POZZOCRETE

What Is Pozzocrete?

          The Pozzocrete Fly Ash range is an artificial pozzolan, specially designed to achieve optimum performance on most cement and concrete applications. In the production of Pozzocrete 40™, 60™, 63™, and 83™ high quality PFA(Pulverized Fuel Ash) were selected and industrially processed in order to obtain maximum performance as a cement replacement product.

          The process used to produce Pozzocrete Fly Ash significantly reduces the amount of unburned particles leading to a virtually carbon free material. Optimum reactivity is obtained by rejecting the less reactive crystalline Fly Ash particles and by increasing the specific surface of the final product.

          Pozzocrete products are available depending on the product type in 25 kg, 30 kg, 50 kg paper or HDP bags as well as in 1 ton big bags. The design of all Pozzocrete grades has taken into account the severe weather conditions in the areas where it may be supplied, in order to allow high cement replacement volumes ensuring adequate strength development and good durability whilst avoiding short-term damage such as bleeding and thermo-hygrometric plastic shrinkage.

          The use of this advanced pozzolanic Fly Ash material provides an effective reduction of the mixing water required and superior strength development, allowing high replacement rates that are not possible with normal cement additions.

          The cement replacement rate depends on cement and admixture type and on type of use. For normal concrete production using OPC,replacement rates in the range of 35%. For each single application,the proper mix design shall be determined by appropriate testing.

         The proper use of Pozzocrete results in long term technical improvements of the final product and at the same time ensures major cost savings to the user.

What Is Fly Ash or Pulverised Fuel Ash (PFA) ?

          At present thermal energy has became the chief source of energy in India and it will continue to be so in near future. This thermal energy can be produced in thermal power stations which generates fly ash as the waste product in large quantity.

          Fly Ash is nothing but the residual resulting from the combustion of bituminous coal or sub bituminous coal in thermal power plants.

          It constitutes about 80 to 85 % of total ash produced. Fly ash is usually collected in silos and it is available in different fineness. The fly ash produced from the burning of pulverized coal in a coal-fired boiler is a fine-grained, powdery particulate material that is carried off in the flue gas and usually collected from the flue gas by means of electrostatic precipitators, baghouses, or mechanical collection devices such as cyclones.The finest fly ash can be readily used in Portland cement. At the same time it is also used in brick manufacturing, mine filling, filling up of low lying areas etc.

          It is available in large quantity and at present over 100 million tons of fly ash is generated in India. Out of this only 20 % i.e. 20 million tons of ash is being utilised, but still we have to find ways for safely depositing over 80 million tons of ash every year.

         Recent investigations on fly ash have indicated great scope for its utilisation as a construction material. Greater utilisation of fly ash will lead to, not only saving of scares construction material but also assist in solving problem of disposal of fly ash. The utilisation of fly ash in cement and concrete is gaining immense importance today, mainly on account of the ecological benefits and the improvements in the long term durability of concrete. Fly ash can be economically used in bricks, concrete, high volume fly ash concrete, roads and embankments.

         The major constituents of fly ash are oxides of silica, aluminium, iron, calcium and magnesium making about 95 % of total composition. Fly ash consists principally of fine glassy spherical particles together with some crystalline matter and varying amount of unburnt carbon.

         The color of fly ash can vary from tan to gray to black, depending on the amount of unburned carbon in the ash. The lighter the color, the lower the carbon content. Lignite or subbituminous fly ashes are usually light tan to buff in color, indicating relatively low amounts of carbon as well as the presence of some lime or calcium.

        Although fly ash or pulverised fuel ash does not generally have much cementitious property of its own, it reacts with lime (CaO) in the presence of water to give hydrated products of calcium aluminosilicates which have cementitous property.

       Pulverized coal fly ash is presently used in most European countries as a constituent of the cementitious material in the production of concrete, mortar and grout. As fly ash properties depend on many different factors associated to the operating conditions of the power stations (coal properties, boiler conditions, mill performance, etc), many power stations have implemented sophisticated quality control/adjustment systems in order to produce fly ashes with stable properties and adequate quality. Simultaneously, several industrial installations designed to improve the quality of fly ash have been built in order to satisfy the increasing quality requirements of customers and to optimize the use of fly ash as an ingredient of the cementitious material. These industrial processes may include several different operations, such as, selection, carbon reduction, classification, grinding, blending, and homogenization.

                                                              

Manufacturing

            Most powders are the result of a comminution process. The design of the comminution process is usually dictated by characteristics such as the hardness or abrasive nature of the feed material. The driver is therefore the physical nature of the material rather than considerations of the process to which the comminuted material will be applied. There is a range of machines available for the comminution process, each having its own particular characteristics depending upon whether it relies on breakage by compression, impact or attrition or any combination. Further, the ways in which materials break down can vary greatly; hence it is unlikely that the optimum size distribution requirements of a particular process will be achieved directly. Often status substantial modification of the size distribution of a powder is necessary, and it is here that classification plays a key role.

             In the case of Pulverised Fuel Ash, the general case described above is even more

complex. At best, PFA is a by-product- and at worst -a waste. The ash-produced PFA by- product- quality varies widely dependant on fuel source and quality variations on plant design combustion and dust collection processes plant engineering plant operational practice.

Principles

            The classification of dry powders using conventional sieving techniques becomes progressively more difficult as the size separation point is reduced, particularly if materials are of low specific gravity or contain a high percentage of ultra-fine material below 10 microns. Such materials have a tendency to agglomerate or build-up on the screen cloth causing blinding of the sieve and the result is a loss in separation efficiency. This is often the case when attempting to screen large quantities of material to below 130 microns and it is in these applications that the air classifier comes into its own.

             Air classifiers work on the principle that wherever relative motion exists between a

particle and a surrounding fluid, the fluid will exert a drag force on the particle. If the individual particle was falling under the influence of gravity in still air, it would accelerate until it reached a constant velocity which is known as the Terminal Settling Velocity. This occurs when the drag force exerted by the air balances the gravitational force exerted on the particle. Should the air be rising with this velocity then smaller particles of lower terminal settling velocity would be entrained and carried upwards. Such velocities can be determined by Stokes law and others, which are well documented. Since the settling velocities of particles in the low micron range are themselves very small, only very low air velocities and volumes in a given space are necessary for entrainment. Further, since the difference in terminal settling velocity between particles in the sub sieve range becomes so small, then a simple gravitational system becomes impractical. To overcome these limitations it is necessary to increase the gravitational force. In this way the air velocity necessary for the entrainment of a

given particle is increased, the difference between particles magnified and high efficiency classification becomes possible in a relatively small space.

In practice this is achieved by causing the carrying air and particles to follow a curvedpath which spirals inwards to a concentric discharge point. In this way forces of several hundred gravities may be generated and consequently high capacity and precise separation achieved in relatively small machines.

For an air classifier to be effective three factors need careful consideration.

· Firstly there must be a clearly defined and stable classification zone where particles can come under the influence of the separating forces and be free from external influences.

· Secondly the particles must be presented into the classification zone at a constant rate and in a specific manner and should remain in the zone just long enough for the desired classification to occur.

· Finally the feed to the classification zone must be suitably dispersed such that the size of the largest agglomerate is below the operating cut size. Failure to achieve these requirements will inevitably lead to a reduction in separating efficiency. Either the quality of the fines fraction will suffer by showing a long “tail " at the top of the size distribution instead of a sharp cut off, or material which should be classified as fines will find its way into the coarse fraction.

Air Separation Equipment

                There is a variety of machines on the market today to cover the air classification range from about 150 microns down to 1 micron. Some of these machines are very limited in their operating size range and in the materials that they can handle, whilst a few can virtually cover the full range and can be adapted to suit most materials.The machines available generally fall into three categories:

· the static types that depend solely on the passage of an air stream and casing design to generate the necessary separating forces;

· mechanical aided types again relying on the passage of an air stream but employing a mechanical rotating element to generate the separating forces

· and finally the unit machines which are completely self-contained generating their own internal air system and separating forces and having provision for collection of the separated fines and coarse material.

Double cone classifier

         

           An example of the static type is shown in Fig. This unit is often referred to as thedouble cone classifier. Here the feed and carrying air are introduced at the bottom ofthe machine and pass up the central vertical feed pipe (1). Coarse classification takes place as the material and air sweep around the baffle (2) at the top of the feed pipe and into the outer cone (4). The oversize material drops out of suspension and collects atthe base of thecone (3) to be discharged under gravity through some form of airlock.The roughly sized material continues upwards between the inner and outer cones withthe air stream and passes into the inner cone (5) via the adjustable vanes (6). Becauseof the angle of these vanes aspinis imparted to the air stream and this induces theforces necessary for classification.Fine particles together with the air are drawn towards the central discharge (8), whilst the coarse particles descend down the innercone pass through flaps (7), across the incoming air stream and descend to the base ofthe outer cone.

            These machines are capable of classifying materials generally within the range 50% to

99% passing 200 mesh B.S.S. and require- a separate air fan and fines/product collection systems.

High efficiency-mechanically aided air-borne fed classifier .

          Fig shows an example of a high efficiency mechanically aided, airborne feedmachine. In this design the feed material and carrying air enter the classifiertangentially to the rotor through a horizontal duct (1).Due to the design of the casing and the rotation of the multivaned rotor (2) aspiralling air system is generated and it is here that classification of the particulate material into two size fractions occurs. As the particles approach the rotor they are subjected to a centrifugal force which may be of several hundred gravities depending upon the speed of rotation of the rotor.This force overcomes or is overcome by the centripetal frictional drag force exerted on the particle by the air as it spirals inwardsthrough the rotor vanes. At the cut point these two opposing forces are in equilibrium and for particles above the cut size the centrifugal force predominates and they fly away from the rotor to impinge on the casing and descend to be collected in the oversize cyclone (3). The particles below the cut size are carried inwards through the rotor finally leaving the machine through duct (4) together with the entraining air. As explained previously, the efficiency of separation of any classifier depends largely on the ability to completely disperse individual particles into the separating zone and with most materials this becomes increasingly difficult as the cut size becomessmaller. Fine particles tend to adhere to one another and as a result are classified in a similar manner to the coarse particles. The volute casing is particularly effective in this respect and to overcome the tendency further a secondary air system has been built into the machine. It consists of an additional induced and controlled air system entering the classifier tangentially through duct (5) and rising to the separating rotor. In this way it is possible to control the residence time of particles in the separating zone and also to sift the reject material as it descends to the coarse cyclone liberating any agglomerated fine particles and returning them to the separating rotor.

          As with the static types, these classifiers require an external product collection system for the fines fraction and suitable suction fan however, with this machine it is possible to classify within the range 85% less than 200 mesh down to 99% less than 5 microns with high efficiency and capacity.

            An example of the last group, the unit machines, is shown in Fig. 3. These machines work on a similar principle to the mechanically-aided classifiers described above except that the feed material enters the machine down a central tube and through a rotating dispersing cage. The air is circulated within the machine by a fan which is close coupled to the separating rotor, and the fines passing through the rotor and the fan are collected in the outer casing as in a cyclone. A secondary air system is incorporated in this machine, however its ultimate performance is limited by its ability to collect the fines fraction once separated. Its range is however, very wide for a unit machine operating from about 90% less than 200 mesh down to 99% passing 10 microns.

Working

           Through contact with water cement particles hydrate leading to the formation of hydrated calcium silicates and aluminates and also of calcium hydroxide. Some of these hydration products, such as the calcium hydroxide become present in the water solution between the particles, leading to an increase of the pH.

           The Pozzocrete particles are dissolved by the alkaline solution, and a reaction takes place, mainly between the calcium hydroxide and the reactive silica from Pozzocrete. This reaction promotes a better and more stable bonding of the matrix, while it also fills the voids between the particles. This way the binder matrix becomes denser, stronger and more durable.

Applications

1) General Structural Concrete:

        The optimum level of replacement of Cement with pozzocrete within the concrete mix will be between 20 – 35% by weight.The addition of pozzocrete will have a plasticizing effect on concrete and so as to achieve the same workability as of Portland cement concrete and to give the guarantee of strength development, the total volume of water in the concrete should be reduced by at least 10%. In addition,reducing the amount of sand by 10% by weight will be beneficial to the final mix. In general, there is a slight slowing down in the setting time of pozzocrete concrete which may lead to a delay in form stripping – for stripping of formwork after 10 days or longer.

2) Mortar for bricklaying:

        Pozzocrete is an ideal product for bricklaying mortar due to the increased cohesiveness and flowability of material when added to the mix.

Mix Design: A replacement level of around 30%-35% of the cement by weight and reduce the water by 10% and 10% of the sand may be removed if the mix becomes too cohesive.

3) Plaster:

a) External Plaster:

        Pozzocrete improves the surface finish of external mortars and reduce the level of surface cracking it will also lead to a decrease in water penetration of the hardened product. External plaster containing pozzocrete will give a more impermeable cover to your building as well as giving a finer surface finish.

MixDesign:
      A replacement of 45% by weight of the cement with a reduction of sand and water of 10%.

b) Internal Plaster:

       Using pozzocrete in internal plaster gives a better surface finish, more time for finishing and a virtually crack-free surface, leading to a higher quality look to all internal walls.

MixDesign:
       A 45% - 50% replacement level of cement by weight and a reduction of sand and water by 10% .

4)Pozzocrete for Pre-cast/Pre-stressed Concrete Products:

         Production of pre-cast concrete products involves intricate, difficult patterns.  Pozzocrete concrete mixes can help pre-casters solve challenges in many areas of production. Pre-cast concrete products can be produced with or without reinforcement, but units typically consist of narrow, deep sections, which are heavily reinforced making concrete placement very difficult. Reinforcement typically includes the use of fibres, conventional reinforcing steel, and pre-stressing steel tendons, either pre- or post-tensioned or combinations thereof.  Mixtures must have enough workability to flow well under vibration and totally fill the form without segregation.  Hand finishing is often required, requiring a mixture workable enough to allow for this kind of manipulation.

         By definition, recast concrete products are cast and cured in other than their final position.  This enables the use of reusable forms, which, due to economic concerns, are cycled as rapidly as possible.  For this reason, these concrete products generally achieve their competitive position in the marketplace by using a limited number of forms with a short production cycle.  Normal production schedules allow for one usage of forms per day; however, 10 to 12 hour schedules are common.  Accelerated curing, typically employed to enhance early age concrete strength for handling, shipping, and product utilization, accelerates the pozzolanic reaction of Pozzocrete to help develop the necessary early strengths.

        Concrete mixtures for these products are proportioned for high levels of performance at early ages.  Compressive strengths of 3,500 to 5,000 Psi (24 to 28Mpa) are typically required at the time of form removal or stripping.  These early concrete strengths are generally achieved with cementitious material contents of 600 to 750 lb/cy (355 to 445 kg/cm).  Conventional  and  high-range  water reducing agents are often employed to attain workability at very low water content.  Non-chloride accelerating admixtures are also used when necessary.  While the early strength gain characteristics of Pozzocrete has generally been considered too slow for use in these mixtures, conditions are changing toward the use of Pozzocrete in these applications.  As is true of all mixtures used in pre-cast concrete work, mixture proportioning and curing procedures used must produce adequate early strength of the turn-around time on forms or molds will be increased.

        Pre-tensioned hollow-core structural slabs are produced with no-slump concrete.  It is consolidated and shaped as it passes through an extrusion machine.  The particle shape of the coarse aggregate and the amount of fine aggregate are very important to workability.  Pozzocrete is widely considered as a beneficial ingredient to increase the workability of these dry, harsh mixes.  Early strength performance of these mixtures using Pozzocrete closely parallel mixtures without Pozzocrete in terms of early compressive strength.  No early reduction is apparent.

        Although most concern is directed at obtaining desired early compressive strength, these concrete products must possess durability to resist destructive attack from numerous environmental factors. Pozzocrete is seen as a major ingredient utilized in the production of durable concrete and as such should be included in any concrete subject to severe environments.

5) Pozzocrete for Pumped Concrete:

         Pumped concrete must be designed to that it can be easily conveyed by pressure through a rigid pipe of flexible hose for discharge directly into the desired area.  Pozzocrete use can greatly improve concrete flow characteristics making it much easier to pump, while enhancing the quality of the concrete and controlling costs. Three mix proportioning methods frequently used to produce pump able concrete are :

 Maximum Density of Combined Materials

 Maximum Density – Least Voids

 Minimum Voids – Minimum Area

         Mixes must be designed with several factors in mind:

 1. Pumped concrete must be more fluid with enough fine material and water to fill internal voids.

2. Since the surface area and void content of fine material below 300 microns control the liquid under pressure, there must be a high quantity of fine material in a normal mix.  Generally speaking, the finer the material, the greater the control.

3. Coarse aggregate grading should be continuous, and often the sand content must be increased by up to five percent  at the expense of the coarser aggregate so as to balance the 500 micron fraction against the finer solids.

       There are many advantages to including pozzocrete in concrete mixes to be pumped. Among them are :

1. Particle Size: Pozzocrete meets IS 3812 Specification with 66% passing the 325 (45-micron) sieve and these fine particles are ideal for void filling.  Just a small deficiency in the mix fines can often prevent successful pumping.

2. Particle Shape: Microscopic examination shows most Pozzocrete particles are spherical and act like miniature ball bearings aiding the movement of the concrete by reducing frictional losses in the pump and pining.  Studies have shown that Pozzocrete can be twice as effective as cement in improving workability and, therefore, improve pumping characteristics.

3.Pozzolanic Activity: This chemical reaction combines the Pozzocrete particles with the calcium hydroxide liberated through the hydration of cement to form additional cementitious compounds which increase concrete strength.

4.Water Requirement: Excess water in pumped mixes resulting in over six inch slumps will often cause material segregation and result in line blockage.  As in conventionally placed mixes, pumped concrete mixes with excessive water also contribute to lower strength, increased bleeding and shrinkage. The use of Pozzocrete in pumped or conventionally placed mixes can reduce the water requirement by 2% to 10% for any given slump.

5.Sand/Coarse Aggregate Ratio: In pumped mixes, the inclusion of liberal quantities of coarse aggregate can be very beneficial because it reduces the total aggregate surface area, thereby increasing the effectiveness of the available cementitious paste.  This approach is in keeping with the “minimum voids, minimum area” proportioning method.  As aggregate size increases, so does the optimum quantity of coarse aggregate.  Unfortunately, this process is frequently reversed in pump mixes, and sand would be substituted for coarse aggregate to make pumping easier.  When that happens, there is a need to increase costly cementitious material to compensate for strength loss.  However, if Pozzocrete is utilized, its unique workability and pump ability properties permit a better balance of sand to coarse aggregate resulting in a more economical, pump able concrete

GUIDELINES

1.Guidelines for fresh Pozzocrete concrete

       Pozzocrete is a fine material with spherical particle shape. When added to concrete it produces a more cohesive mix, which looks drier than normal. Usually less fine aggregate will be added to Pozzocrete concrete in order to produce the best performance from the mix. The exception is ‘Self Compacting Concrete’ where the mix is designed to be high in fine material to reduce mix segregation and ensure it is capable of flowing properly. Following are the main factors that should be taken into consideration:

i.Pozzocrete concrete visually appears more cohesive and less workable than Portland cement only concrete. However, do not add water as this will reduce the strength and durability of the concrete. Because of the rounded shape of Pozzocrete particles, when vibrated the concrete will become highly mobile and should move readily within shutters.

ii.Pozzocrete reduces the rate of bleeding within concrete. Bleed water that collects at the surface of concrete results in a localised increase in the water/cement ratio, reducing the strength and durability. However, because less water rises with Pozzocrete concretes they must be protected from excessive water loss, e.g. in drying windy weather conditions. If the surface of any concrete dries out before sufficient strength has developed early age drying shrinkage cracking may occur. Protection and curing should be carried out earlier to prevent cracking problems.

2.Guidelines for hardened Pozzocrete concrete.

     Pozzocrete concrete is very similar in most respects to Portland cement concrete. The following are a few factors to consider:

 i. Protection - All concrete should be protected from physical damage. The strength development of all concrete takes time and arisses are easily damaged. Prevent access to the area or provide protection to exposed edges, corners, etc.

ii. Making good - No special procedures are required for Pozzocrete concrete. All 'making good' operations should be avoided wherever possible - these are no substitute for care in construction.

iii. Durability - With proper site practice Pozzocrete concrete can be exceptionally durable.

ADVANTAGES

        Using pozzocrete not only gives benefits to the concrete in the early days – but because of the nature of its reaction these benefits are carried on through the whole life of the concrete. The main benefits are:

1) Increased resistance to chemical and atmospheric attack

        Because strength development of pozzocrete in concrete is slower than that of cement the only place where it can grow is within the voids left in the hardened cement matrix - it “plugs the gaps” in the concrete. There is a significant reduction of the amount of voids found in concrete made using pozzocrete – this reduction is so great that there is a measurable increase in the impermeability of the concrete.

        A concrete with improved impermeability will better resist attack from chemicals and salts, which can be carried in water. By reducing the amount of water penetrating the concrete, the salts are unable to reach the binder and attack it. It is because of this characteristic that throughout the world, concrete for use in marine and harsh environments will usually be specified to contain  pozzocrete.

2) Increased protection for reinforcement:

       The highly alkaline environment that concrete produces passivates reinforcing steel. Should the alkalinity of the concrete be reduced – by penetration of CO₂, the steel will start to oxidise and the rust produced will cause expansion within the concrete this will cause the concrete to crack, allowing further attack on the steel resulting ultimately in a total breakdown of the concrete structure. pozzocrete concrete, because of its increased impermeability, will limit the degree of penetration of the gas – providing, that if the cover of the steel is designed correctly, a longer life to the reinforcing steel.

        So pozzocrete improves the concrete both at the start and throughout its life. pozzocrete provides the assurance that concrete well designed and well made will last.

3) Low hydration heat:

        Because only part of the cementitious material reacts immediately with the addition of water the heat generated by that chemical reaction is also reduced. This is particularly important in warm climates as the risk of thermal stress leading to surface cracking is reduced and the surface of the concrete will be more uniform with the mix. A high quality finish can be achieved which with lower level of cracking.

4) Setting Time:

        Pozzocrete, unlike cement, contains almost no lime, yet it needs the lime to start reacting as a cement.Once the cement starts to hydrate with the addition of water it forms a matrix of crystals, which tie together the particles of sand and aggregate, but it also starts to produce an excess of lime that has not been needed in the reaction - this “free-lime” can have a detrimental effect on the porosity of the concrete as it can leech out in water – but it is this very “free lime” that the pozzocrete needs to start its reaction – so the pozzocrete will not show any significant activity immediately, it will not react until the volume of “free-lime” is sufficient for pozzocrete to start acting like a cement.

5) Workability:

        The addition of pozzocrete to a concrete mix will increase the workability of the concrete – this is for two reasons:

1) Pozzocrete is much lighter than cement; when the same weight of pozzocrete replaces the same weight of cement, the volume of powder and therefore the cementitious paste is greater, this allows for more effective coating of the aggregate.  

2) The shape of the pozzocrete; because it is spherical it acts as tiny ball bearings around the other particles, which reduces their drag against each other. For good concrete the minimum volume of water, sufficient for placing and compaction should be used, with pozzocrete the water can be reduced to between 10% and 15% in volume of that required by concrete containing cement only.

         A further advantage of adding pozzocrete for workability is that because of the increase in paste and the particle shape, cohesion within the mix is increased; this reduces significantly the risk of segregation within the concrete both under vibration and in pumping.

6) Contribution to strength:

         For the pozzolanic reaction to become possible, it is first necessary to reach the appropriate levels of Ph and to destroy the glassy structure of the pozzolan particles. When Portland Cement is used the level of pH increases with time during the first few months of the hydration process, depending on the properties of cement (mainly on C₃A and alkali content) and on the cement concentration on the mixing water (water to cement ratio). The speed of the ‘attack’ to the glassy structure depends on the level of pH and on the way the particles are exposed to the ‘aggressive’ agent. As the glassy structure is not absorbing any water before it has been destroyed, the ‘attack’ progresses from the surface of the particle to its inside. For this reason it is very important the exposed surface of the pozzolanic material, meaning its specific surface.

       Normal fly ashes only provide its major contribution to the strength development after several months (generally over one year). In general when cement is replaced by raw fly ash at a proportion of 1:1, if the same water to binder ratio is used, the same strength level compared to the pure cement mix is only achieved after more than 3 months, being that, on a long-term basis, higher strength levels are achieved with the replacement of cement by fly ash.

       

          If the water to binder ration is adjusted to provide the same workability of the mix, then, as in general fly ash reduces the water demand, similar strength levels to the pure cement mix can be achieved after 2 or 3 months. As lower water demand means higher density of the binder matrix, also significantly higher long-term strength is obtained.

        If the raw fly ashes are properly processed, it is possible to improve their contribution to strength development. By reducing the carbon content of the fly ash, it is possible to increase the reactivity and to reduce the water demand, two factors that contribute to achieving higher strength levels. The same is achieved by rejecting the particles with higher concentration of the crystalline phase. By increasing the fly ash fineness it is possible to fasten the pozzolanic reaction, improving the strength development on the early ages (before 28 days).

7) Contribution to chemical stability:

        Pure calcium hydroxide is an unstable molecule, easily soluble in water, and very sensitive to acidic elements, such as Sulphates and Nitrates. For this reason, when hydrated OPC is exposed to pure or acidic waters, the available calcium hydroxide may be leaching out or reacting with the acidic agents, leading to the degradation of the exposed surface, and to an increase of the matrix permeability that will extend this phenomenon in depth.

        If a proper pozzolanic reaction has taken place after the hydration of the ordinary Portland cement, most calcium hydroxide will be in a stable crystalline structure, in combination with silica and alumina. This way it is neither soluble nor susceptible to acidic elements anymore, and so the stability of the hydrated binder is improved.

        On the other hand, although the fly ash contains a significant amount of alkali (mainly potassium), it has been proved that only a small part of this alkali is available (aprox 1/6). This way, replacing cement by fly ash is a good way of reducing the amount of available alkali, reducing also the risk of ASR (Alkali-Silica-Reaction), when reactive aggregates are used.

8) Contribution to durability under severe environments:

       When fly ash products are used, the water to binder ratio (on a volume basis) is significantly lower than with pure OPC binder. For this reason the binder matrix is denser and less pervious to water (or other fluids).In addition to this purely physical effect, there is also a contribution of the pozzolanic reaction to reduce the pore size distribution and to decrease the permeability of the binder matrix.

       The combination of these two effects provides a significant reduction of the permeability of the binder matrix when ‘appropriate’ fly ash products are used.

       If the fly ash has an high carbon content and a poor grading, the physical effect will be of little importance; and if in addition to that the fly ash is not enough reactive, the result may be a binder matrix not as dense as a pure OPC matrix, meaning that the use of such fly ash may increase the permeability figures, mainly during the first few months.

        On the other hand, if high grade processed fly ashes are used, the significant water reduction achieved, together with a significant early age pozzolanic reaction (during the first few weeks), will lead to very low permeability figures, reducing the sensitivity to the penetration of strange elements (Carbon Dioxide, Sulphur, Nitrates, Chlorine, etc.).

      The increase of Chlorine concentration inside cementitious materials does not endanger the cemented material itself, but increases the risk of the steel corrosion in reinforced concrete. When steel corrodes inside the concrete there is an expansion, due to the formation of iron oxide, which results in concrete deterioration. For this reason it is important to assure that Chlorine does not penetrate deeper than a certain depth, which can only be achieved by decreasing the permeability.

      Carbonation is a detrimental reaction that occurs when calcium hydroxide resulting from cement hydration is exposed to CO₂. As calcium carbonate has a low pH (compared to calcium hydroxide), the result of carbonation is a decrease in the pH of the binder matrix to values generally below 9. Low pH values (bellow 10.5 to 11) may lead to the corrosion of the reinforcement steel.

      Usually, at normal air pressure carbonation only takes place on the surfaces exposed to the air, and it progresses a little in depth. In general carbonation starts on the surface when it dries and progresses in depth with the drying, before the material permeability has decreased due to the hydration process. It only stops when the material permeability decreases, avoiding water loss (and CO₂ penetration), or when the Calcium Hydroxide has reacted into a more stable structure.

 

      For this reason, when normal fly ash is used, as the strength gain and the permeability decrease are slower than with pure cement, the carbonation phenomenon progresses to a higher depth. This means that concrete using such low grade fly ash, needs a significant increase of the curing period or, as an alternative higher steel covering thickness.

       This disadvantage on using fly ash may be reduced by processing it, in order to achieve a faster pozzolanic reaction and reduced water to cement ratio. When the used fly ash has been properly processed, the carbonation problem during the early age can be eliminated and after the first few weeks the concrete is even less sensitive to carbonation than OPC concrete.

9) Pozzocrete for Ready-Mix :

A ready mix producer has several Benefits for using Pozzocrete in concrete.

1.Pozzocrete can compensate for fines not found in some sands and thereby enhance pump ability and concrete finishing.

2.Pozzocrete offers flexibility in mix design providing a greater range of mixes from liquid soil at 100 psi to high strength 8,000 – plus psi concrete – produced by the same batch plant without sophisticated equipment.

3.Pozzocrete improves the flow characteristics of the concrete that translates into less wear and tear on all the producer’s equipment from batching facilities to trucks.

4.Pozzocrete enables the producer to customize designs to each customer’s needs, thus providing the producer who uses it with a competitive advantage.

10) Pozzocrete Increases Resistance to Sulphate Attack:

        Soluble sulphates in soils, ground waters, and sewage can destroy Portland cement concrete unless it is produced with Pozzocrete to provide sulphate resistance according to the severity of the attack. Sulphate attack is a two-phased process:

1) Sulphates are combined with calcium hydroxide generated during cement hydration to form calcium sulphate (gypsum).  The volume of this gypsum is greater that the sum of its components causing internal pressure and expansion, which fractures the concrete.

2) Aluminate compounds from Portland cement react chemically with sulphates and calcium to form a compound called ettringite (calcium sulphoaluminate).  Ettringite formation destroys the concrete in the same manner as gypsum formation.

Pozzocrete effectively reduces this sulphate deterioration in three important ways:

1) Pozzocrete chemically binds free lime in cementations compounds rendering it unavailable for sulphate reaction.

2) Pozzocrete activity reduces concrete permeability, keeping sulphates from penetrating concrete.

3) Pozzocrete replaces a portion of Portland cement therefore reduces the amount of reactive aluminates (tricalcium aluminate) available for sulphate reaction.

Studies show that properly proportioned concrete utilizing up to 35 percent will withstand sulphate attack far well than conventional Portland cement.

To ensure the most durable concrete possible, Pozzocrete is an essential ingredient when the project will be vulnerable to attack by sulphates or other aggressive compounds.

11) For Engineers and Architects:

       Engineers and Architects will find the Pozzocrete provides the following  benefits:     

    1.It enables engineers and architects to provide the client with a superior and more durable finished concrete. 

    2.Pozzocrete produces a high strength concrete that accommodates the design of thinner sections.

    3.Pozzocrete permits design flexibility accommodating curves, arches and other pleasing architectural effects.

    4.The addition of Pozzocrete to the mix is a built-in insurance for later-age strength gain in concrete.

    5.Pozzocrete ensures that the concrete will qualify as a durable building material.

    6.Pozzocrete contributes to the aesthetic appearances of the concrete.

12) For Developers, Contractors, Owners:

Pozzocrete concrete provides the following advantages to developers,  contractors and owners .

    1.The workability of Pozzocrete concrete generally ensures that the speed of construction is faster, which   translates into a quicker return on investment.

    2.Pozzocrete in the mix accommodates more creative designs.

    3.Since Pozzocrete concrete is not as vulnerable to deterioration or disintegration as rapidly as concrete without Pozzocrete, it ensures low-maintenance buildings that will retain their value over the long-term.

Using Pozzocrete we achieve cost savings. It increases workability of the mix .

Other reasons for use of Pozzocrete are:

    1) Increased 28 day strength,

    2) Achievement of 3,500 Psi overnight,

    3) Better filling of voids,

    4) Reduction in permeability,

    5) Minimization of shrinkage.

    Pozzocrete may also be valuable as a mineral admixture to enhance product quality.  Pozzocrete used in pre-cast concrete products improves workability, resulting in products with sharp,distinctive corners and edges. Pozzocrete can also provide improved flow characteristics, resulting in products with better surface appearance. Better flow characteristics and workability properties achieved by using Pozzocrete are particularly desirable for products with intricate shapes and surface patterns and for those that are heavily reinforced.  Reduced costs associated with repair of surface defects can be attributed to the use of Pozzocrete.