One-part hybrid cements from fly ash and electric arc furnace slag activated by sodium sulphate or sodium chloride

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An innovative hybrid cement, based on type F fly ash (FA), iron and steel slag (ISS) and Portland cement (OPC), is proposed in the present paper. The precursors were activated by powder-form elements – sodium sulphate or sodium chloride – forming what is known as a ‘one part geopolymer’ cement, which was then evaluated considering both mechanical and microstructural perspectives. Results showed that, in these conditions, the reactivity of the ISS is lower than that of the FA, with or without the presence of the activators. In the ternary pastes formed by 50%ISS +25%FA + 25%OPC and 25%ISS +25%FA + 50%OPC, with no activator included, both the ISS and the FA reacted due to its inherent pozzolanic activity. The presence of the powder-form activators increased the mechanical strength of the pastes (which are known as “hybrid cements”, since an activator and OPC are simultaneously present), relatively to the no-activator pastes. Furthermore, the inclusion of Na2SO4 or NaCl increased and decreased the activation rate, respectively, relatively to the equivalent binder hydrated with water (i.e. without the presence of an activator). Regardless, in both cases, strength values above 40 MPa and 35 MPa, after 28 days curing, were obtained. In the hybrid cements, the activators first reacted with the OPC phases; with the activator anions originating products such as calcium sulphate, ettringite or Friedel's salt (depending on the activator type), which increase the density of the matrix and, therefore, the resulting mechanical strength. Furthermore, in the presence of the Na+ cation, local NaOH is generated, increasing the alkalinity of the medium and, consequently, accelerating the reaction of the ISS and FA, which further improves the mechanical strength. Behind every combination, a C-(A)-S-H gel is the main reaction product.