February 4, 2025 · Education
Concrete vs. Cement — Not the Same
"Concrete" and "cement" are not interchangeable. Cement is just one ingredient — like flour in bread. Portland cement is the binder that undergoes hydration when mixed with water. Concrete is the complete mixture: cement (10-15%), water (15-20%), and aggregates (65-75%), plus admixtures.
The Hydration Process
When cement contacts water, it undergoes an exothermic chemical reaction. Cement particles form calcium silicate hydrate crystals that interlock and bind aggregate into a solid mass. Concrete cures better when kept moist. It reaches ~70% of design strength in 7 days and 99% at 28 days.
Why Mix Design Matters
The ratio of ingredients determines workability, strength, durability, and finish quality. Residential foundations typically use 3,000-4,000 PSI concrete. Higher cement content means higher strength but also more heat and shrinkage — a careful balance.
The Role of Aggregates
Sand and gravel make up the bulk of concrete. Central Texas aggregates include limestone and river gravel, each offering different properties. Gradation is carefully controlled to minimize voids and maximize density.
Admixtures: The Secret Ingredients
Water reducers allow stronger concrete without extra cement. Retarders slow setting time for hot weather — essential in Texas. Air-entraining agents improve freeze-thaw resistance. Accelerators speed curing in cold weather.