The effect of fluorapatite in limestones on the mineral compositions of Portland cement clinkers

In this paper, phosphorus-bearing limestone (PL), an industrial raw material from Angola, was used to prepare portland cement clinker. The influence of fluorapatite in PL on the mineral formation and composition of cement clinker was investigated. Raw meals of cements with 0–5.0% P₂O₅ were prepared by mixing powdered phosphorus-bearing limestones, normal limestones, clays and iron ore tailings, which all were industrial materials. The mineral composition, morphology and phosphorous distribution in cement clinker have been studied by X-ray diffraction (XRD), laser scanning confocal microscopy (LSCM) and also scanning electron microscopy (SEM). Besides, the burnability of raw materials was also investigated by differential scanning calorimetry (DSC). The results showed that both C₃S and C₃A will gradually decrease with the increase of Ca₅(PO₄)₃F, while C₂S and free CaO will correspondingly increase. During calcination process of cement clinker, natural fluorapatite will decompose accompany with the existence of C₂S at 1100 °C. As a result, fluorine will react with CaO and Al₂O₃ to form C₁₁A₇·CaF_2, preventing the formation of C₃A. And the phosphorous as the C₃P will go into the C₂S and form the stable α'-C₂S-xC₃P, inhibiting the formation of C₃S. Overall, the amount of phosphorus in the clinker presented as follows: C₂S(1.92%)＞C₃S(1.02%)＞Interstitial phase(0.52%).