Calcium Chloride Activation of Acid-Washed Sewage Sludge Ash: Hydration Mechanism and Performance as a Supplementary Cementitious Material

Due to the fact that municipal sludge incineration generates a large amount of toxic sewage sludge ash (SSA), cement solidification is becoming an efficient harmless treatment method. However, the large amount of phosphorus in sludge ash can delay the setting time of cement pastes and lead to a decrease in its later strength. Moreover, as an important non-renewable resource, the recycling and utilization of phosphorus is also a current research hotspot. To this end, this paper proposes to remove phosphorus from incinerated sewage sludge ash (ISSA) by the acid washing process, and, in response to the problem of reduced volcanic ash activity in acid-washed sewage sludge ash (ASSA), calcium chloride (CaCl2) is added as an activator to explore the optimal process and hydration mechanism for enhancing ASSA activity. This study removed phosphorus from calcined SSA (800 °C, 2 h) via acid washing (0.3 mol/L H2SO4, L/S = 10:1, 2 h) to obtain ASSA, and used CaCl2 as an activator to improve its pozzolanic activity. Different CaCl2 dosages (1–3%) were tested for paste setting time, 3/7/28 d compressive strength, and characterized by hydration heat, XRD, SEM, etc. Experimental results indicated that the synergistic addition of 2 wt.% CaCl2 and 20 wt.% ASSA shortened the initial and final setting times by 38% and 23% to 142 min and 289 min, respectively. Meanwhile, the compressive strength at 28 days was improved by 24%, with a final strength value of 42 MPa. Microanalysis revealed abundant Friedel’s salt formation and a denser structure (fine mesopores 60%, coarse mesopores 26%), confirming CaCl2 effectively activates ASSA for cement admixture use.