What Is C3S (Alite) in Cement Chemistry?
C3S (alite) is the dominant phase in Portland cement clinker at 50-70% by mass. It drives early strength via rapid hydration to C-S-H gel.
C3S (alite) is the dominant crystalline phase in Portland cement clinker, typically comprising 50-70% of clinker mass, and is the primary source of early compressive strength in hardened cement [1]. It forms at sintering temperature (around 1,450 °C) in the rotary kiln from lime (CaO) and silica (SiO2) and is the phase that most determines how quickly a concrete structure gains load-bearing capacity in its first 28 days. For the full four-phase picture, see what the chemical composition of clinker is.
Common questions about this topic
C3S is tricalcium silicate, written 3CaO·SiO2 in oxide notation and abbreviated C3S in cement-chemist shorthand (where C = CaO, S = SiO2). It forms during sintering in the rotary kiln and constitutes 50-70% of ordinary Portland cement clinker by mass (Taylor, *Cement Chemistry*, 2nd ed., Thomas Telford, 1997) [1]. In plant documentation, C3S and alite are used interchangeably. Real alite is not stoichiometrically pure C3S; it is an impure solid solution containing 3-4% substituent oxides (Al2O3, MgO, Fe2O3), which modify its crystal structure and hydration kinetics relative to the synthetic compound.
C3S reacts rapidly with water to produce calcium silicate hydrate (C-S-H) gel and calcium hydroxide (Ca(OH)2, portlandite). The C-S-H gel is the principal strength-giving product in hardened cement paste. The reaction:
OPC clinker targets 50-65% C3S for standard early and long-term strength balance. High-early-strength cement (ASTM C150 Type III) pushes toward 60-70% C3S to accelerate strength gain for fast-track construction. Sulphate-resistant and low-heat cements deliberately reduce C3S below 50% and shift the balance toward C2S, which hydrates more slowly and generates less heat [3].
Higher C3S targets require a more lime-rich raw mix, meaning a higher lime saturation factor (LSF). This makes the raw mix harder to burn: the kiln needs a hotter burning zone and more residence time to drive the CaO-SiO2 reaction to completion. The result is higher specific fuel consumption and higher process CO2 per tonne of clinker. The Global Cement and Concrete Association (GCCA) Net Zero Roadmap identifies high-belite (low-C3S) clinker as one of the composition-side levers for cement decarbonisation [4]. False air ingress at the kiln inlet and outlet compounds the problem by diluting combustion gases and lowering effective burning temperature; controlling this is the subject of [false air in cement kilns](/en/blog/understanding-false-air-in-cement-kilns).
C3S is routinely estimated using the Bogue calculation applied to XRF oxide data. The Bogue equation gives the "potential C3S," but it tends to understate alite relative to direct measurement because real clinker phases are non-ideal solid solutions. Quantitative XRD (QXRD) with Rietveld refinement provides a direct phase measurement and is the reference method for research-grade work (Stutzman et al., NIST) [5]. For plant quality control, Bogue is adequate; for research or litigation-level accuracy, QXRD is required. The full Bogue equations are covered in [what is the chemical composition of clinker](/en/blog/chemical-composition-of-clinker).
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