Engineering Properties of Xypex-Treated Cement Paste

Bassam S. TERRO

        In order to mimic possible "realcrete", the hcp substrates chosen were highly permeable and unsaturated. The former property was achieved by a combination of a low C₃S OPC and a high w/c ratio of 0.65; the latter by preconditioning the substrates at relative humidities of 100%, 88%, 72%, 56% and in air. Subsequently, the substrates were treated with Xypex and conditioned at approximately 100% RH for periods up to 26 weeks prior to assessment.

        The assessment of Xypex was performed at different depths within the substrate using propan‑2‑ol counter‑diffusion, chloride diffusion, and "water" permeability. Pore structure was characterized using propan‑2‑ol adsorption, whilst the mineralogy of hcp was monitored using Thermogravimetry. In order to isolate the effect of the active ingredient of Xypex a control programme was performed. This included both untreated substrates and those treated with a special Xypexpreparation in which the active ingredient was omitted. An additional set of control substrates were cured continuously in lime water to provide a datum for the potential development of the substrate given ideal curing conditions.

        The crystal growth of Xypex is determined, yet the solubility of the crystals requires further investigation. The crystal growth occurs rapidly in the substrate, yielding improvement in the engineering properties examined. However, the efficacy of Xypex is test‑dependent and strongly depends on the state of substrate prior to the Xypex application. The benefits experienced by the engineering properties follow improvements in the pore structure, a result of a chemical reaction between the active ingredient of Xypex and the cement minerals, together with the consequences of the ingress of water to partially dehydrated substrates, The chemical reaction consumes calcium hydroxide to produce carbonate‑like andCSH‑like phases.

      It was found that only propan‑2‑ol counter‑diffusion is well‑correlated to the pore structure for substrates preconditioned at 100% and 88% RH. It would appear that preconditioning at 72% and 56% RH modifies this correlation due to the effect of other pore parameters such as shape, tortuosity and isotropy. Comparison with published data suggests that the application of Xypex is equivalent to a reduction in w/c from 0.8 to approximately 0.5.