# Immobilization of (Aqueous) Cations in Low pH M-S-H Cement
## 3 Results
### Ex Situ Characterization
#### 3.2.5 TEM  
Due to the nanometre size of the individual M-S-H globules, TEM was used to further investigate the precipitates regarding size, shape, and surface features. Agreeing with the SEM analysis, the TEM micrographs (Figure 5a) depicts pure M-S-H as agglomerates of poorly crystalline (Figure 5a, inset) spherical particles. A closer look (Figure 5b) reveals that the surface of globular particles is formed by a large amount of interconnected foil-like structures, which yield to a rough and porous surface. Similar foils or lamellar structures have been observed in the synthesis of hollow M-S-H spheres from silica globules [26; 52; 53]. EDX elemental mapping confirmed that Mg and Si are homogeneously distributed over the particles (Figure 5c(i-iv)). In general, the morphology of the most M-S-H precipitates obtained in the presence of different secondary metal cations are comparable to the reference samples (Figure 5), however subtle differences were observed. The globular M-S-H precipitates obtained with Fe (Figure 5d-f), Pb (Figure 5g-i), and Cr (results not shown) do not possess a rough foil-like structure but rather irregular cloud-like morphologies. In the case of Cu, Co, and Zn containing samples, both foil-like and cloud-like morphologies were found (Figure S2). The M-S-H precipitates formed in the presence of Cs, Li, Ni, and Ba are analogous to the control M-S-H material. For samples obtained in the presence of Cs, the foil-like structure is more developed (Figure 5j-l) than in the rest of the samples and in control runs, despite  
Figure 4: Scanning electron microscopy (SEM) images of the precipitated crystals (**a**,**b**) pure M-S-H; (**c**,**d**) Barium containing M-S-H and barium carbonate; (**e**,**f**) aluminum-containing M-S-H.  
the homogeneous distribution of the Cs in the material and the low amount incorporated (Figure 5**(iii)). Unlike SEM analysis which uncovered the presence of \"desert-rose-like\" structures, no significant changes were found for samples containing Al observed under TEM (Figure S2). The SAED also reveals an amorphous structure with a homogeneous distribution of Al throughout the material (Figure S2). Figure 5: Transmission electron microscopy (TEM) images of precipitated crystals (**a,b**) pure M-S-H system with selected area electron diffraction (SAED) pattern inset; (**c**(**i**-**iv**)) elemental mapping of the pure M-S-H; (**d,e**) Fe-containing M-S-H samples SAED pattern inset; (**f**(**i**-**iv**)) elemental mapping of Fe-containing M-S-H, red ellipse highlight Fe-rich regions within the M-S-H matrix; (**g,h**) Pb-containing M-S-H samples SAED pattern inset; (**i**(**i**-**iv**)) elemental mapping of Pb-containing M-S-H; (**j**) Cs-containing M-S-H samples SAED pattern inset; (**k**) zoom into the yellow square in (**j**); (**l**(**i**-**iv**)) elemental mapping of Cs-containing M-S-H. In the case of the Fe containing samples, the EDX analysis showed that even if Fe was detected everywhere in the precipitates, there were some elliptical areas (ca. 20-30 nm in length) in which Fe was enriched and significantly lower amounts of Mg and Si were detected (red ellipse in Figure 5a,f (iii)). In addition, the SAED of Fe bearing precipitates on those areas where higher amount of Fe was detected possess additional distinct reflexes evidencing crystalline solids within the M-S-H matrix (Figure 5d, inset). Analysis of the SAED (d-spacings of ca. 1.45 A and 2.6 A) together with XRD and EDX mapping suggest the presence of an iron oxide phase(s) within M-S-H precipitated material [50]. It is worth mentioning, that no iron oxide particles were found outside the M-S-H matrix. Interestingly, HAADF images of Pb and Fe samples both display the existence of ca. 2-5 nm sized particles with higher electron contrast, embedded in the amorphous M-S-H matrix (Figure 5e,h (black spots)). These nanocrystals were uniformly distributed in both samples.