Why was the concrete made by the Romans more than 2,000 years ago stronger than the one used today?

 The strength and durability of ancient Roman concrete, used more than 2,000 years ago, have been a subject of scientific inquiry and fascination. Several factors contribute to the enduring strength of Roman concrete, especially in marine environments:

1. **Use of Pozzolanic Materials:** Roman concrete, often referred to as Roman maritime concrete, incorporated volcanic ash known as pozzolana. The use of pozzolana, which contains reactive silica, contributed to the formation of durable and stable compounds in the concrete.

2. **Chemical Reactions:** The pozzolanic reaction between the lime in the Roman concrete and the silica-rich volcanic ash created a durable binding material called calcium-aluminum-silicate-hydrate (C-A-S-H). This compound is similar to the modern hydrated calcium silicate gel (C-S-H) but with the addition of aluminum, which enhances the strength and durability.

3. **Seawater Exposure:** Roman maritime concrete was often used in marine structures like harbors and breakwaters. The exposure to seawater led to the growth of crystals within the concrete, including aluminous tobermorite and phillipsite, which improved the concrete's strength over time.

4. **Mineral Formation:** The interaction of seawater with the Roman concrete led to the formation of minerals that filled voids and cracks in the material, contributing to increased density and durability.

5. **Low Calcium-to-Silica Ratio:** Roman concrete had a lower calcium-to-silica ratio compared to modern concrete. The incorporation of more silica-rich materials, such as volcanic ash, contributed to the formation of additional binding compounds, enhancing the strength.

6. **Natural Pozzolanic Materials:** The Romans had access to specific natural pozzolanic materials, such as volcanic ash from Pozzuoli near Naples, that played a crucial role in the exceptional properties of their concrete.

While Roman concrete exhibited remarkable durability, it's essential to note that modern concrete has advantages in terms of standardization, ease of production, and adaptability to various construction needs. Researchers are studying the composition and techniques of ancient Roman concrete to gain insights into sustainable and durable construction materials for the present and future. The unique properties of Roman concrete have inspired contemporary efforts to develop more environmentally friendly and resilient building materials.