- Cement Chemistry Notation and Mineral Names
The durability of cement pastes is strongly influenced by: i internal chemistry, and ii paste microstructure. The industrial by-product additives above all influence the development of paste microstructures. In neat OPC pastes, two types of porosity contribute to the total pore volume. Isolated pores are completely enclosed by hydration products so that material transport into and out of the pore is limited. Connected porosity is that through which a continuous pathway between regions of the microstructure exists.
Continuous or interconnected porosity often although not always links the interior of the paste to the outside world so that aggressive chemical species can penetrate and degrade the paste internally having consequences for paste durability. The effect of the blending agents identified above on microstructure is to cause a reduction in the degree of interconnected porosity.
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This is especially true in the case of BFS-containing pastes. Although the overall porosity, as determined by neutron scatterring is still significant, the interconnected porosity as measured by intrusion methods e. MIP is low. Durability includes influences of mechanical damage, e. Sulphate attack : — expansion arising from the reaction between monosulphate 4CaO.
The conversion from the high density phase to the low density one can cause expansion and cracking. The cracking opens up new connected porosity which accelerates the transport of sulphate into the cement paste and the deterioration of the paste.
Try writing out the equations for the monosulphate-ettringite conversion. Delayed Ettringite Formation : — a fairly recently discovered problem relating to degradation in steam-cured products.
15th Interational Congress on the Chemistry of Cement
A number of mechanisms for this have been proposed and at least one major legal conflict has arisen based on assigning liability for failed concrete products. It is proposed that the temperature cycle used in steam-curing produces a non-crystalline ettringite precursor which, on cooling and after some extended time period, crystallises having absorbed the required amount of moisture. Carbonation : — lowering of matrix pH due to the reaction between dissolved CO 2 and the calcerous phases in the paste. Found at surfaces, a zone of carbonated product penetrates towards the interior to a distance which is defined by the porosity of the paste.
Degradation of the principal binding phase therefore can lead to strength loss. A more immediate concern is the loss of high pH in the vicinity of steel reinforcement. A pH of greater than about Loss of pH therefore increases the corrosion risk. Effect of Chloride : — used to be a common additive to cements to accelerate setting but this use is now banned in structural concretes Why do you think CaCl 2 would accelerated cement setting?
Chloride interactions with set concrete are however common, e.
Penetration is via connected porosity so that cover concrete may quickly become heavily loaded with soluble chloride. Steel passivation films are rapidly de-stabilised by chloride locally giving rise to pitting corrosion and rapid deterioration of the steel the corrosion is focused in series of a small areas. Where corrosion can continue by depassivation , expansion of corrosion products at the cement-steel interface and the subsequent spalling of cover concrete can occur.
Many examples of this can be seen in concrete structures. Spalling leads to exposure of previously internal concrete as a fresh site for environmental damage. Generally, flints, opals, cherts and strained quartz have a high degree of reactivity in concretes. The reaction is driven by the high pH pore fluid and the reactive silica and gives rise to a sodium silicate gel product which contains only a small amount of calcium.
The gel imbibes water causing swelling and this gives rise to expansion cracks in affected concretes. The degree of expansion is important with respect to the servicability of affected structures. A recent study on a water inlet tower of a dam in Tasmania, Australia, showed that AAR expansion had increased the diameter of the tower sufficiently so that the inlet valves were unable to stem the flow of water when they were in the closed position.
Glass-fibre Reinforcement Corrosion : Unlike steel reinforcement, glass fibres are introduced in random orientation and throughout the paste. Typically, as filament bundles of around 50 filaments the fibres will be of variable length up to 2 cm typically. As in the AAR, the highly alkaline cement pore fluid attacks the siliceous glass to produce a gel which imbibes water. While this system has definite advantages, it does take some getting used to, since, for example, C normally stands for carbon and H normally stands for hydrogen.
Unfortunately the confusion does not end there, as we also have to deal with chemical and mineral names for solid phases.
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The various ways of describing some of the main solid phases associated with cement chemistry are summarized in Table 3. Bensted and P. Department with academic responsibility Department of Materials Science and Engineering. For more information regarding registration for examination and examination procedures, see "Innsida - Exams".
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