Composite activation of ferronickel slag-calcium carbide slag system and the properties of concrete

Abstract

Ferronickel slag (FNS) is a by-product of ferronickel alloy which causes pollution. Calcium carbide slag (CS) is an industrial waste residue from the hydrolysis of CaC2 with the main components of Ca(OH)2 and CaCO3. The hydraulicity of FNS can be improved by alkali activation to prepare cement-free clinker. Sodium carbonate (SC), sodium sulfate (SS), hemihydrate phosphogypsum (HG) and dihydrate phosphogypsum (DG) are used to study the combined effect with CS on the activation of FNS. The characteristics of FNS are studied firstly, then FNS with the particle size of D50=8.4μm is selected to be the precursor. The main conclusions are as follows: The main chemical composition of FNS is Al2O3, SiO2, CaO, MgO and Fe2O3. Cr is the primary heavy metal in ferronickel slag. The existing MgO phases in ferronickel slag are mainly spinel and forsterite, which is essential for the low grindability and negligible decrease in binding energy. CS and SC can decrease the fluidity and setting time of specimens by increasing the alkali degree of paste. However, SS increases the fluidity and setting time. Although the dissolution of FNS mainly depends on the alkalinity of the pore solution, the heat release and strength development are retarded when high Na2O-E is dosed in the systems. The gel product is believed to be C-(N)-A-S-H gel. SC-activated system showed looser structure and micro-crack in interfacial transition zone (ITZ). Although the larger critical pore diameter and fewer gel products are exhibited in SS-activated system, the compressive strength is higher than SC-activated system at 28d due to the better interfacial transition zone caused by lower autogenous shrinkage and the reasonable hydration products. The formation of CaSO4·2H2O in HG-activated system can build a structure rapidly to decrease the fluidity and setting time sharply. DG shows inert to prolong the setting time, and the decreased degree of fluidity is not apparent. The impurities in phosphogypsum can retard the occurred time of exothermal peak. HG and DG show a positive effect on compressive strength at later ages. But CS shows negative effect on strength development in both systems. High CS content results in a high Ca/Si ratio in ITZ and large crack can be observed. All the HG and DG-activated systems specimens present a stable deformation after a quick expansion increment. SS-activated and DG-activated concrete were studied. The workability and mechanical property of the SS-activated system is better than DG-activated system. The two systems present a decreased strength at 60d but recover after 90d. The carbonization degree of the SS-activated system is much lower than the DG-activated system. The specimens after sulfate curing present higher compressive strength than standard curing. The integrity of DG-activated system is still well after being calcined at 800℃, but SS-activated system shows better high-temperature resistance from 200℃ to 600℃.