Plastic and cement may seem like two materials that aren't a likely combination, but it's often useful to be able to attach the two for the sake of fixing small plastic appliances and decorative fixtures to floors and walls. To securely join the porous, hard concrete with the non-porous, often flexible plastic, use an epoxy glue that's ideally suited for work with both.
Fit the plastic to the cement in preparation for gluing. Press the plastic against the cement and find a good location to seat it; this should be a place where part of the plastic item can press relatively flat against the concrete and cover enough surface area to support its weight. Use the pencil to trace around the plastic object on the concrete where the two touch, making an outline.
Scrub the cement with soap, water and scrub brush. Remove any loose grains of concrete or other sediment that may have built up. These bits of debris can interfere with the adhesion of the epoxy glue, so remove as much of this buildup as you can. Towel dry, then let the concrete fully air dry before you continue.
Mix the epoxy in a disposable container using a disposable stir stick (you will not be able to clean the glue from either of these when finished). Add equal parts of both liquid solutions and stir thoroughly, until the mixture becomes cloudy; failure to mix completely will result in an insecure bond and a mess.
Apply the epoxy to the cement. Spread the glue generously inside the pencil outline you drew in Step 1. Use as much glue as you can keep contained in this area; use more if you're OK with overlapping it. Epoxy is a thick and strongly cohesive glue, meaning that you can build up the glue around the base of the plastic to help hold it in place. Work quickly so that you can attach the pieces before the epoxy sets up.
Fix the plastic piece in place in the epoxy. Hold it firmly against the concrete until the epoxy sets up. Do not disturb the joint until the epoxy has fully cured according the manufacturer's recommendation.