Conference Agenda

The aggregate-cement paste bond strength is one of crucial factors influencing the durability, overall mechanical performance and lifespan of concrete. This study for the first time compared the bond strength to aggregate between reactive magnesium cement (RMC) and traditional Portland cement (PC). The mechanical properties, including compression strength and bond strength, of RMC highly rely on the carbonation curing while carbonation curing slightly reduces the mechanical properties of PC. Even though under carbonation curing, the bond strength of RMC is still inferior to the PC, which is due to limited carbonation depth, as evidenced by XRD and TGA characterizations. The contribution of carbonation to the bond strength was calculated, showing that the RMC holds great potential to achieve greater bond strength compared with PC if the carbonation degree can be further increased.

The recycled concrete is increasingly used in buildings. Lowered properties are claimed from the replacement of natural aggregates with recycled concrete aggregates. However, this reduc-tion depends on the quality of the source concrete and the dosage. This latter should be kept below 50 %, in order to achieve the main mechanical properties, such as compressive strength and modulus of elasticity. In this manner, the cementitious material may be adequate to be used buildings. In the case the old source concrete exhibits a high quality, e. g. compressive strength above 60 Mpa, du-rability parameters can also be attained and the recycled concrete can be used for infrastructures. On the other hand, the behaviour to fire and the cooling process of recycled concrete still needs a detail clarification. The water chemically bond within the old mortar may largely influence the per-formance during fire. Concretes were prepared with a different cement dosage and water to cement ratio. They were crushed and the recycled concrete aggregates were aged up to 2 years. Recycled concrete with a natural aggregate replacement of 25% were prepared. The recycled concretes were exposed to 500 oC air or water quenched. The air coling promoted the formation of fine cracks with the microstructure within the old mortar or along the old mortar-new cementitious matrix interface. The rapid water quenching promoted a general widening of the crack to macrocracking, a partial detachment of the cementitious matrix along both the RCA and NA-cement-based matrix interface. Not rarely grain pull outs and a crack branching was seen in the water cooled specimens. For this latter rapid cooling procedure, the aggregates, particularly the Si-bearing aggregates showed an in-creased expansion and cracking a s compared to the air cooling procedure. A denser microstructure with a lower water to cement ratio and a higher cement dosage created a more susceptible micro-structure with respect to the internal thermal stresses, that causes a widening of the cracking, rather than an increase in the microcracking frequency.

Incorporation of carbon-negative biochar into cementitious materials has been recognized as one of the most promising ways to achieve global carbon neutrality. However, negative effect on mechanical strength occurs when adding high amount (>5% by weight) of biochar to replace cement due to the weak binding of interfacial transition zone between biochar and cementitious matrix. Therefore, it is imperative to develop an approach for enhancing the interfacial transition zone. In this study, we evaluated the carbon dioxide adsorption performance of amine-modified biochar and investigated the effect of biochar (5% replacement of cement by weight) saturated with CO₂ as internal carbonation curing supplier on the mechanical property of cement paste. Diethanolamine (DEA) was employed to modify biochar to enhance the CO₂ adsorption capacity and subsequent constraint on CO₂. Higher CO₂ adsorption (1.84 mmol/g) was achieved in modified biochar compared to pristine biochar (0.33 mmol/g) at 30 ℃, and more CO₂ (80.94% versus 4.33%) was retained in pores of modified biochar after being exposure to the air atmosphere. The group incorporating modified biochar saturated with CO₂ exhibited similar compressive strength even compared to blank group without adding biochar (43.92 MPa versus 41.76 MPa). Improved compressive strength (41.77%, 28 days) of specimens with adding CO₂-saturated modified biochar was observed compared to pristine biochar saturated with CO₂, which can be attributed to the hydration acceleration resulted by filler and nucleation effect. This approach can effectively reduce the negative effect of adding high amount of biochar on the mechanical property of cement paste, further helping achieve global carbon neutrality.