Effects of belite incorporation on mechanical and microstructural properties of OPC mortars cured under carbonation condition

This study investigates the effects of various dosages of belite replacing ordinary Portland cement (OPC) on properties of OPC binders using carbonation curing. Mechanical strength was examined, pore-related test and microstructural/mineralogical analyses including water absorption, scanning electron microscopy (SEM), thermogravimetric analysis and 29Si nuclear magnetic resonance (29Si-NMR) were carried out. Extra attention was paid on the changes in the thickness and porosity of interfacial transition zone (ITZ) of mortars. Compared to normal curing, carbonation curing increased the compressive strength of all mortar mixes, especially given a longer period of curing and/or with belite addition. The greatest increase (73.18%) in the compressive strength was seen in the specimen with 35% of belite after 28 days of carbonation curing compared its peer under normal curing condition. Besides, specimens with 5% and 15% belite obtained higher flexural strength than that of the control after 7 and 28 days of carbonation curing. Carbonation curing led to about 92.75% increment in the flexural strength of the specimen with 15% belite. Regardless of curing conditions, in most cases, belite modified mortars had higher fractural toughness, probably due to the formation of more C–S–H gels instead of Portlandite. Compressive and flexural strength are more sensitive towards the thickness of ITZ than its porosity and carbonation curing strengthened this sensitivity. Carbonation curing densified the bulk OPC binders, especially after a long period of curing incorporated with belite. Microstructural analyses indicate that although in general belite hinders the hydration process of OPC, a suitable amount (5 wt%) improved the hydration degree of OPC when cured in carbonation environment. The polymerization degree of C–S–H gels could be enhanced when belite is added irrespective of curing conditions.