Concrete Additives and Mortar Types: A Detailed Look
Concrete Additives
Air-entraining additives are chemicals that create tiny air bubbles within the concrete mass. These bubbles, with diameters less than 2mm, are incorporated in small doses.
Expanders are products based on aluminum powder that produce gas. This generates a volume increase of about 2% within the concrete mass, reducing segregation and exudation.
Water-repellents include waterproofing incorporators and air-entraining agents. They are prepared on the basis of soaps and petroleum products.
Anti-freeze additives are soluble accelerators that activate cement hydration. The hardening effect occurs before the freezing effects of low temperatures by lowering the freezing temperature of the mixing water.
Hydrophobic mass additives are designed to reduce water penetration through mortar or concrete. They are usually incorporated during mixing, unlike surface waterproofing, which is applied superficially. Water can enter a hardened structure through hydrostatic pressure or capillary action.
Using mold release agents, like Sikaform, on wooden casings improves concrete impermeability. The release agent prevents the mixing water from being absorbed from the concrete surface, leaving it strong and compact.
Other Features of Concrete Additives:
- Improved workability of concrete in vertical and horizontal elements.
- Reduced water-cement ratio.
- Improved adherence to elements where concrete is placed.
- Facilitates emptying and transport.
- Improves compressive strength of concrete and mortars.
Applications:
- Reinforced concrete with high reinforcement density.
- Precast concrete, especially architectural elements.
- Concrete placed by pneumatic pumps.
- Special concrete, such as pre-stressed or post-tensioned concrete.
- Lightweight concrete requiring a high-quality finish.
Retarders: Their main function is to delay the onset of hardening of the cement paste, keeping the setting time and resistance as constant as possible at different ages.
Effect on mechanical resistance: Generally, mechanical resistance is lower in the first three days, but after that, it tends to equal or surpass that at 28 and 90 days. This effect is more noticeable with greater retardation.
Effect on workability: Retarders usually have a plasticizing effect on fresh concrete, improving workability. Retardants are additives that increase setting time; they are usually based on compounds containing sugars, sucrose, or starch.
Accelerators: Setting and hardening accelerators advance the start of setting and increase hardening speed, allowing for higher strengths at early ages. Calcium chloride is commonly used in doses of 1% to 2% by weight of cement to advance setting and increase early-age resistance.
Microglobule air-entraining additives: These chemicals form small air bubbles in concrete. Added in small doses, they create closed beads within the concrete with diameters less than 2 mm.
Key Features of Air-Entraining Additives:
- Increased workability.
- Greater consistency in the concrete.
- Easier and improved placement.
- Reduced resistance due to pores.
- Increased resistance to freeze-thaw cycles, impeding water passage.
- Reduced concrete density.
Applications: Use in works subject to low temperatures and in concrete subjected to freeze-thaw cycles.
Mortar Types
Mortar types can be classified as follows:
By Fine Aggregate Type:
- Regular: Consisting of sand flows.
- Lightweight: Formed by fine aggregates like lime-foam cement.
- Ornamental: Prepared with special fine aggregates (e.g., marble sequin).
By Sand Size:
- Fine: Used for grouting, placed under pressure to repair structures. These require high doses of cement and plastic mortars. They should not hinder the flow of mortar through nozzles. Also used in lightweight mortars with a maximum sand mesh size of No. 30, and up to 10% passing No. 200 mesh.
- Medium: Used in stucco and for grant.
- Coarse: Used in stucco and masonry.
By Water Dose:
- Solids: Those that coalesce under pressure.
- Plastics: Those that are left to mold.
- Fluid: Those that are given by injections.
- Soups: Those that tend to a liquid state.