Unit
- Calcium silicates as main binders: Portland cement consists primarily of four compounds. Tricalcium silicate (C3S) provides early strength development. Dicalcium silicate (C2S) contributes to later strength gain. Tricalcium aluminate (C3A) influences setting time and sulfate resistance. Tetracalcium aluminoferrite (C4AF) affects color and heat of hydration.
- Oxide composition targets: Raw materials must supply specific oxide ratios. Limestone supplies calcium oxide (CaO). Clay or shale provides silica (SiO2), alumina (Al2O3), and iron oxide (Fe2O3). Typical target values are CaO around 65%, SiO2 21%, Al2O3 5%, Fe2O3 3%, and MgO less than 5%.
- Lime saturation factor (LSF): LSF measures the ratio of available CaO to the amount needed to combine with silica, alumina, and iron. A value of 90-95% ensures sufficient C3S formation without excess free lime. Formula: LSF = (CaO) / (2.8*SiO2 + 1.2*Al2O3 + 0.65*Fe2O3).
- Silica modulus (SM) and alumina modulus (AM): SM = SiO2/(Al2O3+Fe2O3) controls the proportion of liquid phase in the kiln. AM = Al2O3/Fe2O3 affects melt viscosity. Typical SM is 2.0-3.0, AM is 1.0-2.5 for ordinary Portland cement.
- Hydration reaction process: When water is added to cement, C3S and C2S react to form calcium silicate hydrate (C-S-H) gel and calcium hydroxide. C3A reacts quickly with gypsum to form ettringite, which regulates setting. The hydration rate determines workability and final strength.
- Role of gypsum in cement: Gypsum (calcium sulfate dihydrate) is interground with clinker at 3-5% to control the rapid reaction of C3A. Without gypsum, cement would flash set within minutes. The optimum sulfate content depends on clinker composition and fineness.
- Raw material impurities and their effects: Magnesium oxide (MgO) above 5% can cause expansion and cracking due to periclase formation. Alkalis (Na2O, K2O) can react with aggregates in concrete (alkali-silica reaction). Chlorides above 0.05% may cause corrosion of reinforcement.
- Modulus calculations for raw mix design: The raw meal is proportioned using three moduli: LSF, SM, AM. Operators adjust quarry feed or add corrective materials (iron ore, sand, bauxite) to maintain target values. A deviation of 1% in LSF can shift clinker quality noticeably.
Process Flow
- Raw material extraction: Limestone and clay are drilled, blasted, and transported from the quarry to the crusher. Selective mining maintains average LSF targets across the deposit.
- Crushing and prehomogenization: Primary crushers reduce material to 100-200 mm fragments. Stacker-reclaimer systems blend material layers to smooth out chemical variations.
- Raw grinding and blending: Vertical roller mills or ball mills grind crushed material to 90% passing 90 microns. Blending silos homogenize the raw meal to within 0.5% LSF variation.
- Preheating and precalcining: Raw meal enters a cyclone preheater at 80% CaCO3 decomposition, then passes to a precalciner where fuel combustion raises temperature to 900C.
- Clinker formation in the rotary kiln: The material reaches 1450C in the burning zone. C3S forms from belite and free lime. The clinker is then cooled rapidly to prevent C3S reversion.
- Final grinding with gypsum: Clinker and gypsum are ground together in a finish mill to produce cement with specific fineness (typically 3500-4000 cm2/g Blaine).
Quick Revision
- Portland cement consists of four main compounds: C3S, C2S, C3A, C4AF.
- Raw mix design uses LSF, silica modulus, and alumina modulus to control clinker quality.
- Gypsum is essential to control setting time by moderating C3A hydration.
- Impurities such as MgO and alkalis must be limited to avoid concrete durability issues.
- Process flow from quarry to finish mill follows six key stages with strict chemical control.