Effect Of Additives On The Setting Time And Early Bonding Strength Of The Spinel-containing Aluminous Cements

With the rapid advance of monolithics, refractory castables are widely used in high-temperature furnaces. The castable used in ladle is an important kind of refractory. The aluminous cements are usually used for the binder. For castable used in ladle, magnesium aluminate (MA) spinel is usually added into the castable in order to improve its thermal shock resistance and corrosion resistance. The spinel-containing aluminous cements are synthesized by introducing proper amount of MA into the aluminous cement, which can increase the refractoriness of aluminous cements and inexpensive. However, the limitation use of the spinel-containing aluminous cements is the long setting time and lower early bonding strength. Consequently, the aim of the present work is to improve the performances of the new spinel-containing aluminous cements.In this work, new spinel-containing aluminous cements were synthesized from light-burned dolomite and industrial Al2O3 by sintering process. The phase constituents of new aluminous cement were inspected by X-ray diffraction (XRD), and the microstructure of hydrated cement was analyzed by scanning electron microscopy (SEM). The effects of some additives (CaCl2, Na2SO4, Na2Si03, NaF, Li2CO3, FDN) on setting time and early bonding strength of the refractory castables were studied specifically.The results showed that the initial setting time of spinel-containing aluminous cement without additive is 46min, the final setting time is 77min. The additives can reduce the setting time of new aluminous cements in varying degrees. The effects are in order of Li2CO3>CaCl2>NaF>Na2SO4>Na2SiO3. The initial setting time are shortened about:31%,22%,17%,2%,2% respectively, and the final setting time are shortened about:24%,16%,10%,8%,2%. We chose CaCl2, Na2SO4, Li2CO3 for the early strength modifier of new aluminous cements. They can increase the early bonding strength of the cements prepared. The effects are in order of Li2CO3>CaCl2>Na2SO4. The bending strengths are increased about:32%,25%,13% respectively, and the compressive strengths are increased about:38%,11%,9%. The setting time of new cement is almost as same as the secar71 from Larfarge when Li2CO3 was added to new cement with 0.01 percent, and the early bonding strength of the refractory castables can be increased obviously. This is because lithium cation has better hydration ability. When the hydrated products results from liquid, the energy will be needed for crystallization process. This energy is barrier to the phase appearance. The accelerating effect of crystallization process in lithium salts added sample is due to a removal of this barrier, caused by an initially fast precipitation of lithium hydrometaaluminate. Accordingly, the hydration speed of samples is increased, and the reduction of liquidity leads to coagulation. Combined with FDN, the effect of additives is more effective. The bending strength is increased about 59%; The compressive strength is increased 65%.