BUILDING GYPSUM CHARACTERISTICS BASIC INFORMATION

Compared with other binding materials, building gypsum has the following characteristics:

1. Fast Setting and Hardening
The setting time of building gypsum changes with the calcination temperature, grinding rate and impurity content. Generally, mixed with water, its initial setting needs just a few minutes at room temperature, and its final setting is also within 30min.

Under the natural dry indoor conditions, total hardening needs about one week. The setting time can be adjusted according to requirements.

If the time needs to be postponed, delayed coagulant can be added to reduce the solubility and the solution rate of building gypsum, such as sulfite alcohol wastewater, bone glue activated by borax or lime, hide glue, and protein glue; if it needs to be accelerated, accelerator can be added, such as sodium chloride, silicon sodium fluoride, sodium sulfate, and magnesium sulfate.

2. Micro-expansion
In the hardening process, the volume of building gypsum just expands a little, and there won’t be any cracks. Thus, it can be used alone without any extenders, and can also be casted into construction members and decorative patterns with accurate size and smooth and compact surface.

3. Big Porosity
After hardening, the porosity of building gypsum can reach 50%-60%, so its products are light, insulating, and sound-absorbing. But these products have low strength and large water absorption due to big porosity.


4. Poor Water Resistance
Building gypsum has low softening coefficient (about 0.2-0.3) and poor water resistance. Absorbing water, it.wil1 break up with the freeze of water. Thus, its water resistance and frost resistance are poor, not used outdoors.

5. Good Fire Resistance
The main component of building gypsum after hardcning is CaS04*2H20. When it contacts with fire, the evaporation of crystal water will absorb heat and generate anhydrous gypsum which has good thermal insulation. The thicker its products are, the better their fire resistance will be.

6. Large Plastic Deformation
Gypsum and its products have an obvious performance of plastic deformation. Creep becomes more serious especially under bending load. Thus, it is not used for load-bearing structures normally. If it is used, some necessary measures need to be taken

BUILDING GYPSUM CHARACTERISTICS BASIC AND TUTORIALS

Compared with other binding materials, building gypsum has the following characteristics:

1. Fast Setting and Hardening
The setting time of building gypsum changes with the calcination temperature, grinding rate and impurity content. Generally, mixed with water, its initial setting needs just a few minutes at room temperature, and its final setting is also within 30min.

Under the natural dry indoor conditions, total hardening needs about one week. The setting time can be adjusted according to requirements.

If the time needs to be postponed, delayed coagulant can be added to reduce the solubility and the solution rate of building gypsum, such as sulfite alcohol wastewater, bone glue activated by borax or lime, hide glue, and protein glue; if it needs to be accelerated, accelerator can be added, such as sodium chloride, silicon sodium fluoride, sodium sulfate, and magnesium sulfate.

2. Micro-expansion
In the hardening process, the volume of building gypsum just expands a little, and there won’t be any cracks. Thus, it can be used alone without any extenders, and can also be casted into construction members and decorative patterns with accurate size and smooth and compact surface.

3. Big Porosity
After hardening, the porosity of building gypsum can reach 50%-60%, so its products are light, insulating, and sound-absorbing. But these products have low strength and large water absorption due to big porosity.

4. Poor Water Resistance
Building gypsum has low softening coefficient (about 0.2-0.3) and poor water resistance. Absorbing water, it will break up with the freeze of water. Thus, its water resistance and frost resistance are poor, not used outdoors.

5. Good Fire Resistance
The main component of building gypsum after hardcning is CaS04*2H20. When it contacts with fire, the evaporation of crystal water will absorb heat and generate anhydrous gypsum which has good thermal insulation. The thicker its products are, the better their fire resistance will be.

6. Large Plastic Deformation
Gypsum and its products have an obvious performance of plastic deformation. Creep becomes more serious especially under bending load. Thus, it is not used for load-bearing structures normally. If it is used, some necessary measures need to be taken.

FIBERS FOR CONCRETE MIXES BASIC AND TUTORIALS

As used in concrete, fibers are discontinuous, discrete units. They may be described by their aspect ratio, the ratio of length to equivalent diameter. Fibers find their greatest use in crack control of concrete flatwork, especially slabs on grade.

The most commonly used types of fibers in concrete are synthetics, which include polypropylene, nylon, polyester, and polyethylene materials. Specialty synthetics include aramid, carbon, and acrylic fibers. Glass fiber-reinforced concrete is made using E-glass and alkali-resistant (AR) glass fibers. Steel fibers are chopped high-tensile or stainless steel.

Fibers should be dispersed uniformly throughout a mix. Orientation of the fibers in concrete generally is random. Conventional reinforcement, in contrast, typically is oriented in one or two directions, generally in planes parallel to the surface.

Further, welded-wire fabric or reinforcing steel bars must be held in position as concrete is placed. Regardless of the type, fibers are effective in crack control because they provide omnidirectional reinforcement to the concrete matrix. With steel fibers, impact strength and toughness of concrete may be greatly improved and flexural and fatigue strengths enhanced.

Synthetic fibers are typically used to replace welded-wire fabric as secondary reinforcing for crack control in concrete flatwork. Depending on the fiber length, the fiber can limit the size and spread of plastic shrinkage cracks or both plastic and drying shrinkage cracks. Although synthetic fibers are not designed to provide structural properties, slabs tested in accordance with ASTM E72, ‘‘Standard Methods of Conducting Strength Tests of Panels for Building Construction,’’ showed that test slabs reinforced with synthetic fibers carried greater uniform loads than slabs containing welded wire fabric.

While much of the research for synthetic fibers has used reinforcement ratios greater than 2%, the common\ field practice is to use 0.1% (1.5 lb /yd3). This dosage provides more cross-sectional area than 10-gage welded wire fabric. The empirical results indicate that cracking is significantly reduced and is controlled. A further benefit of fibers is that after the initial cracking, the fibers tend to hold the concrete together.

Aramid, carbon, and acrylic fibers have been studied for structural applications, such as wrapping concrete columns to provide additional strength. Other possible uses are for corrosion-resistance structures. The higher costs of the specialty synthetics limit their use in general construction.

Glass-fiber-reinforced concrete (GFRC) is used to construct many types of building elements, including architectural wall panels, roofing tiles, and water tanks. The full potential of GFRC has not been attained because the E-glass fibers are alkali reactive and the AR-glass fibers are subject to embrittlement, possibly from infiltration of calcium-hydroxide particles.

Steel fibers can be used as a structural material and replace conventional reinforcing steel. The volume of steel fiber in a mix ranges from 0.5 to 2%. Much work has been done to develop rapid repair methods using thin panels of densely packed steel fibers and a cement paste squeegeed into the steel matrix.

American Concrete Institute Committee 544 states in ‘‘Guide for Specifying, Mixing, Placing, and Finishing Steel Fiber Reinforced Concrete,’’ ACI 544.3R, that, in structural members such as beams, columns, and floors not on grade, reinforcing steel should be provided to support the total tensile load. In other cases, fibers can be used to reduce section thickness or improve performance. See also ACI 344.1R and 344.2R.

FORMS OF SILICA FUMES BASIC AND TUTORIALS

FORMS OF SILICA FUMES BASIC INFORMATION
What Are The Forms Of Silica Fumes?


Silica fume is available commercially in several forms in both North America and Europe:

• As-produced silica fume is silica fume collected in dedusting systems known as bag houses. In this form, the material is very fine and has a bulk density of about 200 to 300 kg/m3, compared with 1500 kg/m3 for Portland cement (Malhotra et al., 1987).

As-produced silica fume is available in bags or in bulk. Because of its extreme fineness, this form poses handling problems; in spite of this, the material can be and has been transported and handled like Portland cement.

• Compacted silica fume has a bulk density ranging from 500 to 700 kg/m3 and is considerably easier to handle than as-produced silica fume.

To produce the compacted form, the as-produced silica fume is placed in a silo, and compressed air is blown in from the bottom of the silo. This causes the particles to tumble, and in doing so they agglomerate.

The heavier agglomerates fall to the bottom of the silo and are removed at intervals. The air compaction of the asproduced silica fume is designed so the agglomerates produced are rather weak and quickly break down during concrete mixing.

Mechanical means have also been used to produce compacted silica fume.

• Water-based silica fume slurry overcomes the handling and transporting problems associated with as-produced silica fume; the slurry contains about 40 to 60% solid particles. Typically, these slurries have a density of about 1300 kg/m3.

Some slurries may contain chemical admixtures such as superplasticizers, water reducers, and retarders. One such product (known as Force 10,000®) has been successfully marketed in North America.

CONCRETE CORROSION INHIBITORS ADMIXTURE BASIC AND TUTORIALS

CONCRETE CORROSION INHIBITORS ADMIXTURE BASIC INFORMATION
What Are Corrosion Inhibitors Admixtures For Concrete?


Reinforcing steel in concrete usually is protected against corrosion by the high alkalinity of the concrete, which creates a passivating layer at the steel surface.

This layer is composed of ferric oxide, a stable compound. Within and at the surface of the ferric oxide, however, are ferrous-oxide compounds, which are more reactive.

When the ferrous-oxide compounds come into contact with aggressive substances, such as chloride ions, they react with oxygen to form solid, iron-oxide corrosion products.

These produce a fourfold increase in volume and create an expansion force greater than the concrete tensile strength. The result is deterioration of the concrete.

For corrosion to occur, chloride in the range of 1.0 to 1.5 lb /yd3 must be present. If there is a possibility that chlorides may be introduced from outside the concrete matrix, for example, by deicing salts, the concrete can be protected by lowering the water-cement ratio, or increasing the amount of cover over the reinforcing steel, or entraining air in the concrete, or adding a calcium-nitrate admixture, or adding an internal-barrier admixture, or cathodic protection, or a combination of these methods.

To inhibit corrosion, calcium-nitrate admixtures are added to the concrete at the time of batching. They do not create a physical barrier to chloride ion ingress. Rather, they modify the concrete chemistry near the steel surface.

The nitrite ions oxidize ferrous oxide present, converting it to ferric oxide. The nitrite is also absorbed at the steel surface and fortifies the ferric-oxide passivating layer.

For a calcium-nitrite admixture to be effective, the dosage should be adjusted in accordance with the exposure condition of the concrete to corrosive agents. The greater the exposure, the larger should be the dosage.

The correct dosage can only be determined on a project-by-project basis with data for the specific admixture proposed. Internal-barrier admixtures come in two groups. One comprises waterproofing
and dampproofing compounds.

The second consists of agents that create an organic film around the reinforcing steel, supplementing the passivating layer. This type of admixture is promoted for addition at a fixed rate regardless of expected
chloride exposure.

WATER REDUCING ADMIXTURES FOR CONCRETE BASIC AND TUTORIALS

WATER REDUCING ADMIXTURES FOR CONCRETE BASIC INFORMATION
What Are Water Reducing Concrete Admixtures?


Water-Reducing Admixtures
These decrease water requirements for a concrete mix by chemically reacting with early hydration products to form a monomolecular layer of admixture at the cementwater interface.

This layer isolates individual particles of cement and reduces the energy required to cause the mix to flow. Thus, the mix is ‘‘lubricated’’ and exposes more cement particles for hydration.

The Type A admixture allows the amount of mixing water to be reduced while maintaining the same mix slump. Or at a constant water-cement ratio, this admixture allows the cement content to be decreased without loss of strength.

If the amount of water is not reduced, slump of the mix will be increased and also strength will be increased because more of the cement surface area will be exposed for hydration. Similar effects occur for Type D and E admixtures. Typically, a reduction in mixing water of 5 to 10% can be expected.

Type F and G admixtures are used where there is a need for high-workability concrete. A concrete without an admixture typically has a slump of 2 to 3 in. After the admixture is added, the slump may be in the range of 8 to 10 in without segregation of mix components.

These admixtures are especially useful for mixes with a low water-cement ratio. Their 12 to 30% reduction in water allows a corresponding reduction in cementitious material.

The water-reducing admixtures are commonly manufactured from lignosulfonic acids and their salts, hydroxylated carboxylic acids and their salts, or polymers of derivatives of melamines or naphthalenes or sulfonated hydrocarbons. The combination of admixtures used in a concrete mix should be carefully evaluated and tested to ensure that the desired properties are achieved.

For example, depending on the dosage of admixture and chemistry of the cement, it is possible that a retarding admixture will accelerate the set. Note also that all normal-set admixtures will retard the set if the dosage is excessive.

Furthermore, because of differences in percentage of solids between products from different companies, there is not always a direct correspondence in dosage between admixtures of the same class. Therefore, it is
important to consider the chemical composition carefully when evaluating competing admixtures.

Superplasticizers are high-range water-reducing admixtures that meet the requirements of ASTM C494 Type F or G. They are often used to achieve highstrength concrete by use of a low water-cement ratio with good workability and low segregation.

They also may be used to produce concrete of specified strengths with less cement at constant water cement ratio. And they may be used to produce self-compacting, self-leveling flowing concretes, for such applications as longdistance pumping of concrete from mixer to formwork or placing concrete in forms congested with reinforcing steel.

For these concretes, the cement content or watercement ratio is not reduced, but the slump is increased substantially without causing segregation. For example, an initial slump of 3 to 4 in for an ordinary concrete mix may be increased to 7 to 8 in without addition of water and decrease in strength.

Superplasticizers may be classified as sulfonated melamine-formaldehyde condensates, sulfonated naphthaline-formaldehyde condensates, modified lignosulfonates, or synthetic polymers.