How To Choose Cement

Table of contents:

How To Choose Cement
How To Choose Cement

Video: How To Choose Cement

Video: How To Choose Cement
Video: Understanding Concrete, Cement, and Mortar | Ask This Old House 2024, March
Anonim
  • Cement making
  • Properties and characteristics of cement

    • Hydration
    • Related videos:
    • Cement and temperature
    • Cement and water
    • Volume change
    • Activity and strength
    • Grinding fineness
    • Density, specific gravity
    • Other qualities of cement
  • Cement types
  • Learning to read the markings

    • GOST 31108-2003
    • GOST 10178–85
  • How to distinguish original cement
  • Related videos:
How to choose cement
How to choose cement

Cement rightfully occupies a leading position among all modern building materials. It is used everywhere, at almost all stages of building construction. Foundations and concrete roofs, piece wall blocks and monolithic enclosing structures, cast at the place of overlap and ready-made slabs of concrete goods, steps and staircases, screeds and lintels, masonry and plaster mortars, tile adhesives, putties, grouting, leveling materials and other dry mixtures - difficult building without cement is almost impossible. But he was not always at the disposal of people.

How to choose cement
How to choose cement

For several millennia in a row, our ancestors, by trial and error, were looking for such a universal astringent, but they could not come close to the ideal. Clay, lime, gypsum almost immediately fell into the field of vision of ancient architects, and they are still actively used to prepare solutions for internal work. However, the only problem was that such binders are not able to adequately withstand the effects of moisture.

There was no alternative for a long time, but at the very beginning of our era, the Romans were able to create a mixture similar to modern concrete - archaeologists found and dated to the first century well-preserved, rather massive monolithic structures. And these were not shards and fragments, but serious structures, for example, the famous Roman drainages. It is believed that the builders of Ancient Rome successfully experimented with volcanic ash, especially notable were the soils in the Pozzuoli area near Naples, which could be seized even under water. Everything would be fine, but over time, the technology for producing pozzolanic binder was irretrievably lost, alchemists again began to invent a magic powder that turns into stone.

How to choose cement
How to choose cement

At the turn of the 18th and 19th centuries, the first cements of modern times appeared. Who should give the palm for the opening, as usual, it's hard to say - everything happened at lightning speed and almost simultaneously. In 1796, James Parker at 800-900 degrees burned a mixture of clay and lime and received "Roman cement" (Roman cement). Louis Veca in 1817 published (without patenting the technology) his research on the hydraulic properties of a mixture of lime and volcanic ash, where he described in detail the temperatures, ratios, conditions, compositions. 1824 Joseph Aspdin refined the theoretical basis of his predecessors and quickly received a patent for "Portland cement" (in some characteristics similar to the rocks from the English town of Portland).

How to choose cement
How to choose cement

Almost in parallel with the Englishman, our compatriot Yegor Gerasimovich Cheliev announces his discovery of cement, but the patent has already been issued to a citizen of the British Empire, and this innovative binder is named “English cement” for a long time. After the World Exhibition in London in 1851, cement gains universal recognition, and its mass production begins in all developed countries. In 1856 the first cement plant was opened in Russia. A real construction breakthrough was achieved with the introduction into life of materials derived from cement - concrete and reinforced concrete, it didn't take long, it happened in the middle of the 19th century.

Cement making

More than two hundred years have passed, but the technology of cement production has not fundamentally changed, except that more advanced aggregates are now used, grinding has become thinner, special additives have appeared, and clear parameters for control and weight manipulations have been established. The process of obtaining cement occurs approximately according to the following scheme:

How to choose cement
How to choose cement
  1. The raw material (chalk / limestone, clay / loam) after extraction is crushed into pieces of no more than 10 cm and mixed - at the exit, a coarse sludge is obtained. Further, it will be crushed several more times, each time everything is thinner.
  2. The sludge is moistened, or vice versa, dried (there are three methods of cement production - wet, dry and combined). With the dry method, the sludge, not moistened to 30–50%, is fed into the furnace, but dry powder.
  3. At a temperature of about 1400–1500 ° C, the sludge is fired for several hours in a rotary kiln until the elements are sintered into a homogeneous "alloy" - cement clinker in the form of balls.
  4. The clinker is ground / ground to a powder state.
  5. About five percent of gypsum is added to the powder, which is designed to control the setting speed. At the same stage, various mineral additives can be introduced into the resulting composition, which improve some properties or reduce the cost.
  6. At the final stage, the quality of the entire batch is checked - samples of products are made from cement mortar, which are tested in laboratory conditions. Based on experimental research, each batch receives its own strength grade.
How to choose cement
How to choose cement

Properties and characteristics of cement

So, cement is an artificially produced, mineral binder, which, after mixing with water, turns into a plastic mass that can subsequently harden and form a stone. Cement differs from other known binders in that it can set and gain strength in wet conditions. It is mainly used for the preparation of mortars and concretes. To learn how to correctly use this unique binder, it is necessary to study its main properties, to understand the processes occurring in cement mortars and concretes.

How to choose cement
How to choose cement

Hydration

Cement is a hydraulic binder, that is, in order to obtain a stone-like body, the powder must be mixed with water. After adding water, an irreversible, but completely controlled process begins.

Water molecules attach to the mineral molecules that make up the cement - the powder dissolves - a plastic mass is obtained - a chemical reaction begins. The phenomenon of formation of water compounds with dissolved material is precisely called hydration.

How to choose cement
How to choose cement

The reaction products (they are responsible for the strength of the future product) gradually saturate the solution, after which the dissolution stops, and the cement paste loses its mobility, turns into jelly - it sets. Setting occurs in the first hours after mixing the solution. The start date of this process (colloidation) and its duration largely depend on the temperature regime, the amount of water, the fineness of the powder grinding (the finer, the faster the setting occurs), the composition of the cement (without gypsum stone, mobility is lost instantly). Delay, stretch the setting process, possibly by stirring the still moving mass, this is the basis of the work of mixer cars that deliver concrete. However, it will not be possible to maintain the plasticity of the solution indefinitely; at a certain moment, the mass will begin to lose its useful properties. It is recommended to test the cement in advance in order to understand the actual setting time, if it is necessary to adjust them, you can use special additives.

How to choose cement
How to choose cement

Sometimes cement sets in just a few seconds after mixing with water, and a large amount of heat is released. This phenomenon is called false setting, the solution can be "revived" by stirring. False setting for cement is a negative property, such a binder is discarded, in extreme cases, the manufacturer must indicate it upon shipment.

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After the solution loses its mobility, the dissolved cement particles begin to crystallize, grow, intertwine, and coalesce into a cement stone. The hardening process is non-linear - in the first week, the mass quickly gains strength, then crystallization slows down. This is due to the fact that different minerals (there are four main ones) enter the final phase of the reaction after different periods of time. Full hardening can last for years, but the design grade of product strength is achieved within 28 days, demoulding under favorable temperature and humidity conditions is possible within 3-5 days.

Cement and temperature

Temperature has a serious effect on the setting and hardening of cement stone. The higher the air temperature, the faster you can get a product of a given strength. For example, two hours after the preparation of the solution, the setting will begin, after another hour it will stop. This is at a normal air temperature of 20 degrees, while at its lower rates, mobility can be maintained much longer (at zero degrees - up to 20 hours). Of course, there is a direct relationship between the temperature regime and the stage of hardening of the solution / concrete, the corresponding table is given in our article “Strip foundation. Part 3: concreting, final operations."

How to choose cement
How to choose cement

So, with a thermometer reading below zero, this process practically stops, but with an artificial increase in temperature, you can very quickly bring the product to the design grade. This is the basis for the technology of steaming reinforced concrete products at a temperature of 80–90 degrees, which gain 60–70 percent strength in 10–12 hours, can be removed from the molds and shipped. Electric heating systems also work to accelerate the growth of strength.

How to choose cement
How to choose cement

Separately, it should be noted that in the process of cement hydration, a certain amount of heat is released. Under normal conditions, this phenomenon must be controlled, since in massive monoliths the outer layers of concrete cool faster, and the inner layers slower, which can cause thermal stresses leading to the formation of cracks. In winter, active heat generation is an undoubted advantage, since the temperature of the laid concrete is maintained at a high level for a longer time (you just need to insulate the working area), and the strength continues to grow. The amount of heat released depends on the fineness of grinding and the mineralogical composition of the cement; heat generation can be adjusted either by selecting a specific type of cement or by using mineral additives.

Cement and water

Moisture is essential for hydration. But in fact, the water demand of cement is relatively small, one part of water to five to six parts of cement is quite enough for this binder to turn into dough and begin to harden. However, mortars with such a water-cement ratio (W / C) are inactive, have low workability, therefore, in practice, a proportion close to 1: 2 is used - water is half as much cement.

A higher W / C ratio leads to the formation of an excessive amount of air pores in the cement stone (excess water will evaporate over time), which significantly reduces the strength of the finished product, the grade of mortar / concrete. It is possible to reduce the amount of water, but at the same time maintain the normal density of the cement paste (LHCT), by adding surfactants to the mixture, which makes it possible to reduce the consumption of cement and increase the strength and durability of concrete. By introducing pozzolanic additives into the composition of the solution, and using a finer grinding of the powder, it is possible to increase the water demand of cement.

How to choose cement
How to choose cement

After 3-5 hours, under the influence of gravity, heavy particles of cement slurry settle, and a layer of water rises to the surface. This is the so-called delamination effect that occurs when a lot of excess moisture is used. Delamination violates the integrity of the concrete mass with a decrease in its strength, especially with multi-layer pouring. The delamination process also takes place inside the mass, when water accumulates under the reinforcement threads and particles of coarse aggregate, naturally, with the destruction of the necessary bonds. Layering of the mortar greatly impairs its workability.

Note that moderate water separation is considered a normal property for most cements (especially in concretes), its intensity directly depends on the characteristics of the raw materials used by the manufacturer. However, if the percentage of water loss at normal W / C significantly exceeds the 15 percent barrier (in relation to the cement paste), then this circumstance may be a sign of poor quality cement.

How to choose cement
How to choose cement

The high water-retention capacity (in fact, this is an increased water demand) of cement is considered a plus, since the concrete from it turns out to be more homogeneous with good adhesion of the cement stone to reinforcement and crushed stone.

The water demand and water separation of specific cements can vary, therefore, when choosing the optimal water-cement ratio, one should be guided not only by the requirements for the mobility of solutions and the strength of products, but also to study the properties of the binder. To assess the water-holding capacity of cement and water separation, it is recommended to carry out a trial pouring. These indicators are corrected … correctly, by the introduction of special additives.

Volume change

During the formation of a cement stone in a monolith, volumetric deformations can be observed - the mass shrinks over time. This process is especially evident in the case of strong heat release. If the shrinkage occurs too much, then excessive internal stress in the concrete can cause its destruction. In accordance with the new GOSTs, cement manufacturers must check the uniformity of deformations in laboratory conditions; according to the norms, they should not exceed a threshold of 10 mm.

How to choose cement
How to choose cement

Activity and strength

Strength is the most important characteristic of cement, on which the strength and durability of concrete depends. Therefore, the choice of cement only by brand or class is quite understandable. To determine the strength, a sample of the product, ripened to the required level (28 days or steamed), is examined - a beam 40x40x160 mm in size, made of a cement-sand mortar in a ratio of 1: 3. The essence of the measurement is to determine the ultimate loads (calculated in MPa) at which destruction occurs. According to the research results, cement is attributed to a certain brand or strength class, we will talk about them later.

How to choose cement
How to choose cement

There is one more correct concept that better defines the strength properties of cement - this is its activity. Considering that the maximum strength of concrete is gained not at all for 28 days, but much longer, then a better brand, a higher class by this time will show a more active cement, which has managed to increase its strength and will allow to remove the formwork earlier and continue construction work. The activity of cement directly depends on its composition and fineness of grinding, and if we talk about "outside" factors, then on the ambient temperature and on the water-cement ratio.

How to choose cement
How to choose cement

During long-term storage, the activity of cement is significantly reduced, the reason for this is atmospheric moisture, which is absorbed by the powder. That is why the storage period for cement is 60 days. It can still be used after this time, but the actual brand will no longer correspond to the declared one - it will be necessary to increase its content in the solution.

Grinding fineness

We have already mentioned this characteristic of cement many times, perhaps it is worth mentioning it separately. Finely ground cement enters the hydration process more actively, since the interaction with water begins from the surface of the particles, the total area of which will be much larger than that of coarse cement. Finely ground cement is characterized by less water separation, it gains strength faster (that is, it is more active and durable), and provides better workability.

How to choose cement
How to choose cement

Some disadvantages of fine grinding include a tendency to shrinkage and rapid loss of activity if stored improperly. Considering all the pros and cons, in many countries standards for grain thickness have been developed and minimum permissible values are indicated. Now the fineness of grinding is measured in the total surface area of the grains (cm 2) per gram of cement - the standard for Portland cement is 2250 cm 2 / g, for fast-setting cement - not less than 3250 cm 2 / g.

Density, specific gravity

Despite the high theoretical specific gravity of cement (3000–3200 kg / m 3), in practice its density is much lower and can vary. It's all about intergranular voidness - air fills the gaps between the cement particles, especially if the powder is electrified after pneumatic unloading. The actual density of fresh cement is about 1100–1200 kg / m 3, but after storage and transportation it will be somewhere around 1500 kg / m 3 and more. Influences the specific gravity of cement not only its condition, but the fineness or coarseness of grinding.

How to choose cement
How to choose cement

Obviously, for the preparation of concrete / mortar, cement must be weighed and dosed only in a mass ratio, as well as water and all aggregates.

Other qualities of cement

Let us note a few more special properties of cement, which in one way or another affect its consumer characteristics.

Corrosion resistance reflects the ability of cement stone (it is the weakest element in concrete) to resist external physical and chemical influences, such as washout or, for example, abrasion. Pozzolanic cement is highly resistant to corrosion.

How to choose cement
How to choose cement

Frost resistance - characterizes cement as capable of freezing and thawing more times than usual. Here, the density of the cement stone is important, in which there will be fewer pores and free water, capable of freezing and destroying the structure of the monolith. The durability of the structure, which is subject to increased requirements for frost resistance, can be optimized by using hydrophobic cement.

Watertightness is essential to prevent concrete washout in particularly humid environments. It is increased by introducing special additives into the cement. By definition, water resistant is an expandable waterproof cement.

How to choose cement
How to choose cement

Resistance to sulfate ions is necessary if the construction of hydraulic structures is underway. It is worth using special sulfate-resistant cements.

A truly high-quality cement is characterized by the constancy of all its inherent useful properties. The compliance of cement with the standards should be considered only in combination.

Cement types

Practice shows that there is no cement “for all occasions” - in certain operating conditions, a binder with special properties is required for a specific type of work. At the moment, there are about thirty types of cement, each of which has its own field of application, may differ in mineral composition, a set of additives, the speed of setting and hardening …

There is a conditional division of cements into general construction (Portland cement, slag Portland cement, Portland cement with mineral additives), and special-purpose cements.

Portland cement is a classic binder, on the basis of which most of the most popular specialty cements are made. It accounts for 99% of all cement production. Portland cement is suitable for the manufacture of monolithic concrete, masonry and finishing mortars, reinforced concrete products with steaming, the manufacture of wall blocks and paving slabs. In the overwhelming majority of cases, for an individual developer, the best option is PC brand 400 or 500.

How to choose cement
How to choose cement

Slag Portland cement, in addition to clinker and gypsum, contains a certain amount of granulated slag. This type of cement, when compared with conventional PC, hardens more slowly, emits less heat (well suited for creating massive structures), has a low degree of volumetric deformation, is more resistant to moisture and high temperatures, and is slightly cheaper. ShPC is used for the production of mortars and concrete; products located underground and in damp conditions; wall blocks and building mixtures.

Fast-setting cement provides a rapid build-up of strength, which allows you to optimize formwork turnover. Effectively used in monolithic construction. Acceleration of the formation of cement stone is ensured by the introduction of mineral additives or ultrafine grinding of the powder.

How to choose cement
How to choose cement

Plasticized cement, thanks to surfactants, has improved mobility and workability. Possesses significant corrosion resistance and frost resistance, is used for the construction of roads, airfields, hydraulic structures.

Hydrophobic cement absorbs less moisture from the air, retains its activity longer, and does not form lumps during storage. It is used to obtain frost-resistant and moisture-proof mortar, excellent for external plaster and other types of building cladding with a wet method.

The stress cement expands during the formation of the cement stone. Excellent for renovation work (filling cracks, seams). Differs in good frost resistance and unsurpassed moisture resistance, therefore it is very successfully used for the construction of foundations and monolithic walls of basements, concrete roofs, swimming pools, tanks.

Alumina cement is gaining design strength very quickly (up to half per day). A lot of heat is generated during curing, so this type of cement is well suited for winter work.

Expanding cement without subsequent shrinkage increases in volume during the formation of a cement stone. It is used for the production of waterproofing mixtures, for the construction of hydraulic structures, for sealing joints of various structures.

Pozzolanic cement has a long hardening period, high corrosion resistance, frost resistance. When solidified, it emits very little heat, therefore it is advisable to use it for the manufacture of massive concrete structures, including underground and underwater ones.

How to choose cement
How to choose cement

Sulfate-resistant cement is used for the construction of concrete structures in water - piles, bridge supports, foundations in areas with variable groundwater levels. This cement can be improved with hydrophobic and plasticizing additives.

Wellbore cement withstands high temperatures well (up to 150 degrees), is characterized by increased bending strength, which is necessary for operation in deep gas and oil wells. It is used to isolate wells from groundwater.

Colored cements are used for the production of decorative monolithic products that do not require subsequent finishing. They are obtained by introducing special pigments into the composition of traditional cement.

How to choose cement
How to choose cement

White cement is made by using white clay and quenching the clinker as it exits the kiln. It has high resistance to external influences, quickly gains strength. It is used for the production of decorative concrete and light-colored plaster mixes. It is the basis for the production of colored cement.

Learning to read the markings

So that the consumer can accurately choose the binder he needs, all cements are subject to mandatory labeling. At the moment, the most common is a conditional description of cement in accordance with GOST 10178–85, but gradually domestic manufacturers are switching to marking cement in accordance with GOST 31108–2003, which, according to the regulations, is as close as possible to EU standards (in particular, EN 197–1). In fact, the cement manufacturing technology has not changed, only the laboratory testing methods have changed. Now, to measure the strength, samples are produced at a fixed water-cement ratio W / C = 0.5, using sand of different fractions.

GOST 31108-2003

The substance composition of cement is characterized by Roman numerals:

  • I - Portland cement
  • II - Portland cement with mineral additives
  • III - slag Portland cement
  • IV - pozzolanic cement
  • V - composite cement

In terms of compressive strength, cements are subdivided into classes - 22.5; 32.5; 42.5; 52.5. These figures indicate the minimum strength in MPa for cement stone samples with an age of 28 days.

How to choose cement
How to choose cement

For cement of class 32.5; 42.5; 52.5 is the letter designation for the rate of strength gain at the age of 2 and 7 days. "N" means normally hardening, "B" - fast hardening.

The amount of additives in the cement is marked as follows:

  • A - mineral additives from 6 to 20%
  • B - mineral additives from 21 to 35%

The type of additive is indicated as follows:

  • P - pozzolan
  • Ш - granulated slag

For example, CEM II / A-Sh 42.5B is a Portland cement with an additive in the form of granular slag (up to 20%), fast-hardening, compressive strength of at least 42.5 MPa.

GOST 10178–85

The composition of the cement is indicated by the following abbreviations:

  • PC - Portland cement
  • ShPTs - slag portland cement
How to choose cement
How to choose cement

Letters indicate the special properties of cements:

  • GF - hydrophobic
  • BC - white
  • VRC - waterproof expanding
  • PL - plasticized
  • SS - sulfate resistant

The compressive strength of the cement stone is indicated by the brand: 300, 400, 500, 550, 600, 700 (samples are tested with arbitrary V / C and monofraction sand).

The presence of mineral additives is indicated as follows:

  • D0 - no additives
  • D5 - additives not more than 5 percent
  • D20 - additives are from 5 to 20 percent

Fast-setting cement is marked with the letter "B".

For example, PC-500-D20-B is a fast-hardening Portland cement of grade 500, which contains mineral additives in an amount of 5 to 20%.

Cements with a strength designation according to both GOSTs can be conditionally compared. Here is a table by which you can understand the actual ratio of class and brand:

CEM I 52.5N PC 600-D0 CEM I 42.5N PC 500-D0 CEM I 42.5N PC 400-D0 CEM I 32.5B PC 400-D0 CEM I 32.5N PC 400-D0 CEM II / A-Sh 42.5N PC 500-D20 CEM II / A-Sh 32.5B PC 400-D20 CEM II / A-Sh 32.5N PC 400-D20 CEM II / V-Sh 32.5N ShPC 400 CEM II / A-K (Sh-P) 42.5N PC 500-D20 CEM II / A-K (Sh-P) 32.5B PC 400-D20 CEM II / A-K (Sh-P) 32.5N PC 400-D20

How to distinguish original cement

Many developers and would-be builders do not pay attention to the quality of the purchased cement, focusing only on its price. Meanwhile, the purchase of cement is a very responsible matter, since the market is filled with all kinds of counterfeit products, ranging from underweight (a bag may lack 10 kilograms or more), to re-grading and dilution of cement with completely unnecessary "additives" (mineral powder for asphalt concrete, slag, fly ash, dolomite dust). Some businessmen even manage to grind and repack the fully set cement stone. By the way, there are serious questions about cheap factory cement imported from abroad, which may initially have a brand lower than the declared one or lose activity due to long-term delivery and improper storage (moisture saturation during sea transportation,strong clumping in high-bay storage …).

How to choose cement
How to choose cement

So, how not to fall into the trap of "bargain" prices, let's look at some rules.

Purchase cement only from trusted reputable manufacturers. Give preference to domestic factories. It makes sense to make a purchase from firms engaged in the production of concrete and reinforced concrete products, or from a large supplier of factory cement.

Avoid cement, the cost of which is already 10–20 percent lower than the market value. Check on the manufacturer's website what the selling prices are at the moment, there can be no miracles - cement should be more expensive in retail.

How to choose cement
How to choose cement

Buy a bag of cement for testing, examine it by all available means. The best option is if you contact the laboratory at the nearest concrete plant, where you can find out the actual strength (grade), grinding fineness, uniformity of volume change, setting time and other nuances.

Buy cement packed in 4-5 ply kraft bags. The fake is packed in simple bags with two or three layers. Pay attention to the integrity of the bags.

How to choose cement
How to choose cement

Weigh the bag, the weight error should not exceed 1 kilogram.

Study the labeling carefully. The bag should contain the following information:

  • manufacturer's name;
  • full name of the cement;
  • name and quantity of additives;
  • strength class / grade;
  • name of the normative document according to which the cement was manufactured;
  • Net weight;
  • certification data.

Ask the seller to submit documents for the cement:

  • certificate of conformity;
  • sanitary and epidemiological conclusion;
  • invoices for a batch of cement (we look at the dates of arrival of goods, points of unloading …);
  • results of laboratory tests.

We check the cement visually and with our own hands:

  • the powder must be "fluid";
  • the normal color of the cement is uniform, dark gray, light gray, gray-green;
  • if there are lumps, but they disintegrate when lightly pressed, this is normal;
  • if the grains are hard, have sharp protrusions - the cement has partially entered the hydration process and is guaranteed to lose activity.
How to choose cement
How to choose cement

We conduct tests at home. We prepare a cement paste based on salt-alkaline mineral water, make a small disc 10-15 cm in diameter out of the mass. A product made of high-quality Portland cement will set in 5-10 minutes and heat up significantly, small cracks may appear on the edges of the body. The counterfeit will harden for a very long time (up to an hour), the disc does not heat up, it can seize in separate areas, crack strongly and not keep its shape at all.

As you can see, the cement issue is not as simple as it seems in reality. But, if you approach the choice of cement with all responsibility, learn how to properly use this seemingly familiar binder, then no problems will happen on your construction site, you can proudly say: "My home is my fortress."

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