In the absence of calcium hydroxide (〖Ca(OH)〗_2), hydrated produces are ettringite and an amorphous alumina gel (AH_3) as follows: C_4 A_3 S ̅+2CS ̅H_x+(38-2x)H→C_6 AS ̅_3 H_32+2AH_3 with x = 0 for anhydrite, x = 0.5 for hemihydrate or plaster and x = 2 for gypsum. When calcium sulfate is fully depleted in the system that absence of calcium hydroxide, the formation of monosulfate (C_4 AS ̅H_12) by tetracalcium trialuminate sulfate hydration becomes the dominant reaction according to the following
Sodium and potassium fluoaluminates Silico-aluminate dissolution in HF. Fluoaluminates, like fluosilicates, occur when spent mud acid (H2SiF6) reacts with the formation. They can also form if seawater or sodium or potassium brines are mixed with spent HF. Aluminium hydroxides and fluorides Clay and feldspar dissolution in HF can cause these precipitates. Iron compounds Iron minerals or iron oxides (rust) can react with HCl-HF to produce these compounds.
These reactions occur at varies rate and varies time. Altogether, these chemical reactions determine how cement harden and gain strength. The main product of hydration of silicate mineral is a calcium silicate hydrate (C-S-H) of colloidal dimension that, at early ages, under 11 | Page scanning electron microscope, usually shows up like an aggregation of very fine grains partly inter-grown together ( Reinhardt, 1995). Beside calcium silicate hydrate (C-S-H), other product of hydration are such as calcium hydroxide, ettringite and monosulfate. Calcium hydroxide is also more soluble and alkaline than the other hydration products.
• For conduction, the particles must be in direct contact with one another. • A temperature gradient must exist. • The rate at which energy is transferred is dependent on parameters of the system which include its geometry, thickness and the material from which it is made. Thermal Convection: • Convection is heat transfer that occurs within a fluid. • Heat is transferred from one part of the fluid of higher temperature to another part of the fluid at a lower temperature.
3.Concrete corrosion and the stages: 3.1 Reinforcement corrosion Reinforcement corrosion refers to the electro-chemical or chemical reaction between the reinforcement bar and the environment, causing its deterioration. The causes of corrosion in concrete can be explained practically or in terms of electrochemistry. Some of the factors affecting corrosion include the presence of aggressive species, for instance chlorides and their quantity. This species breaks down the reinforcing bar protective film thus causing corrosion. Corrosion of reinforced bars is the main cause of concrete degradation but not due to the limitations caused by the concrete itself.
Firstly sulphide mineral is oxidised to form dissolved iron, sulphate and hydrogen, by the following chemical reaction. (Evangelou, V.P, 1998) 2FeS2+7O2+H2O→2Fe2+ +4SO4 2- +4H+ (Evangelou, V.P, 1998) (1) These dissolved chemical products represent an increase in solids and acidity of the water. Due to an exposure to oxidizing environment such as oxygen in the atmosphere, ferrous ions decompose further according to the following chemical reaction: (Evangelou, V.P, 1998) 4Fe2+ + O2+4H+→4Fe3+ +2H2O (Evangelou, V.P, 1998) (2) Fe3+ precipitates as Fe (OH) 3 thus further lowering
Oelkers and D. R. Cole, 2008). Mineral carbonation is the reaction of carbon dioxide with basic minerals to form solid carbonates that are stable and relatively insoluble (E. H. Oelkers, S.R. Gislason and J.M. Matter, 2008). The CO2 is usually reacted with the elements calcium, magnesium, and iron to form compounds such as calcite and magnesite, which means less CO2 in the atmosphere, hence reducing its impact on global warming and climate change (E. H. Oelkers, S.R.
QUESTION 1 (A) Alkali-silica reaction • Chemical reactions that take place between the alkali pore solution of concrete and metastable forms of silica. • The aggregates that react to alkali can be identified by using the petrographic examination or the laboratory test found in SANS 6245. Hydration reaction • Hydration is a chemical reaction in which the major compounds in cement form chemical bonds with water molecules and become hydrates or hydration products. • Heat is given off during the chemical reaction as the cement hydrates. • Fast reaction (C3S + H => C-S-H + C-H) which increases the rate of the heat being released and the development of strength.
The reaction between lime stone (CaCO3) and H2SO4 takes place in the following manner: CaCO3 + H2SO4 CaSO4 + CO2 + H2O. CaSO4 produced in this reaction is water soluble and is easily washed away, leaving behind an eroded and pitted surface. Effects of acid rain Acid rain causes a number of harmful effects. It causes deterioration of buildings especially made of marble e.g. monuments like Taj Mahal.
For flexural strength beams of dimensions 150mm×150mm×700mm were prepared. To check the repeatability of results three beams of each mix were prepared and then tested for 28 days strength in universal testing machine. For split tensile strength cylinders of diameter 150mm and height 300mm were prepared. To check the repeatability of results three cylinders of each mix were prepared and then tested for 28 days strength in compression testing