Carbonation of concrete - Effect of mineral additions and influence on transport properties

Abstract

Fly ash and Ground Granulated Blast-furnace Slag (GGBS), two mineral additions used to partly replace ordinary Portland cement, are residues from coal combustion and steel manufacturing, respectively. Desirable properties for concrete containing these two additions are, for example, lower carbon footprint, lower heat development and higher resistance to acid, sulfate and chloride attacks, but they can also cause unwanted properties, such as increased carbonation and reduced strength. Hence, durability is in some aspects improved, while in other reduced. This study investigated how mineral addition influences carbonation, and how carbonation affects chloride migration and transport properties in mortar. Accelerated carbonation with an elevated CO2 level of 2% and Rapid Chloride Migration (RCM) were used. These, together with compressive strength tests (SS-EN 196-1) and capillary absorption tests (NT Build 368), were carried out as means of a comparative study of mortar mixtures with different levels of mineral addition and w/b ratios. Carbonation rate was increased and compressive strength was reduced, by increased amount of mineral addition, as expected. The results also showed an interdependence between different deteriorating processes. Carbonation reduced the porosity, rate of reaching saturation and connectivity of the pore structure, and it also reduced the resistance to chloride migration. Nearly all the effects caused by carbonation were magnified by the amount of mineral addition, except for some reverse effects on high levels of GGBS. The reverse effects can partly be explained by low degree of hydration of GGBS, caused by short curing time in the test method.