Mass Concrete and Heat of Hydration: Why PPC and PSC Help
Big concrete pours generate a lot of heat. Without control, the temperature difference between core and surface cracks the concrete from within.
Cement releases heat as it hydrates. In thick sections such as rafts, large footings, piers and dams, the core gets much hotter than the surface, and the difference causes thermal cracking. Control it by using low-heat cements like PPC or PSC, minimising cement content through mix design, cooling materials, pouring in lifts or at night, and insulating the surface to reduce the core-to-surface temperature difference (commonly limited to around 20 °C, as specified).
What is heat of hydration?
The chemical reactions between cement and water are exothermic — they release heat. Thin elements lose that heat quickly to the air. In thick sections, the heat is trapped in the core, which can rise far above air temperature while the surface cools.
Why it causes cracks
As the hot core and cooler surface try to expand and contract by different amounts, tensile stresses develop. When they exceed the young concrete's tensile strength, thermal cracks form. Later, as the core cools, restraint from the ground or previous pours can cause more cracking. These cracks can let water reach the steel.
Mass concrete is generally any section thick enough for this to matter — often rafts, pile caps and footings thicker than about 1 m, bridge piers, retaining walls and dams. Your engineer decides when thermal control is needed.
How cement choice helps
| Cement | Relative heat of hydration |
|---|---|
| OPC 53 | Highest, fastest |
| OPC 43 | High |
| PPC | Lower |
| PSC (high slag) | Lower |
| Low-heat Portland cement (IS 12600) | Lowest, specialised |
PPC and PSC replace part of the clinker with fly ash or slag, which react more slowly and release less heat early.
Other control measures
- Lower cement content through design mix and admixtures.
- Larger, well-graded aggregates to reduce paste volume.
- Cool materials — shaded aggregates, chilled water or ice where specified.
- Pour at night or in cool hours.
- Pour in lifts or bays with planned construction joints.
- Insulate the surface with blankets to keep it warm and reduce the core-surface temperature difference.
- Monitor temperatures with embedded sensors on large pours; specifications often limit the core-to-surface difference to about 20 °C.
- Extended curing — keep moist and insulated for the specified period.
Key takeaways
- Thick pours trap heat; core-surface differences cause cracks.
- PPC, PSC and low-heat cements reduce peak temperatures.
- Combine cement choice with mix design, cooling, lifts and insulation.
Frequently asked questions
What is heat of hydration in cement?
It is the heat released when cement reacts with water. It is significant in thick concrete sections where it cannot escape quickly.
Which cement has the lowest heat of hydration?
Low-heat Portland cement (IS 12600) is designed for this. Among common cements, PSC and PPC produce less heat than OPC.
How is thermal cracking in rafts prevented?
By using low-heat cements, reducing cement content, cooling materials, pouring in lifts or at night, insulating the surface and monitoring temperatures.
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This guide is general information for planning and buying. Quantities, grades and methods for your structure must follow your approved drawings, your engineer’s specification and the applicable Indian Standards.
