Revealing the effect mechanism of calcareous aggregate and silica aggregate on the strength of ordinary concrete: Experiments, microscopic characterization, and molecular simulation

Aggregate is a critical component of concrete, but the mechanism by which aggregate composition affects the strength of ordinary concrete remains unclear. This study aims to investigate the influence of calcareous aggregate (CL Agg.) and siliceous aggregate (SL Agg.) on the strength of ordinary concrete through experiments, microscopic characterization, and molecular simulation. Results indicate that the ITZ (Interfacial Transition Zone) microhardness of CL Agg.-Cement is higher than SL Agg.-Cement. CL Agg. and SL Agg. were more likely to adsorb CH (portlandite) than CSH (calcium silicate hydrate), because the oxygen atoms in the aggregate would combine with the calcium in the cement, thus forming CH on the surface. CL Agg. has more oxygen atoms than SL Agg. and is more chemically active and attracts calcium atoms more easily. Adsorption energy per unit area of aggregate-CH is greater than that of aggregate-CSH., resulting in higher splitting tensile strength for CL Agg. concrete. Using CL Agg. in ordinary concrete is recommended to improve its cracking resistance and to use broken limestone and other calcareous rocks for machine sand production in ordinary concrete. The research results can provide a reference value for modifying aggregate and manufacturing machine-made sand.