For each potential cause, there is a short description followed by a brief discussion. In essence, this e- book is a checklist of some possibilities to consider when. Thank you for subscribing to the “Understanding Cement” newsletter! Just save this pdf file to your desktop, or other convenient place, and open it when you. Hydration reaction between water and cement in concrete is primarily dependant on 2 things: 1 W t / M i t. Water / Moisture. – Water is the fuel for the reaction.
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Understanding wm-greece.info - Ebook download as PDF File .pdf), Text File .txt) or read book online. AAC Understanding Cement - Download as Word Doc .doc /.docx), PDF File . pdf), Text File .txt) or read online. AAC Understanding Cement. PDF | The present work describes some new improvements concerning the analysis of cement hydration processes using `pencil-beam' synchrotron X-ray.
Autoclaved aerated concrete can be produced using a wide range of cementitous materials, commonly: The sand is usually milled to achieve adequate fineness. A small amount of anhydrite or gypsum is also often added.
Autoclaved aerated concrete is quite different from dense concrete ie: Dense concrete is typically a mixture of cement and water, often with slag or PFA, and fine and coarse aggregate. In contrast, autoclaved aerated concrete is of much lower density than dense concrete. The chemical reactions forming the hydration products go virtually to completion during autoclaving and so when removed from the autoclave and cooled, the blocks are ready for use.
Autoclaved aerated concrete does not contain any aggregate; all the main mix components are reactive, even milled sand where it is used. The sand, inert when used in dense concrete, behaves as a pozzolan in the autoclave due to the high temperature and pressure. The autoclaved aerated concrete production process differs slightly between individual production plants but the principles are similar.
We will assume a mix that contains cement, lime and sand; these are mixed to form a slurry. Also present in the slurry is fine aluminium powder - this is added to produce the cellular structure. The density of the final block can be varied by changing the amount of aluminium powder in the mix. The slurry is poured into moulds that resemble small railway wagons with drop-down sides.
Over a period of several hours, two processes occur simultaneously: The cement hydrates normally to produce ettringite and calcium silicate hydrates and the mix gradually stiffens to form what is termed a "green cake". The green cake rises in the mould due to the evolution of hydrogen gas formed from the reaction between the fine aluminium particles and the alkaline liquid.
These gas bubbles give the material its cellular structure. There are some parallels between autoclaved aerated concrete production and bread-making. In bread, the dough contains yeast and is mixed, then left to rise as the yeast converts sugars to carbon dioxide.
The dough must have the right consistency; too hard and the bubbles of carbon dioxide cannot 'stretch' the dough to make it rise, but if the dough is too sloppy, the carbon dioxide bubbles rise to the surface and are lost and the dough collapses. With the right consistency, the dough is sufficiently elastic to stretch and expand, but strong enough to retain the gas so that the dough does not collapse.
When risen, the dough is placed in the oven. Although a much more complex process, Aircrete production conditions are precisely-controlled for, in part, somewhat similar reasons. The mix proportions and the initial mix temperature must be correct and the aluminium powder must be present in the required amount and with the appropriate reactivity an an alkaline environment.
All of the materials be be of suitable fineness. A complicating factor is that the temperature of the green cake increases due to the exothermic reactions as the lime and the cement hydrate, so the reactions proceed faster. When the cake has risen to the required height, the mould moves along a track to where the cake is cut to the required block size. Depending on the actual production process, the cake may be demoulded entirely onto a trolley before cutting, or it may be cut in the mould after the sides are removed.
The cake is cut by passing through a series of cutting wires. At the cutting stage, the blocks are still green - only a few hours has passed since the mix was poured into the mould and they are soft and easily damaged.
However, if they are too soft, the cut blocks may either fall apart or stick together; if they are too hard, the wires will not cut them - here too, the process has to be carefully controlled to achieve the necessary consistency. The cut blocks are then loaded into the autoclave. When removed from the autoclave and cooled, the blocks have achieved their full strength and are packed ready for transport.
AAC Composition The essence of aircrete production is that lime from the cement and lime in the mix reacts with silica to form 1. Cement chemistry notation is used below. If you are not familiar with this, see our cement chemistry notation explained page. After autoclaving, tobermorite is normally the principal final reaction product due to the high temperature and pressure. Small amounts of other hydrated phases will also be present in the final product. Additionally, hydrated phases form in the autoclave as intermediate products, principally C-S-H I.
This is a more crystalline form of calcium silicate hydrate than occurs in dense concrete; it can have a ratio of calcium to silicon of 0. The compositions of the hydration products in aircrete are therefore quite different from those in dense concrete cured at normal temperatures ie: Looking at this in a little more detail from when the green blocks enter the autoclave, the main reactions that occur are broadly as follows: C-S-H I is therefore mainly an intermediate compound.
The final hydration products are then principally: SEM image of polished section showing a detail - a cell wall - of a block made with cement, lime and sand mix. Some residual unreacted sand particles remain examples arrowed , often with rims of hydration product showing the size of the original particle. Most of the matrix is composed of tobermorite.
Black areas at top left and bottom right are epoxy resin used in preparing the polished section filling air voids air cells. The objective is to react sufficient silica from the sand to form tobermorite from the available lime supplied by the lime and cement.
This will depend on a range of factors, including the inherent reactivities of the materials, their fineness especially the sand , and the temperature and pressure. If the autoclaving time is too short, the tobermorite content will not be maximised and some unreacted calcium hydroxide will remain and block strengths will be then less than optimum. If the autoclaving time is too long, other hydration products may form which may also be detrimental to strength and an unnecessary energy cost will be incurred.
Mineralised cements characteristically have good early strengths. Figure 5. Combinability depends on composition. Combinability also depends on particle fineness. The composition of the coarse residue is also important. The small particles. For two mixes. Portland cement manufacturing — from raw materials to cement For example. We then repeat this for other mixes. We then decide on a free lime level that we consider represents acceptable combination eg: A and B. Combinability depends additionally on how well the raw materials react together.
We would expect hotter burning to give lower free lime contents. In Figure 5. Any heterogeneities will result in areas of excess lime or silica and these will be difficult to combine. Fineness can mean the blended raw materials collectively.
Assuming we have a suitable range of raw materials to control all three parameters at least four materials individual parameters can be changed to show how each parameter affects the combinability temperature for that mix.
Minor constituents can alter the optimum AR. We would also expect the combinability temperature to increase with increasing SR because.
We would typically expect to find that: We could expect that the combinability temperature would increase with LSF. Alumina Ratios for ordinary Portland cement. Since it is the liquid flux that mainly facilitates combination.
We then repeat. It will be clear that combinability experiments may involve doing tens or even hundreds of individual burns of test mixes and that this is not a trivial exercise.
With decreasing LSF mix heterogeneities become less important. Portland cement manufacturing — from raw materials to cement In order to find out how each of these parameters affects how particular raw materials will combine. Many hours of fun later. Portland cement manufacturing — from raw materials to cement example.
With increasing LSF. With increasing silica becomes progressively harder to achieve ratio. Combination is also affected by the viscosity of the liquid phase.
Liquid viscosity increases with increasing AR and decreases with increasing temperature. Before computers became widely available these calculations. The principle of the proportioning calculation is simple but the actual calculation is tedious when done manually.
Computers can do it instantly by solving simultaneous equations. Much more control would be possible for the lime saturation factor. To calculate the effect of coal ash. Compositions are often a compromise between what is ideally wanted and what can be achieved in reality. Where coal is the fuel for the kiln. Portland cement manufacturing — from raw materials to cement So. Other important factors will include fuel and raw material grinding costs.
Most of the ash will become incorporated into the clinker and the quantity of ash is enough to have a significant effect on clinker composition. Calcination takes place in a few seconds. In a dry process preheater kiln. Surface reactions on siliceous particles will occur to a limited extent.
Portland cement manufacturing — from raw materials to cement 5. In a wet-process kiln. In a dry process kiln with a precalciner. Water evaporation and calcining In isolation. This situation is not ideal because heat transfer has to take place through a large mass of material and water vapour and CO2 have to escape outwards.
Any moisture will evaporate. In a preheater kiln. Calcining takes place after the water has been driven off. The surfaces of siliceous particles will react with lime to a greater extent than in a kiln with a preheater only. A sulfate melt phase may form. Alkali chlorides may be also present. It can cause problems in the presence of solid condensation products at the kiln back end cool end.
The main initial silicate product is belite. Before decarbonation is completed. Other intermediate products that may form include calcium sulfosilicate 2C2S. Some minerals are formed in the kiln at relatively low temperatures compared with the temperature in the burning zone.
The extent of these is dependent on the overall level of sulfate in the clinker. Some calcium aluminate CA and C12A7 and ferrite phases also start to form. Some intermediate compounds assist in forming the final clinker minerals.
Portland cement manufacturing — from raw materials to cement Formation of early and intermediate compounds As lime is calcined. Its composition can be written as 2C2S. Most of these minerals dissociate in the burning zone and are not therefore present in the clinker.
The clinker is in the burning zone for perhaps minutes but in this time a lot happens: A high recirculating volatile load may cause adverse clinker characteristics. The volatilised material passes down the kiln. It can also result in kiln rings when condensed volatiles adhere to the kiln wall causing rings to form.
Clinker sulfate phases will be looked at in Chapter 5. Alite formation Alite forms mainly by the reaction of lime with belite. Evaporation of volatiles Alkalis. The additional liquid causes feed particles to stick together. These can interfere with production if the kiln has to be shut down to remove the blockage.
The liquid content is more than the sum of the aluminate and ferrite phases in the cooled clinker mainly because of dissolved lime and silica. These recycling volatiles are known as the recirculating volatile load. Some alite also forms directly from free lime and silica. Dissociation of intermediate compounds As the temperature rises. The liquid forms from melting ferrite and aluminate phases and some belite. It then travels back towards the burning zone.
A controlled supply of air secondary air is admitted to allow combustion but if oxygen loss in the burning zone is extreme. Iron II can substitute for calcium and so can be present in all the clinker phases. Over-large aluminate crystals can lead to erratic setting characteristics. Slow cooling gives less hydraulically-reactive silicates. The effect on cement made from clinker produced under reducing conditions is generally to reduce cement performance.
White cement clinker is burned deliberately under slightly reducing conditions to reduce iron III to iron II — that way the cement is whiter. As there is little iron present anyway. Fast cooling of clinker is advantageous. Portland cement manufacturing — from raw materials to cement conditions can result. The only exception to this is that of white cement. This adversely affects cement quality. Concrete strengths may be lower and setting times erratic. Sulfides also form.
As it does so. The consequences of this are numerous. Rapid cooling probably makes the clinker easier to grind by inducing thermal cracking. It is unlikely that the amount of available sulfate will exactly equate with the amount of available alkali to make alkali sulfates. Within the kiln. Despite this relatively low proportion of the total clinker.
An excess of available sulfate over available alkali is the more desirable of the two alternatives. In the kiln.
Since sulfates are largely combined with alkalis. When clinker is ground to make cement. Clinker alkali sulfate forms mainly in pores in the clinker Figure 5.
Belite often also contains small inclusions of alkali sulfate. It occurs where there is an excess of alkali over sulfate in the clinker. Normal aluminate has a cubic crystal structure.
Clinker alkalis can enhance early strength development in concrete as they accelerate the rate of alite hydration by increasing the alkalinity of the pore fluid. Calcium langbeinite is an effective set retarder. Alkali and sulfate. The main forms of clinker alkali sulfate are: Prismatic aluminate or alkali aluminate has a higher alkali content. Portland cement manufacturing — from raw materials to cement If there is an excess of sulfate over alkali. Free lime does not normally contain much alkali or sulfate.
Clinker alkalis and sulfates — it is all a matter of balance Suppose we have a clinker which has an excess of available alkali over sulfate. For simplicity. Potassium sulfate K2SO4 consists of about The types of clinker sulfate present are determined by the ratio of clinker alkali to sulfate and by the types of alkali. Suppose now that the clinker sulfate content is gradually increased perhaps by burning a higher-sulfur fuel.
There will be less alkali in the aluminate and belite and more alkali sulfate as arcanite. Combinability and cement performance should improve. Portland cement manufacturing — from raw materials to cement For a clinker with 0. A small excess of available alkali over sulfate is not usually important. With still more sulfate. If the sulfate level is increased further.
With yet more sulfate addition. At some point. The clinker will then contain a mixture of calcium langbeinite and arcanite. As the clinker sulfate increases. If so. A large excess of available alkali over sulfate is to be avoided if possible because: If there is an excess of available alkali compared with available sulfate.
Cement ground to a finer particle size will clearly react more quickly than the same cement milled more coarsely. These methods give similar results. It also may reduce the tendency of the cement to agglomerate during storage.
Natural anhydrite may also be added to fine-tune the rate at which sulfate dissolves when the cement is mixed with water. This is typically determined by an air permeability test.
A large mill might be about 15 metres long and 5 metres in diameter. Diaphragms perforated with holes or slots permit the passage of cement from one chamber to the next. Other test methods may give different values. The fineness of cement is one of the main factors controlling its behaviour when mixed with water.
How easy a particular clinker is to grind. The grinding of clinker requires a lot of energy. As part of the grinding process. Fineness is measured s the specific surface area in square metres per kilogram m2 kg H2O sticking cement particles together. Cement mills need to be cooled to limit the temperature rise of the cement.
On further heating. Solubilities of the different sulfate types in decreasing order are approximately: Over-large crystals can lead to erratic setting characteristics. Since the clinker gets hot in the mill due to the heat generated by grinding. This is done by air-cooling. In controlling the rate of aluminate hydration to regulate the setting of cement.
Problems associated with setting and strength characteristics of concrete can often be traced to one or more of: Changes in soluble sulfate availability can also affect the performance of some concrete admixtures. The relative proportions and different solubilities of these various types of calcium sulfate are of importance in controlling the rate of aluminate hydration and consequently of cement set retardation. Portland cement manufacturing — from raw materials to cement Air-setting is due to small crystals of syngenite calcium potassium sulfate hydrate.
It then forms hemihydrate. To summarise this complex issue of clinker sulfate and added gypsum and other sulfate: This will be governed by many factors. They operate by inhibiting the formation of coatings of powder on the grinding media. While this cost comparison is undeniable. Limestone Small amounts of limestone. Some grinding aids are also designed to affect cement hydration. Grinding aids Grinding aids are small quantities typically up to 0.
Cynics may say that cement companies like to add limestone because limestone is cheaper than cement. Limestone is softer than clinker and so generally grinds preferentially. In the past. This is a relatively recent development in many countries.
By intergrinding limestone. This is because chromium causes skin sensitisation. The added limestone acts as a filler. Fine limestone can also be beneficial by providing nucleation sites on which hydration products can form. For related reasons. The fine filler effect may also improve later strengths by producing a more uniform hydrate microstructure.
Chromium VI compounds are more water-soluble and so are more likely to cause problems. Not all chromium in cement is chromium VI. After making the deduction for sulfate and for free lime if required. This is more complex than for a clinker as we need to consider what other materials may have been added to the cement at the grinding stage.
One approach is to subtract 0. In the European Union. If the purity of the limestone is known. H2O or heptahydrate FeSO4. To allow for limestone in the calculation. For many years. At such low levels of addition. The American standard specification for Portland cement ASTM C contains a modified version of the Bogue calculation that allows for both added gypsum and limestone: By mass of cement.
Some standard specifications permit the addition of other materials also. Portland cement manufacturing — from raw materials to cement Reducing agents are added to control the Chromium VI content of the cement.
Typical reducing agents are ferrous sulfate. As given above, this is a calculation of the potential phase composition, since it does not allow for any free CaO. For any calculations related to that standard specification, the calculation stated in the standard should obviously be followed. As discussed previously Chapter 3. Chapters Cement hydration is the process by which cement and water react together.
When cement and water are mixed in suitable proportions, the result of the reaction is a solid mass, composed of gel and crystalline material, which binds together the constituents of a concrete mix. In a Portland cement composed of clinker and interground gypsum, the reactants in the process are:.
A mix of cement and water forms a hard cement paste. Concrete is a mix of cement, water and aggregate ie: Because it is the cement paste that binds all the constituents together as a single, solid, mass it follows that the properties of the concrete strength and durability for example will be critically influenced by the properties of the cement paste.
When cement is mixed with water, virtually all the alkali sulfate from the clinker dissolves rapidly. Calcium sulfate dissolves until the solution becomes saturated. Of the four main clinker phases, the aluminate phase is the most reactive, followed by alite, then belite. Ferrite is initially very reactive but the hydration subsequently slows appreciably. When cement and water are mixed, exothermic reactions occur. The rate at which heat is evolved varies for the first three days or so and then gradually declines.
The general shape of this plot is useful in understanding the reactions that occur as the cement hydrates Figure 6. The shape of the plot shows a rapid exothermic reaction at first I in Figure 6.
Figure 6. Stages I to V are described in the text. The initial reaction lasts only a few minutes, or less, and is followed by a dormant, or induction, period between I and II in Figure 6. Subsequently, the rate of hydration picks up again; during this second period of heat evolution, the. The main characteristics of the plot in Figure 6. The cement paste sets when solid particles in the mix become tenuously connected by the products of the hydration reactions. Rapid-hardening cement may have similar setting times to normal ordinary Portland cement.
There are laboratory procedures for determining these using weighted needles penetrating into the cement paste. Hydration of cement — chemical and physical properties of cementitious materials concrete sets and gains strength.
As time passes. The rate of heat evolution in this second period reaches a peak III in Figure 6. The rate of hardening. After a few days. During this dormant. CH will also form from the hydration of free lime in the cement. It usually forms as hexagonal plates. Calcium silicate hydrate is the main strength-giving component of hydrated Portland cement and it is therefore usually advantageous to maximise the proportion of C-S-H produced. Hydration of cement — chemical and physical properties of cementitious materials products continue to form.
S and H are intended. The four main clinker minerals produce hydration products broadly as follows: Calcium hydroxide: It may be conveniently shortened to C-S-H in cement chemistry notation. Dense growths of CH can also infill pores and the hexagonal crystal shape is not then apparent.
Alite hydration will therefore result in excess calcium if the C-S-H that forms has a Ca: Si ratio of 2: In contrast. The excess calcium oxide produces calcium hydroxide by reacting with water. Belite C2S in the pure form also has a Ca: Si ratio of about 2: Calcium silicate hydrate: Si ratio of approximately 3: These physical changes can be explained more fully by first considering the hydration products that form as hydration proceeds.
S and H respectively. In Portland cement. The important features in this image are the long. Towards the right of the image. Towards the left of this image. Ettringite forms initially as acicular crystals — thin rods a micron or so in length. This picture illustrates a transitional stage where some regions of the paste remain porous and weak.
These sparse. After another day or so. Monosulfate can be expressed as: Monosulfate then forms instead of ettringite. In discussing cement hydration products. Fine limestone affects cement hydration in two principal ways: Monosulfate typically forms as thin platelets a few microns across. CaSO4 in monosulfate is therefore 1: Monosulfate resembles ettringite in being a hydrated compound containing calcium.
Hydration of cement — chemical and physical properties of cementitious materials Another important phase is: Monosulfate phase: In the early stages of cement hydration. Hydration of cement — chemical and physical properties of cementitious materials 6. In both of these groups. Monosulfate is one of another group termed AFm phases.
Singly-charged anions may be accommodated to a limited extent. Neglecting any iron substitution for aluminium. The general composition of an AFm phase can be written as: The general composition of an AFt phase is: Unless you are doing a PhD in cement hydrate composition. Temperature may also have an effect. AFm phases will be monosulfate and hemicarbonate. Ettringite will be the AFt phase and monosulfate the main AFm phase. There will also be other minor phases. The most significant AFm phases in the context of cement hydration are: Where interground limestone is present in the cement.
Hydration of cement — chemical and physical properties of cementitious materials AFm phases are numerous. Ettringite will be the AFt phase. Another difference between false set and flash set is that a false set may be reversed if continued mixing can break up the gypsum crystals bonding the solids.
The rapid hydration of the C3A results in the evolution of much heat. Concrete strengths where flash setting has occurred may be lower than normal.
The resulting gypsum fragments then slowly dissolve and the concrete should set normally and show fairly normal strength growth. These crystals link up the particles cement.
If an excess of sulfate is present in the form of hemihydrate. Smaller cement grains have fully reacted. A period of lower activity follows the dormant period. The cement reacts in two ways. C-S-H and CH formation in the large water-filled gaps between the cement grains continues also at a slower rate and strength development continues at a slower rate reflecting the slower C-S-H formation. Once the small gap between the anhydrous cement grain and surrounding dense shell has filled.
The hydration products at four weeks are www. A small fluid-filled gap less than a micron across appears between the surface of the unhydrated cement grain and the surrounding dense shell of C-S-H. By about 4 weeks. C-S-H and CH continue to form in the large water-filled gaps between the cement grains.
With this decrease in sulfate availability. Small crystals of ettringite also form. As alite becomes depleted. Continued aluminate hydration produces larger rod-like ettringite crystals. Sulfate from the gypsum and clinker sulfate starts to dissolve and an amorphous gel containing alumina.
The initial set typically occurs at about 2 hours and hydration restarts in earnest after hours. In ettringite. C-S-H forms dense shells around cement grains and a delicate sheet structure bridging the water-filled gaps between the cement grains. By this time. This causes a gradual change in the ratio of available sulfate to aluminate. Look first at Figure 6. After a year. These effects are apparent in the sequence of four images that follow.
This has two direct consequences: The cement paste shown in Figure 6. At this magnification. AFt and AFm phases probably ettringite and monosulfate are also likely to be present but are too small to be seen at this magnification. These particles have reacted to a considerable extent. CH is just visible. The hydration products in Circle A have formed in a region that was occupied by water when the cement and water were mixed.
AFm and AFt are not visible as they are too small. Hydration of cement — chemical and physical properties of cementitious materials Figure 6. CH — calcium hydroxide. Although it looks solid and dense in the image. They appear not to be connected. In this case. Now look at the image in Figure 6. As in Figure 6. The capillary pores black. Two features are immediately clear: The capillary pores in Figure 6. Now that you know what you are seeing in these images of cement paste. The pores became filled with epoxy resin when the polished section was prepared and so the black features are actually epoxy resin.
Portland cement. Most of the unhydrated cement in Figure 6. CH fills some of the pores and occupies extensive areas. As the crystalline components of the hydration product eg: The classical model. Increased permeability will mean that the concrete is less resistant to deleterious processes such as frost damage. Hydration of cement — chemical and physical properties of cementitious materials Additionally.
The Powers-Brownyard model simplifies the cement paste into three components: What follows is a brief summary. As the cement continues to hydrate.
If a cement paste is cured in a sealed container. If additional water is www. Experimental evidence indicated to Powers and Brownyard that the hydration product. Powers and Brownyard used the model to calculate some useful values: As the quantity of hydration product increases. To summarise. Hydration of cement — chemical and physical properties of cementitious materials The model assumes that. This is known as the critical water-cement ratio. At values below the critical water-cement ratio.
Cement wouldn't be so interesting if it weren't for occasional reminders that maybe we don't understand it as well as we thought. In this context.
What follows is the briefest of introductions but references are given where more detail can be found if needed. Two properties are immediately obvious: These three properties are huge subjects in their own right. Other models of cement paste structure have been proposed.
These values are regarded as being approximately correct for most cements but there will inevitably be some variation.
Workability can be defined in terms of the energy required to overcome the friction between the particles in the concrete in order to achieve full compaction. The gel porosity. In the slump test. As discussed above. This includes all the things that are done to wet concrete. A third. Despite the limitations of the model. Higher numbers mean the concrete is more workable.
Hydration of cement — chemical and physical properties of cementitious materials available. Picture to ensure good compaction. Workability is important because it indicates how easily concrete can be compacted. The main factor affecting the workability of a concrete mix is the water content - as more water is added. Obtaining the maximum possible density of the wet mix is important to maximise the strength of the hardened concrete and its resistance to deterioration. Picture courtesy Kirton between the top of the concrete and the Concrete Services Ltd.
Picture courtesy Kirton Concrete Services Ltd. Other factors include: Concrete made from the cement will still be subjected to traditional cube or cylinder tests when delivered to a construction site.
If more water is added to increase workability again. Once the concrete starts to stiffen as it sets at the end of the induction. Hydration of cement — chemical and physical properties of cementitious materials the fineness of the cement and whether any water-reducing admixtures are present. Under the European cement standard EN See Reference 7 for more on workability.
It is clear that. The plot in Figure 6. Concrete is much stronger under compression than it is under tension. Compressive strengths of concrete have been traditionally measured by crushing cubes or cylinders of concrete at particular ages under controlled conditions. The shape of the curve and the strengths attained will be subject to many different factors.
In the first few days after mixing. Paste contains capillary pores and gel pores but it is the system of capillary pores that controls the permeability of the paste. From the images of pastes in Figures 6. See Reference 8 for more on concrete strength.
See Reference 9 for more on concrete permeability.
These are elasticity. Assuming the aggregate to be impermeable and that no cracks are present. The porosity of the concrete is therefore of prime importance. Hydration of cement — chemical and physical properties of cementitious materials Many factors influence concrete strength.
The size of the capillary pores. Permeability is important principally because an increase in permeability is likely to result in an increase in the rate of concrete deterioration due to chemical attack or by freezing.
At early ages of hydration. Assuming the aggregate is of low porosity. Concrete strength increases as its density increases.
Cement and Concrete Research 37 Vol Hydration of cement — chemical and physical properties of cementitious materials While important. Journal American Concrete Institute Proceedings. If you would like to read more on these specifically. Chapter 6 1. A M Neville.
Chapter 6. B Lothenbach and F P Glasser. H F W Taylor. Chapter 4 8. M Rossler and I Odler. Thomas Telford. Prentice Hall. Chapter 9. The most common examples of mineral additions are: It may be interground with the cement clinker or blended later at the concrete plant or on site.
To avoid confusion between the ratio of water to Portland cement. The additional reactive material may be described variously as an extender. While their use may well be beneficial. The use of composite cements can bring technical benefits in terms of improved concrete properties and these will be discussed below in the section for each of the different materials considered. Calcined clay and shale are used as mineral additions.
Examples of pozzolanic materials are fly ash. Volcanic glass is a natural pozzolan and is widely used where it is available and is recognised by some national standard specifications eg: EN They are generally deficient in lime and so need the addition of lime to form calcium silicate hydrate. Crushed fired clay brick. Concrete cured at low temperatures develops strength more slowly than concrete cured at higher temperatures. By using mineral additions to replace some of the Portland cement.
The most commonly-used mineral addition of this type is slag. The strength increase is principally because the mineral additions increase the total amount of calcium silicate hydrate C-S-H in the hydration product. The reason for this is not fully understood but appears to be related to how the microstructure of the paste develops. Composite Cements also be some potential difficulties with their use.
Curing temperature can also have a significant effect. Increasing the AFm content of the paste will evidently bind some lime in the additional AFm. AFm phases also chemically bind some ingressing dissolved anions. Slag and fly ash. This is particularly so at early ages. In slag. Inclusion of any of these reactive mineral additions in cement therefore increases the amount of silica available for the formation of hydration products compared with a Portland cement-only mix.
This increases the proportion of hydrate phases containing aluminium. Mineral additions generally react more slowly than does Portland cement. In low-lime fly ash. Comparison of porosities in pastes made with Portland cement only. AFm phases are beneficial in that they can block capillary pores in the paste. Composite Cements Compared with Portland cement.
The additional silica combines with calcium hydroxide CH in the paste. Pores in Portland cement-only pastes appear to be more continuous than pores in pastes made with composite cements. AFm phases may make some contribution to strength by reducing paste porosity. Microsilica and metakaolin contain little or no calcium. How a paste made with a composite cement can have a higher porosity but a lower permeability is not fully understood. As well as physically blocking pores. The actual savings will obviously depend on the level of replacement of Portland cement.
Taking the United Kingdom as an example. Cements containing other combinations of cementitious materials have intermediate ECO2 figures. Figures for the CO2 emissions for Portland cement production vary widely but are generally gradually declining as the cement producers make efforts to lower them. Material Embodied CO2 kg CO2 tonne-1 Portland cement CEM I Ground granulated blastfurnace slag 52 Fly ash from coal burning power generation 4 Limestone 32 These illustrative figures demonstrate that significant savings in CO2 emissions can be achieved by the use of mineral additions.
Table 7. Most of the above are self-evident. Composite Cements 7. For an equivalent day strength to a concrete made with Portland cement only. If the resulting liquid is tapped off and cooled rapidly. A higher total cementitious content ie: Portland cement plus slag. Much may also depend on the concrete curing temperature. The slower strength gain is important for two reasons: To some extent.
Test data on cubes. Possible exceptions are microsilica and metakaolin. If the liquid is cooled slowly. Limestone is added as a flux during smelting to combine with silica and other impurities in the iron ore. Using mineral additions may make the concrete more susceptible to carbonation Chapter 9. The crystalline fraction is unreactive. When allowed to cool slowly the crystalline content of the slag increases.
Pelletised slag used in concrete generally has a lower glass content than granulated slag. Rapid cooling is generally achieved by one of two methods: An earlier version. The slag breaks into pellets and is propelled through the air in a chamber full of water spray. Composite Cements Slag used in concrete is largely composed of a calcium aluminosilicate glass.
Chemical moduli have been used to evaluate slags. Figure 7. Composite Cements Figure 7. Hanson Cement. The glassy fractions of the two slags cooled at different rates may therefore have different reactivities. In addition. LOI Total Because of the need for precise control of the iron production process. Reactivity will depend particularly on the composition of the glassy fraction.
As used in cement. Whether a slag is suitable for use as a cementitious material in a composite cement depends mainly on how reactive the material is.
Composite Cements Slag is an impure calcium aluminosilicate glass Table 7. The main technical benefits of using slag in a composite cement are: Permeability and durability Mature concrete made with cement containing slag is generally of lower permeability to water than concrete made with Portland cement only. The heat of hydration of a composite cement containing slag is therefore less than that of a comparable PC-only mix. Later strengths should be higher in a suitably-designed mix.
Although slag powder is almost colourless. Chloride and sulfate penetration should be lower. Higher later strengths. Both the rate of heat evolution and the total heat evolved are lower in mixes where Portland cement is partially replaced with slag.
Strengths Partial replacement of Portland cement by slag generally results in lower early strengths. Heat of hydration Compared with Portland cement. Slag does not normally contain ferrite or other strongly-coloured minerals so partial replacement of Portland cement with slag should result in a lighter colour of concrete. In mature concrete. Colour The grey colour of Portland cement is due to the ferrite phase. Alkali silica reaction ASR is also less likely.
This is thought to be due to sulfide from the slag entering the hydration products.