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This technique is based upon organic acids, salts of acids and bases which form substantially insoluble complexes with ions selected from the group consisting of calcium and aluminum ions. When applied to a thoroughly moist concrete surface the active ingredients react with the various concrete minerals forming soluble crystal complexes which fill out all cracks, pores and voids up to a width of at least 500 microns. This crystalline growth will eventually take place deep into the concrete structure and may eventually penetrate it.
The reactions which take place are very complex but can, in broad terms be explained as follows. During the cement hydration process, the crystalline complex reacts with calcium hydroxide and various metal oxides and salts found in the concrete. As a result of these reactions soluble connections or more complex salts capable of binding varying amounts of water are formed.
Due to the consumption of calcium hydroxide the hydration of the cement will extend further than normal. Normally in setting concrete up to 25 per cent of the cement will not become hydrated due to the protection against water which the surrounding tobermorite gel offers. The reason for this is that the cement gel is changed in character. Due to an increase in volume the gel will craze which again allows the entry of more water to the unhydrated cement. This process produces still larger volumes of cement gel, and thus larger capillary tracks will be narrowed or possibly blocked to the passage of water.
Described as re-activated cement it is considered as spherical shell-like crystals due to the large number of polar groups. For stereometrical reasons these crystals have a positive charge on the outside and a negative charge towards the centre. Due to their size, water molecules may force their way into these shells and will there as a result of the negative charges (normally produced by oxygen atoms) be bound to these charges with weak hydrogen linkages (Van do Waal bonds). As the distance between the free positive negative charges increases, the crystals will outwardly become more positive, and the water molecules can now be bound to the outer surfaces of the shells because of the large dipole moment of the water and in this way the volume of the crystals will increase considerably.
Not only will these chemical reactions take place at the surface of the concrete or immediate adjacent area, but will continue deep into the concrete structure. The extent of the penetration and the time factor involved depend first of all on the presence of the moisture and the concrete ingredients. Furthermore it would depend on the physical properties of the concrete such as cement content, density and compaction.
This crystalline reaction spreads in the concrete structure as a result of three individual factors and combinations thereof, osmosis, Brownian movement, and dry particle reaction. When cement bound reagents are applied the chemical potential of the concrete changes where the application takes place, then by means of these factors, the chemicals and thereby the chemical reactions will tend to move from an area of high salinity (chemical potential) to an area of lower salinity until the chemical potential becomes uniform throughout the structure.
Although these crystalline complexes form as a result of reactivation of the cement, they eventually block the capillary systems from passage of water but will allow the passage of vapour. and in this way water which has not been bound in crystal form is allowed to evaporate during the course of the crytallising process until it is halted altogether with a minimum of internal pressure. If the reaction is denied water it will cease until wetted and the remaining capillaries take up free water thus completing Crystallization. It is vital to keep the concrete surface wetted during reactivation and hydration which is akin to curing.
Experience and results over 20 years has shown that even construction joints can be made watertight and withstand pressure in excess of 3 bar in reinforced concrete 200 mm thick and over. If the organic acid and base compound is used in solution with freshly mixed concrete the effect is to retard setting and must be avoided as shutter striking periods will be unnecessarily affected.
Weathering and frost action on cracked, crazed and degraded concrete can be halted by reactivating the concrete gel structure. The cost benefit of this method of enhancing concrete is invaluable and attractive.
The subject of waterproofing concrete has been described in considerable detail by comparing surface membranes with a method of enhancement of the chemical reactions involved with cement. Economics determine the use (coupled with the need for applied finishes) and as concrete is normally thought of as a dense material it beholds the engineer to enhance it during construction.
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