In Japan, various efforts are being made to realize a sustainable society. In the concrete field, from the viewpoint of carbon-neutral, it is considered that the use of industrial by-products such as ground granulated blast furnace slag (GGBS) and fly ash as alternative materials for cement, which emits large amounts of CO2 during production. Research is also underway on CO2 fixation in concrete and aggregates. In addition, from the viewpoint of resource recycling, it is considered that effective reuse of dismantled concrete masses generated by the demolition of concrete structures as recycled aggregate. Recycled aggregates are classified according to quality into three categories: low, medium and high quality. The low-quality recycled aggregates have the advantage of low energy and low-cost production. However, there are problems such as reduced strength and durability on concreate accelerated compared to when ordinary aggregates are used. Our laboratory has developed that low quality recycled aggregate can be modified by carbonation with CO2 gas. When improved aggregate is used in combination with ordinary Portland cement, it is possible to improve strength and durability on concreate. In this study, tests were conducted to understand the strength properties of concrete made with low quality recycled aggregate and GGBS high content cement. As a result, it was found that the strength improvement effect of the aggregate modification varied with the GGBS addition rate. To understand the cause of the problem, we focused on the old paste/mortar portion of the recycled aggregate. Then, multiple verifications were conducted using aggregates with varying moisture content, CH content, and other conditions.