You can neutralize concrete by soaking it in water for about three days. Soaking concrete or cement in water leaches lime through the pores. The benefit of doing this is that some plant types may not thrive, or even could eventually die, due to high alkalinity. You see, lime has a high pH, making it very alkaline. However, plants like most succulents, often thrive in soil with a higher pH and so lime leaching a concrete planter or pot isn't necessary.