Article updated on 14/07/20 by Sarah Moore.
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Concrete is one of the world’s most commonly used materials. All manner of buildings and structures use concrete and it is cheap, abundant, and easy to produce.
Like any material, knowledge of the structure of its components is important for determining the desirable end product. One constituent part of concrete is cement paste, a combination of powdered Portland cement and water. Even though it is a simple enough material, knowledge of its constitution and microstructure is still vital for the overall concrete fabrication process.
Properties of Cement Paste
Cement paste has several properties that are important to produce viable concrete. Cement paste alone is not viable for concrete production: aggregates (stone, sand, etc.) must be added which the cement paste coats. Any type of water can be used to create the paste. The process of hydration causes the cement paste to harden as it works on the unique chemical and physical properties of the cement used.
Achieving a strong and durable concrete relies upon careful proportioning and mixing of the ingredients. Too little paste and the voids between the aggregates will not be filled properly, which creates a porous concrete with rough surfaces. Too much and the resulting concrete is prone to cracking and is not cost-effective. Typically, a good mix is 10-15% cement, 60-75% aggregate and 15-20% water. Entrapped air can make up another 5-8%.
Important properties of cement are fineness, soundness, consistency, setting time, compressive strength, the heat of hydration, and specific gravity. Knowledge of these properties is useful for determining the quality of the cement paste and therefore the finished concrete.
Physical tests can be performed, such as the creation of cement-concrete cubes to test compressive strength, however, several methods can be employed to investigate the atomic structure of cement and cement paste. One of these is X-ray diffraction (XRD.)
XRD – An Overview
X-ray diffraction is a process that can reveal the crystallographic structure, chemical composition, and physical properties of a material. It is one of a series of non-destructive analytical methods that can be employed in the laboratory by researchers and technicians.
The process works on the principle that electromagnetic radiation (such as X-rays) can be scattered (diffracted) by the atoms in a crystalline lattice. The wider field of X-ray crystallography has been instrumental in the development of several related fields.
The incident X-rays are scattered in many specific directions by the atoms and molecules in the crystal lattice, which can then be interpreted by the crystallographer to provide data on the chemical composition of the material. the information provided includes the size of atoms, length and type of bonds, and atomic-scale differences within the material.
There are three stages to a crystallographic experiment: Firstly, a sufficiently large crystal must be formed, which is then subjected to intense X-rays, and finally, the information obtained about the crystalline structure is interpreted, usually with the help of specialized software.
Recent Studies on Cement Paste
There have been several recent studies on cement paste that have utilized XRD. Four of these are listed below.
- A 2015 study on cement paste with synthetic zeolite, a substance used as effective partial substitute for Portland cement, used X-ray powder diffraction alongside other techniques to explore the compressive strength of this type of cement paste. It was observed that the samples with the best compressive strength were composed of 5% synthetic zeolite. Compared to the control samples, the strength of the cement/zeolite mixture was increased by 9%. This study has implications for the cement industry as cement can be saved and industrial waste can be reduced.
- A 2019 study used XRD to investigate the changes in microstructure of Portland cement. Scientists wanted to know how substituting 0-14% of the mass of ordinary Portland cement with differing combinations of metakaolin and red mud would impact the physical, chemical, and strength properties of the cement. They found that while water requirements were lowest when the cement had no addition of metakaolin and red mud, an addition in the ratio 80:20 of these two substances strengthened cement due to evoked pozzolanic reaction, inducing filler effects on the concrete.
- Scientists have also recently been investigating Portland cement with XRD to help develop new applications of the material. A 2017 study used XRD to uncover the mechanical and antimicrobial properties of Portland cement when silver nanoparticles are added to it. They discovered that the material may be useful for future bone substitute materials.
- A 2018 study used XRD to determine the correct decompositional temperatures of cementitious phases in Portland cement paste as well as in GGBFS-blended cement paste, and Ca(OH)2-activated GGBFS binders. The analysis highlighted the decompositional temperature ranges of common phases, and how the composition of the cement paste may impact these.
In conclusion, XRD represents one of the best analytical tools for the study of the chemical composition and by extension, physical properties, of cement paste. Likely, the science of XRD will continue to provide useful information on novel cement-based materials as they are developed in the future.
References and Further Reading
Danute Vaičiukynienė-Palubinskaitė et al. (2015) Blended Cements Produced with Synthetic Zeolite Made from Industrial By-Product Materials Science Vol. 21 Issue 1 - https://www.researchgate.net/publication/274695482_Blended_Cements_Produced_With_Synthetic_Zeolite_Made_from_Industrial_By-Product
How Concrete is Made – Cement.org - https://www.cement.org/cement-concrete-applications/how-concrete-is-made
Portland cement properties – Acivilengineer.com
Guoping Geng et al. (2018) A high-pressure X-day diffraction study of the crystalline phases in calcium aluminate cement paste Cement and Concrete Research Vol. 108 Pgs. 38-45 - https://www.sciencedirect.com/science/article/pii/S0008884617310451
Natural zeolites as pozzolans in concrete - kmizeolite.com - https://www.kmizeolite.com/
Rathan Raj, R., Brijitta, J., Ramachandran, D. et al. (2019). Microstructure Evolution in Ordinary Portland Cement–Metakaolin–Red Mud-Based Ternary Blended Cement. J. Inst. Eng. India Ser. Vol 100, Pgs. 707–718 - https://link.springer.com/article/10.1007/s40030-019-00398-4?shared-article-renderer
Nam KY. (2017) Characterization and antimicrobial efficacy of Portland cement impregnated with silver nanoparticles. J Adv Prosthodont. Vol 9, Issue 3, Pgs - 217-223 - https://pubmed.ncbi.nlm.nih.gov/28680554/
Song, H., Jeong, Y., et al. (2018). A study of thermal decomposition of phases in cementitious systems using HT-XRD and TG. Construction and Building Materials. Vol 169, Pgs 648-661 - https://www.sciencedirect.com/science/article/abs/pii/S095006181830480X