In OPC, C3A and C2AF has high affinity to sulfate (gypsum) to form mono-sulfate or ettrengite. But in contrast, high alumina cement is good at resisting when exposed to SO4- environment. Why?
When sulphate react with alumina in the cement forms Aluminium sulphate, common and popularly called Alum.
Aluminium sulfate is used in water purification and as a mordant in dyeing and printing textiles. In water purification, it causes suspended impurities to coagulate into larger particles and then settle to the bottom of the container (or be filtered out) more easily. This process is called coagulation or flocculation. Research suggests that in Australia, aluminium sulfate used this way in drinking water treatment is the primary source of hydrogen sulfide gas in sanitary sewer systems. An improper and excess application incident in 1988 polluted the water supply of Camelford in Cornwall.
When dissolved in a large amount of neutral or slightly alkaline water, aluminium sulfate produces a gelatinous precipitate of aluminium hydroxide, Al(OH)3. In dyeing and printing cloth, the gelatinous precipitate helps the dye adhere to the clothing fibers by rendering the pigment insoluble.
Aluminium sulfate is sometimes used to reduce the pH of garden soil, as it hydrolyzes to form the aluminium hydroxide precipitate and a dilute sulfuric acid solution. An example of what changing the pH level of soil can do to plants is visible when looking at Hydrangea macrophylla. The gardener can add aluminium sulfate to the soil to reduce the pH which in turn will result in the flowers of the Hydrangea turning a different color (blue). The aluminium is what makes the flowers blue; at a higher pH, the aluminium is not available to the plant.
In the construction industry, it is used as waterproofing agent and accelerator in concrete. Another use is a foaming agent in fire fighting foam.
This is the main reason that high alumina content cement has more resistant against sulphate attack.
Literature mentions that high alumina cement forms dense micro-structure. It also has very hard surface and carbonation further decreases the porosity at the surface. This limits the penetration of SO4- inside the core of paste hence less deterioration. But I am not satisfied with this explanation.
For example, diffusion process should be able to get SO4- penetrated inside the cement paste core. Even small amount of SO4- can cause transformation of mono sulfate to ettrengite causing huge volume increase.
It's all about Ca(OH)2. It reacts well with sulphates. It appears in concrete from alite and belite reactions with water, if portland cement is used. In alumina cements there is no alite and belite (only belite can be present in very small doses), therefore Ca(OH)2 is not formed.
The compound calcium hydroxide - Ca(OH)2 is formed when Portland cement hydrates . Ca(OH)2 is not resistant to sulphate attack. Without getting too technical, a different compound is formed when High Alumina cement hydrates. This compound is resistant to sulphate attack.
Please excuse my trans-Atlantic spelling of sulphate (sulfate)!
C4A3S + 18H → C3A.C$.12H + 2AH3
C4A3S + 2CSH2 + 34H → C3A.3C＄.32H + 2AH3
C4A3S + 6CH +8C$H2 + 74H → C3A.3C$.32H
C2S + 2H → C-S-H + CH
C2S + AH3 + 5H → C2ASH8
C4AF + 12C$H2 + 106H → 4C3(A0.75,F0.25).3C＄.32H +2FH3
Ye'elimite which is C4A3S can hydrate just in presence of water and gives monosulphate. You have mono sulphate and aluminum hydroxide, also called gypsite. When there is the presence of gypsum, you have a formation of Ettringite. And again you will have aluminum hydroxide. Now, when there is enough lime in the system, which may be the case when you have enough of C2S in CSA cement or it is used with Portland cement, you have only Ettringite forming, so no aluminum hydroxide. C2S can react with water and form CSH, calcium silicate hydrate. And the C2S can also react with this aluminum hydroxide which is coming from these two reactions and form stratlingite. So these are the possible reactions which may take place depending on obviously composition of CSA cement. Although C4AF is in minor amount but it can also hydrate in presence of gypsum and form Ettringite. Therefore, CSA cement performs better in sulfate attack compare to OPC.
High Alumina cement offers good resistance to sulfate attack owing to the following reasons: the absence of CH, the presence of a protective coating of alumina gel, and the low reactivity of its hydration product, CAH10, with sulfate ions. However, depending on the temperature at which the hydration process takes place, hexagonal hydrates (CAH10 or C2AH8) or cubic hydrates (C3AH6) form upon hydration of HAC. Even though CAH10 or C2AH8 form as primary products, they are metastable and they convert to more stable and denser C3AH6 with time. This conversion causes a decrease in the volume of hydrated materials resulting in an increased porosity and permeability and a decreased strength of concrete. On the other hand, an expansive reaction is said to take place between C3AH6 (called katoite) and sulfate ions.
This question is very important and attracted me to read all the answers that are well explained and do not contradict each other. Thank you!
This question is very important and attracted me to read all the answers that are well explained and do not contradict each other. Thank you!
can you people can give reference of some article
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