Transformation of Hydraulic Binders Over Time: Analytical Insights

Thermogravimetric analysis, scanning electron microscopy and mechanical strength measurements were used to describe the changes in cementitious mortars caused by the action of liquid water and carbon dioxide over time. Model lime-cement mixtures were exposed to three different regimes of artificial ageing – (1) exposure to CO2 and humidity, (2) exposure to CO2 and (3) exposure to humidity. Although the effect of different ageing regimes showed similar trends (carbonation and subsequent decalcification of clinker residues), the combination of CO2 and humidity had the strongest impact on the samples. Total carbonation and the separation of the binder to newly formed CaCO3 crystals (including metastable vaterite or aragonite) and amorphous silica-rich structures were observed. Flexural strength decreased slightly for all samples. The most significant decrease was determined for mortar affected by both increased humidity and CO2. Apart from artificially aged model samples, the changes in two authentic 120-year-old mortars from the inner parts of Most Legií bridge, Prague, were studied. Their binder was identified as early Portland cement. The mortars showed a different degree of carbonation and the formation of metastable CaCO3 polymorphs (vaterite and aragonite) or imperfectly crystallised calcite. This can be explained by the low CO2 access within the inner structure of the bridge’s construction.