Combined Use of Sodium Bentonite and Recycled Aggregates in Concrete: Microstructural Characterization and Mechanical Performance

The reuse of construction and demolition waste (CDW) and supplementary aluminosilicate materials can reduce the environmental impact of concrete. This study evaluated two formulations produced with fine and coarse recycled CDW aggregates: a control with 0 wt% sodium bentonite and an experimental mixture with 5 wt% sodium bentonite as a partial Portland cement replacement (ACI 211.1-91, target 19.61 MPa at 28 days). Two independent control batches achieved 19.833 MPa (202.24 kg/cm2) and 19.788 MPa (201.78 kg/cm2) (mean 19.810 MPa, 101.01% of target). The 5 wt% bentonite mixture achieved a mean of 15.353 MPa (156.55 kg/cm2, SD 0.060 MPa, CV 0.391%, 95% CI, 15.290–15.416 MPa, n = 6), equivalent to 78.29% of the target strength and 22.50% lower than the control mean. The six specimens containing 5 wt% bentonite were technical replicates obtained from a single batch, whereas control values correspond to two independent batches (one specimen each); therefore, confidence intervals are presented descriptively. Characterization of the 5 wt% formulation at 28 days was performed by XDR, SEM, and EDS. XRD identified calcite, ettringite, quartz, and signals compatible with calcium silicate hydrate (C-S-H) products and tobermorite. SEM reveals a heterogeneous matrix with acicular morphologies compatible with C-S-H and densified regions. EDS showed O 54.8 wt%, Ca 27.8 wt%, Si 10.2 wt%, and Al 2.8 wt% with heterogeneous distribution of Ca, Si, and Al without marked segregation at the observation scale. Characterization was limited to the 5 wt% formulation at 28 days; therefore, observed phases and morphologies cannot be attributed exclusively to bentonite.