Concrete is a product created through cement hydration reaction and subsequent condensation and hardening. The hydration and condensation hardening of cement takes a time process from the surface of the particles to the interior. The hydration speed starts relatively fast and the strength develops relatively quickly, and then gradually slows down. The strength development of different cements is not exactly the same. Portland cement mixed with more active mixed materials has lower strength in the early stage and higher growth rate in the later stage. The setting and hardening process of Portland cement is affected by many factors. So, what are the main factors that affect the hardening of concrete?
(1) Clinker-mineral composition
The mineral composition of Portland cement clinker affects the setting and hardening of cement. The main factors are the hydration and condensation hardening characteristics of the four clinker minerals.
(2) Cement fineness
The thickness of cement particles directly affects the hydration, setting and hardening, drying shrinkage and hydration heat of cement. Generally speaking, the finer the cement particles , the faster and more fully the hydration develops, the faster the setting and hardening, and the higher the early strength. However, if the cement particles are too fine, the standard consistency requires more water, the activity is easily reduced, the shrinkage after hardening is also large, and the grinding energy consumption is high and the cost is high.
(3) Amount of water added during mixing
The larger the water-cement ratio, the slower the setting. The reason is that the greater the water-cement ratio, the less dense the cement slurry system structure is, and the gel is less likely to aggregate.
(4) Maintenance humidity and temperature
Water is a substance that participates in cement hydration and is a necessary condition for cement hydration and hardening. Therefore, the concrete (mortar) after pouring should be maintained Moist state to facilitate the acquisition and development of strength. Increasing the temperature can lead to faster early strength development, and lowering the temperature can achieve higher final strength. But below 0℃, after water turns into ice, the hydration, condensation and hardening of cement will stop.
(5) Curing age
Generally speaking, the strength of cement stone continues to increase with the increase of curing age.
(6) Concrete admixture: polycarboxylate water-reducing agent
Choosing appropriate concrete admixtures can effectively improve the workability of concrete, adjust performance, increase strength, and change Clotting time and other effects.
(7) Storage
After cement gets damp, its strength will decrease. Generally, the strength will decrease by about 10% to 20% after three months of storage, and by 15% to 30% after six months of storage. It decreases by about 25% to 40% a year.
How to solve the problem of concrete hardening too fast
Increase the water-cement ratio and increase the water volume
Adjust the sand rate and reduce the sand
Adjust the sand The water content of the stone is not that high due to dry weather
If it dries too fast, it is best to increase the amount of admixture. The workability, fluidity and slump will be improved. Do not overdo it.
What are the properties of hardened concrete?
Workability: the most important property of concrete mixtures. It comprehensively represents the consistency, fluidity, plasticity, anti-separation and bleeding properties of the mixture and its ease of spreading. There are many methods and indicators for measuring and expressing the workability of a mixture. China mainly uses the concrete on-site slump (mm) measured by a truncated cone slump cylinder and the Vibe time (seconds) measured by a Vibe instrument as the consistency indicator. main indicators.
Strength: The most important mechanical property of concrete after hardening refers to the ability of concrete to resist stresses such as compression, tension, bending, and shear. The water-cement ratio, cement type and amount, aggregate type and amount, as well as mixing, forming, and curing all directly affect the strength of concrete. The strength grade of concrete is classified according to the standard compressive strength (a cube with a side length of 150mm is used as a standard specimen, cured for 28 days under standard curing conditions, and the cube compressive strength with a 95% guarantee rate is measured according to the standard test method) , called labels, divided into C10, C15, C20, C25, etc. The tensile strength of concrete is only 1/13 to 1/8 of its compressive strength. Improving the ratio of concrete tensile strength to compressive strength is an important aspect of concrete modification.
Deformation: Concrete will deform under the action of load or temperature and humidity, mainly including elastic deformation, plastic deformation, shrinkage and temperature deformation. The elastic deformation of concrete under short-term load is mainly expressed by elastic modulus. Under long-term load, the phenomenon of constant stress and continuous increase of strain is called creep; the phenomenon of constant strain and continuous decrease of stress is called relaxation. The volume deformation caused by cement hydration, carbonization of cement stone and water loss is called shrinkage. The deformation of concrete is divided into two categories. One is the stress deformation under the action of load, such as the deformation of monotonous short-term loading, the deformation of long-term load and the deformation of repeated loading; the other type has nothing to do with stress, which is called It is volume deformation, such as concrete shrinkage and deformation caused by temperature changes.
Durability: Under normal circumstances, concrete has good durability. However, in cold areas, especially in engineering parts where the water level changes and when subjected to frequent freezing and thawing in a saturated state, concrete is easily damaged. For this reason, concrete must have certain frost resistance requirements. When used in impermeable projects, concrete is required to have good impermeability and corrosion resistance. Concrete durability includes impermeability, frost resistance, and erosion resistance.
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The company’s sustainable development is inseparable from the continuous suggestions and trust verification of our products by our customers. In response to the needs of our customers in the market, the company will continue to optimize products every year and consider polycarboxylate water-reducing agents. Dominating the market, the company is also constantly researching various types of polycarboxylic acid processes to make the company's products more adaptable in the national market!
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