Powder hydraulic inorganic cementing material. Mud can be hardened in air or water after being mixed with water, and can firmly bond sand, stone and other materials together. Cement is an important building material. Mortar or concrete made of cement is durable and widely used in civil construction, water conservancy, national defense and other projects.
The word "cement" is developed from the Latin caementum, which means crushed stone and flaky. The history of cement can be traced back to the mixture of lime and volcanic ash used by the ancient Romans in construction projects. 1796, an Englishman, J. Parker, fired a brown cement from marl, which is called Roman cement or natural cement. 1824, J. asp in England burnt limestone and clay to make cement. After hardening, the color is similar to the stone used in construction in Portland, England. It was named Portland cement and patented. At the beginning of the 20th century, with the improvement of people's living standards, the requirements for construction projects are increasing. While continuously improving portland cement, we have successfully developed a number of cements suitable for special construction projects, such as high-alumina cement and special cement, and the variety of cement has grown to more than 100.
In the production process of cement, limestone and clay are used as main raw materials, which are crushed, mixed and ground into raw materials, which are sent to a cement kiln to be calcined into mature materials, and then a proper amount of gypsum (sometimes mixture or additive) is added for grinding. According to the purpose and performance, it can be divided into three categories: ① general cement. Used in general civil engineering, such as portland cement (the general name of portland cement with calcium silicate as the main mineral, including ordinary portland cement, slag, volcanic ash, fly ash, mixed portland cement, etc. ) .② Special cement. Used in special projects, such as oil well cement and sand cement. ③ Special cement. It is used in projects that have special requirements for some properties of concrete, such as quick-hardening cement, hydraulic cement, sulfate-resistant cement, expansive cement, self-stressing cement, etc. The performance of cement must meet the requirements of national standards such as fineness, setting time, stability, strength, specific gravity, hydration heat, impermeability, frost resistance, expansion and contraction, heat resistance and corrosion resistance.
Cement classification
Cement is divided into two types according to its use and performance.
1, ordinary cement, commonly used in general civil engineering. General cement mainly refers to six types of cement specified in GB 175- 1999, GB 1344- 1999 and GB 12958- 1999, namely portland cement and ordinary portland cement.
2. Special cement and special cement. For example: G-class oil well cement and road portland cement.
3. Characteristic cement, a kind of cement with outstanding performance. Such as: quick-hardening portland cement, low-heat slag portland cement and expanded sulphoaluminate cement.
According to the name of its main hydraulic substances, cement is divided into the following categories.
(1) Portland cement, commonly known as Portland cement abroad; Aluminate cement; (3) sulphoaluminate cement; (4) iron aluminate cement; (5) Fluoroaluminate cement; (6) Cement with active substances such as volcanic ash or potential hydraulic materials as the main component.
The main technical characteristics of cement indicated in cement naming as required are divided into:
(1) Fast hardening: it can be divided into fast hardening and fast hardening;
(2) Hydration heat: divided into moderate heat and low heat;
(3) Sulfate resistance: divided into two categories: moderate sulfate resistance and high sulfate resistance;
(4) Expansibility: it can be divided into two categories: expansibility and self-stress;
(5) High temperature resistance: The high temperature resistance of aluminate cement is graded according to the content of alumina in cement. Four, the general principles of cement naming:
The naming of cement is based on the main hydraulic minerals, mixed materials, uses and main characteristics of cement according to different categories, and strives to be concise and accurate. Abbreviations are allowed when the name is too long.
General cement is named after the name of the main hydraulic minerals of cement and the name of the admixture or other appropriate names.
Special cement is named after its special use, and can be labeled with different models.
Characteristic cement is named after the main hydraulic minerals of cement, which can be named according to different models or mixed materials.
Cement with active substances such as pozzolan or potential hydraulic materials as its main components can be named after active substances or with characteristic names, such as persulfate cement and lime pozzolan cement.
Definition of main cement products
1. cement: a powdery hydraulic cementing material, which can be mixed with water to form plastic slurry, can be used to cement sand, stone and other materials, and can be hardened in air and water.
2. Portland cement: a hydraulic cementing material made of portland cement clinker, 0%~5% limestone or granulated blast furnace slag, and a proper amount of gypsum, which is called Portland cement and is divided into P.I and P.II. It is commonly known as Portland cement abroad.
3. Ordinary Portland cement: a hydraulic cementing material made of Portland cement clinker, 6%~ 15% admixture and a proper amount of gypsum, which is called ordinary Portland cement (abbreviated as ordinary cement) with code name: P.O. ..
4. Slag Portland cement: a hydraulic cementing material made of Portland cement clinker, granulated blast furnace slag and appropriate amount of gypsum, called slag Portland cement, code name: P.S..
5. Volcanic pozzolanic Portland cement: a kind of hydraulic cementing material made of portland cement clinker, volcanic pozzolanic mixture and appropriate amount of gypsum. It's called pozzolanic portland cement, code name: p.p.
6. Fly ash portland cement: a hydraulic cementing material made of portland cement clinker, fly ash and appropriate amount of gypsum, called fly ash portland cement, code name: P.F. ..
7. Composite Portland cement: a hydraulic cementing material made of Portland cement clinker, two or more specific mixed materials and a proper amount of gypsum, which is called composite Portland cement (hereinafter referred to as composite cement) with code name P.C. ..
8. Medium-heat Portland cement: a kind of hydraulic cementing material with medium hydration heat, which is made by grinding portland cement clinker with appropriate composition and adding appropriate amount of gypsum.
9. Low-heat slag portland cement: a kind of hydraulic cementing material with low hydration heat, which is made by grinding portland cement clinker with appropriate composition and adding appropriate amount of gypsum.
10. Quick-hardening Portland cement: adding appropriate amount of gypsum into Portland cement clinker, and grinding to make cement with high early strength, marked by 3-day compressive strength.
1 1. Sulfate-resistant Portland cement: cement with good sulfate resistance made of portland cement clinker and appropriate amount of gypsum.
12, white portland cement: white cement made of portland cement clinker with little iron oxide content, adding appropriate amount of gypsum and grinding.
13. Road Portland cement: a hydraulic cementing material made of skilled road Portland cement, 0%~ 10% active mixture and appropriate amount of gypsum, which is called road Portland cement.
14. Masonry cement: Low-grade cement, which is mainly used for masonry mortar, is made of active mixed materials, adding appropriate amount of portland cement clinker and gypsum and grinding.
15. Oil well cement: cement made of silicate cement clinker composed of appropriate minerals, appropriate amount of gypsum and mixed materials. It is suitable for oil and gas well cementing engineering at a certain well temperature.
16, persulfate cement: cement made of granulated blast furnace slag as the main component material and adding appropriate amount of gypsum, portland cement clinker or lime.
Types and functions of cement kilns
At present, there are two main types of cement kilns. One is rotary kiln (also known as rotary kiln), the kiln barrel is horizontal (slightly inclined) and can do rotary motion; Another kind of kiln is vertical and does not rotate, which is called vertical kiln. Types and characteristics of cement rotary kiln;
The development of cement industry has different production modes and different types of rotary kilns. According to the preparation methods of raw materials, it can be divided into dry production and wet production, and the rotary kiln suitable for production methods can be divided into dry rotary kiln and wet rotary kiln. Due to the different heat exchange devices at the kiln tail, it can be divided into different types of kilns. The classification of rotary kiln is roughly as follows:
1, type of wet rotary kiln:
The cement kiln used in wet production is called wet kiln, and wet production is to make raw materials into slurry with water content of 32%~40%. Due to the preparation of slurry with fluidity, the raw materials are evenly mixed, the composition of raw materials is uniform and the quality of sintered clinker is high, which is the main advantage of wet production.
2. Types of dry rotary kiln:
Compared with wet rotary kiln, dry rotary kiln has completely opposite advantages and disadvantages. Dry method makes raw meal into raw meal dry powder, and the water content is generally less than 65438 0%, so the heat required for evaporation of water is reduced compared with wet method. Due to the high exhaust temperature of hollow kiln, the heat consumption is not low. Dry production, raw meal is made into dry powder, and its fluidity is worse than mud. Therefore, the raw materials are not well mixed and the ingredients are uneven.
Types and characteristics of cement vertical kiln
There are two kinds of shaft kilns currently used in China: ordinary shaft kilns and mechanical shaft kilns.
Ordinary vertical kiln is manual feeding and manual unloading or mechanical feeding and manual unloading; Mechanical vertical kiln is mechanical feeding and mechanical unloading. The output, quality and labor productivity of mechanical shaft kiln are higher than those of ordinary shaft kiln. According to the technical policy requirements of building materials, small cement plants gradually replace ordinary vertical kilns with mechanized vertical kilns.
Quality control and standards in cement production
There are two main aspects of cement production quality management: on the one hand, the main equipment-kiln and mill are controlled within the index control range.
Normal operation of; On the other hand, it is to manage the quantity and quality of raw materials, coal, raw materials, clinker, cement and other warehouses, grasp the warehousing and warehousing situation, and ensure the normal operation of production. It is very important to determine quality control points and control indicators. We must work out a reasonable and feasible scheme according to the specific conditions of our factory's technological process and equipment in order to better guide production.
The main contents of the revision of China cement standard
Compared with the old standard, there are two main changes in the new cement standard in China: first, GB/T176711999 cement mortar strength test method (ISO method) is adopted to replace the current GB 177-85 cement mortar strength test method; Secondly, six general cement standards in China were revised based on ISO strength.
(a) GB/T17671-1999 "Cement mortar strength test method (ISO method)" standard formulation.
GB/T17671-1999 was formulated in China by adopting the international standard ISO 679-1989, which was promulgated on February 8, 1999 and came into effect on May 8, 1999.
GB/T17671-1999 and GB 177-85 belong to the "softening method" for testing the strength of cement mortar, that is, plastic sand. We use 4X4X 160m prism specimens to test the flexural strength of the specimens first. The core difference between them lies in the different composition of mortar. ISO method adopts moderate water-cement ratio and moderate lime-sand ratio, especially graded standard sand, so the strength value obtained by ISO method is closer to the application effect of cement in concrete than GB- 177 method.
(B) the main contents of the revision of the six major cement standards
1. The strength test method of cement mortar was changed to GB/T17671-1999.
GB/T17671-KLOC-0/999 is adopted as the strength test method of cement mortar in the six major cement product standards, and GB 177-85 is no longer used. Therefore, the method of GB/T17671-1999 rose to mandatory method, while the method of GB177-85 fell to recommended method.
2. Change the cement label to strength grade.
The six old cement standards are all marked with Kgf/cm2, such as 32.5, 42.5, 42.5R, 52.5 and 52.5R, etc.
The six new standards of cement implement the strength grades expressed in Mpa, such as 32.5, 32.5R, 42.5, 42.5R, 52.5R, 52.5 r, etc. Therefore, the value of strength grade is the same as the lowest value of 28-day compressive strength index of cement.
The new standard also plans the strength grade of cement in China. Portland cement is divided into three grades and six types, 42.5, 42.5R, 52.5, 52.5R and 62.5r. The other five cements are also divided into three grades and six types, which are 32.5, 32.5R, 42.5, 42.5R, 52.5r and 52.5r respectively.
3. Strength age and strength index setting for each age
The cement strength age stipulated in the six new cement standards is 3 days and 28 days, and each age has the requirements of flexural and compressive strength index.
Selection of cement
Main technical performance indexes of cement:
(1) Specific gravity and bulk density: the specific gravity of ordinary cement is 3: 1, and the bulk density is usually 1300 kg/m3.
(2) Fineness: refers to the thickness of cement particles. The finer the particles, the faster the hardening and the higher the early strength.
(3) Setting time: The time required for cement to start setting after mixing with water is called initial setting time. The time from adding water and stirring to the completion of coagulation is called the final coagulation time. The initial setting time of portland cement is not earlier than 45 minutes, and the final setting time is not later than 12 hours.
(4) Strength: The strength of cement should meet the national standards.
(5) Volume stability: refers to the performance of uniform volume change of cement during hardening. There are many impurities in cement, which will produce uneven deformation.
(6) Hydration heat: The interaction between cement and water will produce exothermic reaction. In the process of cement hardening, the heat released continuously is called hydration heat.
Commonly used cement varieties:
(1) Portland cement: Portland cement clinker with calcium silicate as the main component mixed with appropriate amount of gypsum and ground.
(2) Ordinary Portland cement: It is made of Portland cement clinker, adding appropriate amount of gypsum and admixture, and grinding.
(3) Slag Portland cement: it is made of Portland cement clinker mixed with proper amount of granulated blast furnace slag and gypsum.
(4) Pozzolanic Portland cement: It is made of Portland cement clinker, pozzolanic material and gypsum by mixing and grinding in proportion.
(5) Fly Ash Portland Cement: It is made of Portland cement clinker and fly ash, mixed with appropriate amount of gypsum and ground.
Commonly used cement label:
No.225, No.275, No.325, No.425, No.525 and No.625 have different tensile strengths, and the MPa values are between 2.8-4.5 and 3.4-8.0.
Decorative cement varieties
Decorative cement is often used to decorate the surface of buildings, with simple construction, convenient modeling, easy maintenance and low price. There are the following varieties:
(1) white portland cement: made of calcium silicate as the main component, with a small amount of iron clinker and a proper amount of gypsum.
(2) Colored Portland cement: white Portland cement clinker and high-quality white gypsum, with pigments and additives, are ground together. Commonly used color pigments include iron oxide (red, yellow, brown and black), manganese dioxide (brown and black), chromium oxide (green), cobalt blue (blue), ultramarine (indigo), Kong Quelan (navy blue) and carbon black (black).
Decorative cement is similar to portland cement, with the same construction and maintenance, but it is easy to be polluted, and the instruments and tools must be clean.
Application and selection of cement mortar
In home decoration, cement mortar is used for sticking and masonry of floor tiles and wall tiles, which can not only enhance the adsorption capacity of surface materials and grass-roots units, but also protect the internal structure, and can be used as a leveling layer for rough surfaces of buildings, so cement mortar is an essential material in decoration engineering.
Many customers think that the greater the proportion of cement in the whole mortar, the stronger its cohesiveness, so they often disagree with the decoration company on the amount of cement. In fact, if the cement label is too large, when the cement mortar sets, the cement will absorb a lot of water. At this time, the tiles on the surface layer absorb too much water and are easy to crack, shortening the service life. Generally, cement mortar should be mixed according to the ratio of cement: sand = 1: 2 (volume ratio).
At present, there are many kinds of cement on the market, including portland cement, ordinary portland cement and slag portland cement. Portland cement is usually used for home decoration.
Selection principle of cement and sand
■ In order to ensure the quality of cement mortar, we must pay attention to whether the cement is 425# Portland cement produced by a large factory.
■ Sand should be selected, and the particle thickness of medium sand is very suitable for use in cement mortar. Many customers think that the finer the sand, the better the mortar, which is actually a misunderstanding. Too fine sand has weak adsorption capacity, can't produce large friction force, and can't stick the tiles firmly.
Example of cement production process flow
After the raw materials and fuel enter the factory, they are sampled, analyzed and tested by the laboratory, homogenized according to quality and stored in the raw material shed. Clay, coal and pyrite powder are dried by the dryer to the process index value, and lifted to the corresponding raw material warehouse by the hoist. After two-stage crushing, limestone, fluorite and gypsum are sent to their respective warehouses by hoists. According to the quality of limestone, clay, anthracite, fluorite and pyrite powder, the laboratory calculates the process formula. Raw meal is mixed with black raw meal through microcomputer batching system and ground by raw meal mill. Sampling and testing the percentage content of calcium oxide and iron oxide and the fineness of raw meal every hour, and adjusting them in time to make all the data meet the requirements of process formula. The ground black raw meal is lifted to the raw meal bin by the bucket elevator. According to the quality of raw meal after grinding, the laboratory homogenized the raw meal through multi-bin proportioning and mechanical dumping, and then lifted it to two raw meal homogenizing bins by elevator. The raw meal is lifted to the pelletizing bin by the cooperation of two homogenization bins. The pre-added water pelletizing control device installed on the kiln surface of the vertical kiln is used for the proportion of raw meal and water, and the raw meal is pelletized through the pelletizing tray. The raw meal balls are distributed to different positions in the kiln by the vertical kiln distributor for calcination, and the calcined clinker is sent to the clinker crusher through the discharge pipe and the weighing machine for crushing, and the laboratory samples every hour for chemical and physical analysis. According to the quality of clinker, it is put into the corresponding clinker warehouse by the hoist. At the same time, according to the requirements of production and operation and the building materials market, the laboratory mixes clinker, gypsum and slag with cement through the clinker microcomputer batching system, and the cement mill grinds 425 and 525 ordinary portland cement respectively, and samples are taken every hour for analysis and inspection. The ground cement is lifted by bucket elevator to three cement warehouses. According to the quality of grinding cement, the laboratory adopts the way of multi-storehouse collocation and mechanical dumping to homogenize cement. It is sent to two cement homogenizing bins by a hoist and then matched with the two cement homogenizing bins. The packaging machine is controlled by microcomputer to package cement, and the packaged bagged cement is stored in the finished product warehouse. After the test and sampling inspection are qualified, the cement factory notice will be issued.
Eight taboos of using cement
First, avoid getting wet and hardening.
Cement hardened by moisture will reduce or even lose its original strength, so the specification stipulates that cement that has been shipped for more than 3 months should be re-tested and used according to the test results. Cement caked or hardened by moisture must be screened before use. The screened aggregates are generally used for masonry mortar or plastering mortar in secondary projects after rubbing or grinding. For cement blocks that can be pulverized by one touch or one pinch, the strength grade can be appropriately reduced.
Second, avoid exposure and quick drying.
Concrete or plaster will be exposed to the sun after operation, and its strength will be reduced or even completely lost with the rapid evaporation of water. Therefore, before construction, the base must be strictly cleaned and fully wetted; After construction, it should be strictly covered and watered and maintained according to regulations.
Third, avoid negative temperature freezing.
After mixing concrete or mortar, if it is frozen, its cement can't hydrate, and water freezes and expands, then the concrete or mortar will be destroyed by the gradually deepening powder cake from the outside to the inside, so the construction should be carried out in strict accordance with the Code for Winter Construction of Building Engineering (JJ104-97).
Fourth, avoid high temperature and heat.
If the cured mortar layer or concrete member is often in high temperature and extremely hot state, it will lose its strength, because calcium hydroxide in cement stone will decompose at high temperature; In addition, some aggregates will decompose or expand in volume at high temperature.
For long-term high temperature occasions, ordinary mortar or concrete can be isolated and protected by refractory bricks. When the temperature is high, special heat-resistant concrete should be used for pouring, or a certain amount of ground heat-resistant materials should be mixed in the mud.
Five, avoid dirty and soft at the grass-roots level
Cement can be firmly bonded or wrapped with hard and clean base, but its bonding strength is related to the flatness of base surface. During the construction of smooth base, it is necessary to cut hair, smash hemp and brush clean in advance, so that the cement can be firmly bonded with the base.
Dirt, grease, acid and alkali on the grass-roots level will play an isolation role, and must be carefully removed and cleaned, and then a layer of plain cement slurry is brushed first, and then mortar or concrete is poured.
Cement will shrink during solidification, and it will not adapt to the volume change of loose and weak base in the process of dry, wet, cold and hot changes, which will inevitably lead to empty drums or cracks and make it difficult to bond firmly. Therefore, wood, slag cushion and lime-soil cushion cannot be firmly bonded with mortar or concrete.
Six, avoid aggregate impurities
As the aggregate of concrete or cement mortar, if there are dust, clay or other organic impurities, it will affect the bond strength between cement, sand and stone, thus ultimately reducing the compressive strength. So if the impurity content exceeds the standard, it will be used after all.
Seven, avoid water more gray and thick.
People often ignore the influence of water consumption on concrete strength. In order to facilitate pouring, sometimes the mixture ratio is not carefully implemented, and the concrete is mixed very thinly. Because the water required for hydration is only about 20% of the cement weight, excess water will leave many pores in concrete after evaporation, which will reduce the strength of concrete. Therefore, on the premise of ensuring the pouring density, try to reduce the mixing water.
Many people think that the more cement is used for plastering, the stronger the plastering layer will be. In fact, the more cement is used, the thicker the mortar is, the greater the volume shrinkage of the plastering layer is, and the more cracks are produced. In general, when plastering, the leveling layer should be plastered with 1: (3-5) coarse mortar first, and then the thin surface layer should be plastered with 1: (1.5-2.5) cement mortar, and excessive cement is not allowed.
Eight, avoid acid corrosion
Acidic substances in cement and calcium hydroxide will undergo neutralization reaction, and the product will be loose and expand, and it will be easily hydrolyzed and pulverized when it meets water. As a result, the concrete or plaster layer is gradually corroded and decomposed, thus protecting the cement from acid corrosion.
Acid-resistant mortar and acid-resistant concrete should be used for occasions or containers in contact with acidic substances. Slag cement, pozzolan cement and fly ash cement all have good acid resistance, so these three cements should be preferred to prepare acid-resistant mortar and concrete. Ordinary cement is not allowed to be used in projects with strict requirements on acid and corrosion resistance.