High-temperature, high-pressure torsion experiments have been conducted on Solnhofen limestone and synthetic calcite--quartz aggregates to investigate the evolution of mechanical strength and microstructure to large strains. Hot isostatic pressing of powders containing 1--30 wt% quartz particles produced synthetic two-phase aggregates containing trace amounts of wollastonite. The experiments were performed at constant twist rate, 300 MPa confining pressure, and temperatures ranging from 900 to 1300 K in the stability fields of calcite plus quartz and wollastonite plus carbon dioxide. The mechanical data from torsion tests of most samples show a pronounced peak stress at shear strains 5. A distinct shape and lattice preferred orientation developed in Solnhofen limestone and in the calcite matrix of the synthetic aggregates. However, at large shear strains, flattened porphyroclasts were almost entirely consumed by recrystallization. Elongation parallel to the specimen axis at temperature below 1100 K is possibly due to rotation of minerals into a preferred crystallographic orientation. Stress exponent and activation energy for creep of the calcite--quartz aggregates increase substantially with increasing quartz content. The strength of the two-phase aggregates increases with quartz fraction up to 20 wt% by a factor of 2--5 depending on temperature and finite strain. Continuum models underestimate particle strengthening of the calcite--quartz aggregates. An alternative microphysical mechanism for the observed strengthening may be related to the reduced mobility of dislocations through diffusion of silicon into dislocation cores.