TEQ Flashcards
TEQ: Please make a list of all the phases contained in your hardened portland cement and shortly describe the role of each phase, with special care to degradation.
C-A-H
Products of hydration of aluminites
Loss of workability
Ettringite
Formation from reaction with gypsum
Skin on the surface of C-A-H surface to extend workability window to delay the setting
Ca(OH)2 = portlandite
From hydration of silicates
Problems
Chemical degradation
C-S-H crystals
development of mechanical properties
Tricalcium aluminate = celite
Loss of workability - liquid phase
Bicalcium silicate = belite
Mechanical properties
Tricalcium silicate = alite
Mechanical properties
Iron containing phase
For the liquid phase
All four present from before the reaction
Make a list of the major physical (biological) mechanisms of weathering of stone and select one of them in order to describe the degradation.
The major physical mechanisms of weathering of stone material are the following:
Micro traumas
Coupling or contact with other stone materials, especially when there is difference in the thermal expansion coefficient
Too high loads/mechanical stresses
Wind and water effects: erosion, removal of material
Light effect: change of color
Thermal shock effects: disintegration, flaking, exfoliating, swelling
Micro cracks can occur when treating the stone surface with different methods such as bush hammering or trimming since there will appear micro cracks on the surface. These micro cracks can become even bigger when for example load is applied which will make water enter into the stone material. The consequence of cracks and water entering can be tremendous. For example if water contains salts it can crystallize which expands and forms pressure inside the stone. This can cause flaking. Water in combination with temperature differences can cause gelivity, which means that the water can freeze and since ice expands this will also create pressure inside of the stone and can break the structure.
Make a list of the main types of chemical weathering for stones and select one of these mechanisms for a more detailed description.
The major chemical weathering for stones are chemical weathering for carbonatic stones and chemical weathering for silicate based stones.
The mechanisms for chemical weathering of carbonatic stones are the following:
Bicarbonate
Nitrification
Sulfation
The process of sulfation occurs when the stone is exposed to the atmosphere since sulfouros oxide comin form industrial emissions form sulfuric oxide and react with water to form sulfuric acid. When it comes in contact tih the carbonatic rock, this acis react with the stone material to form calcium culfate. The calcium sulfate along with soot and other particles form dark crusts.
Dark crusts increase the volume of the salt with about 20%. The thermal expansion coefficient of the salt is about 5 times higher than of the rock so stresses will definitely be induced. Swelling and detachment will occur. To remove the dark crust you need to be careful, the best is to soften the dark crust first.
The mechanisms for chemical weathering of silicate stones are a bit more complicated, but one example is for granite:
GRanite is composed by feldspar, quartz and mica. The chemical weathering of granite primarily involves the hydrolysis of feldspar, oxidation of biotite and other ferromagnesian minerals, hydration and carbonation. These processes lead to the breakdown of the original minerals in granite, forming clay minerals, oxides and hydroxides, as well as dissolved ios. This results in increased porosity, weakening of the rock structure and the gradual disintegration of granite over time.
TEQ: describe how we can consolidate a stone material.
If we need to consolidate a stone material we can use several different consolicating agents. It all depends on if the material is exposed to the outdoor environment, what type of climate the stone is in, and what type of consolidating the stone needs to be prevented with.
A stone can be consolidated with inorganic or organic consolidating agents. The inorganic agents are typically lime, calcium bicarbonate, barium hydroxide, TEOS. These are good since they are very compatible with the substrate, they can be water soluble which can be good if we want to remove this consolidating agent later. The inorganic consolidating agents can thus be irreversible, have poor capability to penetrate into the stone material and also poor capability of repairing large fractures.
The inorganic materials can be applied using several methods, such as water based methods, mechanical based, laser technology etc.
The organic consolidating agents are of polymers such as polyester, epoxy resins, polyurethanes, silicones, acrylics. These agents are good since they are easy to penetrate into the stone, they have good adhesive properties, and they can improve the mechanical properties of the stone. They are more likely to degrade tho and are not as optimal substrate to the substrate as the inorganic ones are.
The organic agents can be applied using methods such as bruch or painting, spraying, immersing, capillary rising, under vacuum. The different polymers listed above can be used for different things. For adhesive problems the polyester, epoxy, acrylic and polyurethanes can be used. The epoxy should be used outside, one thing to consider then is that the epoxy does not have a good resistance to UV and water, so it should be coated with a protection layer of polyacrylic. If the environment has high temperature the silicones are good to use since they have high temperature resistance, and also to chemical attacks and water. If we need to fill a stone material we can use polyester, polyurethanes, acrylic.
If we need to protect the material, meaning to protect the surface form aggressive agents, we can use the same agents as for consolidation → epoxy resins, acrylic resins, silicone. The approach are different depending on if the material is compact and non permeable or porous and permeable. If they are compact we can use coating, if they are porous we can impregnate with monomers that can polymerize in situ.
TEQ: Please discuss the advantages of Portland cement blends … from the viewpoint of degradation.
Portland cement blends are a type of hydraulic portland cement of clinker + gypsum but with the addition of additives in the mixture before the hydration phase. The portland clinker can be replaced with other materials, which makes this product cheaper, more recycling friendly and also mor reusable for specific applications in for example concrete.
Portland cement blends delivers a lower heat during hydration due to a low amount of tricalcium aluminate, and this makes the thermal conductivity problem decrease. The thermal conductivity problem is a problem when their is a thermal gradient between the core and the surface of the cement and therefore the surface tends to shrink more easily and propagate cracks in the material and thus degrade.
If we use pozzolanic cement, we add even more additives in the form of volcanic ash, fly ash, clay furnace etc. The pozzolan can be added up to 55% wt, and makes the cement silica rich material. The siliceous rich cement gives very high mechanical properties during the hydration. But the silica also forms Portlandite during the hydration. Pozzolanic material reacts with portlandite and reduces the amount of it. The reduction of portlandite reduces the pH and this can lead to corrosion in reinforced concrete. One positive thing when the pozzolanic material reacts with portlandite is that it reduces the problem of fluent water inside of the cement. Since Portlandite is soluble in water, this reacts with CO2 to form calcium carbonate and this increases the porosity even more and weakens the material.
The pozzolanic cement is good in the viewpoint of the hydration process, during the hydration it is not so high heat produced which lowers the crack risk due to the lower thermal conductivity problem. This type of cement is also good for mass production, like bridges, damms etc, since they have very high mechanical strength.
TEQ: You have a certain brick wall or whatever and discuss the possible degradation mechanisms of the building and the possible strategies to solve the problems.
If we have a certain brick wall outside, these bricks are exposed to the outside environment. Some weathering mechanisms for bricks in general are gelivity and crystallization of soluble salts.
Gelivity can become a problem when moisture enter the pores in the brick and represents the liquid-solid transformation due to the change in the temperature. The process can be reversible which is when rein enters and follows by the sun. But the more dangerous gelivity problem is when it is irreversible, meaning that moisture enters the pores and the temperature gets low the water can freeze inside the pores. Ice expands inside the pores and puts pressure on the pore walls which can cause expansions and cracks in the material.
Another weathering mechanism is crystallization of soluble salts. This is just like for stones. When water evaporates and leaves behind salt crystals inside the material this can cause internal cracking. The salts can come from rain, snow, capillary rising, materials inside of the brick itself.
How to mitigate the problem of degradation of a brick in a wall? This can be done by solving the problem of getting water inside of the brick by using
A hydrophobic coating
A impermeable coat like a plastic sheet underground to avoid direct contact between wall and solid
A kap on the top
Render = puts, can be applied on walls to join different parts together.
TEQ: Further for the brick wall outside, explain how the water can be removed from the brick wall when degradation has happened and restoration needs to be done.
To remove water from the brick wall we can use electro osmosis. This is a way of creating an electric current to make the water go back down to the ground again.
The process starts with capillary rise of water, since this is the natural flow of water transporting inside of a material with pores, due to a negatively charged surface and a positively charged surface in the ground. By inserting an electrode application, we can make the surface to a positively charged anode and the ground to a negatively charged cathode, making the water travel down to the ground again. The movement of the water dehydrates the wall and making it dry again, and a new plaster, lika an impermeable coat, can be applied between the ground and the wall to protect it from water penetrating up again after the restoration is done.
