Cement stacking requirements

Cement hardened by moisture will reduce or even lose its original strength, so the specification stipulates that cement shipped for more than 3 months should be retested 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 and grinding. For cement blocks that can be crushed by one touch or one pinch, the strength grade can be appropriately reduced.

(2) Avoid exposure and quick drying.

Concrete or gypsum will be exposed to the sun after operation, and its strength will decrease or even be 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.

(3) Avoid freezing at negative temperature.

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 powder cake that gradually deepens 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).

④ Avoid high temperature and extreme heat.

The solidified mortar layer or concrete member will lose its strength if it is often in high temperature and extremely hot state, 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 with refractory bricks. When the temperature is high, special heat-resistant concrete should be used for pouring, and a certain amount of ground heat-resistant materials can also be mixed into the concrete.

⑤ Avoid messy and soft grass roots.

Cement can be firmly bonded or wrapped with hard and clean base, but its bonding strength is related to the flatness of base surface. In the construction of leveling the base, it is necessary to cut hair, smash hemp and brush it clean in advance, so that the cement can be firmly bonded with the base.

Dirt, grease, acid and alkali on the grass-roots 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. In the process of dry, wet, hot and cold changes, it is extremely unsuitable for the volume change of loose and weak base, which will inevitably lead to empty drums or cracks, making it difficult to bond firmly. Therefore, wood, slag cushion and lime-soil cushion cannot be firmly bonded with mortar or concrete.

[6] Avoid impure aggregate.

As an 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. Therefore, if the impurity content exceeds the standard, it must be cleaned before use.

Avoid water that is too thick and gray.

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 needed for hydration only accounts for about 20% of the cement weight, the excess water will leave many pores in the concrete after evaporation, thus reducing the strength of the concrete. Therefore, under the premise of ensuring the pouring compactness, the mixing water should be reduced as much as possible.

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. Generally speaking, 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.

(8) Avoid acid corrosion.

Acidic substances and calcium hydroxide in cement will undergo neutralization reaction, and the product will expand loosely and be easily hydrolyzed and pulverized when it meets water. Therefore, the concrete or plaster layer is gradually corroded and decomposed, so the cement is protected 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.

Extended data:

Main technical indicators of cement:

(1) Specific gravity and bulk density: the specific gravity of standard 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 to stirring to solidification is called final setting time. Portland cement, ordinary portland cement initial setting time is not earlier than 45 minutes, the final setting time is not later than 6.5 hours; The final setting time of pozzolanic cement, fly ash cement and composite portland cement shall not be later than 10 hour. In fact, the initial setting time is 1~3h, while the final setting time is 4 ~ 6h. The setting time of cement is determined by a special setting time tester.

(4) Strength: The strength of cement should meet the national standards.

(5) Volume stability: refers to the uniform expression of 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. The heat continuously released in the process of cement hardening is called hydration heat.

(7) Standard consistency: refers to the consistency when the cement paste has certain resistance to the sinking of the standard test bar.

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