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How Polycarboxylate Superplasticizer and Fly Ash Control Temperature Field and Strength of Mass Concrete

Mass concrete structures – such as dam foundations, nuclear reactor bases, and high‑rise building raft slabs – are essential in modern construction. However, their large volume creates a serious challenge: heat accumulation from cement hydration. Because concrete is a poor conductor of heat, the core temperature can rise far above the surface temperature, generating thermal stresses that lead to cracking. These cracks compromise durability, safety, and service life.

According to the Chinese standard GB50496‑2018, any concrete structure with a minimum dimension of≥1 m requires temperature control measures. Among the most effective strategies are the use of polycarboxylate superplasticizer (PCE) and fly ash (FA).

Why Do Mass Concrete Structures Crack?

The root cause is the heat of hydration. Cement minerals – especially tricalcium aluminate (C₃A) and tetracalcium aluminoferrite (C₄AF) – react rapidly with water, releasing large amounts of heat. In a large structure, the core cannot dissipate this heat as fast as the surface. The resulting temperature gradient induces tensile stresses on the surface. If these stresses exceed the early‑age tensile strength, cracks propagate.

For example, in the control mix of this study (no PCE, no fly ash), the core temperature peaked at 51.4 °C, while the surface temperature was only 26.7 °C – a difference of 24.7 °C – well above recommended limits. Such a gradient dramatically increases the risk of cracking.

How Polycarboxylate Superplasticizer and Fly Ash Work Together

Polycarboxylate Superplasticizer (PCE)

Polycarboxylate superplasticizer (PCE) is a high‑range water reducer, but it also retards the early hydration of cement. Its long side chains and carboxylate groups adsorb onto cement particles, forming a physical barrier that slows water access to the mineral surfaces. This delays the peak heat release and reduces the rate of temperature rise. In addition, PCE improves the uniformity of the cement paste, resulting in a denser interfacial transition zone (ITZ) and, ultimately, higher late‑age strength.

Fly Ash

Fly ash (a pozzolanic material) partially replaces cement. It reacts more slowly than cement – its pozzolanic reaction consumes calcium hydroxide to form additional C‑S‑H gel. This reduces the total heat released because less cement is present, and the fly ash reaction is less exothermic. However, excessive fly ash can slightly reduce early strength. The key is to find the optimal replacement ratio that balances temperature reduction and strength development.

Combined effect: PCE delays and flattens the hydration exotherm, while fly ash reduces the total heat generated. Together, they provide synergistic temperature control.

Experimental Design

Four concrete mixes were prepared with a constant water‑to‑cement ratio of 0.42. The details are given in Table 1.

Mix	Cement (kg/m³)	Superplasticizer	SP dosage (%)	Fly ash replacement (%)	W/C
A (control)	412.5	Naphthalene‑based	0.6	0	0.42
B	412.5	PCE	1.0	0	0.42
C	392.5	PCE	1.0	5	0.42
D	382.5	PCE	1.0	10	0.42

Tests performed:

Adiabatic temperature rise (0–168 h) – measured core and surface temperatures, calculated temperature difference.
Compressive strength at 3, 7, 14, and 28 days (100 mm cubes, standard curing).
Finite element simulation using Midas Civil (model size 2.0 m × 2.0 m × 1.5 m, hexahedral mesh, convective boundary conditions).

Results and Discussion

Temperature Field

Mix	Peak core temperature (°C)	Maximum core‑surface ΔT (°C)
A (control)	51.4	24.7
B (PCE only)	39.5	18.6
C (PCE + 5% FA)	37.5	17.5
D (PCE + 10% FA)	35.6	16.5

PCE alone reduced the peak core temperature by 23.2% (from 51.4 to 39.5 °C) and the ΔT by 24.7% (from 24.7 to 18.6 °C).
Adding 5% fly ash further lowered the peak to 37.5 °C (a 27% reduction from control) and ΔT to 17.5 °C.
10% fly ash gave even lower temperatures (35.6 °C), but with a small trade‑off in strength (see below).

Compressive Strength

Age (d)	A (control)	B (PCE)	C (PCE+5% FA)	D (PCE+10% FA)
3	32.5	34.6	34.2	33.7
7	40.2	43.6	43.1	42.5
14	49.3	51.0	50.8	50.2
28	52.3	54.7	54.0	53.5

At 3 days, PCE mixes (B, C, D) all showed slightly higher strength than the control – meaning the retarding effect did not sacrifice early strength. This is attributed to improved dispersion and a denser microstructure.
At 28 days, all PCE‑containing mixes exceeded the control. Mix B gave the highest strength (+4.6%), followed by C (+3.3%) and D (+2.3%).
Mix C (PCE + 5% FA) achieved the best balance: a 27% reduction in peak core temperature and a 3.3% increase in 28‑day strength compared to the control.

Finite Element Validation

The Midas Civil model predicted core temperatures and surface temperatures for each mix. The correlation coefficients between simulated and measured values are shown below.

Mix	Core temperature correlation	Surface temperature correlation
A	0.95	0.96
B	0.93	0.95
C	0.92	0.94
D	0.92	0.94

All correlation coefficients were ≥0.92, demonstrating excellent agreement. The model also captured the decreasing trend in the hydration function coefficients (K and a) with increasing fly ash content, confirming its reliability for practical temperature-control design.

Recommended Mix and Temperature Control Measures

Based on the results, Mix C (1% PCE + 5% fly ash replacement) is recommended as the optimal formulation. It offers:

Peak core temperature: 37.5 °C (27% lower than control)
Core‑surface ΔT: 17.5 °C (within safety limits)
28‑day compressive strength: 54.0 MPa (3.3% higher than control)

Practical temperature control measures for construction:

Curing: Immediately after pouring, cover the surface with wooden formwork and insulation boards to slow surface cooling.
Monitoring: Measure temperature at least every 4 hours. If the ΔT approaches the limit (usually 20–25 °C depending on the standard), increase insulation.
Formwork removal: Do not remove forms until the concrete has gained sufficient strength AND the surface‑to‑air temperature difference is ≤20 °C.
Adjustment: Admixture dosages may need fine‑tuning based on local materials and ambient conditions.

Conclusion

Polycarboxylate superplasticizer alone (1% by cement mass) significantly reduces the peak hydration temperature and the core‑surface temperature difference in mass concrete, without harming early strength.
Combining PCE with 5–10% fly ash further lowers the temperature field. The 5% fly ash mix achieved the best balance: 27% lower peak temperature and 3.3% higher 28‑day strength compared to a conventional naphthalene‑based superplasticizer mix.
The finite element model (Midas Civil) accurately predicted temperature evolution (correlation ≥ 0.92), providing a reliable tool for designing temperature-control strategies for large pours.
The recommended formulation (1% PCE + 5% fly ash), together with proper curing and monitoring, constitutes an effective, practical solution to prevent thermal cracking in mass concrete structures.