Comprehensive Study Notes on Binding Materials and Portland Cement Technology

Classification and Varieties of Portland Cement

Portland cement can be classified according to its chemical composition and its specific functional properties. Based on the mineralogical composition of the clinker, the primary types include ordinary, alite-rich, belite-rich, aluminate, aluminoferrite, and ferrite cements. When categorized by usage and specific properties, Portland cement is divided into ordinary, rapid-hardening (high-early-strength), special rapid-hardening, plasticized, hydrophobic, and oil-well (tamponaj) types.

Rapid-hardening Portland cement is characterized by its significant strength gain in the early stages of hydration. One day after mixing, its compressive strength reaches approximately $20\,MPa$, and by three days, the strength increases to nearly $30\,MPa$. To achieve these properties, the specific surface area of the cement must be finely ground to a range of $350-450\,m^2/kg$. Plasticized or hydrophobic Portland cements are considered hydraulic binding agents produced by finely grinding the clinker together with specific plasticizing or hydrophobic additives. Plasticized cements are manufactured in several strength grades, including 300, 400, 500, and 600. The primary components for manufacturing plasticized cement include Portland cement clinker, gypsum, and a plasticizing additive. To enhance the strength and durability of the cement, special hydrophobic agents such as asidol, asidol-milonaft, milonaft, oleic acid, or oxidized petrolatum are added.

Specialty Cements and Production Methods

Sulfate-resistant Portland cement is produced by grinding a clinker with a standardized mineralogical composition together with gypsum. The essential raw materials for this type are clinker, gypsum, slag, and active mineral additives. It is typically produced in strength grades 300 and 400. A distinctive feature of its production technology compared to ordinary cement is that the clinker is often used in a prepared state, meaning the burning process is sometimes omitted if the correct clinker is sourced.

White and colored Portland cements are manufactured from high-purity carbonate and sand-clay raw materials, specifically using pure kaolin and its industrial waste. The clinker for these cements is fired at temperatures ranging from $1600-1650^\circ C$. To ensure a white color, specific cooling methods such as water-based or gas-based processes are applied. White Portland cement is produced by grinding low-iron clinker with gypsum and active mineral additives like diatomite. Colored Portland cements are obtained by grinding white or colored clinker with pigments, diatomite, and gypsum. During the grinding process in a ball mill, coloring oxides can be introduced through the fuel ash, the steel lining of the mill, or the grinding media (steel balls). For instance, cobalt oxide ($CoO$) is used to give Portland cement a blue (azure) color. Based on light reflection values, white cements are graded into Superior (Oliy), BS-1, and BS-2.

Expansive cements are unique because they increase in volume during the setting process rather than shrinking. To produce expansive cement, the components are usually mixed in a ratio of 85% aluminous cement, 10% gypsum, and 5% lime. These cements, especially those that are rapid-setting and expansive, are available in grades 300, 400, 500, and 600.

Mineral Additives and Slag-Based Binders

Active mineral additives are classified into natural and artificial types. Natural additives include diatomite, tripoli (trepel), opoka, and clay-like substances. Artificial additives consist of active silica waste, burnt clays, fuel ash, and various slags. Volcanic additives, such as volcanic ash, tuff, and pumice (sea foam), are also used. Burnt clays are produced by firing clays containing kaolinite ($Al_2O_3 \cdot 2SiO_2 \cdot 2H_2O$) at temperatures between $600-800^\circ C$.

Puzzolanic Portland cement is produced by grinding clinker together with mineral additives and gypsum. Its strength grades are defined as M200, M250, M300, M400, and M500. Slags are products formed by the high-temperature chemical reaction of raw material components during metallurgical processes. Metallurgical slags are divided into ferrous (black) and non-ferrous (colored) types. Ferrous slags result from the smelting of pig iron (cho‘yan) and steel. Non-ferrous slags are by-products of smelting copper ($Cu$), nickel ($Ni$), and lead ($Pb$). On average, producing 1 ton of pig iron yields $0.6-0.7\,t$ of slag. The chemical composition of blast furnace slag includes $CaO$, $Al_2O_3$, $SiO_2$, $MgO$, $FeO$, $CaS$, and $MgS$. The hydraulic activity of slag is measured by its basicity module and activity module; for high activity, the basicity module should be greater than or equal to 1. Granulated electro-thermophosphorus slag is a by-product of obtaining phosphorus from phosphorites and apatites using electro-thermal treatment. To increase the activity of slags used for binders, they are rapidly cooled with water, steam, or air.

Slag Portland cement is made by grinding Portland cement clinker with blast furnace or electro-thermophosphorus slag and gypsum ($CaSO_4 \cdot 2H_2O$). Slag content typically ranges between 21% and 60% by mass. Gypsum is added to accelerate the setting time. For rapid-hardening slag Portland cement, the specific surface area is ground to $4000-5000\,cm^2/g$. This cement has a density of $2.8-3.0\,g/cm^3$ and is available in grades 150, 200, 250, 300, 400, and 500. It is primarily used for large-scale structures, hot workshops, and hydraulic engineering projects, though it is not recommended for structures subjected to frequent freezing-thawing or alternating dry-wet cycles.

Lime and Gypsum Binders

Hydraulic lime-based binders include lime-slag, lime-ash, and lime-puzzolanic binders. Lime-slag binder is produced by grinding unslaked lime together with granulated blast furnace slag and gypsum. The production process involves six stages. During production, lime and gypsum are crushed in hammer or impact crushers to a particle size of $5-10\,mm$. Lime-ash binders, used for plastering and bricklaying, have strength grades of 50, 100, 150, and 200. Sulfate-slag cement contains 80-85% slag, along with anhydrite, Portland cement clinker, and lime, yielding a compressive strength of $300-400\,kgf/cm^2$.

Aluminous (high-alumina) cement is produced from bauxite, a sedimentary rock. Its chemical composition includes $Al_2O_3$, $CaO$, $SiO_2$, and $Fe_2O_3$. If the $CaO$ content exceeds 40%, it is considered high-lime. Mineralogically, it consists of low-basic calcium aluminates ($CaO \cdot Al_2O_3$ and $CaO \cdot 2Al_2O_3$). Bauxites are graded as B-2, B-3, and B-7, and are fired at $1800-2000^\circ C$. Aluminous cement is used in special applications requiring rapid setting and high heat resistance. Unlike Portland cement, it requires significant water during hydration, with a water-cement ratio exceeding $0.5-0.6$.

Gypsum binders are classified by their processing temperature into low-temperature and high-temperature varieties. Building gypsum (Plaster of Paris) is produced by heating natural gypsum stone ($CaSO_4 \cdot 2H_2O$) at $150-170^\circ C$ to form semi-hydrated gypsum ($CaSO_4 \cdot 0.5H_2O$). High-grade (Grade 1) gypsum requires raw stone with 90% purity, while Grade 2 requires 65%. The setting of building gypsum typically starts after 4 minutes and concludes by 30 minutes. According to Academician A.A. Baykov, the hardening process involves the dissolution of semi-hydrate to form a gel-like state of di-hydrate. Gypsum is valued for its rapid setting and fire resistance but suffers from low strength and poor water resistance. It is graded by fineness (coarse, medium, fine) and setting speed (fast, medium, slow). High-strength gypsum consists primarily of semi-hydrated calcium sulfate and is used in architecture and metallurgy.

Magnesial, Organic Binders, and Plastics

Caustic magnesite is produced by firing natural magnesite ($MgCO_3$) at $750-850^\circ C$ and grinding it into a fine powder. Natural magnesite is a sedimentary rock with a chemical composition of 47.82% $MgO$ and 52.18% $CO_2$. It has a density of $3.1-3.4\,g/cm^3$. The hardening process occurs in three stages, starting with the hydration of magnesium oxide. Setting should begin after 20 minutes and conclude within 6 hours. Caustic dolomite is obtained from natural dolomite ($MgCO_3 \cdot CaCO_3$), which has a Moos hardness of 3.5-4.5 and a compressive strength of $40-130\,MPa$.

Organic binders include bitumen and resins (qatron). Bitumens are derived from natural sources, petroleum, or shale. Petroleum bitumens are categorized by their production method as oxidized, residual, or cracked. Bitumen quality is determined by its viscosity (measured via a penetrometer), softening point (measured using the "Ring and Ball" apparatus), and flash point. Resins are obtained by the dry distillation of coal, peat, or wood at high temperatures in reactors or kilns.

Plastics are composed of polymers, fillers, plasticizers (to improve molding), hardeners (to accelerate setting), pigments, and stabilizers (to increase durability). Polymers can be natural (proteins, nucleic acids, rubber) or synthetic (derived from coal, petroleum, and gas). Based on their behavior under heat, they are thermoplastic (soften when heated, harden when cooled) or thermosetting (harden under heat/pressure and do not soften upon reheating). Common synthetic polymers include polyethylene, polypropylene, and phenol-formaldehyde. Plastics have a density of $0.8-1.8\,g/cm^3$ and a compressive strength of $100-150\,MPa$. Their main disadvantage is low heat resistance and aging.

Portland Cement Production Technology

Portland cement is manufactured in rotary kilns, which are installed at a slope of $5^\circ$ and rotate at a speed of $1-2\,rpm$. The kiln is divided into six thermal zones. In the Drying zone, temperatures range from $600-700^\circ C$. The Heating zone reaches $700-800^\circ C$. The Decarbonization zone ($CaCO_3 \rightarrow CaO + CO_2$) operates between $700-1100^\circ C$. The Exothermic reaction zone is between $1100-1250^\circ C$, leading to the firing zone where clinkerization occurs at approximately $1450^\circ C$. Finally, the clinker exits the Cooling zone at $1000-1100^\circ C$.

Finished clinker is stored in covered silos with walls 3-6 meters high, equipped with bridge cranes. The clinker is then ground into cement powder with particle sizes ranging from $5-10\,mm$ down to $30-40\,\mu m$. Grinding efficiency is often improved by adding surface-active agents like milonaft or petrolatum. The finished cement is stored in reinforced concrete silos with diameters of $8-18\,m$ and heights of $25-40\,m$.

There are three primary recognized theories regarding the hardening of Portland cement: the crystallization theory by Le Chatelier, the colloid theory by Michaelis, and the combined theory by A.A. Baykov. Portland cement clinker primarily consists of four minerals: tricalcium silicate ($C_3S$, 45-60%), dicalcium silicate ($C_2S$, 15-37%), tricalcium aluminate ($C_3A$, 7-15%), and tetracalcium aluminoferrite ($C_4AF$, 10-18%). The setting time and quality of the cement are influenced by the presence of alkali oxides ($Na_2O$ and $K_2O$).