Pozzolan

A pozzolan is a material which, when combined with calcium hydroxide, exhibits cementitious properties. Pozzolans are commonly used as an addition (the technical term is "cement extender") to Portland cement concrete mixtures to increase the long-term strength and other material properties of Portland cement concrete, and in some cases reduce the material cost of concrete. Pozzolans are primarily vitreous siliceous materials which react with calcium hydroxide to form calcium silicates; other cementitious materials may also be formed depending on the constituents of the pozzolan.
The pozzolanic reaction may be slower than the rest of the reactions that occur during cement hydration, and thus the short-term strength of concrete made with pozzolans may not be as high as concrete made with purely cementitious materials; conversely, highly reactive pozzolans, such as silica fume and high reactivity metakaolin can produce "high early strength" concrete that increase the rate at which concrete gains strength.
The first known pozzolan was pozzolana, a volcanic ash, for which the category of materials was named. The most commonly used pozzolan today is fly ash, though silica fume, high-reactivity metakaolin, ground granulated blast furnace slag, and other materials are also used as pozzolans.
A pozzolan is a siliceous or aluminosiliceous material, which is highly vitreous. This material independently has few/fewer cementitious properties, but in the presence of a lime-rich medium like calcium hydroxide, shows better cementitious properties towards the later day strength (> 28 days). The mechanism for this display of strength is the reaction of silicates with lime to form secondary cementitious phases (calcium silicate hydrates with a lower C/S ratio) which display gradual strengthening properties usually after 7 days.
The extent of the strength development depends upon the chemical composition of the pozzolan: the greater the composition of alumina and silica along with the vitreous phase in the material, the better the pozzolanic reaction and strength display.
Many pozzolans available for use in construction today were previously seen as waste products, often ending up in landfills. Use of pozzolans can permit a decrease in the use of Portland cement when producing concrete; this is more environmentally friendly than limiting cementitious materials to Portland cement. Due to knowledge gained since the 1990s, current practice may permit up to a 40 percent reduction of Portland cement used in the concrete mix when replaced with a carefully designed combination of approved pozzolans. When the mix is designed properly, concrete can utilize pozzolans without significantly reducing the final compressive strength or other performance characteristics.

Pozzolanic reaction.

At the basis of the Pozzolanic reaction stands a simple acid-base reaction between calcium hydroxide, also known as Portlandite, or (Ca(OH)₂), and silicic acid (H₄SiO₄, or Si(OH)₄). Simply, this reaction can be schematically represented as follows:

Ca(OH)₂ + H₄SiO₄ → Ca²⁺ + H₂SiO₄^2- + 2 H₂O → CaH₂SiO₄ · 2 H₂O

or summarized in abbreviated notation of cement chemists:

CH + SH → CSH

The product of general formula (CaH₂SiO₄ · 2 H₂O ) formed is a calcium silicate hydrate, also abbreviated as CSH in cement chemist notation. The ratio Ca/Si, or C/S, and the number of water molecules can vary and the above mentioned stoichiometry may differ.
As the density of CSH is lower than that of portlandite and pure silica, a consequence of this reaction is a swelling of the reaction products. This reaction may also occur with time in concrete between alkaline cement porewater and poorly-crystalline silica aggregates. This delayed process is also known as alkali silica reaction, or alkali-aggregate reaction, and may seriously damage concrete structures because the resulting volumetric expansion is also responsible for spalling and decrease of the concrete strength.

 BENEFITS AND ADVANTAGES OF THE NATURAL POZZOLAN

•   Lithification: Once the Natural pozzolan-lime mixture is hydrated, the pozzolanic reaction begins immediately and continues for many years. Eventually, the mass will reach complete lithification, forming a rocky material similar to plagioclase with some content of magnetite. The compressive strength as well as the flexural strength will continue to increase for a long time. This unique characteristic is one of the main reasons many great ancient structures have lasted for over two thousand years.

• Autogenous Healing: A unique characteristic of Natural pozzolan is its inherent ability to actually heal or re-cement cracks within the concrete by means of the continuation of pozzolanic reaction with the calcium hydroxide freed from the cement hydration reaction. This results in the filling up of most of the gaps inside the hardened concrete matrix

• Reduced Permeability and Voids: The leaching of water-soluble calcium hydroxide produced by the hydration of Portland cement can be a significant contributor to the formation of voids. The amount of "water of convenience" used to make the concrete workable during the placing process creates permeable voids in the hardened mass. Natural pozzolan can increase the fluidity of concrete without "water of convenience," so that the size and number of capillary pores created by the use of too much water can be minimized.

• Reduces Expansion and Heat of Hydration: Experiments show that replacing 30% Portland cement with Natural pozzolan can reduce the expansion and heat of hydration to as low as 40% of normal. This may be because there is no heat produced when Natural pozzolan reacts with calcium hydroxide and    that the free calcium oxide in the cement can hydrate with natural pozzolan to form C-S-H. Natural pozzolan decreases the heat generated by cement hydration and delays the time of peak temperature. The graphic pattern of Natural pozzolan - Portland cement mixture is extended longer and lower to form a much more moderate curve than the heat of hydration curve of Portland cement itself.

• Reduces Creep and Cracks: While concrete is hardening, the "water of convenience" dries away. The surface of the hardening mass then begins to shrink as the temperature goes down from outside. This results in the formation of creep and cracks. Natural pozzolan moderates the expansion and shrinkage of concrete. It also helps to lower the water content of the fresh concrete. Therefore, the creep and cracks can be significantly reduced without the process of water cooling.

• Reduces Microcracking: The expansion and shrinkage mentioned above also create microcracks inside the hardened C-S-H paste and in-between the aggregate and the C-S-H paste. These microcracks significantly contribute to concrete permeability as well as other concrete defects. The Natural pozzolan- Portland cement mixture expands these shrinks so moderately that there is no microcracking inside the C-S-H paste after drying.

• Increases Compressive Strength: The pozzolanic reaction between natural pozzolan and calcium hydroxide happens after the C3S and C2S in the cement begins to hydrate. At the early stage of curing, 30% Natural pozzolan substituting Portland cement mixture is slightly lower than reference OPC [Ordinary Portland Cement} in regard to compressive strength. As time goes by, natural pozzolan continues to react with the calcium hydroxide produced by cement hydration and increases the compressive strength by producing additional C-S-H. After 21 curing days, the 30% Natural pozzolan/ 70% Portland cement mixture begins to exceed reference OPC in compressive strength. After 28 days, it exceeds reference OPC by about 15%. The pozzolanic reaction continues until there is no free calcium hydroxide available in the mass and the compressive strength exceeds the reference OPC by 30-40%.

• Increases Resistance to chloride Attack: Concrete deterioration caused by the penetration of chloride occurs quickly when chloride  ions react with calcium. The expansion of hydrated calcium oxy-chloride enlarges the microcracks and increases the permeablity that causes quicker chloride penetration and more damage from freezing and thawing action. The 30% Natural pozzolan added into cement can react with almost all the free calcium hydroxide and form a much denser past. Thus, the penetration of chloride can be minimized and the few penetrated chloride ions cannot find free calcium hydroxide with which to react.

• Increases resistance to sulfate attack: There are three chemical reactions involved in sulfate attack on concrete: 1) Combination of free calcium hydroxide and sulfate to form gypsum (CaSO4-2H2O).    2) Combination of gypsum and calcium aluminate hydrate (C-A-H) to form ettringite (C3A-3CaSO-32H2O).    3) Combination of gypsum and calcium carbonate with C-S-H to form thaumasite (CaCO3-CaSiO3-CaSO4-15H2O).

        All these reactions result in the expansion and disruption of concrete. Thaumasite in particular is accompanied by a very severe damaging effect which is able    to transform hardened concrete into a pulpy mass.

• Reduces alkali-aggregate reaction: Because Natural pozzolan is shattered into such a fine particle size resulting in dramatically increased reactive surface area, it can react quickly with calcium hydroxide and can trap the alkali inside the cement paste. Thus, it helps to form a denser paste with almost no alkali aggregate reaction at all.

• Protects steel reinforcement from corrosion: The preceding discussions make it very clear that concrete made from 30% Natural pozzolan/ 70% Portland cement mixture can protect steel reinforcement because it creates an environment so densely packed that no liquids or gases can penetrate through it to cause corrosion to the steel.

• Increases abrasion resistance: Natural pozzolan increases the compressive strength of concrete and makes the concrete matrix stronger and more dense. It also prevents the formation of pulpy, crispy, or water-soluble materials created by chemical attack. Therefore, it helps the concrete to durably resist abrasion.

• Lowers water requirement with high fluidity, self-leveling, and compression: In normal operations, the bulk volume of concrete in the constructions are placed and compacted by use of high frequency poke vibrators. The rapid vibration induces segregation phenomena of all orders of magnitude in the fresh concrete, e.g., stone segregation, internal bleeding giving bonding failures, and inhomogeneous cement paste and air-void systems. Under proper use of vibratory compaction, Natural Pozzolan minimizes or eliminates these problems due to the amorphous structure of the pozzolan particles.

• Improves Durability: The benefits and characteristics of Natural Pozzolan mentioned above clearly explain why the ancient structures built by the Greeks have survived over 2000 years of weathering.