Influence of lead nitrate on the early hydration of high alumina cement that contains spinel

The effect of 0.5 wt.% lead nitrate on the early hydration of high alumina cement (HAC) that contains spinel was examined by isothermal calorimetry, XRD, TG-DSC and SEM. Lead nitrate affects the hydration and both the initial setting time and final setting time were extended. Lead nitrate also reduced and delayed the maximum rate peak of hydration of HAC. The pH value of HAC paste was significantly lower in the early stages when compared with the addition of lead nitrate. It was found that an NO3-hydrate phase (Ca4Al2(NO3)(2)(OH)(12) center dot 4H(2)O, NO3-AFm) was formed at 6 h. Although the characteristic value of the hydrated calcium aluminate (CAH(10)) was decreased, the characteristic values of the hydrated dicalcium aluminate (C(2)AH(8)) and tricalcium aluminate (C(3)AH(6)) were increased significantly. That means that lead nitrate promotes the early generation of NO3-AFm in HAC, which will delay early hydration. However, after 18 h lead nitrate promotes C(2)AH(8) or transformation of the metastable needle-shaped or hexagonal CAH(10) into stable cubic C(3)AH(6).