Calcium Hydroxide Treatment of Construction Aggregates for Improved Cementation Properties

Laboratory studies have shown beneficial effects from calcium hydroxide (Ca(OH)₂) treatment of crushed aggregate materials commonly utilized in construction. Two contrasting cementation systems were investigated which demonstrate how Ca(OH)₂ treatment of aggregates may be effectively used to improve the cementing properties of construction materials.

Highway base course materials composed of crushed limestone aggregates have been observed to increase in strength with time after compaction and is thought to be due to natural carbonate cementation processes. Addition of Ca(OH)₂ to crushed limestone base course materials before compaction caused significant increases in laboratory bearing strength values after continuous soaking in water. It is believed this is due to carbonation of the Ca²⁺ ions, producing additional calcium carbonate cementation of the particles. The amount of strength developed is a function of calcite/quartz ratios for the compacted materials. Scanning electron microscope (SEM) investigations suggest this is due to the better bonding properties of carbonate cement-carbonate particle systems versus carbonate cement-silicate particle systems.

In portland cement concrete, carbonate aggregates appear to react chemically with the cement hydration products, forming a good interfacial bond. In contrast, cracks are often more prominent at the cement paste-silicate aggregate interface, suggesting this is a weak link in both strength and durability of concrete. Treatment of crushed granite aggregates with a Ca(OH)₂ solution, followed by the subsequent drying of the aggregates, resulted in increased compressive strength for 28 day moist cured portland cement concrete mixes. SEM studies of concrete prepared with untreated and treated granite aggregates suggest this is due to improved interfacial bonding between the cement paste and treated granite aggregates. The proposed mechanism for improved bonding is disruption of the hydrated silicate surface by drying, allowing for stronger adsorption of Ca²⁺ ions onto negative charge sites. This appears to provide a bridging mechanism for bonding cement hydration products to the silicate aggregate surface.