The Invention Of Portland Cement Biology Essay

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Since 1824 the invention of Portland cement, concrete material has been 180 years of history. Currently, the world's annual production of concrete has reached more than 30 billion cubic metres, and is widely used in civil engineering, water conservancy, construction engineering, marine, transportation, highway and railway projects, and even aerospace engineering and so on. It can be said that concrete materials made ​​a great huge contribution towards the development of human civilization.

In recent years, many literatures reported due to the lack of durability of concrete structures causing huge economic loss. The United States had a loss of 700 billion U.S. dollars in 1975 due to corrosion of concrete structure, and in 1985 it reached 

1680 Billion; it cost the United States 91 billion Dollars only to repair damaged bridge due to lack of durability; the United Kingdom spent up to 200 million pounds each year for the cost of repairing reinforced concrete structure. 

De Sitter's "law of five" stated that one dollar spent in getting the structure designed and built correctly is as effective as spending $5 when the structure has been constructed but corrosion has yet to start, $25 when corrosion has started at some points, $125 when corrosion has become widespread.

At present, the UK is experiencing more and more extreme weather during winter and there is a wide range of chloride ion corrosion to the road and bridge structures as a result of de-icing gritting, and the existence of acid rain in some industrial areas also cause concrete structures to suffer. Therefore, the corrosion of these structures is particularly important, and the durability of concrete has become one of the most focused issues in relation to the water conservancy, transportation and building construction.

1.2 durability of concrete

The AC1201 Committee defines the durability of concrete as the resistance of climate, chemical attack, abrasion and other damage process. Concrete structures are vulnerable in the course of the natural environment or human activity; in fact aging is another factor which is liable for concrete materials to damage over time, resulting in decreased bearing capacity and durability.

There are a number of causes responsible for reducing the durability of concrete structures, the carbonation of concrete, reinforcement corrosion, chloride ion penetration, freezing and thawing, alkali attack and some times due to micro-organisms. However there is a lot more issues related to durability, the impact factors and failure mechanism is very complex. The following briefly describes several effect, respectively, the failure mechanism and impact. 

1.2.1 Carbonation of concrete

Carbonation of concrete is one of the important factors that are influencing the durability of concrete structures. CO2, SO2 and other acidic gases or liquids penetrate into the concrete, triggering the chemical reaction with alkaline substances and lowering the pH value. This brings a neutralisation of concrete, where CO2 induced the neutralization process that is known as concrete carbonation.

The mechanism of carbonation are: CO2 penetrate into the concrete pore and start chemical reaction with Ca(OH)2 producing CaCO3 and H2O. On the other hand cement hydration also takes place and 3CaO · 2SiO2 · 3H2O (C-S-H) is generated which gets involved in carbonation reaction. The main chemical reaction as follows:

CO2 + H2O → H2CO3

Ca(OH)2 + H2CO3 → CaCO3 + 2H2O

3CaO · 2SiO2 · 3H2O + 3H2CO3 → 3CaCO3 + 2SiO2 + 6H2O

2CaO · SiO2 · 4H2O + 2H2CO3 → 2CaCO3 + SiO2 + 6H2O

When the carbonation causes the pH value to fall down to about 8.5, it has been generally believed that concrete is carbonised. Steel's passive film loses its steady state when the pH value of concrete is less than 11.5. When the carbonation depth exceeds concrete's its protective layer, given the appropriate temperature, air and moisture conditions, and the steel inside of the concrete rust easily, thus affecting the durability of concrete structures. Meanwhile, during the carbonation process, the reduction of alkali gives a chance to allow cement hydration products to decompose and release crystallised water, initiating irreversible shrinkage of concrete. If this process is carried out under the constraints, often lead to the formation of micro-cracks on the concrete surface, which further accelerate the carbonation reaction. Furthermore, carbonation also makes concrete to be brittle and its ductility deteriorated.

1.2.2 Steel corrosion

Carbonation and reinforcement corrosion are the main reasons lead to loss of durability of reinforced concrete structures. Discussed above is the mechanism of carbonation of concrete, and steel corrosion mechanism is briefly described below.

Under normal circumstances, concrete pore solution appears to be alkaline, and the pH value is between 12 and 13. In this strongly alkaline environment, a layer of dense oxide (nFe2O3 · H2O) forms on the steel surface to protect the steel from corrosion. However, if the protective layer is wholly or partially damaged, a large potential difference in different parts of the surface will be formed, and the formation of cathode and anode. In the presence of moisture and oxygen, steel electrode reaction occurs and starts corrosion process. Steel corrosion is usually divided into natural electrochemical corrosion and stray current corrosion. Corrosion of reinforcing steel bar in concrete is generally the natural electrochemical corrosion, the electrochemical reaction as follows:

At the anode, Fe2+ along with OH- form insoluble Fe (OH) 2, and further oxidised to produce Fe (OH) 3, the reaction is as follows:

The generated forcing its inner layer of iron into the cathode, there by further development of corrosion. Some of the  may also be subjected to water loss, forming FeOOH (oxy iron hydroxide), and some will generate Fe3O4 · nH2O due to insufficient oxygen for reaction, create a layer of loose rust on the steel surface. As a result of corrosion, the volume of steel reinforcement could expand about 3 to 7 times of its original, which would have exerted a large tensile stress to the surrounding. Since concrete is not good against tension, cracks may form and will accelerate the corrosion of steel even more, leading to a vicious cycle. Therefore, the result is caused by steel corrosion, cracking of concrete up to the protective layer, reducing the effective area of ​​reinforcement, and ending up in structural damage.

1.2.3 Chloride Corrosion

Chloride ions are the most important and one of the common media that are liable for destroying the steel passivation layer. During the concrete manufacturing process, its mix materials or added admixtures may contain chloride ions, while concrete can be exposed to water or de-icing salt gritting environment. These factors can make the chloride ion concentration on the steel surface to reach a critical point where corrosion of reinforcement takes place.

When chloride ion dissolves in water its penetrating ability increases dramatically, that can be easily adsorbed and kept on the steel surface, leading to greater corrosion. The reaction is as follows:

1.3 durability and permeability of concrete

1.3.1 The relationship between concrete's durability and permeability

From the above mechanism of various types of corrosion, it can be seen that H2O, CO2, O2 and chloride ions play a key role, and in particular the role of water:

H2O and CO2 penetrate into the concrete leading to the formation of H2CO3, which is a necessary condition for carbonation of concrete. Carbonation of concrete would not continue without the presence of those two elements.

The presence of O2 and H2O make it possible for steel corrosion, and if neither is dispensable to the formation of the cathode, therefore not to generate Fe(OH)3 and rust layer.

Chloride ions could only penetrate concrete easily when dissolved in water and it catalyses the corrosion of passivation layer. If without the presence of water, the external chloride ions could not enter the concrete system, or accumulate within the concrete structure to reach a critical concentration.

External H2O, CO2 and chloride ions have been able to enter the concrete, mainly depends upon permeable of concrete. Large number of studies show that the permeability of concrete and its durability is closely related (Figure 1.1). Permeability is generally considered to be a crucial indicator to evaluate concrete's durability. As some experts commented, concrete with lower permeability generally has better durability.

The figure below concisely reflects the relationship between the durability and permeability.

Figure 1.1 relationships between the durability and permeability of concrete.

1.3.2 Dynamics Affecting Permeability of Concrete 

To reduce the permeability of the concrete, we must first understand the features affecting permeability of concrete.

Concrete is solidified by cement hydration with sand and gravel aggregate, and existing as heterogeneous porous material. The permeability subjected the speed of proliferation or migration of gas, liquid, and ions under pressure, chemical potential or the electric field in the Concrete. A point of view from the microstructure is that concrete is porous, and its permeability is closely related to porosity, porosity distribution and pore continuity.

The sources of pores in concrete can be divided into two major categories: constructive pores and structural pore. Constructive pores are formed during the manufacturing process owing to insufficient degree of vibration, resulting in unsatisfactory density, therefore leaving pore spaces inside of concrete. Structural pores are formed during concrete's hydration process, due to excessive moisture evaporation. The pores can also be classified according to their size:

Gel pore- that is, the pores existing in the grid structure of cement gel, cement hydration produces porous calcium silicate gel, counting for 28% of gel volume, and its size is generally 1.5nm to 3.0 nm, slightly bigger than water molecules in terms of magnitude, and the permeability coefficient is small, about 10-6cm/s to 10-9cm/s, is one of the smallest structural pore and is generally considered impermeable.

Capillary pore - that is, space left in the concrete after evaporation of excess water during the cement hydration process. The size of it varies from 10nm - 100nm. The water content needed for cement hydration is around 20%-25% of cement, but the actual quality of water is much larger than this value. Some part of the pore volume in cement hydration process will be occupied when the cement gel formed (cement hydration generated solid volume of cement is 2.1 times the original), but the cement hydrates are still unable to completely fill the space originally occupied by water. Therefore, the greater the water-cement ratio, the more the pores left behind after water evaporation, the larger the pore space and the worse the resistance to permeability.

Deposition pore - aggregate particles in concrete are mostly surrounded by cement, so the greatest impact on the permeability of concrete is permeable cement, whereas aggregate particles themselves have little impact on permeability. However, condensation in the concrete hydration process leads to the formation of pores due to sand and aggregate settlement and inconsistent deformation of sand and aggregate materials and surface water evaporation also lead to the formation of connected pores which are even larger than the capillary pores. Those are the main reasons for permeability of concrete.

Other than the existence of pores, the impact of crack must not be ignored. Cracks in concrete can be caused mainly from two aspects, plastic cracking and shrinkage cracking. when the concrete at the plastic stage, a large amount of water evaporate from its surface, the surface tension acts to force concrete surface shrinking and this results in formation of plastic shrinkage crack. The other type of cracking happens during the cement hydration process, chemical reaction generate heat and this makes a temperature gradient inside and outside the concrete, causing uneven expansion of concrete volume, thus causing cracks forming. When the crack size reaches 0.1mm, it will be vulnerable to water penetration.

In addition, exposing concrete to the weathering and erosion in the natural environment also reduces its impermeability. It is worth mentioning a chemical reaction takes place in concrete between tricalcium silicate, dicalcium silicate and water to produce calcium silicate hydrate and calcium hydroxide, which calcium hydroxide has greater solubility in water. When water goes inside the concrete, calcium hydroxide seeps through it leaving the concentration out of balance, and calcium silicate, calcium aluminate and calcium oxide that have already reacted with water will be decomposed into the water and washed away, leading to structural pores to increase in size and quantity, and loosening the concrete structure, increasing its permeability and eventually reducing its performance. 

From the above review we can see there are three main reasons accountable for concrete permeability: (l) concrete pore spaces; (2) internal cracks; (3) concrete erosion by weathering forming water channel.

Due to variety of components of concrete, complex hydration reaction, the environment the concrete is surrounded by and poor weather conditions, the solution to the impermeability of concrete is very difficult.

1.3.3 Methods to improve the impermeability of concrete structures

There are four conventional methods that are used to improve the impermeability of concrete:

Material grading

Through the different gradations of gravel, concrete can be made with improved density, but the procedure is very demanding to the gravel gradation, strictly conformed to the sieve size distribution curve. This method not only increase costs, but difficult for implementation, thus not commonly used in the UK.

Mineral admixture

Such as fly ash added to concrete, silica fume, granulated blast furnace slag and other admixtures, not only can improve the structural interface of concrete and cement paste pore structure, but also help improve the flow, reduce water consumption and increase the impermeability and Durability . However this technology is heavily dependent on the source raw material.

Incorporation of additives

Concrete mixed with additives, such as water-reducing agent, air entraining agent, expanding agent and water repellent and so on, to appropriately reduce water consumption as well as water-cement ratio, in order to improve the workability of concrete and to change the pore structure in the hardened concrete. Thus obtaining significant decrease in pore size and porosity, the pores are more subtle, more uniform which makes the impermeability and density of concrete to be improved. Incorporation of additives to improve the impermeability of the concrete has obvious and effective results, but there are also a number of downsides, while only suitable for new construction.

Use of Waterproof membrane and waterproof coating outer cover

Waterproof membrane and waterproof coatings can be used for new or existing projects. Waterproof membrane in terms of classes and categories include asphalt waterproofing membrane, flexible polymer bitumen waterproofing membrane, plastic bitumen waterproofing membrane, and ternary PVC waterproofing membrane. Waterproof coatings are usually acrylic, polyurethane cool class, cementitious capillary crystalline and silicone based waterproof coating.

From technical point of view of the four different methods described above: the most direct, simple, and effective way to improve the impermeability of concrete is use of waterproof materials. In addition, the waterproof coating and waterproofing membrane not only appropriate for newly constructed buildings, but also particularly suitable for protective measures of the existing construction.

1.4 Silicone Silane

Silicone based silane is one category of the broad term waterproof coating, and it differs from other waterproof coating by: the former makes a good use of the permeability of concrete, with a certain depth of penetration into the concrete surface, providing an immersion protection, whereas the later completely blocks the pores on the surface. We can clearly tell from the comparison above that silicone based silane delivers impregnated protection, not only greatly reduce concrete's capillary water absorption but also prevent freeze-thaw damage, steel corrosion due to carbonation and chloride ions attack, ensure its long-term durability (alkali resistance, UV resistance). Since silane maintains the permeability of concrete and external appearance, so it will not happen the same way as other film-forming agent which causes cracking. Its features are especially useful for roads and bridge structures that need resistance to constant abrasion and corrosion. 

Figure 1.2 Ways of sealing pores on concrete surface

Filling the pores b) Film coating c) Silicone Silane

By applying silicone silane through brush or spray, siloxane hydrophobic membrane is formed on the silicate substrate surface, so that it changes the nature of the substrate surface. When the angle, at point of contact between substrate surface and water becomes greater than , the entry of water into pore space is prevented, thus achieving the waterproof effect.

Figure 1.3 the effect of contacting angle (angle of contact,)

Figure 1.4 the effect of contacting angle (angle of contact,)

1.4.1 Category of Silane

Silicone based waterproofing products usually contain silane alkyl, siloxane, alkyl silicon alkoxide and hydrogen silicone oil as the main active ingredient. They are composed differently for various purposes, some products contain 100% of the active material, and some have their active ingredients dissolved in a solvent by a certain percentage. They can be divided into the following categories:

Water-soluble silicone based penetrant

The main component of water-soluble silicone penetrant is methyl-silicate solution and is generally in the form of yellow or transparent liquid. The methyl silicate is susceptible to weak acid and is easily decomposed when in contact with water and carbon dioxide in the air, forming a waterproof poly-methyl-ether silicon membrane and soon making a good bonding with the concrete surface and through the internal layer to improve the penetration resistance of mortar.

Advantages:

Relatively cheap and easy to use

Disadvantages:

Slow reaction with carbon dioxide with 24h of curing required

Can only be used under correct circumstances since it is still vulnerable to rain and frost few hours after application, causing unreacted alkali metal methyl silicate to leave concrete surface and losing its functionality

Solvent based silicone based penetrant

The main component of solvent-based silicone penetrant is siloxane isobutyl silane, and organic solvents are added as a carrier. Active siloxane, especially the alkylation poly-siloxane, its polymer molecular chain contains a certain number of active groups such as hydroxyl, carboxyl, amino and so on. As such agent is sprayed on to the concrete surface, with catalyst or its self-introduced amino it solidifies, forming a hydrophobic layer. During the formation of hydrophobic membrane, it does not require the introduction of carbon dioxide from the air, or the production of alkaline carbonate substances which are harmful to the concrete.

Advantages:

Product is stable and therefore easy for storage

Durability of the hydrophobic membrane is higher than the methyl silicone alkoxide , alkyl hydrogen silicone oil. 

When applied to a surface of concrete, solvent evaporates very quickly, depositing a thin layer of film covering pores on the concrete surface.

Since the layer is so thin that it almost does not change the natural appearance of concrete. 

Silicone solvent is also less influenced by the outside environment than the methyl sodium silicate, therefore providing better resistance to water penetration.

It is suitable for reinforced concrete, marble and other low porosity substrate thanks to its decent durability, good penetrating ability.

Disadvantages:

The treatment surface must be dry when application.

Due to demand of organic solvents as a carrier, there may be some threads toward the environment, and the problem has raised a growing social concern, so it needs further research to resolve.

Silicone cream penetrant

In recent years, solvent-based silicone acrylic resin is under more stringent environmental regulatory restrictions, the high performance, low-pollution water-based acrylic silicone coating gradually becomes a new focus of attention.

Silicone cream is a water repellent organic polymer latex (such as acrylic acid, acrylic polymer cream, etc.) and active silicone cream copolymerization of a structure coating. It can form a transparent organic polymer cream membrane that has good adhesion to the concrete surface, but it is relatively poor in heat and weather resistance. The active silicone cream contains a crosslinking agent and catalyst compositions, and water loss can be carried out at room temperature, forming a poly siloxane network structure of the elastic membrane. This membrane has excellent resistance against high and low temperature and water repellence. But to an extend, not performing well in adhesion for some filler.

Furthermore, in order to overcome the serious loss of material when applying the silicone water repellent products, to lengthen the contact time with the concrete surface, and to increase the penetration depth, scientists have further developed paste, gel, silica powder and so on to help achieve the competent performance.

1.4.2 Study and application of Silane

Organic silicon compounds born in the 19th century, but until the mid-20th century, the United States and European countries first began to use it for waterproofing of building structures. In the past two decades, organic silicon has been a widespread concern in the field of concrete structure waterproofing research and application.

On going research in the world

In 1990, WACKER CHIEMIE GMBH (DE) patented the EP0442098. Huhn, Dr.Karl and many others who introduced the organic poly-siloxane silane with an average size less than 0.3. It can be made using a mixing device at the right turbulent and pressure of 0.01 - 1MPa (HBS) under the pressure to form a concentrated solution of organic poly-siloxane, and then diluted with water under similar conditions  to achieve desired concentration, which can also be adjusted by adding acid to attain a pH of around 3 - 7.

In 1991, Robert L. Cuthbert and Edwin P. Plueddemann from the Dow Corning Corporation patented US 5073195.

In 1993, Dr. Goebel Thomas Michel Rudolf and Alff Harald from DEGUSSA patented EP 0538555 and in which they presented that they have achieved less than 1 droplet of the silane cream.

In 1994, Dr. Montigny Armand and Kober Hermann patented EP0616989.

In japan, Hamasaka Mitsuo from the TOYO ink MFG co Ltd patented JP 09-202875. He introduced the composition which had better performance with long term storage, and stability, which could penetrate into capillary pores on the concrete surface, through bonding reaction to form a hydrophobic layer to improve water resistance.

In 1998, WACKER CHEMIE GMBH (DE) patented the EP0819665, enabling it to be publically used for buildings' waterproof treatment.

In 1999, Dow Corning Corporation introduced its new patents US 6103001 and WO 00/3406.

In 2000, Hwang In Dong from Korea improved the stability and penetrating ability of silicone based silane, achieving a penetrating depth greater than 4mm, and water absorption less than 0.1.

1.4.3 Products of silane protective Coatings

Silane

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