Silica sand can be hardened through several methods, depending on the desired application and final properties. The most common method is CO2-hardening, where carbon dioxide reacts with sodium hydroxide (NaOH) present in the sand, forming sodium carbonate (Na2CO3) and increasing the sodium silicate modulus. This subsequently loses water and hardens.
Methods for Hardening Silica Sand
-
CO2 Hardening: This process involves introducing carbon dioxide gas into the silica sand, which reacts with existing sodium hydroxide to solidify the sand. This is a widely used industrial method.
-
Polymeric Sand: This commercially available product is not just silica sand but a blend of 85-90% quartz (crystalline silica) and polymeric binders. The binding agents cause the sand to harden upon wetting. This method is commonly used for paving applications, as detailed on this website.
-
Sodium Silicate Binding: Sodium silicate acts as a binder for silica sand. After application, the mixture hardens as it dries and loses water, a process that can be accelerated by heat. However, curing times vary depending on environmental conditions and thickness of the application. The evenness of curing can be affected as seen in this forum discussion.
-
Drying: While not hardening the sand in itself, drying is crucial for many hardening processes. Removing excess moisture is essential for successful binding with sodium silicate or polymeric agents. This is highlighted in various sources discussing the importance of drying silica gel (this Reddit post) and sand for resin applications (this article).
Considerations for Hardening Silica Sand
The choice of method depends on the specific application. For example, CO2 hardening is used in industrial settings, while polymeric sand is suited for paving. The properties of the final hardened product, such as strength and durability, will also vary depending on the chosen method and the specifics of the process. The use of cement is mentioned for paver sand hardening; however, simply using dry brushing to set the sand in the joints is also suggested (this Quora answer). Note that the term "silica sand" often gets used interchangeably with other types of sand, which may not contain enough silica to react in the same way (this Moparts forum post).
