Influence of phosphorus from phosphogypsum on the initial hydration of Portland cement in the presence of superplasticizers

Phosphogypsum is widely used for the total or partial substitution of natural gypsum in the production of Portland cement. However, contaminants from the phosphogypsum, such as chemicals that contain phosphorus, may affect the performance of the binder, especially when it is applied to concrete that uses chemical admixture. The goal of this study is to evaluate the impact of successive increments of Na2HPO4 on the hydration of cements that are produced with low or high phosphorus concentrations in the presence of ether-polycarboxylate-based (PCE) and sulfonated-naphthalene-formaldehyde-based (SNF) superplasticizers. Two binders with the same clinker size were produced in the laboratory with natural (phosphorus-free) gypsum and phosphogypsum (contaminated). The isothermal calorimetry and thermogravimetry (TG/DTG) techniques were used to evaluate the heat flow behavior and the formation of portlandite at the end of the induction period (Pi), at the maximum heat flow (Pmax), and at the final 72-h stage (P72). The results indicate that the greatest impact on hydration occurs at phosphorus concentrations between 0.83% and 1.64% in the form of P2O5 in the phosphogypsum and especially at a concentration of 1.13%. Nonetheless, in all cases, the formation of portlandite after 72 h is very similar.