The difference and use of Portland cement and composite cement
It is made of Portland cement clinker, two or more specified mixed materials, and an appropriate amount of gypsum. The hydraulic cementitious material is called composite Portland cement (referred to as composite cement). The total amount of mixed materials in cement should be greater than 15% and not more than 50% in terms of mass percentage.
It is suitable for industrial and civil construction projects, port and navigation projects, underground tunnels, etc. The product has stable performance, large strength increase rate in the later period, good workability, small drying shrinkage, low hydration heat, and high resistance to corrosion. It has good corrosion resistance.
Composite Portland cement should be used with caution in water conservancy projects. It is difficult to determine the reaction of the admixture. Our environment here is classified as a severely cold area, and frost-resistant concrete is what I am studying now. Regarding the key points, I have consulted with experts from the higher-level Academy of Water Conservancy Sciences. There is no clear rule in the regulations that it cannot be used, so it is difficult to draw a conclusion. However, compared with ordinary silicon, composite silicate has high early strength in 7 days and slow strength growth in 28 days. If the construction mix ratio is not well controlled with the surplus coefficient, some of the test blocks submitted for inspection will be unqualified. It is generally recommended not to use composite < /p>
On July 1, 2008 this year, the National Development and Reform Commission has issued a notice that ordinary silicate 32.5 grade will cease to be used and produced. Composite cement can be used without ordinary silicate 32.5. However, the workability of the concrete mixed with it is not as good as that of ordinary silicate. This is certain. The difference between composite cement and Portland cement. Can composite cement be used in the main structure?
The difference is that the proportion of mixed materials and clinker is different
Composite PC cement, proportion Large
Ordinary PO cement, small proportion
can be used for the main body. What is composite cement? Classification of composite cement Composite Portland cement is composed of Portland cement clinker, two The hydraulic cementitious material made of one or more specified mixed materials and an appropriate amount of gypsum is called composite Portland cement (referred to as composite cement), codenamed P.C. The total amount of mixed materials in cement should be greater than 20% and not more than 50% in terms of mass percentage.
It is divided into three numbers: 325, 425 and 525.
Cement should be numbered and sampled according to the same variety and label before leaving the factory. Each number is a sampling unit. The cement number is specified according to the annual output of the cement plant.
More than 1 million tons, not more than 1,000 tons is a number;
More than 500,000 tons - 1 million tons, not more than 800 tons is a number;
< p> Above 300,000 tons - 500,000 tons, no more than 600 tons is a number;Above 100,000 tons - 300,000 tons, no more than 400 tons is a number;
< p> Above 40,000 tons - 100,000 tons, no more than 200 tons is one number;Below 40,000 tons, no more than 100 tons and 3 days output is one number.
Sampling should be representative. It can be taken continuously, or equal amounts of samples can be taken from more than 20 different parts, with a total of at least 14kg. Is composite Portland cement considered composite cement or Portland cement?
In the "GB175-2007 General Portland Cement Standard", Portland cement and composite Portland cement are two different varieties. The clinker of both uses Portland cement. Clinker, the difference lies in the variety and proportion of mixed materials. What is the difference between composite Portland cement and ordinary Portland cement? and applicable conditions
Please refer to "GB175-2007 General Portland Cement Standard".
Ordinary Portland cement
Features: high early strength, fast setting and hardening, good frost resistance, strong wear resistance, good construction performance; high heat of hydration, It has poor heat resistance and poor resistance to chemical erosion by acids, alkalis and silicates;
This cement is suitable for: above-ground projects, underground projects that are not affected by erosion, and projects that are not affected by water pressure. , frozen projects in non-corrosive water, projects with better early strength, projects that need to develop faster under low temperature conditions.
This cement is not suitable for the water part of concrete projects, large-volume concrete projects and projects subject to chemical attack.
Composite Portland cement
Features: high early and late strength, good product workability, low bleeding, good water retention, low heat of hydration, and the prepared concrete is not easy to crack , good construction performance, normal setting time, and the strength and performance of cement are basically the same as ordinary Portland cement.
It can be widely used in urban and rural buildings and can replace ordinary cement (the new standard for general Portland cement was implemented in 2007, and ordinary Portland cement has canceled P.O32.5R and P.O32.5 Grade cement, P.C32.5R and P.C32.5 can now be used instead). What are the uses of cement and the development history of cement?
The English name of cement is cement
Powdered hydraulic inorganic cementitious material. Add water and stir to form a slurry, which can harden in air or water, and can firmly bind sand, stone and other materials together. Cement is an important building material. Mortar or concrete made of cement is strong and 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 gravel and flakes. The history of cement can be traced back to the ancient Romans who used a mixture of lime and volcanic ash in construction projects. In 1796, the Englishman J. Parker used marl to bake a kind of brown cement, which was called Roman cement or natural cement. In 1824, Englishman J. Asputin fired limestone and clay into cement. The hardened color was similar to the stone used for construction in Portland, England. It was named Portland cement and patented. right. At the beginning of the 20th century, with the improvement of people's living standards, the requirements for construction projects were increasing day by day. While continuously improving Portland cement, a batch of cements suitable for special construction projects were successfully developed, such as high alumina cement, special cement, etc. , cement varieties have developed to more than 100 types.
The cement production process uses limestone and clay as the main raw materials, which are crushed, batched, and ground into raw materials. The raw materials are then fed into the cement kiln to be calcined into clinker, and an appropriate amount of gypsum is added (sometimes mixed with materials or additives) ground into fine pieces. It is divided into three major categories according to its use and performance: ① General cement. Used in general civil construction projects, such as Portland cement (general name for cement composed of calcium silicate as the main mineral, internationally collectively known as Portland cement, including ordinary Portland cement, slag, volcanic ash, fly ash , mixed Portland cement, etc.). ②Special cement. Used for certain special projects, such as oil well cement, molding sand cement, etc. ③Special cement. It is used in projects that have special requirements for certain properties of concrete, such as fast-hardening cement, hydraulic cement, sulfate-resistant cement, expansion cement, self-stressing cement, etc. The performance of cement must comply with the fineness, setting time, stability, strength, specific gravity, heat of hydration, impermeability, frost resistance, expansion and contraction, heat resistance and corrosion resistance stipulated in national standards.
Cement classification
Cement is divided according to its use and performance
1. General cement, cement commonly used in general civil construction projects. General cement mainly refers to the six categories of cement specified in GB175-1999, GB1344-1999 and GB12958-1999, namely Portland cement, ordinary Portland cement, slag Portland cement, pozzolanic Portland cement, powder Fly ash Portland cement and composite Portland cement.
2. Special cement, cement for special purposes. Such as: G-class oil well cement, road Portland cement.
3. Characteristic cement, a cement with outstanding performance. Such as: fast hardening Portland cement, low heat slag Portland cement, expanded sulfoaluminate cement.
Cement is divided according to the name of its main hydraulic substance
(1) Portland cement, commonly known as Portland cement abroad; aluminate cement; (3) Sulfur Aluminate cement; (4) Ferroaluminate cement; (5) Fluoroaluminate cement; (6) Cement with volcanic ash or potential hydraulic materials and other active materials as the main component.
Cement is divided into two categories according to the main technical characteristics indicated in the cement naming:
(1) Rapid hardening: divided into two categories: rapid hardening and extra rapid hardening;
< p> (2) Heat of hydration: divided into two categories: medium heat and low heat;(3) Sulfate resistance: divided into two categories: medium resistance to sulfate corrosion and high resistance to sulfate corrosion;
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(4) Expansion: divided into two categories: expansion and self-stress;
(5) High temperature resistance: The high temperature resistance of aluminate cement is graded by the alumina content in the cement. 4. General principles for naming cement:
Cement is named according to the main hydraulic minerals, mixed materials, uses and main characteristics of cement according to different categories, and strives to be concise and accurate. When the name is too long, it is allowed There are abbreviations.
Universal cement is named after the name of the main hydraulic mineral of the cement and the name of the mixed material or other appropriate name.
Special cement is named for its specialized use and may be branded with different models.
Characteristic cement is named after the main hydraulic mineral name of the cement and the main properties of the cement, and can be named by different models or mixed materials.
Cement with pozzolanic or latent hydraulic materials and other active materials as the main components is named after the name of the main component followed by the name of the active material, or it can also be named by the characteristic name, such as Gypsum slag cement, lime pozzolan cement, etc.
Definition of main cement products
1. Cement: A powder that is mixed with water to form a plastic slurry that can bind sand, stone and other materials and can harden both in the air and in water. Hydraulic cementitious material.
2. Portland cement: a hydraulic cementitious material made from Portland cement clinker, 0% to 5% limestone or granulated blast furnace slag, and an appropriate amount of gypsum. It is called silicon. Salt salt cement is divided into P.I and P.II, which are commonly known as Portland cement abroad.
3. Ordinary Portland cement: a hydraulic cementitious material made from Portland cement clinker, 6% to 15% mixed materials, and an appropriate amount of gypsum. It is called ordinary Portland cement. Cement (ordinary cement for short), code name: P.O.
4. Slag Portland cement: a hydraulic cementitious material made from Portland cement clinker, granulated blast furnace slag and an appropriate amount of gypsum. It is called slag Portland cement, code name :P.S.
5. Pozzolanic Portland cement: a hydraulic cementitious material made from Portland cement clinker, pozzolanic mixed materials and an appropriate amount of gypsum. It is called pozzolanic Portland cement, code name: P.P.
6. Fly ash Portland cement: a hydraulic cementitious material made by grinding Portland cement clinker, fly ash and appropriate amount of gypsum, called fly ash silicate Cement, code name: P.F.
7. Composite Portland cement: a hydraulic cementitious material made of Portland cement clinker, two or more specified mixed materials and an appropriate amount of gypsum. It is called composite. Portland cement (referred to as composite cement), code-named P.C.
8. Medium-heat Portland cement: a hydraulic cementitious material with medium heat of hydration made by grinding Portland cement clinker with appropriate components and adding an appropriate amount of gypsum.
9. Low-heat slag Portland cement: a hydraulic cementitious material with low hydration heat made by grinding Portland cement clinker with appropriate components and adding an appropriate amount of gypsum.
10. Fast-hardening Portland cement: Add an appropriate amount of gypsum to Portland cement clinker and grind it into high-strength cement labeled with 3-day compressive strength.
11. Sulfate-resistant Portland cement: A cement with good sulfate corrosion resistance made by grinding Portland cement clinker and adding an appropriate amount of gypsum.
12. White Portland cement: White cement made by grinding Portland cement clinker with low iron oxide content into appropriate amount of gypsum.
13. Road Portland cement: a hydraulic cementitious material made from road Portland cement, 0% to 10% active mixed materials and an appropriate amount of gypsum. It is called road silicic acid. Salt cement, (referred to as road cement).
14. Masonry cement: Low-grade cement mainly used for masonry mortar is made from active mixed materials, adding an appropriate amount of Portland cement clinker and gypsum, and grinding them finely.
15. Oil well cement: Made from Portland cement clinker composed of appropriate minerals, an appropriate amount of gypsum and mixed materials, it is suitable for oil and gas well cementing projects under certain well temperature conditions. cement.
16. Gypsum slag cement: Cement made by grinding granulated blast furnace slag as the main component and adding an appropriate amount of gypsum, Portland cement clinker or lime.
Types and functions of cement kilns
There are currently two main types of cement kilns. One is that the kiln barrel is placed horizontally (with a slight slope) and can perform rotary motion. It is called a rotary kiln (also called a rotary kiln); another type of kiln whose cylinder is vertical and does not rotate is called a vertical kiln. The types and characteristics of cement rotary kilns:
In the development process of the cement industry, different production methods and different types of rotary kilns have emerged. According to the method of raw material preparation, it can be divided into dry production and wet production. , Rotary kilns suitable for production methods are divided into two categories: dry rotary kiln and wet rotary kiln. Due to the different heat exchange devices at the end of the kiln, the kiln can be divided into different types of kilns. The classification of rotary kilns is roughly as follows:
1. Types of wet-process rotary kilns:
The cement kiln used in wet-process production is called a wet-process kiln, and wet-process production is the process of producing raw materials. The material is made into a slurry with a water content of 32% to 40%. Since it is prepared into a fluid slurry, the raw materials are well mixed and the ingredients of the raw material are uniform, so that the quality of the fired clinker is high. This is the main advantage of wet production.
2. Types of dry rotary kilns:
Compared with wet rotary kilns, dry rotary kilns have opposite advantages and disadvantages. The dry method is used to make raw meal into dry raw meal powder, and the moisture is generally less than 1%, so it reduces the heat required to evaporate water than the wet method. Due to the high temperature of the exhaust gas, the heat consumption of the hollow kiln is not low. Dry production produces raw materials into dry powder, which has poorer fluidity than mud. So the raw materials are not mixed well and the ingredients are uneven.
Types and characteristics of cement shaft kilns
There are two types of shaft kilns currently used in my country: ordinary shaft kilns and mechanical shaft kilns.
Ordinary vertical kilns use manual feeding and manual unloading or mechanical feeding and manual unloading; mechanical vertical kilns use mechanical feeding and mechanical unloading. Mechanical shaft kilns operate continuously, and their output, quality and labor productivity are higher than ordinary shaft kilns. According to the technical policy requirements for building materials, small cement plants use mechanized shaft kilns to gradually replace ordinary shaft kilns.
Quality control and standards in cement production
There are two main aspects of cement production quality management: one is to control the host equipment - kiln and grinding mill within the index control range
The normal operation of the warehouse; on the other hand, it is to manage the quantity and quality of various warehouses, raw materials, coal, raw materials, clinker, and cement, and control the entry and exit of the warehouse to ensure the normal operation of production. . Determining quality control points and control indicators is a very important task. We must start from the specific conditions of the factory's process flow and equipment and formulate reasonable and feasible plans to better guide production.
The main contents of the formulation and revision of my country's cement standards
Compared with the old standards, my country's new cement standards have two main changes: First, the adoption of GB/T 17671-1999 " "Cement mortar strength test method (ISO method)" replaces the current GB177-85 "Cement mortar strength test method"; secondly, my country's six general cement standards are revised based on ISO strength.
(1) GB/T 17671-1999 "Cement mortar strength test method (ISO method)" standard formulation
GB/T 17671-1999 is my country's equivalent to the international standard ISO 679-1989, issued on February 8, 1999, and came into effect on May 1, 1999.
GB/T 17671-1999 and GB177-85 both belong to the "soft training method" for testing the strength of cement mortar, which uses plastic sand and a 4X4X160m prism test body. The test body is first subjected to a flexural strength test. After breaking, the two half-section specimens were subjected to a compressive strength test. The core difference between the two is that the composition of the mortar is different. The ISO method uses a moderate water-cement ratio and a moderate lime-sand ratio, especially the graded standard sand. Therefore, the strength value obtained by the ISO method is closer to that of the GB-177 method. The effect of using cement in concrete.
(2) Main contents of the revision of the six major cement standards
1. The strength testing method of cement mortar is changed to the GB/T 17671-1999 method
The six major cement product standards all quote the GB/T 17671-1999 method as the strength testing method of cement mortar, and GB 177 is no longer used. —85 method. Therefore, the GB/T 17671-1999 method has been upgraded to a mandatory method, while the GB 177-85 method has been reduced to a recommended method.
2. Cement labels are changed to strength grades
The six old cement standards implement cement labels expressed in Kgf/cm2, such as 32.5, 42.5, 42.5R, 52.5, 52.5R, etc.
The six new cement standards implement strength grades expressed in MPa, such as 32.5, 32.5R, 42.5, 42.5R, 52.5, 52.5R, etc., so that the value of the strength grade is consistent with the 28-day compressive strength index of cement. The lowest value is the same.
The new standard also unified the strength grades of cement in my country. Portland cement is divided into three grades and six types, 42.5, 42.5R, 52.5, 52.5R, 62.5, 62.5R, and the other five major cements It is also divided into 3 levels and 6 types, namely 32.5, 32.5R, 42.5, 42.5R, 52.5, 52.5R
3. Strength age and strength index settings for each age
The cement strength age specified in the six new cement standards are 3 days and 28 days, and each age has bending resistance and resistance. Pressure strength index requirements.
Cement purchase
Main technical performance indicators of cement:
(1) Specific gravity and bulk density: Ordinary cement has a specific gravity of 3:1, and bulk density is usually used 1300kg/cubic meter.
(2) Fineness: refers to the thickness of cement particles. The finer the particles, the faster they harden and the higher their early strength.
(3) Setting time: The time required for cement to start setting after mixing with water is called initial setting time. The time required from adding water and stirring to the completion of condensation is called the final setting time. The initial setting time of Portland cement shall not be earlier than 45 minutes, and the final setting time shall not be later than 12 hours.
(4) Strength: The strength of cement should comply with national standards.
(5) Volume stability: refers to the uniformity of volume changes of cement during the hardening process. Cement contains a lot of impurities, which can cause uneven deformation.
(6) Heat of hydration: The interaction between cement and water will produce an exothermic reaction. During the hardening process of cement, the heat continuously released is called heat of hydration.
Commonly used cement varieties:
(1) Portland cement: Portland cement clinker with calcium silicate as the main component, and an appropriate amount of gypsum added and ground.
(2) Ordinary Portland cement: It is made from Portland cement clinker, adding an appropriate amount of gypsum and mixed materials and grinding it.
(3) Slag Portland cement: Made from Portland cement clinker, mixed with an appropriate amount of granulated blast furnace slag and gypsum and ground into fine pieces.
(4) Pozzolanic Portland cement: It is made by mixing and grinding Portland cement clinker, pozzolanic materials and gypsum in proportion.
(5) Fly ash Portland cement: It is made by mixing Portland cement clinker and fly ash with an appropriate amount of gypsum and then grinding them into fine pieces.
Commonly used cement labels:
No. 225, No. 275, No. 325, No. 425, No. 525, No. 625, etc. The tensile strength is different due to different varieties and different labels. MPa values ??are between 2.8-4.5 and 3.4-8.0.
Varieties of Decorative Cement
Decorative cement is often used to decorate the surface of buildings. It has simple construction, convenient shape, easy maintenance and low price. The varieties are as follows:
(1) White Portland cement: It is made of calcium silicate as the main component, adding a small amount of iron clinker and an appropriate amount of gypsum.
(2) Colored Portland cement: Made from white Portland cement clinker and high-quality white gypsum, mixed with pigments and admixtures and ground together.
Commonly used color additives include iron oxide (red, yellow, brown, black), manganese dioxide (brown, black), chromium oxide (green), cobalt blue (blue), ultramarine blue (indigo), peacock blue (sea blue) ), carbon black (black), etc.
Decorative cement is similar to Portland cement, and the construction and maintenance are the same, but it is easier to contaminate, and the equipment and tools must be clean.
The use and purchase of cement mortar
In home decoration, cement mortar is used for pasting floor tiles, wall tiles and masonry. It can not only strengthen the surface material and base layer It has excellent adsorption capacity, can also protect the internal structure, and can be used as a leveling layer on the rough surface of the building. Therefore, cement mortar is an essential material in decoration projects.
Many customers believe that the greater the proportion of cement in the entire mortar, the stronger its adhesion, so they often disagree with decoration companies on the amount of cement used. In fact, this is not the case. Take the pasting of ceramic tiles as an example. If the cement label is too large, when the cement mortar sets, the cement will absorb a large amount of water. At this time, the surface tiles will easily crack if the moisture is excessively absorbed, shortening the service life. Cement mortar should generally be mixed at a ratio of cement:sand=1:2 (volume ratio).
Currently there are many varieties of cement on the market, including Portland cement, ordinary Portland cement, slag Portland cement, etc. Portland cement is commonly used for home decoration.
Principles for purchasing cement and sand
■ In order to ensure the quality of cement mortar, when purchasing cement, you must pay attention to whether it is 425# Portland cement produced by a large factory.
■ Sand should be selected. The particle size of medium sand is very suitable for use in cement mortar. Many customers think that the finer the sand, the better the mortar, but this is actually a misunderstanding. Sand that is too fine does not have strong adsorption capacity and cannot produce large friction to adhere to the ceramic tiles.
Examples of cement production process
After raw materials and fuel enter the factory, they are sampled, analyzed and inspected by the laboratory. At the same time, they are 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 then lifted to the corresponding raw material storage warehouse through the elevator. After two-stage crushing, limestone, fluorite and gypsum are sent to their respective storage warehouses by elevators. The laboratory calculates the process formula based on the quality of limestone, clay, anthracite, fluorite, and pyrite powder, and uses the raw meal microcomputer batching system to batch the all-black raw meal, which is ground by a raw meal mill and sampled every hour. Test the percentage content of calcium oxide, ferric oxide and fineness of primary raw materials, and make timely adjustments to ensure that each data meets the requirements of the process formula. The ground black raw meal is lifted into the raw meal warehouse through a bucket elevator. Based on the quality of the ground raw meal, the laboratory homogenizes the raw meal through multiple warehouse combinations and mechanical dumping methods. The elevator lifts it into two Raw meal homogenization warehouse, the raw meal is matched through two homogenization warehouses, and the material is lifted to the pelletizing tray silo. The pre-water pelletization control device located on the surface of the shaft kiln performs the proportioning of material and water. The balling tray is used for balling the raw materials. The resulting balls are calcined by the vertical kiln distributor to distribute the raw material balls at different positions in the kiln. The fired clinker is sent to the clinker crusher through the discharge pipe and scale machine for crushing, and is sampled every hour by the laboratory. Carry out chemical and physical analysis of clinker at one time. According to the clinker quality, the elevator will put it into the corresponding clinker warehouse. At the same time, according to the production and operation requirements and the building materials market situation, the laboratory will carry out cement proportioning of clinker, gypsum and slag through the clinker microcomputer batching system, and the cement mill will Grind No. 425 and No. 525 ordinary Portland cement respectively, and take samples every hour for analysis and inspection. The ground cement is lifted into three cement warehouses via bucket elevators. The laboratory homogenizes the cement through multiple warehouse combinations and mechanical warehouse dumping based on the quality of the ground cement. It is sent into two cement homogenization warehouses through the elevator, and then matched with the two cement homogenization warehouses. A microcomputer-controlled packaging machine is used to pack the cement. The packaged bagged cement is stored in the finished product warehouse. After passing the laboratory sampling inspection, Issue cement factory notice.
· Eight taboos when using cement
1. Avoid getting hardened by moisture
Cement that gets hardened by moisture will reduce or even lose its original strength, so the specification stipulates, Cement that has been out of the factory for more than 3 months should be re-tested and used according to the test results. Cement that has become lumpy or hardened due to moisture must be screened before use. The screened out lumps are generally used for masonry mortar or plastering mortar in secondary projects after being kneaded or ground finely. For cement clumps that pulverize when touched or pinched, the strength level can be appropriately reduced for use.
2. Avoid exposure to the sun and quick drying
If concrete or plaster is exposed to the sun after operation, its strength will be reduced or even completely lost as the water evaporates rapidly. Therefore, the base must be strictly cleaned and fully moistened before construction; it must be covered strictly after construction and watered and maintained in accordance with specifications.
3. Avoid freezing at negative temperatures
After concrete or mortar is mixed, if it is frozen, the cement cannot hydrate, and the water freezes and expands, and the concrete or mortar will freeze. It will be damaged by gradually deepening powder from the outside to the inside. Therefore, construction should be carried out in strict compliance with the "Winter Construction Regulations for Construction Projects" (JGJ104-97).
4. Avoid high temperatures and extreme heat
If the solidified mortar layer or concrete components are often exposed to high temperatures and extreme heat, there will be a loss of strength. This is because under high temperature conditions, the cement stone will The calcium hydroxide in it will decompose; in addition, some aggregates will also decompose or expand in volume under high temperature conditions.
For situations where the temperature is high for a long time, refractory bricks can be used to isolate and protect ordinary mortar or concrete. When encountering higher temperatures, special heat-resistant concrete should be used for pouring, or a certain amount of finely ground heat-resistant materials can be mixed into the mud.
5. Avoid dirty and soft base layers
Cement can be firmly bonded or wrapped with hard and clean base layers, but its bonding strength is not consistent with the smoothness of the base layer. related to the degree. When constructing on a smooth base, it must be chiseled, hemped, and brushed in advance to ensure that the cement and the base can be firmly bonded.
Dirt, greasy, acid and alkali and other substances on the base will act as an isolation agent and must be carefully removed and washed. Then, a layer of plain cement slurry should be applied before applying mortar or pouring concrete.
Cement will shrink during the solidification process, and during the changes in dryness, humidity, cold and heat, it is extremely incompatible with the volume changes of the loose and weak base layer, and hollowing or cracks will inevitably occur, making it difficult to Strong bond. Therefore, wood, slag cushion, lime soil cushion, etc. cannot be firmly bonded with mortar or concrete.
6. Avoid impure aggregates
If sand and gravel are used as aggregates for concrete or cement mortar, if there are dust, clay or other organic impurities, they will affect the relationship between cement, sand and stone. The bond strength between them will eventually decrease the compressive strength. Therefore, if the impurity content exceeds the standard, it must be cleaned before use.
7. Avoid too much water and gray consistency
People often ignore the impact of water consumption on the strength of concrete. In order to facilitate pouring during construction, sometimes they do not carefully implement the mix ratio and mix the concrete. Very thin. Since the water required for hydration is only about 20% of the weight of cement, the excess water will leave many pores in the concrete after evaporating, and these pores will reduce the strength of the concrete. Therefore, on the premise of ensuring dense pouring, mixing water should be minimized.
Many people believe that the more cement used in plastering, the stronger the plastering layer will be. In fact, the more cement is used and the thicker the mortar, the greater the volume shrinkage of the plaster layer, resulting in more cracks. Under normal circumstances, when plastering, you should first use 1: (3-5) coarse mortar to plaster the leveling layer, and then use 1: (1.5-2.5) cement mortar to plaster a very thin surface layer. Do not use too much cement.
8. Avoid acid corrosion
Acidic substances will react with calcium hydroxide in cement. The resulting product will be loose and swollen, and will easily hydrolyze and pulverize when exposed to water. . As a result, the concrete or plaster layer will gradually be corroded and disintegrated, so cement should not be corroded by acid.
Acid-resistant mortar and acid-resistant concrete should be used in situations or containers that are exposed to acidic substances. Slag cement, pozzolana cement and fly ash cement all have good acid resistance, and these three cements should be given priority to prepare acid-resistant mortar and concrete. Ordinary cement is not allowed to be used in projects that strictly require acid corrosion resistance. Video on the difference between the use of composite cement and ordinary Portland cement
Composite cement is mixed with volcanic ash and fly ash mixtures. Its nominal strength is the same as other types of cement of the same grade. Only the water-cement ratio is different, so when using composite cement, the water-cement ratio must be strictly controlled during construction. Otherwise, during the construction process, especially after the concrete is vibrated by the vibrator, water will separate and remain on the concrete surface, which will destroy the surface concrete structure and cause the surface to disappear. Strength, peeling, sanding, etc.