Advanced concrete technology book download

Your email address will not be published. Save my name, email, and website in this browser for the next time I comment. The effects of natural aggregates on the properties of concrete Concrete is defined as a workable mixture of a cementations material mostly cement mixed in suitable proportion with fine and coarser aggregate along with water. Download Advanced Concrete Technology Concrete has specific advantages over other structural materials like timber and steel structures.

The Content is for Members Only!!! Chapter 2 provides the knowledge of raw materials used for making concrete, covering aggregates, binders, admixtures, and water. Chapter 3 discusses the properties of fresh concrete, including workability and the corresponding measurement methods.

Chapter 4 focuses on the structure of concrete at different scales, especially the calcium silicate hydrate at nanometer scale. Chapter 5 covers the properties of hardened concrete, including strength, durability, stress—strain relation, and dimension stability. Chapter 6 provides updated knowledge on various cement-based composites, including self-consolidation concrete, ultra-high-strength concrete, and extruded and engineered cementitious composites.

Chapter 7 focuses the fracture behavior of concrete and provides the basic knowledge of fracture mechanics of concrete. Chapter 8 covers the essential knowledge of non-destructive testing of concrete engineering, including wave propagation theory in 1-D case, detecting principles of different NDT methodologies and techniques of different NDT methods. In Chapter 9, the issues regarding the future and development trend of concrete have been discussed.

Although the book is designed and written primarily to meet the teaching needs for undergraduate students at senior level and graduate students at entry level, it can serve as a reference or a guide for professional engineers in their practice.

Introduction to Concrete 2. Materials for Making Concrete 3. Fresh Concrete 4. Structure of Concrete 5. Hardened Concrete 6. The secondary raw material, which provides the necessary silica, alumina and iron oxide, is normally shale or clay.

Small quantities of sand or iron oxide may be added to adjust the levels of silica and iron oxide in the mix. When proportioning the raw materials, an allowance must be made for ash incorporated into the clinker from the fuel that fires the kiln. Most cement plants worldwide use finely ground pulverized coal as the primary fuel.

Increasingly, by-product fuels such as the residue from oil refining petroleum coke and vehicle tyres are being used to partially replace some of the coal. Typical contents of the four principal oxides in a simplified cement making operation utilizing only two raw materials are given in Figure 1.

Note that the ratio of CaO to the other oxides is lower in the clinker than in the raw mix. This is a result of the incorporation of shale from the coal ash. The levels of the oxides are also increased as a result of decarbonation removal of CO2. In the second half of the twentieth century significant advances were made which have culminated in the development of the precalciner dry process kiln.

The precalcination of the feed brings many advantages, the most important of which is high kiln output from a relatively short and small-diameter rotary kiln.

Almost all new kilns installed since have been of this type. Figure 1. The raw materials are ground to a fineness, which will enable satisfactory combination to be achieved under normal operating conditions. The homogenized raw meal is introduced into the top of the preheater tower and passes downwards through a series of cyclones to the precalciner vessel. The raw meal is suspended in the gas stream and heat exchange is rapid. Material residence time in the rotary kiln of a precalciner process is typically 30 minutes.

The air which cools the clinker is used as preheated combustion air thus improving the thermal efficiency of the process.

As will be discussed in section 1. If coal is the sole fuel in use then a modern kiln will consume approximately 12 tonnes of coal for every tonnes of clinker produced. The high fuel loading in the static precalciner vessel reduces the size of rotary kiln required for a given output and also reduces the consumption of refractories.

A wider range of fuel types for example, tyre chips can be burnt in the precalciner vessel than is possible in the rotary kiln. The composition of the minerals and their normal range of levels in current UK and European Portland cement clinkers are summarized in Table 1. It is the two calcium silicate minerals, C3S and C2S, which are largely responsible for the strength development and the long-term structural and durability properties of Portland cement. However, the reaction between CaO lime from limestone and SiO2 silica from sand is very difficult to achieve, even at high firing temperatures.

The sequence of reactions, which take place in the kiln, is illustrated in Figure 1. This is the lower limit of thermodynamic stability of alite C3S. The ease with which the clinker can be combined is strongly influenced by the mineralogy of the raw materials and, in particular, the level of coarse silica quartz present.

The higher the level of coarse silica in the raw materials, the finer the raw mix will have to be ground to ensure satisfactory combination at acceptable kiln temperatures. Coarse silica is also associated with the occurrence of clusters of relatively large belite crystals around the sites of the silica particles. Figures 1. The belite present in the clusters is less reactive than small well-distributed belite and this has an adverse influence on cement strengths. As the clinker passes under the flame it starts to cool and the molten C3A and C4AF, which constitute the flux phase, crystallize.

Slow cooling should be avoided as this can result in an increase in the belite content at the expense of alite and also the formation of relatively large C3A crystals which can result in unsatisfactory concrete rheology water demand and stiffening. The formula for LSF has been derived from high-temperature phase equilibria studies. This procedure is discussed further in section 1.

Normally, if the LSF is increased at a particular cement plant, the raw mix must be ground finer, i. Proportions of clinker minerals Burning temperature required to Wt. The silica ratio, SR, is the ratio of silica to alumina and iron oxide. Of greater significance, with regard to clinker manufacture, is that the higher the SR, the less molten liquid, or flux, is formed.

This makes clinker combination more difficult unless the LSF is reduced to compensate. The flux phase facilitates the coalescence of the clinker into nodules and also the formation of a protective coating on the refractory kiln lining.

Both are more difficult to achieve as the SR increases. The alumina ratio, AR, is normally the third ratio to be considered.

Fortunately, the AR ratio is relatively easy to control by means of a small addition of iron oxide to the mix. However, they give a good guide to cement properties in terms of strength development, heat of hydration and sulfate resistance. When calculating the compound composition of cements rather than clinkers the normal convention is to assume that all the SO3 present is combined with Ca i.

The total CaO is thus reduced by the free lime level and by 0. Examples of the calculation for cements are given in Table 1. The influence of minor constituents on cement manufacture and cement properties has been reviewed Moir and Glasser, ; Bhatty, Table 1.

The crystallization products depend on the relative levels of the two alkali oxides and the level of SO3. If there is insufficient SO3 to combine with the alkali metal oxides then these may enter into solid solution in the aluminate and silicate phases. C2S can be stabilized at temperatures above oC thus impeding the formation of C3S. A deficiency of SO3 in the clinker is associated with enhanced C3A activity and difficulties in achieving satisfactory early age concrete rheology.

Fluorine occurs naturally in some limestone deposits, for example in the Pennines in England, and has a beneficial effect on clinker combination. It acts as both a flux and mineralizer, increasing the quantity of liquid formed at a given temperature and stabilizing C3S below oC.

Minor constituents also have to be controlled on account of their impact on cement properties and also concrete durability. Related to this, the levels of alkalis, SO3, chloride and MgO are also limited by national cement standards or codes of practice. These aspects are reviewed in section 1.

If clinker stocks are low then the clinker may be ground to cement without the opportunity to cool further during storage. These reactions are considered in detail in section 1. Macs can be helpful in optimizing cement rheological properties.

The vast majority of cement produced throughout the world is ground in ball mills, which are rotating tubes containing a range of sizes of steel balls. A closed-circuit milling installation is illustrated in Figure 1. Closed-circuit mills normally have two chambers separated by a slotted diaphragm, which allows the partially ground cement to pass through but retains the grinding balls.

The first chamber contains large steel balls 60—90 mm in diameter , which crush the clinker. Ball sizes in the second chamber are normally in the range 19—38 mm.

The mill operates in a closed circuit in which the mill product passes to a separating device where coarse particles are rejected and returned to the mill for further grinding. The final product can thus be significantly finer than the material that exits the mill. The efficiency of the clinker grinding process is very low. Modern mills are equipped with internal water sprays, which cool the process by evaporation.

This has some advantages but the level of dehydrated calcium sulfate has to be controlled to optimize the water demand properties of the cement. This aspect is discussed in section 1. The low efficiency of the grinding process has resulted in considerable effort being directed to find more efficient processes.

Some of these developments are listed in Figure 1. As a general rule the more efficient the grinding process, the steeper the particle size grading. The range of particle sizes is smaller and this can result in increased water demand of the cement, at least in pastes and rich concrete mixes.

This is because with a narrow size distribution there are insufficient fine particles to fill the voids between the larger particles and these voids must be filled by water.

One compromise, which lowers grinding power requirement without prejudicing product quality, is the installation of a pre-grinder, such as a high-pressure roll press, to finely crush the clinker obviating the need for large grinding media in the first chamber of the ball mill.

The hydration process has been comprehensively reviewed Taylor, The reactions are summarized in Table 1. This is considerably lower than the ratio in C3S and the excess Ca is precipitated as calcium hydroxide CH crystals.

C2S hydration also results in some CH formation. This is of practical significance because it allows concrete to be placed and compacted before setting and hardening commences. The most favoured is that the initial reaction forms a protective layer of C—S—H on the surface of the C3S and the dormant period ends when this is destroyed or rendered more permeable by ageing or a change in structure.

Reaction may also be inhibited by the time taken for nucleation of the C—S—H main product once water regains access to the C3S crystals. However, the findings should be interpreted with caution as the composition of the aluminate phases in industrial clinker differs considerably from that in synthetic preparations and hydration in cements is strongly influenced by the much larger quantity of silicates reacting and also by the presence of alkalis.

This is a rapid and highly exothermic reaction. The role of gypsum dehydration is considered further in section 1. When the available sulfate has been consumed the ettringite reacts with C3A to form a phase with a lower SO3 content known as monosulfate. The iron enters into solid solution in the crystal structures of ettringite and monosulfate substituting for aluminium. In order to reflect the variable composition of ettringite and monosulfate formed by mixtures of C3A and C4AF they are referred to respectively as AFt alumino-ferrite trisulfate hydrate and AFm alumino-ferrite monosulfate hydrate phases.

A simplified illustration of the development of hydrate structure in cement paste is given in Figure 1. When cement is first mixed with water some of the added calcium sulfate particularly if dehydrated forms are present, and most of the alkali sulfates present see section 1. If calcium langbeinite is present then it will provide both calcium and sulfate ions in solution, which are available for ettringite formation.

The supply of soluble calcium sulfate controls the C3A hydration, thus preventing a flash set. Ground clinker mixed with water without added calcium sulfate sets rapidly with heat evolution as a result of the uncontrolled hydration of C3A.

The cement then enters a dormant period when the rate of loss of workability is relatively slow. Setting time is a function of clinker mineralogy particularly free lime level , clinker chemistry and fineness. The finer the cement and the higher the free lime level, the shorter the setting time in general. Setting is largely due to the hydration of C3S and it represents the development of hydrate structure, which eventually results in compressive strength.

The C—S—H gel which forms around the larger C3S and C2S grains is formed in situ and has a rather dense and featureless appearance when viewed using an electron microscope. The structure of the outer product is strongly influenced by the initial water-to-cement ratio, which in turn determines paste porosity and consequently strength development. The progress of the reactions can be monitored using the technique of isothermal conduction calorimetry Killoh, In some cements with a low ratio of SO3 to C3A it may be associated with the formation of monosulfate.

The heat release is advantageous in cold weather and in precast operations where the temperature rise accelerates strength development and speeds up the production process.

The data were obtained using the equipment described by Coole The extent of hydration is strongly influenced by cement fineness and in particular the proportions of coarse particles in the cement. Elevated temperature curing, arising from either the semi-adiabatic conditions existing in large pours or from externally applied heat, is associated with reduced ultimate strength.

These dehydrated forms of gypsum are present in commercial plasters and it is the formation of an interlocking mass of gypsum crystals which is responsible for the hardening of plaster once mixed with water. An inadequate supply of soluble calcium sulfate can result in a rapid loss of workability known as flash set. This is accompanied by the release of heat and is irreversible.

However, if too high a level of dehydrated gypsum is present, then crystals of gypsum crystallize from solution and cause a plaster or false set. This is known as false set because if mixing continues, or is resumed, the initial level of workability is restored.

The cement manufacturer thus needs to optimize the level of dehydrated gypsum in the cement and match this to the reactivity of the C3A present. This concept is illustrated in Figure 1. Many natural gypsums contain a proportion of the mineral natural anhydrite CaSO4 — not to be confused with soluble anhydrite which is produced by gypsum dehydration. This form of calcium sulfate is unaffected by milling temperature and dissolves slowly in the pore solution providing SO 2— 4 ions necessary for strength optimization but having no potential to produce false set.

Cement—admixture interactions are complex and some admixtures will perform well with certain cements but may perform relatively poorly with others. Thermal analysis The most commonly applied technique is thermo-gravimetric analysis. The technique enables the proportion of certain hydrates present, such as ettringite and Ca OH 2 to be determined quantitatively. X-ray diffraction This technique is rapid but provides limited information as many of the hydrates present, notably C—S—H gel and calcium aluminate monosulfate are poorly crystalline and give ill- defined diffraction patterns.

Scanning electron microscopy SEM This is a powerful technique, particularly when the microscope is equipped with a microprobe analyser. It involves techniques akin to X-ray fluorescence to determine the chemical composition of hydrates in the field of view. The high resolution of the SEM enables the microstructure of the hydrated cement paste in concrete or mortar to be studied.

However, caution must be exercised when interpreting the images as specimen preparation and the vacuum required by most microscopes can generate features, which are not present in the moist paste. A typical example is given in Table 1. It can be seen that the dominant phase present by volume is C—S— H with approximately equal quantities of calcium hydroxide and monosulfate.

These hydrate proportions are changed significantly when Portland cement is blended or interground with pozzolanas such as fly ash or granulated blastfurnace slag. These reactions and the hydration products are discussed briefly in section 1. They also describe the test procedures to be used to determine cement composition and cement properties. Although it is now rather out of date, particularly in relation to the standards in place in European countries, the publication by Cembureau, provides a useful review of cement types produced around the world.

The objective of this standard in common with standards for other materials is to remove barriers to trade. In order to meet this objective, existing national standards in the above countries were withdrawn in It can be expected that this European standard and the supporting standards for test methods EN and for assessment of conformity EN will have a strong influence on national or regional cement standards in the future.

The descriptions given in the table are very general. Low heat properties at high fly ash levels Portland slag Composite Clinker, granulated All types of construction. Protection against alkali silica reaction recognized in some countries. Low heat properties at high slag levels Portland Composite Clinker, limestone of In Europe all types of limestone specified purity and construction cement calcium sulfate Pozzolanic Composite Clinker, natural pozzolana All types of construction.

In this chapter the term composite is applied to all cements containing clinker replacement materials other than a minor additional constituent. Many countries have national specifications for sulfate- resisting Portland cement e.

The European Committee responsible for cement standardization has been unable to reach agreement on the maximum C3A level required to ensure sulfate resistance and sulfate- resistant Portland cements are not recognized as a separate cement type in EN The frequent changeovers disrupt the normal production process and have an adverse effect on the lifetime of the refractory bricks, which line the kiln. Sulfate-resisting concrete can also be produced by ensuring an appropriate level of fly ash or blastfurnace slag.

This can be achieved either by purchasing a factory-produced cement or where national provisions permit by blending fly ash or ground slag with cement. The production of white cement clinker requires careful selection of materials and fuels to ensure the minimum content of iron oxide and of other colouring oxides such as chromium, manganese and copper.

In order to achieve the best possible colour the clinker is normally fired under conditions where there is a slight deficiency of oxygen resulting in reduction of the colouring oxides to lower oxidation states, which have a lesser detrimental effect, and the clinker is quenched rapidly with water to prevent oxidation.

All these measures increase the production cost and white cement sells at a significant premium over grey Portland cement. Composite cements Composite cements are cements in which a proportion of the Portland cement clinker is replaced by industrial by-products, such as granulated blastfurnace slag gbs and power station fly ash also known as pulverized-fuel ash or pfa , certain types of volcanic material natural pozzolanas or limestone.

The gbs, fly ash and natural pozzolanas react with the hydration products of the Portland cement, producing additional hydrates, which make a positive contribution to concrete strength development and durability. Massazza, ; Moranville-Regourd, It is introduced to assist in the control of cement strength development and workability characteristics.

The proportion of composite cements in the UK is at present very much lower, and the UK also differs from most European countries in that the addition of ground granulated blastfurnace slag ggbs and fly ash at the concrete mixer is well established.

Further details are given in section 1. The partial replacement of energy-intensive clinker by an industrial by-product or a naturally occurring material not only has environmental advantages but also has the potential to produce concrete with improved properties including long-term durability. The characteristics of the constituents of composite cements are summarized in Table 1. Occasionally glass zeolite type material Pozzolanic materials contain reactive usually in glassy form silica and alumina, which are able to react with the calcium hydroxide released by hydrating cement, to yield additional C—S—H hydrate and calcium aluminate hydrates.

The nature of additions and the factors determining their performance are reviewed in detail in Chapter 3. Fly ash has a significant advantage over natural pozzolanas as a result of the spherical shape of the glassy particles. These normally have a positive influence on concrete workability enabling concrete water contents to be reduced and thus offsetting the early age strength reduction.

Fly ash performance may be improved by either removing coarse particles using a classifier similar to that employed in closed-circuit cement grinding or by co- grinding the ash with clinker. Blastfurnace slag is latently hydraulic and only requires activation by an alkaline environment to generate C—S—H and calcium aluminate hydrates.

While limestone constituents do not contribute significantly to strengths at 28 days, they do accelerate Portland cement hydration, and the reduction in early strength is normally less than in the case of fly ash. The most important characteristic of a limestone constituent is that it should comply with the purity requirements of the relevant standard BS EN In Figure 1.

The British Standard for sulfate-resisting cement, BS , will continue until such time as agreement is reached on a European Standard for sulfate-resisting cement.

For example, Portland burnt shale cement requires a particular shale type, which is only found in southern Germany. Free lime up to 2. Similarly the requirements for fly ash are essentially the same as those in the British Standards for Portland pulverized-fuel ash cement BS and for pulverized-fuel ash BS Part 1 although there are minor differences related to maximum LOI and CaO content. Standards for concrete additions are reviewed in greater detail in Chapter 3. Compressive strength is determined using the EN mortar prism procedure, which is outlined in section 1.

Setting times are determined by the almost universally applied Vicat needle procedure and soundness by the method first developed by Le Chatelier in the nineteenth century. These methods are described in EN In the UK the established practice is to add ground-granulated slag to BS or pulverized-fuel ash to BS Part 1 direct to the concrete mixer and to claim equivalence to factory-produced cement. In addition, some UK cement standards include cements with strength classes and properties outside the scope of BS EN for common cements.

Both of these standards will be withdrawn when European Standards covering the same scope are eventually published. The chemical requirements of EN cements are summarized in Table 1. The LOI ensured cement freshness and the IR limit prevented contamination by material other than calcium sulfate and clinker. A higher level of assurance of consistency of performance is provided by the much more rigorous performance tests, which must be performed on random despatch samples at least twice per week.

The upper limit for SO3 features in all cement standards and its purpose is to prevent expansion caused by the formation of ettringite from unreacted C3A once the concrete has hardened. This expansive reaction, which occurs at normal curing temperatures a few days after mortar or concrete is mixed with water, should be distinguished from the phenomenon of delayed ettringite formation DEF.

The cement factors which increase the risk of DEF have been identified by Kelham The cement SO3 level has a positive influence on cement strength development, particularly at early ages, and over the past 20 years there has been a trend to raise the upper limit. The purpose of the upper chloride limit is to reduce the risk of corrosion to embedded steel reinforcement.

Although the upper limit is 0. The concrete producer must, of course, consider all sources of chloride water, aggregates, cement and admixtures when meeting the upper limit for chloride in concrete. EN also describes the testing frequencies and the method of data analysis required to demonstrate compliance with the requirements of the standard. Note that the values given in Tables 1.

A given percentage of the results obtained on random despatch samples may lie above or below these values. The spot samples taken at the point of cement despatch are known as autocontrol samples and the test results obtained as autocontrol test results.

Certificates confirming compliance with the requirements of the standards can be issued by EU Notified Certification Bodies e. As EN is a harmonized standard, the certification body can issue EC certificates of conformity which permit the manufacturer to affix the CE marking to despatch documents and packaging. The CE marking indicates a presumption of conformity to relevant EU health and safety legislation and permits the cement to be placed on the single European market.

Failure to consistently meet the requirements of the standard may result in withdrawal of the EC certificate. The solution may be a laboratory performance test but difficulties have been experienced in achieving a satisfactory level of reproducibility.

The chemical composition of raw materials for cement making, clinkers and cements can be determined by so-called wet chemical analysis methods. BS EN , The analysis requires between 2 and 6 hours to complete, depending on the facilities available, to determine the levels of the oxides S, A, F and C.

Wet techniques also require highly skilled staff, if reliable results are to be obtained. Fortunately, a rapid method of analysis known as X-ray fluorescence became available in the late s and this technique is almost universally applied at cement works around the world.

Sample preparation and the principles of the technique are outlined in Figure 1. The sample of raw meal, clinker or cement is inserted in the analyser, either in the form of a pressed disc of finely ground material or after fusing into a glass bead. In the analyser, the specimen is irradiated with X-rays, which cause secondary radiation to be emitted from the sample.

Each chemical element present emits radiation of a specific frequency, and the intensity of the radiation is proportional to the quantity of that element present in the sample. There is also a trend towards fully automated laboratories where the processes outlined in Figure 1. The level of uncombined lime free lime present in clinkers is normally determined by extracting the CaO into hot ethylene glycol and titrating the solution with hydrochloric acid.

Free lime can also be determined using the technique of X-ray diffraction and this is finding increasing favour as it is easier to automate than the glycol extraction method.

Close control over cement milling is essential to ensure a product with consistent properties. Although cement particle size distribution can be determined directly using laser diffraction techniques there have been difficulties with stability of the measurements over a period of time and the main methods most commonly used on cement plants for fineness determination are surface area SA determined by air permeability and sieving. The time taken for a fixed volume of air to pass through the cell is a function of the cement surface area.

The proportion of coarse particles present, as indicated by the micron or for finely ground cements micron sieve residue has a greater influence on cement day strength than the surface area and it is important to monitor this parameter closely. The technique most commonly use is air-jet sieving where a stream of air passes through the sieve, agitating the material above the sieve and greatly speeding up the passage of sub-sized particles through the sieve.

In European plants, cement strengths are determined using the EN mortar test procedure. This utilizes a mortar, which consists of 3 parts by weight sand to 1 part cement. The dry sand, which is certified as meeting the requirements of the standard, is supplied in pre-weighed plastic bags, which simplifies the batching and mixing procedure.

The prisms can be broken in flexure prior to compressive strength testing but this stage is normally omitted and the prisms simply snapped in two using a simple, manually operated, breaking device. Normally three prisms are broken at each test age and the result reported is the mean of six tests. A typical class 42,5 cement will give a strength of 55—59 MPa at 28 days. Most UK cement plants should achieve an annual average standard deviation for the day strength results of main products in the range 1.

The setting time of cement paste is determined using the Vicat needle according to EN Consequently there is a trend to introduce automation and equipment can be purchased which will determine the setting time of a number of cement samples e. If the raw mix is variable then no amount of adjustments to subsequent control parameters will eliminate product variability.

Specialist thermal analysis equipment is required to determine the forms of calcium sulfate present. Although some cement plants have this equipment it is more generally found at central laboratories.

The reduced water demand can be mainly attributed to the glassy spheres present in fly ash which lubricate the mix. The lower density of the ash compared to cement also increases the cement paste volume. Blastfurnace slag grinds to yield angular particles. The slag is unreactive during initial hydration and generally has a neutral influence on water demand. Limestone can have a positive influence on water demand particularly when compared to a more coarsely ground pure Portland cement of the same strength class.

This is because the fine limestone particles result in a more progressive optimized cement particle size distribution with a lower proportion of voids, which must be filled with water. The effects of a steeper size distribution are much more apparent in paste than in concrete and concretes made from the same cement may not exhibit the same magnitude of extension in setting time.

Setting time may also be extended by the presence of certain minor constituents. An example is fluorine. An increase in clinker fluorine level of 0. Both fly ash and slag will increase setting time while a Portland limestone cement may have a slightly shorter setting time than the corresponding pure Portland cement. While surface area is a good guide to the early rate of hydration of cement and thus early strengths, it is a less reliable guide to late strengths and, in particular, to day strengths.

This is because under standard curing conditions clinker particles which are coarser than approximately 30 microns are incompletely hydrated at 28 days. For a given SA the lower the micron residue, the higher the day strength. In EN mortar an increase in micron residue level of 1. In general, more modern milling installations, and in particular those with high efficiency separation, will yield cements with steeper size distributions.

The steeper size distribution may require the introduction of a minor additional constituent mac or filler to control day strength and thus remain within a certain strength class Moir, Note that in some cases the base level may be slightly increased by CaCO3 present as an impurity in the calcium sulfate. The influence of LOI when a cement contains a calcareous mac is much less clear. In cement with several sources of LOI more sophisticated techniques such as X-ray diffraction, or direct determination of carbon can be used to determine the CaCO3 level.

Clinker alkalis and SO3 As described in section 1. In this form they are readily soluble and quickly dissolve in the gauging water modifying strength development properties. Soluble alkalis accelerate early strength development and depress late day strengths Oesbaek, Jons, Note that in these clinkers there was sufficient SO3 to combine most of the alkali present as sulfates.

A similar reduction in late strength can be achieved by adding alkali sulfates to cement; the increase in early strengths is normally less than when the sulfates are present in clinker. At early ages the dissolved alkali sulfate accelerates C3S hydration.

With high alkali cements the pH level may exceed If the level of clinker SO3 is insufficient to combine with the alkali then the accelerating effect is reduced but the depression of day strength is almost the same.

This is because the alkali held in solid solution in the clinker minerals is released as hydration progresses. Cements produced from high alkali clinker tend to give a better performance with slag and fly ash than equivalent low-alkali clinker. This is partly because the pH of the pore solution is reduced both by dilution and by absorption of alkalis in the lower calcium C— S—H formed Taylor, but also because of the more aggressive attack on the glassy phases present in the slag and fly ash.

Free lime Cement-free lime levels should normally lie in the range 0. Late strengths are normally maximized by a low free lime level as this maximizes the combined silicate content but, as discussed in section 1.

Compound composition Strengths at ages of 1, 2 and 7 days are almost linearly related to C3S content. At 28 days C2S makes a significant contribution but the contribution does depend on C2S reactivity, which is determined by clinker microstructure and by impurities present in the crystal lattice solid solution effects.

For example, the presence of belite clusters associated with coarse silica in the raw mix will reduce the contribution from C2S at 28 days. Results are illustrated in Figure 1. The influence on day strength is less certain but will normally be slightly positive.

This effect is more likely to be seen if the C3A content increases as a result of an increase in AR rather than a decrease in SR. A decrease in SR will also reduce the total silicate content. The positive influence of C3A is attributed to its higher reactivity compared to C4AF, which results in a greater volume of hydrates and thus lower cement paste porosity at 28 days.

SO3 level and forms of SO3 present While the forms of calcium sulfate present can have a marked influence on the water demand of cement in concrete it is the total cement SO3, which has the primary influence on strength properties. Anhydrite, if present, will dissolve during the first 24 hours of hydration and be available to participate in the strength-forming hydration reactions in the same manner as calcium sulfate from gypsum.

It is also important to ensure that cements with different SO3 levels are compared on a meaningful basis. For example, at constant SA the higher SO3 cements will normally have higher micron residues and consequently lower day strengths. Most clinkers will show a significant increase in early strength when the cement SO3 level is increased from 2. The influence on day strength is generally much less but still positive.

Typical results from laboratory tests are shown in Figure 1. In the example shown the adverse influence of cement SO3 level on concrete slump could have been reduced by replacing a proportion of the gypsum by natural anhydrite. In most countries the opportunity to optimize cement SO3 is restricted by the upper limits for SO3 in the relevant standards.

One important difference between a factory-produced composite cement and a concrete mixer blend is that in the latter it is not possible to control SO3 level.

With high slag blends in particular, the cement SO3 level will be much lower than that of a factory- produced cement. A large quantity of data have been generated concerning the relationship between mortar and concrete strengths Moir, This phenomenon is illustrated in Figure 1.

Thus no single test procedure, whether mortar or concrete, can reliably predict cement performance across a range of cement contents. However, Table 1. EN Additional cement types drawn from the options available in BS EN may become commercially significant in the future.

The choice of cement for structural concrete should be in accordance with BS after implementation on 1 December when it will supersede BS BS recognizes the sulfate-resisting properties of slag and fly ash concretes and the reduced risk of asr if appropriate levels of these materials are present. It must be recognized that in many applications that cement choice has a much lesser influence on concrete long-term performance than the practical aspects of mix control, cement content, water content, aggregate quality, compaction, finishing and curing.

However, when cement is mixed with water a highly alkaline solution is produced. The pH quickly exceeds 13 and is highly corrosive to skin. In addition to the acute burns described above exposure to moist mortar or concrete over a period of time may result in irritant contact dermatitis.

In Scandinavian countries ferrous sulfate has been added to cement during manufacture to precipitate the soluble hexavalent chromium as the insoluble trivalent form. Although the clinical evidence for the effectiveness of this measure is inconclusive there are moves in Europe to adopt this approach more widely, at least in bagged if not bulk cement.