
equilibrium is obtained at the temperature of formation of clinker and maintained during cooling. (In practice, the formulas overestimate the C3A and C2S)
BOGUE FORMULAS (Composition Potential):
If Al2O3 / Fe2O3 ≥ 0.64: \u200b\u200b
C3S = 4.071CaO - 7.6SiO2 - 6.718Al2O3 - 1.43Fe2O3 - 2.852SO3
C2S = 2.867SiO2 -
0.7544C3S
C3A = 2.65Al2O3 - 1.692Fe2O3
C4AF = 3.04Fe2O3
< 0.64 se forma (C4AF+C2AF) y se calcula:
If Al 2O3/Fe2O3
(C4AF + C2AF) = 2.1Al2O3 + 1.702Fe2O3
and whose case is calculated as Tricalcium Silicate :
C3S = 4.071CaO - 7.6SiO2 - 4.479Al2O3 - 2.859Fe2O3 - 2.852SO3
(These no C3A cements so the sulfate resistance is high, the C2S is calculated as)
variants in the proportions of these compounds are those that define the types of cements that we will see later, and the practical importance of the Bogue formulas is that which will be to assess the likely potential composition and compare it with standard values \u200b\u200bfor each type of cement, trends can be estimated in terms of behavioral characteristics we are interested from the standpoint of the concrete, such as development of resistance over time, hydration heat, resistance to chemical aggression etc.
mechanism of hydration of cement.
hydration is called the set of chemical reactions between water and cement components, which carry the state change the hardened plastic, with the inherent properties of the new products formed. The components mentioned above, to react with water forming calcium hydroxide and complex carbohydrates.
The rate at which hydration takes place is directly proportional to the fineness of cement and inversely proportional to the time, so it is very rapid initially and decreases gradually with the passage of days, but never get to stop.
Contrary to what was thought years ago, the reaction with water joins the cement particles but each particle is dispersed in millions of particles of hydration products disappearing initial constituents. The process is exothermic generating a flow of heat to the outside called heat of hydration.
Depending on the temperature, time, and the relationship between the amount water and cement react, you can define the following states that have been set arbitrarily to distinguish the stages of the hydration process:
a) .-
Union Plastic water and cement powder to form a moldable paste . The lower the water-cement ratio, the higher the concentration of particles compacted cement paste and thus the structure of hydration products is much more resistant.
The first element is the C3A react, and then the silicate and C4AF, characterized by the scattering process of each grain of cement in million particles. Counteracts the action of gypsum speed reactions and in this state produces what is called the latent period or rest in which the reactions are attenuated, and lasts between 40 and 120 minutes depending on temperature and the cement particle. In this state they form calcium hydroxide which contributes to significantly increase the alkalinity of the dough reaches a pH of about 13.
b) initial set
.- Status of cement paste in which speed up chemical reactions, begin the hardening and loss of plasticity is measured in terms of resistance to deformation. It is the stage where the exothermic process is evident where it generates the aforementioned heat of hydration which results from chemical reactions described.
a porous structure is formed gel called calcium silicate hydrates (CHS or Torbemorita), colloidal consistently intermediate between solid and liquid to be stiffened more and more to the extent that continue to hydrate the silicates.
This period lasts about three hours and produced a series of chemical reactions that are causing the gel CHS more stable over time.
At this stage the paste can remix without permanent deformations or alterations in the structure that is still in training.
c) .-
final set is obtained after completing the stage of initial set, characterized by significant and permanent deformation hardening. The gel structure consists of the final assembly of hardened particles.
d) Curing .-
occurs from the final set and is the state to hold and increase with time resistant properties. The predominant reaction is the continuing hydration of calcium silicates, and in theory continue indefinitely.
is the final state of the paste, which fully demonstrate the influence of cement composition. The strong hydration express very low solubility so that the tightening is still feasible under water.
setting There are two phenomena that are different from those described, the first is the so-called "Curing False" that occurs in some cements due to heating during the grinding of clinker with gypsum, resulting in the partial dehydration of the resulting product for that by mixing cement with water, crystallization and hardening occurs apparent during the first 2 minutes of mixing, but remixing the material plasticity is recovered, without generating heat of hydration and resulting negative consequences. The second phenomenon is the "set violence" that occurs when during manufacture is not added in sufficient amount of gypsum, which produces an immediate tightening, development violent heat of hydration and permanent loss of plasticity, however it is very unlikely at present that this phenomenon occurs, because with modern technology added gypsum is controlled very accurately.
HYDRATED CEMENT STRUCTURE.
During the hydration process, the external volume of the paste is relatively constant, however, internally the solid volume increases steadily over time, causing a permanent reduction of porosity, which is inversely related to resistance of the hardened paste and directly to the permeability.
To produce complete hydration is required enough water for the chemical reaction and provide the structure of empty or space for hydration products, the proper temperature and time, getting rid of this fundamental concept of cure, which is in essence these three elements to ensure that the process is complete.
A basic concept that will allow us to understand the behavior of concrete, is that the volume of hydration products is always less than the sum of the volumes of water and cement that arise because of the chemical combination of water volume decreases by about 25%, which brings result in contraction of the hardened paste. The hydration products need a space of about twice the volume of solid cement to produce complete hydration.
Another important concept to be taken into account is that it is shown that the lower value of the water-cement ratio to produce complete hydration of cement is about 0.35 to 0.40 in normal weight and additive mixed depending on the precise relationship of each individual case. In
, illustration can be seen as a typical pattern of the structure of cement paste and water distribution, distinguishing the following parts:
a) .- Cement Gel
Formed by strong hydration (calcium silicate hydrate), the water in the gel, water is called combination, which is not to be intrinsic evaporable chemical reaction.
b) Gel Pores .-
small space between solids that do not allow moisture inside the formation of new solid hydration. The water contained within these pores is called water gel, which can evaporate under special conditions of exposure.
c) capillary pores
.- Formed by the spaces between groups of solid hydration dimensions that provide space for the formation of new hydration products, which are called capillary water contained in them.
To better understand how different components are distributed in the structure of hydrated cement paste, we will establish some relationships that allow us to calculate in a particular case, for which we initially consider a system where there is no loss evaporative water or additional water enters cure:
Sea:
Pac = weight of water
combination Pch = Weight of cement hydration
Vac = Volume of water in combination = Pac / Pch
It must: Pac
Pch = 0.23 ......................( 1)
(average ratio determined experimentally)
Sea:
Cv = Contraction in volume due to hydration
Ga = Specific gravity of water
We mentioned that the water in combination shrinks 25% then:
Cv = 0.25 x Pac / Ga = 0.25 x 0.23 Pch / Ga
Cv = 0.0575 Pch / Ga ..................... (2)
Sea:
sh = Volume of solids of hydration = Pch / Gc Gc = Gravity
specific cement has to be:
sh = Pch / Gc + Vac - Cv ..........( 3)
Replacing (1) and (2) in (3) gives:
sh = (1/Gc + 0.1725/Ga) Pch ..........( 4)
other hand:
Po = porosity of the hydrated paste
Vag = Volume of water gel
defined:
Po = Vag / (sh + Vag) (5)
Replacing (4) to (5) and solving gives:
Vag = [(Po / (1-Po)) x (1/Gc +0.1725 / Ga)] Pch (6) Sea
:
Vad = Volume of water available for hydration
We have:
Vad = Vac + Vag ............................ ........( 7)
Replacing (1) and (6) in (7) and solving follows:
Pch Vadx1 =.. / [(0.23/Ga + (Po (1-Po) ) x (1/Gc +0.1725 / Ga)] (8).
Finally, we define:
VCSH = unhydrated cement volume
Pcd = Weight of cement available
VCV = volume of capillary voids
And it has that:
VCSH = Pcd / Gc - Pch / Gc ...........................( 9)
VCV = Pcd + Vad - sh - Vag - VCSH ............( 10)
With these relationships we have developed which shows the changes in the components of the structure of the paste 100 gr. cement with various amounts of water available for hydration having assumed the following typical parameters:
Gc = Specific gravity of cement = 3.15
Ga = Specific gravity of water = 1.00
Po = porosity of the paste hydrated = 0.28
can see that for very low values \u200b\u200bof the water / cement hydration stops from lack of water to hydrate fully the amount of cement available, remaining unhydrated cement and empty capillaries are capable of allowing entry of additional water and space to develop more solid hydration.
their hydration products, then there are water-cement ratio for which extra water for more than that we provide, there will be total cement hydration.
We also see that for normal conditions as those assumed in which the pasta has only the initial mixing water, you need a water-cement ratio of 0.42 minimum order, and if it provides extra hydration water, the minimum is order of 0.38.
Con los valores de la se han elaborado las 5 donde se gráfica a título explicativo el % de hidratación y el % de cemento no hidratado en función de la relación Agua/Cemento, así como los vacíos capilares obtenidos.
Hay que tener presente, que pese a que para relaciones Agua/Cemento inferiores a las que producen el 100 % de hidratación, aún queda cemento sin hidratar, la estructura es mas compacta con menor cantidad de vacíos, por lo que se obtienen en la práctica características resistentes mas altas pese a no contarse con toda la pasta hidratada; sin embargo para lograr la hidratación máxima que es posible alcanzar con relaciones Agua/Cemento muy bajas, se necesitan condiciones that merit special mixing and pressure increase in compaction energy and otherwise does not moisturize as planned. In practice, with normal mixing conditions was possible to achieve water-cement ratio paste minimum of about 0.25 to 0.30 depending on the type of cement and the conditions of temperature, humidity, pressure and mixing technique. Under special conditions, have got to get pasta in laboratory water-cement ratio as low as 0.08 (Ref.3.6)
TYPES OF CEMENT AND ITS APPLICATIONS MAIN.
Types of portland cement standard that we can qualify, because their manufacture is regulated by requirements are specific (Ref. 3.5): Type I
.- For general use, where special properties are not required.
Type II .- moderate sulfate resistance and moderate heat of hydration. Structures for use in harsh environments and / or mass emptied.
Type III .- Rapid development of resistance to high heat of hydration. For use in cold weather or where you need to advance the commissioning of the structures.
.- Type IV Low heat of hydration. Massive concrete.
Type V - High resistance to sulphates. For very aggressive environments.
When the first three types of cement were added the suffix A (eg Type IA) means they are cements which have been added air entraining in its composition, keeping the original properties. Interestingly
cements referred to as "mixed or added" because some of them are used in our midst:
Cement Type IS .- which was added between 25% to 70% of blast furnace slag referred to total weight. Type
ISM .- Cement which has added less than 25% of blast furnace slag on the total weight.
.- Cement Type IP that pozzolan has been added at a rate ranging between 15% and 40% of the total weight.
IPM .- Cement type that has been added pozzolan at a rate up to 15% of the total weight.
These cements have variants that entrained air is added (suffix A), induced moderate sulfate resistance (suffix M), or moderate heat of hydration (suffix H).
pozzolans are siliceous inert materials and / or alumina, which individually have almost no binding properties, but finely ground and chemically react with calcium hydroxide and water resources are cementitious properties. Pozzolans are usually obtained calcined clay, diatomaceous earth, tuff and volcanic ash, and industrial waste as fly ash, pulverized brick, etc.
The particularity of replacing part of cement by these materials, is to change some of its properties, such as the increasing durations of the above conditions, delay and / or slow the development of resistance over time, reduce permeability , the more water holding capacity, greater cohesiveness, increased water requirements to form dough, lower heat of hydration and better performance against chemical aggression.
must be borne in mind that the variation of these properties will not always be appropriate depending on the particular case, so that you can not take pozzolanic cement or the addition of pozzolan as a panacea, as they are very sensitive to temperature variations and construction processes curing conditions.
end mix design must take into account that the standard cements have a specific gravity of around 3.150 kg/m3 and pozzolanic cements are lighter in specific gravity between 2.850 and 3.000 kg/m3.
in which you can see typical behaviors of basic cement, on the development of resistance over time and heat of hydration.
can be seen in the physical and chemical requirements of manufacturing standards established by ASTM C-150 for standard cements appointed and in the. statistics are reported variation of the components of the various types of normal cement in USA and England, where it is concluded that the elasticity in manufacturing standards supports variations but should not influence the final strength required, if they can cause time-varying behavior.
PERUVIAN CEMENTS AND THEIR CHARACTERISTICS.
currently manufactured in Peru cement Type I, Type II, Type V, Type IP and Type IPM.
can be seen in the physical and chemical characteristics of domestically produced cement provided by the manufacturers, except for cement Rumi, the producer did not agree to provide them, despite our insistence, so an analysis is entered at the request of the author at the Catholic University of Peru in connection with the use of this material during construction of the airport in Juliaca, in which the results are quite irregular a Cement Type I, which nevertheless must be taken with caution because only a sample. In
, Resistances are plotted versus time for different Peruvian cements based on information supplied between January and April of 1.993.
is interesting to note that in general the national cement are typical behaviors in the long run it is reasonable to expect similar cement manufactured in abroad, but experience in using them and the variability that can be seen in the analysis and graphics shown us that the underlying property in the short term do not always keep constant parameters, so you should never rely on them a priori without performing tests of control to the case of works of some importance.
On the other hand, local manufacturers have much experience in the manufacture of cement, but none of them is in the practical application of this material in concrete production because very rarely collect data, or do research in particular, so which is very little information can make in this regard and also, there is usually reluctance to provide results of its quality control routinely. However, we must acknowledge the assistance provided by the producers who agreed to provide and include in this book data provided.
No information is published periodically by the manufacturers on basic issues such as the variation of resistance development time variation of hydration depending on environmental conditions, characteristics of the employing pozzolans in blended cements, statistics interlaboratory controls made, etc.
information would be extremely useful to users and researchers to avoid many situations that behavior is expected by extrapolation outside information or local information incomplete and you get another for lack of reliable data.
As an additional comment would have to say that the introduction of pozzolanic cement and pozzolan modified in our environment has brought benefits from the point of view that have advantages concerning durability as well as being profitable for the manufacturer to replace it cheaper pozzolana cement costs and selling prices experienced some reduction, but these benefits are not fully exploited since it has not been enough research, outreach and educational efforts regarding the considerations for dosing, which results in deficiencies in its use by the user.
normally assumed that these cements designs require the same amount of water than normal, which in practice is not true, as some of them require up to 10% more water and have a consistency that deserves greater cohesive energy compaction so that eventually the economy is not so alleged.
in the Appendix includes copies of the original data supplied by manufacturers in 1993 and 1996, including additional information to that contained in the tables and can be useful for anyone interested in learning more about these issues.
CONTROL AND STORAGE CONDITIONS ON SITE AND ITS CONSEQUENCES.
I already mentioned in relation to the national cement makes us reflect on the need to address if possible to do in work time statistical monitoring and storage conditions and quality of cement used.
A good practice is the run chemical analysis in a reliable laboratory every 500 tons of cement for great things, and seek regular certified manufacturers with results of quality control. In any case the sample is obtained must be less than 5 kg
For storage conditions, is often recommended to clean deposit metal silos especially in high humidity climates, because there is partial hydration of cement adhering to the walls, and using the results that emerge silo hardened pieces and mix with fresh concrete causing problems uniformity of concrete production.
In the case of cement in bags, the concept is similar as to protect from moisture, either isolating or protecting soil indoors.
A practical way to assess whether there has been partial hydration of cement stored, a sample is to sift through the mesh No 100, according to ASTM C-184, weighing the detainee, which the total weight, gives an order magnitude of the portion hydrated. The percentage retained without hydration usually ranges between 0 and 0.5%.
If you remember the concepts regarding the mechanism of hydration can estimate that if we use partially hydrated cement, will be replaced in practice by adding a portion of hardened cement strength characteristics uncertain and definitely below the sand and stone, which will cause areas weak structure, whose significance will be greater the larger the proportion of these particles.
can be estimated that the use of cement hydration by 30% on total weight, with granules of not more than 1 / 4 "results in a reduction resistance to 28 days of the order of 25%, depending on the cement in particular. It is obvious that higher percentages hydrated, with particles larger than 1 / 4 "will cause more negative damage resistance and durability.
Finally, we should clarify that in terms of storage, the proper criterion for evaluating the quality of cement is not time that has been stored but the conditions of hydration of cement within that period, so it is advisable to take provisions to prevent or delay the hydration from the start, instead of letting time pass without caution and then enter in the complications of evaluating whether it will fit or not for use.
REFERENCES
Read Frederik .- "The Chemistry of Cement and Concrete" .- Edward Arnold Publishers - London 1988. Adam Neville
.- "Concrete Technology" .- Mexican Institute of Cement and Concrete - Mexico 1977.
S. Kosmatka, Panarese W. .- "Design and Control of Concrete Mixtures," Portland Cement Association - USA 1988. Sandor Popovics
.- "Concrete: Making Materials" .- Edit. Mc Graw Hill - 1979.
ASTM Standard C-150 .- "Standard Specification for Portland Cement" -1986.
ACI SCM-22 .- .- Troubleshooting Concrete Construction Seminar Course Manual.USA 1990.
ASTM Standard C-595.- "Standard Specification for Blended Hydraulic Cements"-1986.
U.S. Bureau of Reclamation.- "Concrete Manual" Eight Edition- Revised - 1988.
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