Self-compacting mortars produced with fine fraction of calcined waste foundry sand (WFS) as alternative filler: Fresh-state, hydration and hardened-state properties

Abstract

This work investigated the use of the <150 μm fraction of a calcined waste foundry sand (WFS) as filler in self-compacting mortars. WFS was calcined at 600–900 °C, referred to as calcined foundry sand (CFS). Self-compacting mortars were produced with cement replacement levels of 0–30 vol% with CFS and a limestone filler. WFS and CFS were characterized by X-ray diffraction (XRD), scanning electron microscopy with energy-dispersive spectrometry (SEM-EDS) and thermogravimetric analysis (TGA). The fresh-state properties (mini slump flow and mini V-funnel), cement hydration (isothermal calorimetry and XRD), compressive strength and microstructure (SEM-EDS) of the mortars were evaluated. EDS showed that WFS calcination reduced its carbon content and XRD indicated the loss of the montmorillonite crystalline structure present in the clay material. CFS-containing mortars required superplasticizer contents from 2% to 10% higher than the reference to reach the target flow range (240–260 mm), while increased the mini V-funnel from 4.9 (reference) to 10.3 s (30% CFS). Nonetheless, all the mortars met EFNARC’s requirements. Calorimetry and XRD indicated that CFS enhanced the early age hydration of cement, increasing the main heat flow peak from 6.85 to 7.81 mW/g of cement and 72-h cumulative heat from 407 to 462 J/g of cement compared to limestone, with higher calcium hydroxide and lower alite contents at 3 days. CFS-containing mortars showed strength values about 14% higher than those containing limestone, for the same replacement level and age. Cement replacement with 10% CFS resulted in strengths equivalent to those of the reference mix at all ages (about 39, 45 and 54 MPa, respectively at 3, 7 and 28 days). SEM-EDS showed no significant differences between the microstructure of the mortars containing CFS and limestone. Overall, 10% CFS incorporation reduced 76 kg of cement and incorporated 58 kg of WFS per m³ of mortar, with equivalent performance.