Portland cement is a binder, a substance used for construction that sets, hardens, and adheres to other materials like sand or gravels to bind them together. It is a backbone material for any kind of masonry construction works like household foundations, multistoried building, roads, bridge construction etc. So quality checking of cement either in factory site or in laboratory is a necessary action to ascertain its strength. Key ingredient of this cement is clinker which is a high temperature calcined product of a number of raw materials like limestone, clay etc. resulted from the complex series of reactions in a kiln at 1400°C-1500°C. After the formation of clinker, it is cooled followed by the addition of gypsum and/or other supplementary cementitious materials which together determine the fate of the resultant product cement. When this cement is mixed with water, a transient plastic like smooth paste is formed and as the reaction between water and cement continues, cement paste hardens. All these properties strictly depend on the composition of cement. Basically hydration of its major component clinker is responsible for the hardening of cement. The four main mineral phases of the clinker are silicates and oxides namely, C3S: Alite (3CaO·SiO2), C2S: Belite (2CaO·SiO2), C3A: Tricalcium aluminate (3CaO·Al2O3), and C4AF: Brownmillerite (4CaO·Al2O3·Fe2O3). These silicates are responsible for the cement’s mechanical properties whereas tricalcium aluminate and brownmillerite are responsible for the formation of the liquid phase during the sintering process of clinker at high temperature. Alite and belite are hydrated to produce calcium silicate hydrates (C-S-H), lime (C-H) and heat: This calcium silicate hydrate mainly contribute to the mechanical strength of the cement paste like strength development and durability of concrete.
Again addition of some supplementary cementitious materials like gypsum, limestone, slag, fly ash, furnace ash etc. beyond a particular limit cause observable/non observable changes like color, texture, phase pattern, thermal nature, mechanical strength, reactivity which in turn can degrade the quality of cement and even cause structural failure. So phase compositional analysis of cement-clinker is an essential task.
Here, powder X-ray diffraction and thermogravimetric techniques were carried out for the rapid interpretation of their phase composition analysis and corresponding impacts. For these purpose, three clinker samples along with standard certified reference material (SRM 2688) and three commercial cement samples were selected as representative.
Results and Discussion
Figure 1: Powder X-ray diffraction pattern of different Clinker samples.
Figure 2: Comparative powder X-ray diffraction pattern of different commercial cement samples along with the other components.
Figure 3: Characteristic thermal decomposition and X-ray diffraction pattern of commercial cement sample A.
Figure 4: Characteristic thermal decomposition and X-ray diffraction pattern of commercial cement sample B.
Figure 5: Characteristic thermal decomposition and X-ray diffraction pattern of commercial cement sample C.
- Powder X-ray diffraction pattern of different clinker samples as shown in Figure 1 reveals the presence of fundamental mineralogical phases like alite, belite (structure), brownmillerite and tricalcium aluminate in all the samples. Presence of minor amount of lime can also be observed in case of commercial clinker samples. This may be due to the incomplete conversion to clinker from the raw materials at kiln while its manufacturing process.
- Comparative diffraction study of different commercial cement samples as shown in Figure 2 reveals the presence of signature peaks of different components like gypsum and blast furnace slag along with major component clinker in cement composition.
- Characteristic XRD peaks of gypsum, calcite, dolomite and portlandite can be clearly observed in different commercial cement samples which can be further ascertained by the thermal decomposition studies as shown in Figure 3-5. Noticeable presence of portlandite as highlighted in XRD pattern of cement C indicates the deterioration of that cement quality.
- Phase composition analysis, supplementary cementitious additives detection and overall purity analysis of Portland cement is utmost important as it is the key ingredient for any kind of masonry construction.
- Semi-quantitative analysis of different composition as done by PXRD is a mandatory task as presence of gypsum or free lime beyond a particular limit can be a fatal issue as they control the setting time and compressive strength of cement. PXRD study also helps to evident the expiration of cement.
- On the other hand, prior to cement manufacturing, phase composition analysis of clinker material is also essential as it will regulate the temperature and CO2 emission as environmental footprint. It is noteworthy to mention that proportion of alite to belite phase controls the reactivity and strength development property of the cement which carries a valuable commercial impact.