Label, type and validity period

1. Label:

New 32.5 = old 425, new 42.5 = old 525. 32.5 means that the strength of cement is 32.5mpa after 28 days of setting, and so on. The international common cement grades are 32.5 and 42.5. The larger the grade, the higher the cement strength. 32.5 cement is enough for home decoration. 42.5 cement is used for concrete pouring of buildings above 6 floors. If the cement brick is too high, it will be easy to crack.

2. Species:

According to the quality of cement, it can be divided into

(1) Portland cement: code P Ⅱ, this kind of cement shield stone only has 42.5.

(2) Ordinary portland cement: Code Po, can be used for home decoration.

(3) Portland slag cement: Code PC;

(4) Composite Portland cement.

3. Validity period:

The validity period is 3 months from the date of delivery. Generally speaking, as long as the cement does not agglomerate, there is no problem.

4. Setting test:

Water + cement can be activated into a thick congee, initial setting after 2 hours, and final setting after 8 hours. If the cement can be as hard as stone, the quality of cement is good; poor cement is not.

R-type cement belongs to fast hardening type and has higher requirements for 3-day strength

For example, the three-day strength requirement of 32.5 is compressive strength of 11.0 and flexural strength of 2.5

The three-day strength requirement of 32.5R is 16.0 compressive strength and 3.5 flexural strength

Cement and concrete have always been divided into two industries, there is a serious isolation phenomenon. The contradictions and problems between the two industries (each other) have become increasingly prominent. In addition, influenced by traditional concepts and conformity thinking, many misunderstandings and puzzles on development problems have been produced. Some unilateral understanding, guidance and even wrong views and actions have seriously hindered cement and concrete The healthy development of concrete industry seriously affects the engineering quality.

Correct understanding of high performance concrete and improving the applicability of cement

Many authoritative experts repeatedly pointed out that: high performance concrete is to durability as the main index of design, mainly for durability, not strength. However, it can not be ignored that there are still many misunderstandings about the understanding and application of HPC by many domestic practitioners. Among them, the most prominent is to equate high-strength concrete with high-performance concrete or take high-strength as an indispensable part of high-performance concrete, but it is not the case.

China's standard cecs207-2006 "technical specification for application of high performance concrete" and "construction standard [2014] No. 117 document issued by the Ministry of housing and urban rural development of the people's Republic of China, high performance concrete is defined as: it is produced by conventional materials and processes, and has high durability, high workability and high volume stability required by concrete structure, and meets the specific requirements of construction projects. The main characteristics of "three high" do not have high strength; American ACI definition is: high performance concrete is in line with the requirements of special performance combination and homogeneity. It is not a variety of concrete, but the emphasis on the "performance" or quality, state and level of concrete. Different engineering requirements have different emphasis, that is, special performance combination.

American cement standard astmc1157-2011 "cement standard performance specification" has no regulations and restrictions on cement composition, chemical composition and fineness, but only specifies the performance, adding durability and other selective indicators, which divides the general Portland cement into six varieties. Only the common type (GU), high sulfate resistance (HS) and low heat type (LH type) have specified 28 day strength index, which are only 28 MPa, 25 MPa and 21 MPa respectively. Among them, the 7-day and 28-day strength indexes of LH type are 11 MPa and 21 MPa respectively (since there is no systematic comparison and analysis of cement samples between China and the United States in recent years, after consulting the authoritative experts of cement standard development unit, they are equivalent to 14.8 MPa and 28.5 MPa respectively). The other three varieties, namely early strength type (he type), medium heat type (MH type) and medium sulfur resistant type (HS), only require 1-day or 3-day and 7-day strength, but no 28 day strength requirement is specified. The 3-day and 7-day compressive strength indexes of MH type are 5 and 11 MPa respectively, and the water binder ratio of 0.5 is used. This loose and practical requirement provides a lot of room for the application of large amount of admixture and improving the later performance and durability of concrete. Astmc1157 directly and succinctly embodies the essence and function of cement standard, overcomes the shortcomings of standard lag, and can open up the development of materials and technology.

On the strength of cement, concrete and the use of admixture of traditional thinking and one-sided understanding!

It is one-sided that pure silicon cement is good, cement mixed with admixture is not good; there are still many technical personnel in the industry, soil and cement plants subjectively believe or advocate that high-performance and high-strength concrete can be prepared directly by using high-grade cement and aggregate instead of admixture. Due to the influence of some traditional concepts and conformity thinking, there are still many misunderstandings about the concept of cement and concrete strength and the use of admixtures in the industry.

Lian Huizhen, a famous concrete scholar and professor of Tsinghua University, pointed out in his book "the influence of traditional thinking and conformity thinking on the progress of concrete technology". At present, the traditional thinking or misunderstanding that has the greatest impact or hindrance on the progress of concrete technology is "strength first (or even the only one)". The strength level is regarded as the difference between cement and concrete "good" and "poor", and mistakenly believes that the compressive strength is high Concrete, its performance is good; high early strength cement or concrete is good, high strength or high performance concrete must use high strength (grade) cement, it is recommended to cancel low-grade cement; pure cement concrete is genuine, admixture is fake; pure Portland cement and ordinary cement are genuine, others are inferior.

Professor Lian Huizhen repeatedly stressed: "it is a misunderstanding caused by traditional thinking to evaluate the" good "and" bad "of cement and concrete simply by strength. This traditional concept should be corrected, that is, the high limit of cement strength is more important than the low limit, the crack resistance is more important than the strength, and the most important thing is the quality uniformity. Good concrete has all kinds of properties that meet the specific requirements of the project, has the maximum homogeneity, and has stable volume without cracking

For example, in many areas of China, the price of mineral powder is higher than that of cement clinker, but in order to adjust the performance of cement and concrete, the cement plant and mixing plant must also use mineral powder. Admixtures can compensate for the disadvantages of Portland cement, such as reducing the temperature rise of concrete, improving the ability of chemical corrosion resistance and later strength, especially the crack resistance of fly ash. We should reject fake and shoddy products. Proper use of admixtures can greatly improve the performance and durability of concrete. If no admixture is used, the durability of concrete will be difficult to guarantee.

At present, the hydraulic cementitious material of concrete prepared in domestic mixing plant is actually prepared through the secondary batching process. The cement grade or apparent strength of the concrete prepared by mixing clinker with admixture and gypsum is also prepared. If the cement and concrete industry is integrated or the conditions are met, and the division of labor and cooperation between the two industries in some regions is good, the final cement components in concrete can of course be prepared in the cement plant. In this case, it is another misunderstanding to excessively pursue or emphasize the high strength and high grade of cement prepared by the factory without fully considering the application conditions.

Objective view the concrete and cement product structure present situation!

The cement proportion of 42.5 and above in China is almost equal to that of ready mixed concrete. If the cement proportion of 42.5 and above is increased, the key point is to develop ready mixed concrete.

Domestic high-strength concrete such as C50 and above accounted for a small proportion of the actual application, from the current overall situation of our country, the largest amount of commercial concrete is around C30. The "preference" of concrete grade in different areas depends on the floor height of the local buildings. In some areas, the average value of C25 is the most, while in some areas, C30 is the most, or C35 in some urban areas is slightly more. Therefore, to promote the application of high performance concrete, on the one hand, it is necessary to comprehensively improve the grade of concrete used in structural engineering. In addition, how to make ordinary concrete (C25 ~ C35) with high performance is the key point, which will contribute more and more urgently to the society. An important content or direction of structural adjustment of concrete products is to solve the problem of homogenization.

In his personal blog, Bill Gates made a shocking comparison in order to give people a direct understanding of China's "gobbling up" of cement consumption in the 21st century. In 2011-2013, China consumed 6.6 billion tons of cement, more than the United States consumed in the entire 20th century. But look at the projects that the United States built between 1901 and 2000: skyscrapers, intercontinental highways, Hoover Dam, and so on, which consumed only 4.5 billion tons of cement.

In recent years, the cement industry in Europe, the United States and Japan has significantly reduced the output of ordinary portland cement and reduced the proportion of clinker in cement products. At present, European Portland composite cement accounts for about 65% of the total output.

China has a large population base, and its per capita income is far lower than that of developed countries. However, the annual consumption of cement per capita is 1.775 tons (converted into 42.5 ordinary cement according to the comparable cement output, and the annual consumption of cement per capita is 1.61 tons), which is far more than the level of 700 kg per capita per year in the rapid development period of developed countries. In addition to large-scale construction factors, unscientific and unreasonable use of cement is the main cause of waste One of the reasons.

Why is the price of 32.5 cement higher than 42.5 cement in many areas? For example, in a 5000 ton production line in Zhuzhou, Hunan Province, the total profit of 32.5 cement accounted for 40% of the sales products was higher than 42.5 cement accounting for 60%. The most important factor is the production structure and the application conditions of the enterprise. Therefore, China's cement high-grade mark factory is gradual, only relying on the efforts of cement enterprises, its role is very small, at present, the result is only to transfer more part of the admixture to the mixing station, so as to transfer the benefit from the cement plant to the mixing plant.

Redispersible latex powder manufacturers explain the basic role of redispersible latex powder in cement mortar and concrete

Improve the compressive strength and flexural strength of mortar.

Redispersible latex powder is dispersed into film and plays a reinforcing role as the second adhesive;

The protective colloid is absorbed by the mortar system (it will not be destroyed by water or "secondary dispersion" after film forming);

As a reinforcing material, the polymer resin is distributed in the whole mortar system, which increases the cohesion of mortar;

The addition of redispersible latex powder can improve the elongation of mortar, thus improving the impact toughness of mortar, and giving the mortar good stress dispersion effect.

Redispersible latex powder can reduce the elastic modulus of mortar, improve the deformation ability and reduce the cracking phenomenon.

Redispersible latex powder can improve the bonding performance of mortar. The bonding mechanism depends on the adsorption and diffusion of macromolecules on the adhesive surface. Meanwhile, the rubber powder has a certain permeability. It can fully infiltrate the surface of the base material together with cellulose ether, so that the surface performance of the base course is close to that of the new plastering, so as to improve the adsorption property and greatly increase its performance.

Redispersible latex powder can improve the wear resistance of mortar. The improvement of wear resistance is mainly due to the existence of a certain amount of glue on the surface of the mortar. The rubber powder plays a bonding role. The mesh membrane structure formed by the rubber powder can pass through the holes and cracks in the cement mortar, so as to improve the adhesion between the base material and the hydration products of cement, so as to improve the wear resistance.

Redispersible latex powder gives mortar excellent alkali resistance

Mixing station

There are disadvantages in adding admixtures

One of the outstanding problems caused by the isolation of cement and concrete industry is that it is difficult to mix uniformly by the current process due to the mixing of admixtures in the mixing plant at present, which makes the non-uniformity in essence increased. The mixing time for traditional concrete mixture to reach the stability of coefficient of variation is about 75 seconds. For today's mixture with admixtures and admixtures, the whole mixing time (including dry mixing before adding water) is 150 seconds. At present, the mixing time of ready mixed concrete in China is usually 30 seconds, and the longest one is only 60 seconds. Prolonging the mixing time is bound to affect the output, so it is basically impossible to do so.

Gypsum can regulate the setting time and stimulate the activity of admixtures and cement clinker. When mineral admixtures are used in cement for mixing plant, the gypsum content in cement is diluted, and it is impossible to optimize the gypsum content. Therefore, the gypsum in cement is diluted and the water demand, deformation, compressive and tensile strength values are affected.

In developed countries, admixtures are usually mixed by stages in developed countries. Half of the additives are mixed into the mixer before adding the rest automatically through the sensors in the mixer on the conveyor car. In our country, admixtures are dissolved into mixing water and put into concrete mixing plant together, which is the worst way to use admixtures.

Professor Lian Huizhen strongly advocates that admixtures should be added by cement plants, and emphasizes that: in terms of the actual needs of modern concrete with complex components, the cement materials needed for concrete should be put back to the factory for production, and the raw materials should be homogenized in the common grinding process, so that the homogeneity of concrete can be greatly improved without increasing the current mixing time. The overall cost of the mixing plant will not increase, but will produce obvious benefits to save energy and resources and reduce the cost of quality management. On the whole, it can not only achieve a win-win situation between cement plant and mixing plant, but also benefit the quality and durability of construction projects. The cementitious materials needed by cement plant to produce concrete are in line with the objective law.

Due to better production control, management system and other conditions, the homogenization facilities such as pre homogenization, mixing, air mixing and warehouse dumping are more perfect, and the common grinding method can be adopted to achieve better homogenization effect. Meanwhile, the proportion of admixture, cement clinker, admixture and even concrete ratio can be adjusted at any time according to the raw materials, clinker quality and harmful components More uniform, more reasonable particle size distribution, more guaranteed quality. Therefore, admixtures from cement plant is the development direction, and the extension of cement industry and the integration of ready mixed concrete industry are the development trend.

At present, the high-grade cement produced by cement enterprises is still an intermediate product. Most of the concrete, such as C30 and mixing plant, actually adds about 2.5 times of the high-grade cement produced by the factory. It is not conducive to improving the quality of concrete, and it is difficult to achieve the effect expected by some people. On the contrary, the benefit of cement industry is also reduced.

Why is the quality of cement bad now?

At present, the problems of poor concrete durability (impermeability, frost resistance, carbonation resistance and corrosion resistance) caused by early concrete cracking, strength shrinkage and adaptability of cement concrete to engineering are very common.

First of all, the early cracking and strength shrinkage of concrete are more related to the use of fine ground ordinary cement and early strength high strength cement. Some construction units believe that the higher the early strength of cement, the faster the development of concrete strength, the more guaranteed its quality; and the higher the early strength of cement is, the faster the construction speed can be, and the cost can be greatly reduced. The early strength cement is mainly achieved by increasing the proportion of clinker, increasing the early strength minerals and specific surface area of clinker. The visible and invisible internal defects formed by early hydration of this cement are exhausted due to the early hydration products of cement, and there is no later hydration products to repair. In addition, the uneven heat release of hydration, temperature and humidity and other environmental factors vary Which leads to structure