Key factors in preventing the hardening of cement-based castables to meet construction performance requirements

The early strength of castable refractory is mainly provided by cement hydration, and the hardening speed of cement directly determines the stripping time of the castable refractory.

　　As we all know, the mineral composition of cement clinker is a key factor affecting cement setting. The higher the content of tricalcium silicate and tricalcium aluminate in the clinker, the shorter the hardening time of the cement. This is because after these minerals undergo hydration reactions with water, they form an interconnected cement stone structure. Different proportions of mineral composition will lead to different hydration reaction rates. In addition, the fineness of cement clinker is also an important factor affecting cement hydration. The finer the particle size, the faster the hydration reaction and hardening speed.

　　In addition to the constituent minerals of cement clinker themselves, many external factors also affect the hardening of cement castables.

　　Among them, the most intuitive influencing factor is temperature. Generally speaking, during winter construction, we need to take measures such as insulation and adding accelerators to promote the hardening of the castable refractory; in summer, retarders need to be added to slow down its hardening speed. This is because when the temperature rises, the hydration reaction and setting rate of cement will accelerate; conversely, when the temperature decreases, the hydration speed of cement will also slow down.

　　Water, as one of the participants in cement hydration, also plays a crucial role. To ensure the construction performance of the castable refractory, we will add an appropriate amount of water, ensuring that the castable refractory has good fluidity while ensuring that its strength is not affected. However, if too much water is added, during the cement hydration process, excess water will form capillary pores in the cement stone, thereby reducing the strength of the cement stone and affecting the hardening time of the castable refractory.

　　As mentioned earlier, during seasonal changes, United Rongda technicians will add admixtures to the castable refractory to adjust the hardening time. These admixtures are mainly divided into retarders and accelerators.

　　Retarders include organic substances such as citric acid, sodium citrate, and gluconic acid. Most of these retarders have surface activity. They either adsorb on the solid-liquid interface, changing the surface properties of solid particles; or they form a water film protective layer by adsorbing a large number of water molecules through the hydrophilic groups in the molecule, hindering crystal contact and inhibiting the hydration process. Some retarders inhibit the hydration process by generating insoluble calcium salts with free Ca2+ and adsorbing them on the mineral surface, thereby achieving a retarding effect. In addition, inorganic substances such as phosphates ionize in water to form a double electric layer, slowing down the hydration reaction and thus delaying the hardening of the castable refractory.

　　Accelerators, such as Ca(OH)2, sodium fluoride, and Li(OH), can react with gypsum in cement clinker to neutralize the retarding effect of gypsum on cement, thereby accelerating cement hydration.

　　In addition, the fine powder added to the castable refractory, such as silica fume, has high activity and will form negatively charged colloids when mixed with water, which can adsorb Al3+ and Ca2+ ions released during hydrolysis. When the colloids are uncharged, they will aggregate, thus producing bonding.

　　The hardening of castable refractory is a relatively complex process with many influencing factors. It is necessary to adjust according to different site environments to achieve a reasonable hardening speed and meet the requirements of construction performance and strength.