Effect of Colloidal Nano-Silica on Early- and Later-Age Compressive Strength and Microstructure of Pumice Aggregate Lightweight Concrete

This study investigates the effect of a 30 wt.% solid-content colloidal nano-silica (CNS) suspension, incorporated at 0–6 wt.% of cement, on the early-age (7 and 14 days) and later-age (28 and 56 days) compressive strength and microstructure of pumice aggregate lightweight concrete (PALC). The corresponding effective solid nano-silica content ranges from 0 to 1.8 wt.% of cement. Compressive strength increased with CNS dosage up to 5 wt.%, after which a plateau behavior was observed. At 7 days, compressive strength increased from 19.93 MPa to 26.81 MPa, corresponding to an improvement of approximately 34.5%. Although the 6 wt.% mixture showed slightly higher strength at early age, this trend was not sustained at later ages. The highest compressive strength at 56 days was obtained at 5 wt.% CNS (39.68 MPa), with a slight decrease at 6 wt.% CNS. X-ray diffraction (XRD) analysis indicated a reduction in calcium hydroxide (CH) peak intensity with increasing CNS content, suggesting the occurrence of pozzolanic reactions; however, this interpretation remains qualitative. Scanning electron microscopy (SEM) observations revealed a denser and more homogeneous matrix structure at 5 wt.% CNS, corresponding to improved mechanical performance. Slump values decreased from 9.0 cm to 6.6 cm with increasing CNS dosage, indicating reduced workability, while water absorption values slightly decreased from 18.51% to 17.20%.