**6. Conclusion**

**5. Morphology of C-S-H for drug delivery**

86 Cement Based Materials

the enhanced width.

Outside the framework of construction industry, C-S-H also offers numerous benefits as drug carriers, such as high bioactivity and the enhanced affinity toward certain organic drug molecules owing to the presence of calcium ions on the surface [31–33]. Wu et al. synthesized near-spherical, mesoporous C-S-H particles using the surfactant-free, sonochemical method with tetraethylorthosilicate (TEOS) and calcium nitrate as the silicate and calcium source, respectively [33]. Each particle exhibited a 3D-network produced by the assembly of nanosheets, leading to the presence of meso- and macropores (**Figure 4**). The resultant large surface area later facilitated the subsequent loading and unloading of Ibuprofen, utilized as the model drug. Zhang et al. applied the similar sonochemical technique but along with CTAB to produce hollow CSH microspheres [34]. The authors sonicated the mixture of sodium silicate and calcium hydroxide or calcium nitrate, thereby investigating the effect of two distinct calcium-bearing precursors on the final morphology, and also tried two separate mixing techniques, simple stirring and sonication. It was found that well-defined spherical shapes were only produced under sonication while stirring induced the formation of irregular agglomerated nanosheets. Furthermore, calcium hydroxide was found to be a more effective calcium source in inducing hollow spherical morphology than calcium nitrate, owing to the higher dissociation speed of the latter. The fast release of calcium and nitrate ions led to the increased combination rate of calcium and silicate ions prior to the attachment of silicate ions onto the CTAB micelle, thereby resulting in a greater proportion of nonspherical particles. Wu et al. also synthesized ultrathin calcium silicate nanosheets for use as adsorbents for drugs and metal ions [35]. The solvothermal treatment using the as-synthesized nanosheets at 180°C for 24 hours increased crystallinity of the products and induced the nanobelt-like morphology with the thickness of around 5 nm. Further extending this reaction time to 120 hours produced the similar nanobelt-like C-S-H with

**Figure 3.** Morphology map as a function of surfactant concentration and the initial Ca/Si molar ratio [27].

Calcium silicate hydrate, the most commonly renowned as the glue of concretes, has now found widespread potential applications encompassing cementitious and insulation materials, drug delivery, water treatment and bone-tissue engineering. In addition to the inherent benefits including high strength, high bioactivity and high biodegradability, a long list of various C-S-H members accompanying different stoichiometric ratios implies that there exists a room for the attainment of diverse morphologies. Consequently, a large number of efforts have been directed toward achieving specific, well-defined morphologies, which can optimize the functions. For example, enhanced mechanical properties of cementitious materials arising from cubic building blocks and large drug-loading capacity stemming from the large surface area of mesoporous spherical shapes have been achieved. Based on the rapidly advancing nanofabrication techniques, greater diversity regarding the shapes of C-S-H will be accomplished in future.
