Quick-Setting Cement

Quick-Setting Cement

Cement characterized by a rapid increase in strength during the initial hardening period. It is used chiefly in fabricating precast reinforced-concrete structural members and items. The enhanced mechanical strength of quick-setting cement during the early hardening period is due to the specific mineralogical composition and the microstructure of the clinker, the proportioning of additives, and the fineness to which the cement is ground.

The types produced include quick-setting Portland cement with compressive strength after three days of 25 meganewtons per sq m (250 kilograms-force per sq cm), particularly quick-setting Portland cement, and also quick-setting slag-Portland cement.

Air entraining cement

Air entraining cement

Air is entrained in the concrete in the form of bubbles which modify the properties of fresh concrete i.e. workability, segregation, bleeding and finishing characteristics. It also modifies the properties of hardened concrete i.e. resistance to frost action and permeability. Air bubbles acts as flexible ball bearings so that friction between the various ingredients of concrete is reduced and workability is automatically improved without additional water. The desirable air content is 3-6%. The various air entraining agents generally used are:

(a)  Alkali salts of wood resin

(b)  Synthetic detergents

(c)  Calcium ligno-sulphate

(d)  Aluminum powder, animal fats etc.

These are also available under commercial names such as vinsol resin, airalon, darex, teepol etc. Just for clarity, entrained air is different from entrapped air. The difference between them is given in the Table

Difference between entrained and entrapped air

Entrained Air	Entrapped Air
1- Entrained air is intentional and it is distributed evenly. 2-The size of bubbles in entrained air is very small ranging from 5to 80µ. 3-The shape is nearly spherical. 4-It is a desirable process.	1-Entrapped air is due to insufficient compaction and unevenly distributed. 2-The size of entrapped air pockets may vary from 10µ to 1000µ. 3-Entrapped air may be of any shape as it follows the contours of aggregate surface. 4-It is an undesirable inclusion.

Air entraining cement is not very popular and it is not covered by Code.

Portland Pozzolana Cement (PPC)

Portland Pozzolana Cement (PPC)

Pozzolana essentially means a silicious material having no cementing properties in itself. But in finely divided form it reacts with Ca(OH)2 in presence of water at ordinary temperature and forms compounds possessing cementing properties. In this process Ca(OH)2

is also utilized which otherwise is destined to leaching out of concrete leaving a porous and weaker concrete.

 The chemical reaction which takes place is as given below:

 2C3S + 6H = C3S2H3 + 3 CH

Ca(OH)2 + SiO2 + Al2O3 = C3S2H3+ other compounds

 Generally fly-ash, shale, volcanic ash, opaline, diatomaceous earth etc. are used as Pozzolanas. It is easier to grind together OPC clinkers with pozzolana than mixing the pozzolana afterwards at the time of mixing. Mixing afterwards is difficult because of Pozzolana being finer than cement. Fineness of Pozzolana is 300 m2/kg as compared to 225 m2/kg for OPC.

 Fly-ash is a waste product of thermal plants which is collected by electro-statically precipitating the exhaust fumes of coal-fired power stations. The use of fly-ash is an eco-friendly process as precipitation of fly-ash reduces the pollution. The quality of fly ash is tested. The code specifies that percentage of silica plus alumina should be at least 70% and maximum loss on ignition should be maximum 12%. Generally much better quality is available from power plants as compared to the codal specifications. The obvious advantage of using PPC is that impermeable and denser concrete is produced by blending OPC with fly-ash.

The early strength of PPC is contributed by OPC fraction and Pozzolana also starts contributing after sometime. This early strength should normally be not less than 22 MPa in 7 days and not less than 31 MPa in 28 days. Other properties of PPC like soundness, setting time etc. are same as OPC.

 Advantages of Pozzolana cement

 1.   Less heat of hydration

2.   Reduced leaching of Ca(OH)2 as it is utilized in reaction with Pozzolana.

3.   It can be used at almost all the places where OPC is used.

Blast furnace slag cement

Blast furnace slag cement

This cement is also known as ground granulated blast furnace slag (GGBFS). It is produced by blending OPC clinkers with blast furnace slag in suitable proportion (generally 25-65%) and grinding together. A small quantity of gypsum is also added which acts as a retarder. The slag can be separately grinded and mixed with OPC in the mixer during concreting also. But in the conventional drum mixers, it is difficult to achieve proper mixing.

The slag is a waste product in the manufacture of steel from open hearth blast furnace. Earlier this waste was thrown outside the plants and huge stacks of slag were lying unused creating disposal problem. It was later found that the constituents of slag were similar to that of cement and the so called “waste product” was rechristened as “by-product”. Its extensive use now has resulted in saving of enormous energy and raw mineral in addition to prevention of pollution.

Limitations of slag cement

 The only word of caution while using this cement is that the early strength is less as compared to OPC. Therefore the form-work removal should be adjusted accordingly. The early strength of blended cement is due to OPC fraction only and the slag fraction joins in strength contribution later on.

Earlier there was a considerable resistance and suspicion in the minds of the users in using the slag cement mainly because the good quality of slag was not available. In fact earlier the thrown out air-cooled slag was being used which is not considered good quality for concrete. Instead of air cooling if the slag is rapidly cooled by pouring water over it, then the process of crystallization is prevented and it solidifies as granules. This slag is ideal for cement. That is the reason why it is now called granulated slag.

 Physical properties of slag cement are similar to those of OPC in terms of fineness, setting time, soundness and strength etc, but the initial reaction with water is slower due to which the heat of hydration is low. It is a better resistant to sulphate, alkali and acid also.

 Recommended use of Slag cement

 (a)In sewage disposal works

(b)In water treatment plants

(c)Where low heat cement is required

(d)Where high chloride and sulphate attack is there

Sulphate Resistant Cement (SRC)

Sulphate Resistant Cement (SRC)

The normal OPC is susceptible to sulphate attack particularly magnesium sulphate. The sulphate present in the soil or surrounding environment reacts with free Ca(OH)2 available in the concrete and CaSO4 is formed. There is no dearth of free Ca(OH)2 as it is available in abundance in the set cement. The CaSO4 thus produced reacts with hydrate of calcium aluminate and form an expansive compound called calcium sulpho-aluminate which causes expansion and cracks in the set cement. Sulphate attack is further accelerated if it is accompanied by alternate wetting and drying also, which normally takes place in marine structures of the tidal zone.

The cement can be made sulphate resistant if quantities of C3A and C4AF are controlled as given below :

 C3A < 5% (Normally it is 8-12% in OPC) and 3 C3A + C4AF < 25%

 While controlling the quantities of C3A and C4AF, a precaution is to be taken when chloride attack is also perceived in addition to sulphate attack. In this situation, the quantity of C3A should be limited to 5-8%. Similarly when only chloride attack is there and no sulphate attack, then sulphate resistant cement (SRC) should never be used.

 The quantity of C3A in OPC can be controlled simply by blending with slag cement.

 SRC is recommended in

(1)Marine condition where both sulphate and chloride attacks are there

(2)Construction of sewage treatment plants

(3)Foundations and basement in soil having sulphate attack

(4)Chemical factories

Rapid Hardening Cement (RHC)

Rapid Hardening Cement (RHC)

It is also called Early Strength Cement because its 3 days strength is almost equal to 7 days strength of OPC. It is different from quick setting cement which only sets quickly whereas in RHC, strength development is very fast. This is because of following reasons:

(1)Higher fineness of cement. The specific surface of this cement is increased to 320 m2/kg as compared to 225 m2 /kg for OPC.

(2)Higher quantity of C3S in cement as compared to C2S. C3S is more reactive in comparison to C2S.

As a result of these changes, the rate of reaction in the early period is considerably increased and so is the heat generated. Therefore, this cement should be used only where the early strength requirement is there. At the same time it should not be recommended where surface/volume ratio is less and the large heat that will be librated, may not be dissipated effectively. If it is not ensured then there are chances of temperature cracks.

This cement is recommended :

(1)In pre-fabricated construction

(2)When the form work is to be removed early

(3)In road repair works(4)In cold weather where heat generation will be helpful in preventing freezing.

Ordinary Portland Cement (OPC)

Ordinary Portland Cement (OPC)

Ordinary Portland Cement (OPC) is the most common cement used in general concrete construction when there is no exposure to sulphates in the soil or groundwater.

What is OPC? 

Ordinary Portland Cement (OPC) is the most common cement used in general concrete construction when there is no exposure to sulphates in the soil or groundwater.

In OPC category, some grades of cement available in market.

Manufacture

The raw materials required for the manufacture of OPC are calcareous material such as limestone or chalk and argilaceous materials such as shale or clay. A mixture of these materials is burnt at a high temperature of approximately 1400 0C in a rotary kiln to form clinker. The clinker is then cooled and grounded with a requisite amount of gypsum into fine powder known as Portland cement.

OPC is a gray colored powder. It is capable of bonding mineral fragments into a compact whole when mixed with water. This hydration process results in a progressive stiffening, hardening and strength development.

 Compatibility 

OPC manufactured is compatible with admixtures meeting for normal dosages and conditions. It can be blended for use with other Portland cement.

Different types of cement

Mastou Ready Mix, we understand that our ready mix concrete and cement products, are the most unique and versatile building materials.

Same type of cement may not be suitable for different locations and climatic conditions. Therefore various types of cement have been developed as per the actual requirements. The necessary changes have been achieved by different methods like :

(a)   Changing oxide composition

(b)   Changing fineness

(c)   Using additives or mineral mixtures like slag, fly-ash or silica fumes etc.

The various types of cements generally used in various locations are as given below:

1. Ordinary Portland Cement (OPC)

2. Rapid Hardening Cement (RHC)

3. Sulphate Resistant Cement (SRC)

4. Blast furnace slag cement

5. Portland Pozzolana Cement (PPC)

6. Air entraining cement

7. Quick Setting Cement

8. Expansive Cement

9. High alumina cement

Composition of modern cement

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The modern cement is also called  “Ordinary Portland Cement”  because after setting it resembles in color and quality with the “Portland Stone” quarried in Dorset-UK

Portland cement primarily consists of

(1) Calcareous Material : Limestone or Chalk

(2) Argillaceous material : Clay / Shale

(3) Silicious Material : Silica

The process of manufacturing of cement essentially involves the following activities:

(A) Proportioning of raw material

(B) Grinding intimate mixing

(C) Burning in a large rotary kiln at 1400-1450

Due to intense heat, 20 to 30 % of material fuses together forming spherical balls known as “clinkers:. The “clinker” is only one step short of cement. The cement is obtained by grinding the clinkers up to the desired fineness.

Production of concrete

Production of concrete

The production of concrete involves two distinct but equally important activities. One is related to ‘material’ required for concrete and the other to ‘process’ involved in its production.


Material

The activity related to materials involves their

(1)       Selection

(2)       Proportioning

 Process

The activity related to process involved in production of concrete involves :

(1)                Mixing

(2)               Transportation

(3)               Placement

(4)               Compaction and

(5)               Curing

Out of above activities, more often than not, it is the ‘process’ which is responsible for quality of the concrete, though the cost of the concrete is mainly governed by the cost of the materials. The selection of materials and their proportioning is usually well taken care of at higher levels but the process is left to the lower levels. The expenditure incurred on materials goes waste if the process is not taken care of. Ignorance and lack of appreciation of good practices are the main reasons for the poor quality of concrete. Therefore, if we are able to control the process, we can obtain far better quality of concrete at no extra expenditure.

Ingredients of concrete

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The basic ingredients of concrete are as given below

Cement

It is the most important and costliest ingredient of concrete. The mix-design of concrete indirectly means optimizing the use of cement for obtaining the desired properties of concrete in green as well as hardened state. It affects the overall economy of the structure too. Different types of cements are available for different type of structures and different types of locations. Judicious selection of cement is necessary for the longevity of the structure.

Aggregate

The aggregates give volume to the concrete because these occupy maximum space in the total volume of concrete. Efforts should be made to usemaximum quantity of aggregates as these increase thevolumetric stability of concrete and make the mix-design

more economical.

Water 

It is indispensable because it is required for reaction of hydration. But its use should be restricted to minimum as possible considering the requirement for chemical reaction with cement and workability only. Any excess water is destined for evaporation, leaving capillary-pores in the concrete. Eventually, strength and durability both will be adversely affected when water is excessive.

Admixture

It is an optional ingredient which is used only for some specific purpose. It is used to modify some of the properties of concrete like setting time, workability or surface finishing characteristics etc. But admixture shouldnot be used to compensate for bad quality of concrete instead it should be used as a supplement to good construction practices. Though the newer versions of concrete i.e. HPC, RMC and SCC, the use of admixtures has become indispensable.

Superplasticizers & Other Admixtures

Superplasticizers & Other Admixtures

Superplasticizers, or high range water reducers, are admixtures used to make high slump flowing concrete. They can do this when they are added to low to medium slump, and low water to cement ratio concrete. The resulting highly fluid concrete is workable, needs little compaction and vibration, and  bleeding and segregation are reduced. Fluid concrete is used in placing thin sections, unopen spaces, obstructed reinforced steel, underwater tremie pipe, and pumped concrete. Superplasticizers, entrained air, alginates, and cellulose derivatives are workability agents that lubricate or increase the slump of stiff concrete. Other types of admixtures used in concrete are corrosion inhibitors, damp proofing agents, permeability reducing agents, colouring admixtures, pumping aids, alkali reactivity reduction admixtures, bonding admixtures and agents, grouting agents, gas forming agents, air detrainers (air reducers), and fungicidal, germicidal and insecticidal admixtures.

How to Figure Compression Strength of Concrete

How to Figure Compression Strength of Concrete

Building professionals perform concrete compression tests to identify the compressive strength of concrete used in structures, pavement and other applications. Compressive strength is a quality control indicator used to ensure uniform quality and adequate strength for its intended application. Design engineers specify acceptance criteria of compressive strength testing, based on established industry standards. Project personnel send samples to off-site facilities where certified technicians determine compressive strength under controlled conditions.

Instructions

1.    Compressive Strength Test Procedure

1-  Obtain concrete cylindrical core samples. Prepare each core for compression testing. Measure the length of each core. Adjust the core length to obtain a length-to-diameter ratio of 2.0. Prepare core ends so that no projections exceed 0.2 inches, the core is perpendicular to the longitudinal axis, within a 0.5 degree tolerance. This is about 1/16 inch per 12 inches.

2-  Measure the diameter of each cylinder with calipers. Measure the core at mid height in two directions, at right angles to one another. If the two measurements vary by more than 2 percent, the cylinder should not be tested. Add measurements together and divide by two to obtain the average diameter (d). Ensure the diameter of each end is no more than 0.1 inch from the mean diameter of the core. Cap the core with a sulphur or neoprene cap, as directed. Ensure that core remains moist after capping.

3-  Verify that the testing machine is power operated, calibrated and can apply a continuous load. Check that the initial setting is zero before testing. Adjust as needed. Ensure the upper bearing block moves freely and rotates to contact core.

4-  Wipe clean the lower and upper bearing block surfaces and bearing faces of the core. Place the core in the lower bearing block, centering its vertical axis under the upper bearing block. Verify that the machine load indicator is zero. Adjust as needed.

5-  Apply compressive load continuously and without shock. Hydraulic machines should apply loads at 35 plus or minus 7 pounds per square inch per second (psi/s). Apply load until machine indicates decreasing load and the core demonstrates complete failure by a defined fracture.

6-  Record the maximum load (L) attained prior to core failure. Record the type of fracture as directed.

2.     Compressive Strength Calculation

7-  Determine the average cross-sectional area of the cylindrical core. Multiply the average diameter of the core obtained earlier(d) by 0.5 to obtain the average radius(r).

r=0.5d

8-  Multiply the average radius by itself to obtain its square. (r * r)

9-  Multiply this amount by π, or about 3.14. This is the average cross-sectional area. (A)

10-  Calculate the compressive strength (C) of the core by dividing the maximum load (L) by the cross sectional area (A). C=L/A

11-  Record the calculated compressive strength to the nearest 10 pounds per square inch (psi).

Conditions Affecting Concrete Air Volume

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Reducing the aggregate size increases the volume of air in concrete. More air entraining admixture is needed when fly ash, pozzolan, slag and silica fume are added to the concrete. High alkali cements entrain more air than low alkali cements. Increasing the mixing water makes more water available to generate air bubbles in the concrete. Algae in water increases air content. Air content increases then decreases as slump increases. Vibrating the concrete decreases air content. Less air is entrained when the carbon black colouring agent is added to the concrete or the concrete curing temperature is high. Proper mixing increases air entrainment. 1-2 percent of air is lost during the transport of  concrete to the job site. At the job site, the air volume remains constant unless long distance pumping or conveying of the concrete occurs. Up to 2.5 percent of the air can be lost by long distance transport.

How to Use Ready Mix Concrete

Ready mix concrete, Ready mix concrete industry, Ready mix concrete construction

Concrete can be an enormous hassle to mix and pour. Whether it’s for a home renovation project or for use on a construction site, it’s often too much trouble to make yourself. Ready mix concrete comes pre-mixed and ready to pour. The ready mix supplier makes the concrete and drives a truckload to the project site where it is then poured and set up by skilled workers. It saves you time and you can avoid the trial-and-error process of pouring the concrete yourself.

Instructions

1- Use a concrete calculator to calculate how much concrete you’ll need for your project. You can find a link to an online concrete calculator in the Resource section below.

2- Make a list of possible suppliers for your ready mix concrete. You can check in your local Yellow Pages under “Concrete – Ready Mixed”, or online at yellowpages.com. There is also a link in the Resource section to concrete supply companies across America.

3- Determine your price range before ordering. A budget keeps you from overspending. According to the Concrete Network, you’ll want to consider the price of the concrete, grading, subbase, forms and finishing and the reinforcements.

4- Check that a truck can access the area where you need the ready mix concrete poured. The site should not have extremely soft ground where the vehicle will get stuck, be at a steep angle the truck cannot climb or descend, or have a path or road that is too narrow for a large truck.

5- Call your chosen suppliers and discuss what your needs are. Ask them what their recommendations are for your project, how long the job will take, when they can start the job, and what they estimate the job will cost.

6- Choose a company from those you called and schedule a time for them to visit. They will send someone to the building site to evaluate the project.

7- Go over your needs again with them during the visit and compare your results from the concrete calculator with their estimates of how much concrete you’ll need. Ask again about pricing, and discuss the type of concrete mixture needed for your project. Ask what their “lead time” is. Lead time is the number of days they’ll need to prepare before they can make your delivery.

8- Check the weather forecast and schedule the delivery for a clear, sunny day.

9- Order your ready mix concrete delivery when you are satisfied with the supplier and your choice of concrete. You’ll need to tell them how much you need, what type, when and where it should be delivered, and how far apart the trucks should arrive if you need more than one.

Aggregate & Concrete Testing

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Sets the standard for concrete and aggregate testing. Testing of concrete and aggregate provides engineers and contractors with vital information about the composition and performance of the concrete they will be using.

What Are Aggregate and Concrete?

Simple concrete is composed of water, aggregate and cement. Additional additives can be used to cause the concrete to dry faster, improve strength, change the color or improve water resistance. Aggregate can be sand, gravel or some other stone. Cement forms crystals as it dries, binding together the aggregate to form a solid mass.

Why Test?

The composition of concrete varies from batch to batch as well as between manufacturers. A concrete and aggregate testing facility ensures that all concrete used in a construction is consistent and meets current standards. Testing also guarantees that the concrete ordered by the contractor is consistent with the project’s needs.

Types of Testing

Concrete and aggregate inspection involves both onsite and laboratory testing. Qualified laboratories will take samples of mix components from the manufacturer and samples of the mixed product before and after it is delivered. Tests include strength, curing speed, type and quality of additives, composition and amount of entrained air.

Concrete Admixtures – Waterproofing Systems

Concrete Admixtures – Waterproofing Systems

Though Concrete Admixture is add-on materials put in the Concrete Mixer, it is not a substitute for sticking to high quality concreting practices. Accelerators, retarders, super plasticizers and air-entraining agents are usually the commonly sold admixtures by Concrete Admixture Manufacturers. Additional admixtures for bonding, shrinkage reduction, damp proofing and coloring are available with select Concrete Admixture Companies.

Admixtures for Concrete have special instructions of use with cement-concrete mixture. This is a simple process to produce desired properties. It will not affect any feature for installing the required concrete. Appropriate admixture use advice is based on years of testing and Concrete Repair Methods necessary for desired concrete properties.

Use of Concrete Additives is essential to achieve the right building substance as demanded by climatic conditions and project constraints based on geographical locations.

Our Waterproofing Systems provide adhesion, flexibility to improve work process by reducing water ratio but enhancing compression strength. Products of Waterproofing for Terrace are designed to handle hydrostatic pressures needed in the construction industry. High build properties ensure quick application.

Waterproofing Solutions provide homogenous mixes for easy application are tough and have low temperature pliability. High resistance to atmospheric agents and bonding properties are only two attractive features. Waterproofing Materials offer high usability and effectiveness in building or tight schedule construction projects.

Products used for Basement Waterproofing give adhesion, flexibility and high structural strength. The components of Waterproof Coating are usually mixed to form homogenous consistency. This gives ease of application with a brush, trowel or airless spray gun. Such treated surfaces must not receive moisture for the prescribed hours.