Influence of pulp industry dregs on the physical and mechanical properties of mortar

This study examined the constituents and evaluated the effects of incorporating a pulp industry residue (dregs) on the physical and mechanical properties of mortar. The properties of the mortar constituents were evaluated using thermogravimetry-differential scanning calorimetry (TG-DSC), energy-dispersive spectroscopy (EDS), X-ray Fluorescence spectroscopy (XRF), X-ray diffraction (XRD), and tests specified in standards NBR 16605 (2017), NBR 11579 (2013), and NM 18 (2012). The hardened mortar was evaluated through mechanical testing (NBR 7215 2019), water absorption testing (NBR 9778 2005), and mercury intrusion porosimetry. XRD data from dregs treated at different temperatures were analyzed using the Rietveld refinement method. The specific gravities of the cement, dregs, and sand were 3.13, 2.58 and 2.62 g/cm³, respectively. The main chemical elements detected in the dregs (> 1 atom%) were Ca, Mg, Mn, Si, Na, S, and Al. Thermal analysis revealed two endothermic events: the evaporation of sulfur-containing compounds and the decomposition of calcium magnesium carbonate. Lattice parameters were obtained for the Ca_0.87Mg_0.13(CO₃)₂ phase, observed in the dregs treated at 100 and 500 °C, and for the CaO and MgO phases, observed after treatment at 750 °C. To evaluate compressive strength, water absorption, and porosity, mortars were prepared under three distinct conditions: one without dregs; four with dregs added to the mixture, keeping the cement and sand contents constant; and four with dregs as a partial replacement for Portland cement, keeping the sand content constant. Compressive strength decreases with increasing dregs content, with a more pronounced reduction when Portland cement is replaced by dregs than when dregs are added. Incorporating 40 wt% dregs reduced compressive strength by 73% under the Substitution condition and 52% under the Addition condition, relative to the reference mortar without dregs. This reduction in compressive strength is associated with increased water absorption and porosity as the dregs content increases.