A new consolidating treatment for limestone, based on the reaction between diammonium hydrogen phosphate (DAP) and calcite in the stone to form hydroxyapatite (HAP), has recently been proposed. Thanks to the formation of HAP at grain boundaries, so that grains are more effectively bonded, HAP-treated limestone undergoes a significant improvement in mechanical properties. Considering that the HAP treatment has the advantage of not substantially altering the transport properties of treated stones, unlike many other consolidating products currently used, in this paper, the effectiveness of HAP as a consolidant for sandstones with varying carbonate content and porosity was investigated and compared with that of a TEOS-based consolidant. Two lithotypes were used: Giallo Siena (a highly porous calcareous sandstone) and Pietra Serena (a quartzitic sandstone with low porosity). After preliminary artificial weathering according to previously developed methodologies, each lithotype was treated: (i) with a 1 M aqueous solution of DAP; (ii) with a solution of TEOS oligomers in isopropyl alcohol, each applied by brushing until apparent refusal. The effects of the two consolidating treatments were assessed by comparing microstructural, physical and mechanical properties of treated and untreated stones. HAP-treated specimens experienced an increase in dynamic elastic modulus and tensile strength substantially similar to that achieved by TEOS-treated ones. However, differently from samples consolidated with TEOS, HAP-treated stones experienced very low variations in porosity and pore size distribution, leading to substantially no variation in the rate of water sorption and only a small decrease in water vapor permeability. Considering the good performance on both lithotypes, comparable to that achieved by a TEOS-based treatment, HAP is confirmed as a promising consolidating treatment, which has the advantage of being effective after just two days, not substantially altering the transport properties of treated stones and not being hazardous for human health, thanks to its aqueous solvent.