Bio-waste, which includes food waste and yard waste, represents 32.1% (3,477 tons/day) of the municipal solid waste (MSW) deposited in Hong Kong. Pyrolyzed from bio-waste under 500–900 ℃, biochar is a potential adsorbent for indoor gaseous pollutants, especially for CO2 and toluene. However, the high internal moisture has an impact on adsorption ability of biochar due to higher polarity of H2O molecular (μH2O = 1.84) than those non-polar molecules (μ = 0). Reactive magnesia cement (RMC) can bring out moisture from ambient air to hydrate, which will lower down the moisture in the air. This work compares the CO2 adsorption behavior of pristine biochar and embedded biochar within CRMC-based matrix under ambient environments. The adsorption tests are conducted under ambient CO2 level (1000 ppm) within our customized adsorption experimental set up, which enables modifying the inlet gas with varied temperature (20–50℃) and relative humidity (45–95%R.H.). Specifically, the compressive strength and dry density are detected after 90-d pre-curing. The CO2 adsorption behaviors of embedded biochar are investigated via CO2 breakthrough experiment and the adsorption ability and rate are quantitatively analyzed for the first time. The effect of relative humidity on CO2 adsorption of both embedded biochar and pristine biochar are comparatively investigated. The results indicate that the adsorption capacity of embedded biochar is well preserved (almost 100% or even higher), even though adsorption rate is rather slow (over 60% decreased) compared to that of pristine biochar. The relative humidities shows less significant effects on CO2 adsorption ability of embedded biochar than that of pristine biochar due to the water shielding effect of unhydrated RMC.