Main minerals of Portland cement: calcium trisilicate (3CaO·SiO2, C3S), calcium disilicate (2CaO·SiO2, C2S), calcium trialuminate (3CaO·Al2O3, C3A), calcium ferroaluminate (4CaO·Al2O3·Fe2O3, C4AF).
Setting and hardening of cement:
1) 3CaO·SiO2+H2O -- CaO·SiO2·YH2O (C-H-S gel) +Ca (OH) 2;
2) 2CaO·SiO2+H2O -- CaO·SiO2·YH2O (C-H-S gel) +Ca (OH) 2;
3) 3CaO·Al2O3+6H2O -- 3CaO·Al2O3·6H2O (calcium aluminate hydrate, unstable);
3CaO·Al2O3+3CaSO4·2H2O+26H2O -- 3CaO·Al2O3·3CaSO4·32H2O (ettringite, calcium trisulfide hydrated aluminate);
Generally mixed into cement mortar or concrete can accelerate cement mortar or concrete hardening, improve concrete strength, especially the early strength of the admixture known as early strength agent.Early strength agent is an admixture that can accelerate the early strength development of concrete. Its main function is to shorten the curing period, speed up the project progress, and improve the turnover rate of engineering operations. Its main mechanism is to accelerate the hydration speed of cement, accelerate the early crystallization and precipitation of hydration products, so as to promote the early strength development of concrete.
According to its chemical composition, it can be divided into inorganic, organic and composite early strength agents. A large number of studies have shown that there are many types of early strength agents for concrete, which have different mechanisms for promoting cement hydration reaction and different effects for improving early strength of concrete.Based on the research of the mechanism of early strength agent early strength, can be explained theoretically early strength agent is how to improve the cement hydration reaction speed, accelerate the setting and hardening of cement paste, from theory to guide the new research of early strength agent and the preparation of compound early strength agent, give full play to the advantages of all kinds of early strength agent, achieves the best - the best combination of the goal, to maximize the use effect of all kinds of early strength agent.
Chloride early strength agent is generally considered to be the best early strength agent, and its mechanism of action is mainly as follows: firstly, chloride can react with C3A in cement to generate hydrated chloro-aluminate which is insoluble in water, accelerating the hydration of C3A in cement;Secondly, chloride can also react with calcium hydroxide to form calcium chlorate that is insoluble in water, thus reducing the concentration of calcium hydroxide in the liquid phase.
The hydration of C3S is accelerated, and the compound salt generated can also increase the proportion of slurry solid phase and accelerate the formation of cement stone.The rechloride is soluble salt with salt effect, which can increase the solubility of cement clinker in water and accelerate the hydration of cement clinker.
Sodium sulfate early strength agent is the most widely used sulfate early strength agent, its mechanism of action is mainly: (1) sodium sulfate is a strong electrolyte, can increase the ionic strength in the liquid phase, the diffusion of double layer compression, promote cement condensation and hardening;(2) sodium sulfate can react with free calcium hydroxide to generate gypsum and sodium hydroxide, the alkali generated can improve the pH value of the liquid phase, increase the plastic strength of cement slurry, the new generation of secondary gypsum than cement grinding gypsum added to the activity of greater, more can promote the formation of hydrated calcium sulfide aluminate;(3) sodium sulfate hydrolysis with cement produced by calcium hydroxide and cement C3A, SO42- reaction to produce ettringite, reduce the concentration of calcium hydroxide, accelerate the hydration of C3S.
Sodium silicate as the silicate system commonly used in the early strength agent, early strength agent, its function mechanism is mainly due to the hydrolysis of sodium silicate with silicate cement mineral hydrolysis of CH reaction, calcium silicate hydrate generating poorly soluble in water, destruction of C3S and C2S hydrolysis equilibrium, promote the hydration of C3S and C2S, acceleration generated a lot of calcium silicate hydrate, so as to improve the early strength of filling body.
The early strength effect of lithium salt early strength agent is mainly as follows: first, Li+ has the characteristics of small radius, strong polarization and large hydration radius, so as to accelerate the rupture of hydration protective film, shorten the hydration induction period, and improve the hydration capacity of C3S and C2S in cement.Secondly, lithium salt can promote the formation of Aft ettringite crystal and significantly improve the coagulation speed and early strength.
The mechanism of action of inorganic calcium salt early strength agent is that inorganic calcium salt can make Ca(OH) 2 reach saturation and crystallize quickly, which makes the content of Ca2+ in the liquid phase drop sharply, reduces the pH value of C3S - H2O system, thus accelerating the hydration of C3S, and then accelerating the hydration and hardening of cement.
The early strong action mechanism of Al3+, Fe3+ and other high-priced cations mainly lies in the compressive effect of high-priced cations on the diffusion double-layer of c-h-s colloidal particles, which can accelerate the condensation of c-h-s colloidal particles, thus reducing their concentration in the liquid phase, accelerating the hydration reaction of C3S and C2S, and thus accelerating the hardening process of cement and concrete.
The early strong action mechanism of calcium formate is mainly due to the weak acidic ionization of calcium formate in water, which can reduce the pH value in the system, accelerate the hydration of C3S, and accelerate the setting and hardening of cement.In the early hydration stage of sulphoaluminate cement clinker, a large amount of ettringite will be formed, so that there will be a better connection between hydration products and the densification of cement stone structure.
Inorganic class early strength agent, its mechanism is to reduce the cement clinker particles, the surface tension of contact with water to increase its solubility in water, and at the same time by adding early strength agent to reduce the cement hydrolysate concentration in the water, so as to promote C3S and C2S, C3A, C4AF cement components such as dissolution rate of increase, accelerate the ettringite, C - H - S gel, etc. The formation of hydration products, speed up the setting and hardening of cement.