Factors Affecting the Setting and Hardening of Portland cement
(1) The Impact of Cement’s Composition
The mineral composition of cement and their ratios are the main factors affecting the setting and hardening of cement. As mentioned above, various mineral components will reveal different characteristics when reacting with water. For example, the increase of C3A can speed up the setting and hardening rate of cement, and the heat of hydration is high at the same time.
Generally speaking, if mixed materials are added into the cement clinker, the anti-erosion will increase, and the heat of hydration and the early strength will decrease.
(2) The Mixing Amount of Gypsum
Gypsum is called the retarding agent of cement which is mainly used for regulating the setting time of cement and is an indispensable component. Without gypsum, cement clinker can condense immediately by mixing with water and release heat. The major reason is that C3A in the clinker can dissolve in water quickly to generate a kind of calcium aluminate hydrate, a clotting agent, which will destroy the normal use of cement. The retardation mechanism of gypsum is: when cement is hydrated, gypsum reacts with C3A quickly to generate calcium sulfoaluminate hydrate which deposits and forms a protection film on the cement particles to hinder the hydration of C3A and delay the setting time of cement.
If the content of gypsum is too little, the retardation affect will be unobvious. Too much gypsum will accelerate the setting of cement because gypsum can generate a clotting agent itself. The appropriate amount of gypsum depends on the content of C3A in the cement and that of SO3 in gypsum, and it also related to the fineness of cement and the content of SO3 in clinker. The amount of gypsum should account for 3%-5% of the cement’s mass. If the content of gypsum exceeds the limit, it will lower the strength of cement and it can even lead to poor dimensional stability, which will cause the expanded destruction of cement paste. Thus, the national standard requires that the content of SO3 should not be more than 3.5%.
(3) The Impact of Cement’s Fineness
The size of cement particles directly affects the hydration, setting and hardening, strength and heat of hydration.
The finer the cement particles are, the larger the total surface area is and the bigger the area contacting with water is. Thus, the hydration will be quick, the setting and hardening will be accelerated correspondingly, and the early strength will be high.
However, if the cement particles are too small, it is easy for them to react with the water and the calcium dioxide in the air to destroy the storage of cement. If the cement is too fine, its shrinkage is large in the hardening process. Thus, the finer the cement is ground, the more energy will lose and the higher the cost will be. Usually, the grain size of the cement particles is within 7-200pm (0.007-0.2mm).
(4) The Impact of Curing Conditions
The curing environment has sufficient temperature and moisture which is conducive to the hydration and setting and hardening process of cement and benefits the development of the early strength. If the moisture of the environment is very dry, the water in the cement will evaporate, leading to insufficient hydration and ceasing of the hardening. Serious cracks will happen sometimes.
Usually, the temperature rises at the time of curing, and the hydration of cement and the development of early strength become fast. If the hardening process occurs at a low temperature, the final strength won’t be affected though the development of the strength is slow. But if the temperature is under 00C, the hydration of cement will stop and the strength will not only stop growing but also destroy the structure of cement paste due to the condensation of water.
In actual projects, the setting and hardening process of cement products is accelerated by steam curing and autoclave curing.
(5) The Impact of Curing Age
The hydration and hardening of cement is an ongoing process in a long period. With the increase of the hydrating degree of various clinker minerals in cement particles, gels will grow and capillary porosities will decrease, which enables the strength to rise with the increase of age. It is proved that cement develops rapidly within 28d and slowly after 28d.
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(6) The Impact of the Mixing Water Content
If the cement consumption is unchanged, the increase of the mixing water content will enhance the amount of capillary porosities, lower the strength of cement paste, and extend the setting time. Therefore, in practical projects, the amount of water and cement will be changed without modifying the water-cement ratio (the minimum amount of cement is regulated to ensure the durability of concrete) when the liquidity of cement concrete is adjusted.
(7) The Impact of Admixture
Hydration, setting, and hardening of Portland cement are constrained by C3S, C3A. And all the admixtures that affect the hydration of C3S, C3A can change the performance of the hydration, the setting and hardening of Portland cement. For example, the accelerator agents (such as CaC12, Na2S04) can accelerate the hydration and the hardening of cement and improve its strength. On the contrary, the retarding agents (such as calcium lignosulphonate) can delay hydration and hardening of cement and affect the development of the early strength.
(8) The Impact of Storage Conditions
The inappropriate storage will expose cement to moisture. The particle surfaces agglomerate because of hydration which seriously reduces the intensity. Slow hydration and carbonization will happen due to the impact of the water and C02 in the air, even though the storage is good.
The strength decreases by 10%-20% after 3 months, by 15%-30% after 6 months, by 25%-40% after 1 year, so the effective storage period of cement is 3 months and the cement should not be stored for a long time.