# Study on the Properties of Cement-Based Cementitious Materials Modified by Nano-CaCO\({}_{3}\)
## 4 Results and Discussion
### SEM Microscopic and Mechanism Analysis  
The hydration morphology of the samples with 0 and 2.0% Nano-CaCO\({}_{3}\) at the curing age of 28 d is shown in Figure 9.  
It can be seen from Figure 9 that the cement paste of the base group with 0% Nano-CaCO\({}_{3}\) content had several large pores, whereas the sample with 2.0% Nano-CaCO\({}_{3}\) had a lesser amount of internal pores. The cement paste with 2.0% Nano-CaCO\({}_{3}\) content was more compact, in which a large number of high-strength c  
Figure 8: Hydration heat of cement-based materials with different amounts of Nano-CaCO\({}_{3}\) at different curing ages.  
Figure 9: SEM image of a cement stone hydrated with 2.0% Nano-CaCO\({}_{3}\) (**A**) at 28 d and a blank sample (**B**).  
formed. It can also be seen from Figure 10 that the content of ettringite (AFt) in the sample with 2% Nano-CaCO\({}_{3}\), which could inhibit the hydration reaction, was less than that in the blank sample. The hydration reaction was more complete and the hydrated cementitious material was more predominant than in the blank sample, hence, the structural density was better and the mechanical properties were stronger.  
The main way for Nano-CaCO\({}_{3}\) to affect the strength of cement-based materials is to improve the strength of cement paste [29] through the small-size effect and the surface effect [33]. Combined with the SEM test results, we could infer that Nano-CaCO\({}_{3}\) could accelerate the hydration of cement, optimize the interface structure of cement stone, and improve the compactness [34]. Finally, the strength of cement paste was improved.  
The promotion mechanism of Nano-CaCO\({}_{3}\) on hydration reaction in concrete is shown in Figure 11. The high specific surface area and surface energy of Nano-CaCO\({}_{3}\) brought high surface activity. In the initial stage of hydration, C\({}_{3}\)S was hydrated and released a large amount of Ca\({}^{2+}\). The surfaces of Nano-CaCO\({}_{3}\) would physically adsorb Ca\({}^{2+}\)[35], promoting the nucleation of nearby Nano-Ca(OH)\({}_{2}\) and grow in large quantities on the surface of Nano-CaCO\({}_{3}\) particles, so as to accelerate the hydration of C\({}_{3}\)S, which was particularly significant in the early stage of hydration [16]. At the same time, Nano-CaCO\({}_{3}\) could further bind to the cement hydration product, C-H-S, and form ettringite (AFt) with a microcrystalline nucleus of itself. Monosulfide hydrated calcium sulphoaluminate (AFm) and the C-S-H gel phase strengthened the structure and shape of the micro interface layer, and effectively enhanced the mechanical properties of the hardened cement paste [36; 37; 38; 39].  
In the later stage of hydration, a large amount of Nano-CaCO\({}_{3}\) coating around the clinker particles would react with the aluminum phase (C\({}_{3}\)A) in the cement to form the hydration product single carbon hydrated calcium carboaluminate (3 CaOAl\({}_{2}\)O\({}_{3}\)CaCO\({}_{3}\)12H\({}_{2}\)O), which hinders the hydration reaction by reducing the direct contact with clinker and water. The more the content percentage, the more obvious the hindering effect was. Therefore, Nano-CaCO\({}_{3}\) could not promote the later hydration of cement-based materials [40], and the strength of cement-based materials would decrease with the increase of Nano-CaCO\({}_{3}\) content. In addition, too much Nano-CaCO\({}_{3}\) would hinder the transformation of AFt to AFm, and the expanded internal stress produced by a large amount of AFt would destroy the structure of hardened cement slurry and reduce the strength of the cement-based materials.  
Figure 10: XRD pattern of Cement stone hydrated with 2.0% Nano-CaCO\({}_{3}\) (**A**) for 28 d and the blank sample (**B**).