Pozzolanas are natural or artificial materials which contain silica and/or alumina. They are not cementitious themselves, but when finely ground and mixed with lime, the mixture will set and harden at ordinary temperatures in the presence of water, like cement.
Pozzolanas can replace 15 to 40 % of portland cement without significantly reducing the long term strength of the concrete.
Most of the pozzolanic materials described here are by-products of agricultural or industrial processes, which are produced in large quantities, constituting a waste problem, if they remain unused. Even if there were no other benefits, this aspect alone would justify an increased use of these materials. But compared with the production and use of portland cement, these materials contribute to cost and energy savings, help to reduce environmental pollution and, in most cases, improve the quality of the end product.
Types of pozzolanas
• There are basically two types of pozzolanas, namely natural and artificial pozzolanas.
• Natural pozzolanas are essentially volcanic ashes from geologically recent volcanic activity.
• Artificial pozzolanas result from various industrial and agricultural processes, usually as by-products. The most important artificial pozzolanas are burnt clay, pulverized-fuel ash (pfa), ground granulated blast furnace slag (ggbfs) and rice husk ash (RHA).
• The first natural pozzolana to be used in building construction was the volcanic ash from Mt. Vesuvius (Italy), found closeby in the town Pozzuoli, which gave it the name.
• Although the chemical compositions are similar, the glassy material formed by the Violent projection of molten magma into the atmosphere is more reactive with lime, than the volcanic ash formed by less violent eruptions.
• The occurrence of suitable natural pozzolanas is therefore limited to only a few regions of the world.
• Good pozzolanas are often found as fine "rained ashes, but also in the form of large particles or tuffs (solidified volcanic ash), which have to be ground for use as a pozzolana. However, the qualities of such pozzolanas can vary greatly, even within a single deposit.
• Natural pozzolanas are used in the same way as artificial pozzolanas.
• When clay soils are burnt, the water molecules are driven off, forming a quasi-amorphous material which is reactive with lime. This is also true for shales and bauxitic and lateritic soils. This was discovered in ancient times and the first artificial pozzolanas were made from crushed pottery fragments, a traditional technology that is still being widely practiced on the Indian subcontinent, Indonesia and Egypt, using underfired or reject bricks. (In India it is called "surkhi", in Indonesia "semen merah", and in Egypt "homra").
• Alternatively, as reported from a project in India, soils which contain too little clay and too much sand for brickmaking, are cut and removed in blocks, forming circular pits. The blocks are then replaced in the pits, together with alternate layers of firewood. The residue obtained from firing is very friable and needs no pulverization. This is used as masonry mortar by just adding it to lime putty and mixing it, without sand or cement (Bibl. 05.10).
• A similar technique is reported from Java, Indonesia, where clay blocks are burnt in a clamp, disintegrated, sieved and used with lime and sand, sometimes also cement (Bibl. 05.11).
• The qualities of these traditional methods are very variable, but improved methods of calcination have been developed to produce pozzolanas of higher quality and uniformity.
• The illustration shows a vertical shaft kiln (after Thatte and Patel) developed in India. The feed consists of a mixture of clay lumps 50 to 100 mm in size and coal slack (comprising 48 % ash, 31 % fixed carbon and 20 % volatiles). Calcination takes place at 700° C for 3 hours, with the temperature monitored by thermocouples and controlled by an air blower and feed input. The capacity is 10 tonnes per day. A fluidized bed process has been developed by the National Buildings Organization, New Delhi, by which the clay feed is calcined within a few minutes, thus achieving high output rates in a continuous process (Bibl. 08.07).
Pulverized-Fuel Ash (Fly Ash)
• By comparing the production processes of pulverized-fuel ash (pfa), commonly known as fly ash, and ordinary portland cement (OPC), it becomes clear, why pfa can be used as partial replacement of the latter.
• Finely ground coal is injected at high speed with a stream of hot air (about 1500° C) into the furnace at electricity generating stations. The carbonaceous content is burnt instantaneously, and the remaining matter (comprising silica, alumina and iron oxide) melts in suspension, forming fine spherical particles on rapid cooling while being carried out by the flue gases.
• In the production of OPC, limestone and clay, finely ground and mixed, are fed into an inclined rotary kiln, in which a clinker is formed at 1400° C. The cooled clinker is finely ground and mixed with gypsum to produce OPC.
• Depending on the type of coal, pfa contains varying proportions of lime, low-lime pfa being pozzolanic and high-lime pfa having cementitious properties itself. As with other pozzolanas, the lime liberated by the hydration of OPC combines with the pfa to act as a cementitious material.
• The glassy, hollow, spherical particles of pfa have the same fineness as OPC, hence no further grinding is needed. The addition of pfa makes fresh concrete more workable (probably due to the ball-bearing effect of the spherical particles) and homogeneous (by dispersing the cement floes and evenly distributing the water).
Other advantages of using pfa are:
• With increasing age, higher strengths than concrete without pfa are developed.
• Pfa does not adversely influence the structural performance of concrete members.
• Compared to OPC concrete, pfa concrete is lighter, less permeable (due to denser compaction) and with a better surface finish.
• Pfa concrete is also more resistant to sulphate attack and alkali-silica reaction.
• Concretes in which 35 - 50 % by weight of OPC is replaced by pfa have shown satisfactory performances.
• Aggregates derived from fly ash show excellent bonding in pfa concretes, contributing favourably to their performance and durability.
Ground Granulated Blast Furnace Slag
• Blast furnace slag is a molten material which settles above the pig iron at the bottom of the furnace. It is produced from the various input constituents in the furnace when it reaches 1400° to 1600° C.
• Slow cooling of the slag produces acrystalline material, which is used as aggregate. Rapid cooling with air or water under pressure forms glassy pellets (expanded slag > 4mm, suitable as lightweight aggregate) and granules smaller than 4 mm, which possess hydraulic properties when finely ground.
• The ground slag is blended with OPC to produce portland blast furnace cement (PBFC), whereby the slag content can reach 80 %. However, since PBFC is slower to react than OPC, the reactivity is reduced the higher the percentage of slag.
• Although the early strength of PBFC concretes is generally lower than OPC concretes, the final strength is likely to be higher. The slower reactivity of PBFC develops less heat and can be advantageous in situations where thermal cracking is a problem.
• Apart from improving the workability of fresh concrete, PBFC has high resistance to chemical attack, and its capability of protecting steel reinforcement makes it suitable for use in reinforced and prestressed concrete.
Rice Husk Ash
• The combustion of agricultural residues removes the organic matter and produces, in most cases, a silica-rich ash. Of all the common agricultural wastes, rice husks (also called paddy husks) yield the largest quantity of ash - around 20 % by weight - which also has the highest silica content - around 93 % by weight. It is this high silica content that gives the ash its pozzolanic properties.
• However, only amorphous (non-crystalline) silica possesses these properties, which is why the temperature and duration of combustion are of importance in producing rice husk ash (RHA). Amorphous silica is obtained by burning the ash at temperatures below 700° C. Uncontrolled combustion of rice husks, eg when used as a fuel or in heap burning, usually at temperatures above 800° C leads to crystallization of the silica, which is less reactive.
• The illustrated incinerator, first developed by the Pakistan Council of Scientific and Industrial Research (PCSIR) and later improved by the Cement Research Institute of India (CRI), is made of bricks with many openings to allow good air flow through the rice husk mass. The inner surface is covered with a 16 gauge fine-wire mesh. The husks are filled in from the top and the ash removed from the bottom discharge door. A pyrometer monitors the temperature, which can be controlled by shutting or opening the holes, maintaining a temperature around 650° C for 2 - 3 hours.
• The reactive ash is dark grey to white, depending on the residual carbon in it, which has no negative effects if below 10 %. To improve its reactivity, the ash is ground in a ball mill for about one hour, or longer if it contains crystalline silica. The ash can replace up to 30 % of cement in mortar or concrete. Alternatively, it can be mixed with 30 to 50 % of hydrated lime to be used like cement in mortars, renderings and unreinforced concrete.
• In another process, the ash obtained from heap burning or the production of parboiled rice, is mixed with about 20 to 50 % (by weight) of hydrated lime. This is ground for 6 or more hours in a ball mill to produce ASHMOH, a hydraulic binder suitable for masonry, foundations and general concreting work other than reinforced concrete. A variation of this is ASHMENT, in which the lime is substituted by portland cement (Bibl. 08.04).
• A method has also been developed, using waste lime sludge obtained from sugar refining. This is dried and mixed with an equal amount (by weight) of crushed rice husks and some water. Tennis ball sized cakes are made by hand and sun-dried. These are fired on a grating in an open clamp, to produce a soft powder, which is ground in a ball mill. The hydraulic binder is used in the same way as ASHMOH.
• A variation of this method utilizes soils with at least 20 % clay content instead of lime sludge. The resulting binder can be used as a 30 % mixture with portland cement to make portland pozzolana cement. Tests have shown that the pozzolana is best if the clay is bauxitic.
• At the National Building Research Institute, Karachi, Pakistan:
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