Air lime mortar is an important material for restoring ancient buildings, but its application is limited because of its low compressive strength, high shrinkage, and high porosity. In this work, improvement of the compressive strength of lime mortar by adding carboxymethyl cellulose (CMC) solution has been investigated. We characterized fine particles of Ca(OH)(2) and lime mortars by scanning electron microscopy, infrared spectroscopy, the adsorption capacity, and the zeta potential. The experimental results show that the addition of CMC improves the compressive strength of lime mortar. Chemisorption of CMC to Ca(OH)(2) particles leads to CMC bridging Ca(OH)(2) particles, which is the key to form a more compact structure according to the porosity of the lime mortar. It was found that CMC acts as a skeleton in lime mortar because of the bridging effect of the molecules and the extended conformation of the molecules at high pH, which contributes to improvement of the compressive strength of lime mortar.