Compressive strength evolution of concrete incorporating hyperalkaline cement waste-derived portlandite

This study evaluated the strength development in hyperalkalized concrete mixtures incorporating cement waste-derived Portlandite (CDP), synthesized from two variants of cement kiln dust (CKD). The source CKDs (coded CKD1 and CKD2) and synthesized CDPs (coded CDP1 and CDP2) were characterized physically and mineralogically, after which the CDPs were used as cement replacement, from 0 to 60 %, in the mortar phase of a base concrete mixture at w/b of 0.5 and 0.425. After in-mold curing, the mortar mixtures were subjected to 12 h of accelerated carbonation curing (ACC), and then moist–cured until 28 days. The strength development and alkalinity of mortar mixtures were investigated using the strength activity factor (SAF) and pH analysis, respectively. The incorporation of more alkaline and fine-grained CKD1 (D50 = 14.74 µm) was found to increase the water demand and setting time of CKD-cement paste blends, compared with the coarse-grained CKD2 (D50 = 27.51 µm). Additionally, the use of CDP reduced water demand but increased the setting time of paste blends. Further, the incorporation of CDP increased the early-age compressive strength of mortar, but its effect diminished as the mixtures matured, with SAF reduction of over 50 % in most cases. Finally, CDP1 mixtures exhibited about 30 % higher early–age compressive strength than CDP2 counterparts but the difference evened out at a later age, with the studied mixtures exhibiting 28-day compressive strengths varying between 23 and 54 MPa.