Spontaneous hierarchical surface engineering of minerals through coupled dissolution‐precipitation chemistry

Peculiar hierarchical microstructures in creatures inspire modern material design with distinct functionalities. Creatures can effortlessly construct sophisticated yet long‐range ordered microstructure across bio‐membrane through ion secretion and precipitation. However, microstructure biomimicry in current technology generally requires elaborate, point‐by‐point fabrication. Herein, a spontaneous yet controllable strategy is developed to achieve surface microstructure engineering through a natural surface phenomenon similar to ion secretion‐precipitation, that is, coupled dissolution‐precipitation. A series of hierarchical microstructures on mineral surfaces in fluids with tunable morphology, orientation, dimension, and spatial distribution are achieved by simply controlling initial dissolution and fluid chemistry. In seawater, long‐range ordered film of vertically aligned brucite flakes forms through interfacial dissolution, nucleation, and confinement‐induced orientation of flakes with vertically grown {110} plane, on the edge of which, fusiform aragonite epitaxially precipitates. With negligible initial surface dissolution, prismatic aragonite epitaxially grows on a calcite polyhedron‐packed surface. By tuning fluid chemistry, closely packed calcite polyhedron and loosely packed calcite micro‐pillars are engineered through rapid and retarded precipitation, respectively. Surprisingly, the spontaneously grown microstructures resemble those deliberately created by human or found in nature, and tremendously modulate surface functionality. These findings open new possibilities for facile and customizable engineering of microstructural surfaces, hierarchical heterostructures, and biomimetic materials.

Calcium silicate is a family of minerals that can occur naturally or be artificially formed as the reaction products of calcium oxide and silica with various ratios. [20]Its hydraulic, cementitious, and non-toxic nature make it widely applicable in bone regeneration, dental filling, and building construction.In these applications, calcium silicate particles undergo hydration, dissolution, and precipitation to bind together and gain strength as a bulk material.The resulting bulk surface still contains unreacted calcium silicate, with relatively high dissolution rate ranging from ≈10 −5 to ≈10 −4 mol m −2 ⋅s −1 . [21,22]25] The high dissolution rate of calcium silicate allows for efficient dissolution-precipitation processes, making it suitable as a model mineral for exploring surface engineering through dissolution-precipitation chemistry.[35] Hence, the understanding of dissolution-precipitation chemistry, particularly its microstructural consequences, on calcium silicate-based mineral surface is vital, which not only provides fundamental knowledge for mineral surface engineering, but also offers practical insights into building durability and carbon sustainability.
The dissolution-precipitation process on mineral surface is notoriously complex because it is dynamically coupled and intricately linked to the surface dissolution into the solution, ion transfer between interfacial and bulk solution, and precipitation on the parent surface (Figure 1A).To decouple dissolution and precipitation, advancements have been made via real-time visualization of the initial dissolution and early-stage precipitation behavior of various minerals, including calcium silicate, calcium aluminate, calcite, siderite, and brucite. [1]However, most of these studies have focused on the early reaction stages, where the product phase has not fully covered the surface or developed hierarchical structures, thus limiting comprehensive insights into surface engineering.[38] An important knowledge gap exists regarding the complete mechanistic understanding of the coupled dissolution-precipitation process for surface engineering and the associated evolution of surface microstructure.
Here, we present the first study on surface engineering using dissolution-precipitation chemistry, by employing calcium silicate-based cement as the model mineral.Spontaneous and programmable growth of diverse surface microstructures is achieved via dissolution-precipitation chemistry in a fluid environment, which can be versatilely mediated by the original surface dissolution and fluid chemistry.][13][14] Furthermore, the mechanistic panorama of the entire dissolution-precipitation process is revealed by decoupling the whole process through comprehensively analyzing the parent surface chemistry to determine the initial dissolution, the bulk solution chemistry indicating ion transfer, and the depth profiling of the microstructural product recording the precipitation history.Inspired by similar coupled processes observed in biomineralization, we employ depth profiling methodology analogous to the elemental profiling of coral skeleton, which is widely used to trace ocean chemistry (Figure 1B). [13,40]Interestingly, the revealed crystallographic mechanism on the manmade material surfaces can reciprocally decipher the microstructural motifs observed in naturally grown coral skeleton and nacre due to their microstructure resemblance.Moreover, the surface microstructure engineered by dissolution-precipitation chemistry can essentially modulate further functionality.Proof-of-concept experiments show that following the same silane hydrophobizing treatment, different microstructural surfaces exhibit diverse surface wettability, ranging from hydrophilic to dynamically water-repellent, which, arises from the synergistic effects of distinct surface reactivity and morphology.This study offers a new paradigm for facile and versatile surface engineering of mineral materials through coupled dissolution-precipitation chemistry, which opens new possibilities for designing hierarchical microstructures and biomimetic materials, and provides insights into the secondary modification of the as-engineered microstructural surfaces.

Dissolution-precipitation reaction in fluids
Coupled dissolution-precipitation reaction is allowed to proceed on surfaces of cement cubes with varied hydration ages in different fluidic solutions.The composition and microstructure of the parent cement surface dynamically evolve with the hydration age.Thus, its initial hydration age for fluid exposure, namely, the exposure age, could affect the initial dissolution behavior, the coupled precipitation, and the final microstructure, which is considered as a key factor in this study.The initial hydration ages of cement specimens are chosen as 1 day, 14 days, and 28 days, and the corresponding specimens are named as C1d, C14d and C28d, respectively.The exposure medium for C1d, C14d, and C28d surfaces is artificial seawater (SW, NaCl ≈ 24.53 g L −1 , MgCl 2 ≈ 5.20 g L −1 , Na 2 SO 4 ≈ 4.09 g L −1 , CaCl 2 1.16 g L −1 , KCl 0.695 g L −1 , NaHCO 3 ≈ 0.201 g L −1 , pH 8.2), the fluid where a wide range of cement-based constructions served, including platforms, pipeline, and subsea structures.The effect of fluid chemistry is further explored by allowing the C1d surface to react in saline solution (referred as monovalent saltwater, MSW, with pH ≈ 8.2) and in pure water (labelled as W when referring to specimens).In MSW, the divalent salts, namely CaCl 2 and MgCl 2 , in the recipe of artificial seawater, are absent.In pure water, which has a pH of approximately 5.6 due to the solvation of CO 2 in the air, is the commonly employed curing media.The duration of all reactions is controlled as 13 days, during which the dissolution-precipitation reaction reaches equilibrium (Note S1, Figures S1 and S2).As characterized by X-ray Diffractometer (XRD), compared with the parent cement surfaces (Figure 1C), the dissolution-precipitation reaction dramatically alters the surface composition (Figure 1D,E).

Exposure age mediates initial surface dissolution
With the progressive hydration of cement, the chemical composition and microstructure of cement surface evolves.The composition results (Figure 1C) suggest that the unhydrated tricalcium silicate (3CaO⋅SiO 2 , C 3 S) is the main dissolvable mineral in the parent cement surfaces, whereas calcite (the most stable polymorph of CaCO 3 ) is the carbonation product formed in curing with very limited solubility.The content of dissolution-initiating C 3 S on C1d surface is relatively high, which evidently decreases with hydration age of 14 days and becomes discernable on C28d surface.
As to the surface microstructures, the apparent morphology is characterized by scanning electron microscope (SEM, Figure S3), while the permeability by fluid is evaluated by water absorption coefficient (A w ) obtained through water absorption test (Note S2, Figure S4) and water contact angle (WCA) measured by goniometer (Figure S5).C1d surface shows microporous morphology (Figure S3a and Figure 3D), in which the calcite crystal has not fully developed and cannot be differentiated with the unhydrated C 3 S.Such porous morphology results in high degree of water infiltration, demonstrated by the high A w of ≈26.5 × 10 −3 kg (m −2 ⋅ s −1∕2 ) (Figure S4a) and transient absorption (≈1 s) of water droplet with WCA of ≈0 • (Figure S5a).For both C14d and C28d surfaces, calcite crystals develop to polyhedron and form more compact surface layers with similar thickness of ≈4 µm (Figures S3e and Figure 3F).Such calcite layers still do not fully seal the parent surface, leaving pores connecting to the cement matrix for fluid penetration (Figures S3b and Figure 3C, Note S3).As such, the C14d and C28d surfaces shows much lower A w of 3.7 × 10 −3 and 3.2 × 10 −3 kg (m −2 ⋅ s −1∕2 ) (Figure S4a), respectively, and higher contact angle (≈10 • , Figure S5b and Figure 5C).Considering the decreased content of dissolvable C 3 S and lower degree of water infiltration under higher exposure age, initial dissolution is supposed to be modulated by exposure age that it should be most severe at 1 day and almost diminish at 28 days.The dissolved ions should mainly include Ca 2+ and OH − based on the hydration reaction formula of C 3 S:

Ion transfer between the interfacial and bulk seawater solutions
As shown in Figure 1A, the ions dissolved from the parent surfaces to interfacial solution may precipitate with the ions pre-existing at the interface as the product phase or transfer into the bulk solution.Reversely, the consumption of the pre-existed ions in the interfacial layer for precipitation may draw ions from the bulk into the interfacial solution.Thus, the ion transfer between the interfacial and bulk solution can be estimated by comprehensively analyzing the product composition and monitoring the ionic condition (Mg 2+ , Ca 2+ , and pH) of the bulk solutions, which are replaced every two days for a total 13 days of reaction.
Figure 1D shows the surface composition of the C1d, C14d, and C28d surfaces after reaction in seawater, namely, C1d-SW, C14d-SW, and C28d-SW.C1d-SW and C14d-SW surfaces are mainly composed of brucite (Mg(OH) 2 ) and aragonite (metastable polymorph of CaCO 3 ), with brucite content in C14d-SW substantially lower.It should be noted that on C1d-SW surface, the Bragg intensity ratio of (110)/(001) ascribing to brucite (≈23.61) is significantly higher than that of powder XRD pattern of brucite (≈0.27,JCPDS No. 86-0441), marking the preferential orientation of the (110) plane of brucite grains parallel to the bulk surface of the sample.For C14d-SW surface, the preferential orientation level of (110) mitigates while still persists with intensity ratio of (110)/(001) decreasing to 3.68.As to C28d-SW, brucite diminishes while calcite and aragonite coexists on the surface.The cause of the crystallographic orientation and the formation of stable (calcite) or metastable (aragonite) polymorph of CaCO 3 would be discussed in next section.
The brucite and aragonite in the product surface of C1d-SW and C14d-SW suggests both a proportion of the Ca 2+ and OH − dissolved from the parent surfaces precipitates with preexisted Mg 2+ and HCO − 3 /CO 2− 3 in the interfacial solution, respectively.While when it comes to the transfer behavior, substantial transfer of Ca 2+ from the interface to bulk solution contrasted with negligible transfer of OH − is observed based on the evidently elevated Ca 2+ concentration yet stable pH of the bulk solution (Figure S1).Such observation suggests the kinetically preferential precipitation of brucite over aragonite at the interface (Note S4).The precipitation rate difference should be due to the triumph of the one-step brucite precipitation in competing for OH − , compared with aragonite precipitation that takes two steps: The dissolution-precipitation continues a duration of ≈8 to ≈10 days (Note S1), until the depletion of C 3 S in the surface.The overall lower degree of Mg 2+ consumption of C14d-SW than C1d-SW confirms the lower dissolution degree of Ca(OH) 2 with increased exposure age.As to C28d-SW, substantiated by the discernible brucite on surface, the dissolution of Ca(OH) 2 is even less and becomes negligible.In the case, Ca 2+ concentration in bulk solution decreases during the first few days of reaction, which suggests the interfacial precipitation of CaCO 3 , yet its polymorph identity, that is, solely aragonite, or both aragonite and calcite, requires further investigation.

2.4
Crystallographic feature and developing mechanism of the microstructural surfaces engineered in seawater Detailed depth profiles of the product surfaces after the reaction in fluidic seawater were acquired.Due to the distinct surface composition of C1d-SW and C14d-SW compared with C28d-SW, their microstructural development mechanism is discussed respectively.For C1d-SW (Figure 2A) and C14d-SW surfaces (Figure S6), the cross-sectional SEM images show similar hierarchical heterogeneous microstructure that on the parent surfaces, brucite films form with thickness of ≈60 µm and ≈38 µm, respectively, followed by the growth of fusiform aragonite aggregates on the top.The closer formation of brucite to the parent surface than aragonite further confirms the kinetically slower precipitation of aragonite than brucite.
Higher resolution SEM imaging and energy-dispersive Xray spectroscopy (EDS) mapping show that the brucite film, interestingly, is long-range ordered and composed of brucite flakes, in which mostly flakes align vertically to the parent surface (Figure 2B) and form a flat top surface (Figure 2A), while the others assemble radially as micro-flowers with Carich core (Figure 2C) and some of the flowers growing out of top film surface (Figure 2A).All the flakes are closely packed face-to-face.The crystallographic orientation of the brucite flake is surveyed by selected area electron diffraction (SAED). [41]The SAED pattern of the cleaved few-layered brucite flakes (Figure 2D, Note S5) shows hexagonal scalenohedral spots related to the {100} and {110} planes of brucite, which should be parallel to the electron beam and perpendicular to the flake surface.Consequently, the {001} planes, which are perpendicular to both {100} and {110} planes, should be exposed on the flake surface.Combined with the XPS results that the {110} planes of brucite preferentially orient parallel to the bulk surface (Figure 1D), thus it should be the {110} planes of the vertically aligned brucite flakes to be parallel to the bulk surface, and the deduced plane orientation in a single vertical flake is shown in Figure 2E.Therefore, the long-range ordered vertically assembled brucite flakes should form due to the directional growth of their {110} planes perpendicular to the parent surface, while the formation of the scattered micro-flowers should be because that a small proportion of brucite flakes develop their {110} planes radially away from the Ca-concentrated cores.The lower preferential orientation level of {110} plane in C14d-SW surface than C1d-SW observed by XRD (Figure 1D) then can be ascribed to the thinner vertically aligned brucite film (Figure S6a).
On the top surface of the brucite films, it can be observed that the edges of the grown-out flakes are round and terraced (Figure 2F,G, Figure S6b and Figure 6C), thus the exposed The microstructure with flakes vertically assembled is particularly desirable in electrode, membrane, and capacitor because the vertical channels between packed flakes can greatly shorten the ion transporting pathway and enhance efficiency. [42,43]Previously, scalable alignment of as-prepared flaky materials has been achieved via elaborate design and precise process control, for example, freeze casting, [44] magnetic field modulation, [42] liquidcrystal suspension shearing. [39]While in this study, in situ crystallization, spontaneous vertical alignment, and directional plane growth of crystal flakes to form long-range ordered film is observed, which is unprecedented to our best knowledge.The understanding of underlying mechanism driven by dissolution-precipitation chemistry is envisaged to guide the fabrications of diverse vertically assembled flakes-based materials in a brand-new, facile, and versatile approach.
As to the subsequently precipitated aragonite on the top of brucite film, a closer observation of their boundary reveals the selective nucleation of aragonite grains on the edge of the brucite flakes (Figure 2H,I, Note S6), suggesting possible epitaxial relationship.Such hypothesis is also supported by the radial assembly of brucite flakes with the edges toward to the Ca concentrated core (highly likely to be aragonite).The co-occurrence of brucite and aragonite on the surface of C 3 S based construction mate-rials in marine environment has been observed as early as 1980s, [38,45] yet the detailed microstructural feature and crystallographic development mechanism remain unaddressed, with their potential epitaxial relationship unexplored.Coincidently, in early 20th century, the co-existence of brucite and aragonite in coral skeleton has also been observed, [46,47] the architecture-isolated brucite flakes or their assembly mixed with aragonite needles-surprisingly resembles with the microstructure observed in this study.Only till 2018, the developed "coral-on-a-chip" technique allowed the live imaging of skeleton construction and revealed the guiding role of an Mg-rich lattice (highly possible to be brucite, Note S4) in the growth of aragonite, [7] and pointed out that the role of Mg-rich component in calcification-the precipitation and stabilizing of metastable aragonite-is the central question in the construction of coral skeleton.The coincidence of brucite flake-aragonite heterostructure in both artificially and naturally built architectures hints a shared crystallographic mechanism, which, if unraveled, can provide insights into not only the design and engineering heterogeneous microstructures, but the biomineralization and bio-inspired material synthesis.
Based on the detailed depth profiling results, the mechanistic picture of the brucite-aragonite heterogeneous microstructure induced by coupled dissolution-precipitation is revealed by solving the following three important questions based on crystallographic basis.Firstly, the flake-like crystal habit of brucite is ascribed to the lowest growth rate of {001} planes.Based on basic crystallography, the flake-like crystal habit is created when there is one family of planes grows evidently slower than the others, then such planes would be the exposed face as the flake surface. [48]The growth rate of a certain crystal plane in aqueous media mainly depends on the diffusion behavior of the depositing ions from solution onto the crystal surface and the bonding energies during their integration into the crystal lattice. [48]In brucite (Figure 3A), edge-sharing MgO 6 octahedra layers stack in the [001] direction.The hydroxyl group at each apex of octahedra is close to three H atoms in the opposite layer, between which only non-specific and weak interactions exist. [49,50]Contrastingly, for the planes with growth direction perpendicular to [001] direction, that is, [110] and [100] as shown in Figure 3B, covalent bonds exist between layers of the planes.Thus, for the exposed hydroxy-lated {001} planes, the driving force of OH − ions in solution to diffuse onto the plane surface and the integration bond energy to form the next layer should be much lower than the planes vertical to the {001} planes, which then become the exposed flake surface due to slowest growth (Figure 3C).
Next, we attribute the long-range assembly of vertical aligned brucite flakes and the unique vertical upward growth of {110} planes to the confined space of nucleation and growth.In diluted bulk solution (Figure 3C), the orientation of the flake-like crystals should be random, and the growth of its edges along any radial directions should be free.Yet in the interfacial layer on the parent surface, the dissolved ions supersaturate the interfacial solution, resulting in the simultaneous nucleation of concentrated nuclei of brucite flakes (Figure 3D).Thus, the radial growth of the flakes is significantly hindered in the lateral direction due to the confined space.Consequently, only the plane of which the growth direction is nearly vertical to the parent surface could grow continuously, forming the elongated flake morphology.Based on the previous discussion, the {110} planes of the flakes are the only plane family that preferentially grow vertically to the bulk surface, which suggest that the growth of {110} planes should be advantageous and rapid, further orienting their growth to the vertical direction for more growth space.Thus, it is the lateral space confinement that leads to the spontaneous orientation of flakes and the directional growth of {110} planes, forming the long-range assembled film of vertically aligned brucite flakes.The lateral space confinement induced by supersaturation is also supposed to result in the close face-to-face packing of the brucite flakes and retard the thickening of flake along the [001] direction.Compared with C1d, the milder supersaturation of brucite on C14d surface due to less content of dissolvable C 3 S may lead to fewer nucleation sites, giving more space for the flake thickening.Such hypothesis is substantiated by the experimental observation that the thickness of brucite flakes on C13d-SW is higher compared with C1d-SW.Between the packed flakes, channels exist to transport the dissolved ions, which enable the growth at the front of the brucite flakes and the thickening of the macroscopic film.Until the depletion of dissolvable C 3 S, the growth of brucite flakes ceases to form the top surface of the brucite film, on which the exposed flake edges are spherical due to the released space restriction.The microscopic flake thickness and macroscopic film thickness are then suggested to be modulated by the supersaturation related to the dissolution rate of C 3 S and the total content of C 3 S.As such, we unraveled the mechanism of the in situ growth, vertical orientation, and directional plane development of brucite flakes, and envisage our findings can guide the spontaneous growth of diverse vertically assembled flakes-based materials with programmable microstructures.
Further, the possible epitaxy of aragonite on the edge of brucite flake is inspected by scrutinizing the lattice mismatching of different face couplings.Though the triagonal brucite (Figure 3A) and orthorhombic aragonite (Figure 3E) show different symmetry, at the single layer of (110) plane in brucite, rectangular repeating pattern like that at the (010) plane of aragonite is identified (Figure 3F).The lattice mismatches along [ 110] Brucite //[001] Aragonite and [001] Brucite //[100] Aragonite are calculated as ≈5.4% and ≈4.0%(Figure 3F,G), respectively.Based on that the general limit of lattice mismatch for heterogeneous epitaxy is ≈5-10%, [51,52] the epitaxial relationship is proposed as Brucite(110)//Aragonite(010) and Brucite[001]//Aragonite[100].The prosperity of using tolerable lattice mismatch as the criterion in the rational design and controlled synthesis of a library of hierarchical heterostructures lends credence to the hypothesized epitaxial relationship, [53][54][55][56] which is further validated by the experimental observation of the seed of aragonite on brucite flake edges including exposed (110) plane (Figure 3H) and the direction of brucite flake edge toward the Ca concentrated core (highly possible to be aragonite, Figure 3C).By substantiating the epitaxial relationship between aragonite and brucite, a statement is made that both the directed growth of aragonite grains on the brucite and the self-assembly of brucite grains mediated by aragonite is possible to be achieved via epitaxy, providing profound insights for the construction of both manmade and natural architectures.
As to C28d-SW surface, it is composed of aragonite and calcite crystals.The aragonite on the surface of C28d-SW possesses a distinct morphology compared to C1d-SW and C14d-SW, as no assembled fusiform crystals are observed.Instead, the C28d-SW surface is composed with polyhedron grains with overlain prismatic pillars (Figure 4A).In this case, the polymorph of aragonite and calcite cannot be differentiated via the SEM-EDS characterization due to identical formula.By using confocal Raman microscopy to reveal the polymorph distribution (Figure 4B) based on their different absorption bands (Figure 4C), the overlying hexagonal prisms are identified to be solely composed by aragonite, which should be the only polymorph to precipitate in seawater, then the underneath calcite polyhedron is assigned to the parent surface.The distinct morphology of aragonite on calcite compared with that on brucite further confirms the essential role of brucite in directing the fusiform aragonite crystallization, which also suggests that other factor is involved in regulating the aragonite-calcite heterogeneous structure (Figure 4E).Between the (001) planes of orthorhombic aragonite and triagonal calcite (Figure 4F), a pseudohexagonal supercell coincidence is observed with the lattice mismatches of [110] Calcite //[100] Aragonite and [ 110] Calcite //[010] Aragonite being evaluated as ≈0.6% and ≈7.9%, respectively.The epitaxial growth of aragonite (001) plane on calcite (001) plane is then proposed that is also experimentally evidenced by the coincidence between the pseudohexagonal supercell and the hexagonal prism of aragonite.
The hexagonal aragonite-calcite heterogeneous microstructure coincides with shell built by molluscs via the similar ion secretion-precipitation process, during which hexagonal aragonite platelets orderly precipitate on calcite. [57][60] In previous studies, most attentions paid to the effect of the polymeric component in directing the formation of the hexagonal aragonite tablets. [61]The role of underneath calcite in directing aragonite growth had long been neglected until 2022, [62] when the potential epitaxial growth of aragonite on calcite was proposed based on theoretical calculation, yet until this study, the experimental observation of epitaxial growth of hexagonal aragonite on calcite is finally achieved.More broadly, the observed precipitation of metastable aragonite as the only polymorph of CaCO 3 on brucite and calcite in this study, is also tremendously relevant to the longstanding problem that metastable aragonite preferentially precipitates over stable calcite in seawater, which attracts wide interests from fields of biomineralization, paleogeometry, carbon capture, and controlled crystallization. [8]reviously, homogeneous nucleation of metastable aragonite in seawater has been disclosed to be driven by Mg 2+ , and extensive efforts have advanced the underlying theory development, [8,63,64] yet its heterogeneous nucleation is rarely discerned, despite the co-existence of aragonite with brucite/calcite in marine lives. [11,14,46,47]Here in the case of heterogeneous nucleation, we demonstrate that the Mg 2+ -driven preferential precipitation of aragonite is still valid-aragonite remains the only polymorph of CaCO 3 to precipitate on brucite/calcite in seawater.Therefore, we suggest the possible extendibility of the previously developed theories in this scope of heterogeneous nucleation. [8,63,64]n the top of that, the discovery of the epitaxial growth of aragonite with morphology readily modulated by the base material, that is, fusiform aragonite on brucite flake edge and prismatic aragonite on calcite, is unprecedented.By exploiting the unraveled epitaxial relationship, various heterogenous microstructures are possible to be facilely engineered and delicately programmed by customizing the parent surfaces to exert control over the dissolution-precipitation behavior, providing insights into mineral surface engineering, crystal engineering, and the development of biomimetic heterogeneous materials.
The described results reveal that initial dissolution plays a key role in determining surface microstructures, including the product composition, crystallographic orientation, grain morphology, and thus the surface pattern.This is the consequence of "Butterfly effect" that the initial dissolution can profoundly affect the interfacial supersaturation, hence the subsequent ion transfer and precipitation.The fluid chemistry that can influence the interfacial supersaturation thereby should also affect the surface microstructure, which is investigated in the following section.

Dissolution-precipitation mediated by fluids and the associated microstructural development
Fluid chemistry effect is surveyed by allowing C1d surfaces to react in fluidic MSW and pure water, namely, C1d-MSW and C1d-W.Though with initial dissolution of Ca(OH) 2 similar to C1d-SW, the coupled precipitation is highly affected by fluid chemistry, thus the subsequent dissolution-precipitation routes occurring on C1d-MSW and C1d-W surfaces are significantly altered, resulting in diverse surface microstructures.Due to the absence of Mg 2+ in MSW and water, no brucite is produced and calcite is the only polymorph of CaCO 3 as the product phase on C1d-W and C1d-MSW surfaces (Figure 1E). [8]The morphologies of calcite on the surface of C1d-MSW and C1d-W are compactly assembled polyhedron with round edges (Figure 5A) and loosely packed micropillars with sharp edges (Figure 5B), respectively.
On the C1d surface in water, tremendous transfer of the dissolved Ca 2+ ions from the interfacial layer to the bulk solution is observed in the first 2 days, which then mitigates but persists during the whole reaction (Note S7).Contrastingly, for C1d-MSW, negligible Ca 2+ ions fulfill the transfer to the bulk solution, which should be due to the quick precipitation at the interface.Consequently, a compact calcite product layer forms (Figure 5A,B), which is supposed to seal the underneath cement phase better.Such assumption is substantiated by the lower surface penetrability evaluated by lower A w of C1d-MSW surface (0.8 × 10 −3 kg (m −2 ⋅ s −1∕2 ), Figure S4) compared with those of standardly cured C14d (A w , 3.7 × 10 −3 kg (m −2 ⋅ s −1∕2 )) and C1d-SW surfaces (A w , 2.0 × 10 −3 kg (m −2 ⋅ s −1∕2 )) with same hydration age.Such observation suggests when no Mg 2+ ions competitively react with OH − , the two-step reaction between Ca 2+ and the pre-existed HCO − 3 at the interface that consumes OH − (Equations ( 2) and ( 3)) is still quick enough to achieve almost complete interfacial precipitation of calcite (Figure 5C).As to the case with pure water as the reaction fluid, there is negligible HCO − 3 pre-existing at the fluid/parent phase interface.The CO 2 in air should first undergo slow reaction with OH − to supply HCO − 3 ions (Figure 5D), which then migrate to the interface to allow the precipitation of calcite, leading to low precipitation rate and the escape of Ca 2+ to the bulk solution.The synergistic effect of the slow precipitation kinetics and fluid flushing should be the reason to form the loosely packed product layer with A w as 5.1 S4), which enables the continuous dissolution of deeper C 3 S in the parent phase and the gradual growth of the calcite micropillars (Figure 5D).
In this context, it is demonstrated via simply varying the fluid chemistry, the interfacial supersaturation of the precipitating product phase can be modulated; consequently, different microstructural surfaces can be engineered through dissolution-precipitation chemistry.

Effect of the as-engineered surface microstructures on surface functionality
Considering the dominative role of water permeability in affecting the durability of cement-based constructions, and the generally high water absorption level of unprotected cement-based materials, for example, ≈0.94 kg m −2 for 24 h immersion of C28d specimen, hydrophobic functionalization was used as the proof-of-concept example to survey the surface microstructure in determining surface functionality.Representative microstructural surfaces with same total hydration age of 14 days and different surface textures were chosen, including C14d surface with loosely packed submicron calcite polyhedron (Figure S3b and Figure 3E), C1d-SW surface with clusters of aragonite-brucite microspheres dispersed on flat film (Figure 2A), C1d-MSW surface with closely packed calcite micro-polyhedron (Figure 5A), and C1d-W surface with loosely packed calcite micro-pillars (Figure 5B).A simple and the same surface treatment method-immersing the samples in ethanol solution (10 vol% of H 2 O) of a typical hydrophobizing silane agent (HDTMS, 3 vol%)was employed to functionalize these surfaces.After the functionalization, the HDTMS contents on each surface (elevated C─C/C─H content characterized by XPS, Figure 6A) are of evident difference and follow the trend of C14d-HDTMS (50.96%) > C1d-W-HDTMS (41.93%) > C1d-MSW-HDTMS (15.16%) > C1d-SW-HDTMS (6.76%).
The top surface of C14d, C1d-W, and C1d-MSW are mostly composed of calcite, yet their reactivity for functionalization is distinctive, suggesting the involvement of additional catalyzing factor in the reaction system.Since the hydrolysis, condensation, and bonding of silane are all alkaline-catalyzed, the distinctive activities are most likely regulated by the Ca(OH) 2 dissolution behaviors of the specimen surfaces in the water-containing treating solution.
During the functionalization, water may penetrate through the product surface, reach, react with the unhydrated C 3 S in the sublayer, and release trace catalytic OH − .The trend of A w characterizing the water penetration capability, that is, C1d-W > C14d > C1d-SW > C1d-MSW (Figure S4), is similar to that of functionalized HDTMS content, except for the C1d-SW surface.Such coincidence justifies the alkaline catalyzing hypothesis, while the exception of C1d-SW might be due to the OH − scavenge by the excessive Mg 2+ in brucite.In this context, dissolution is an essential factor in affecting surface reactivity, and to be considered in developing effective functionalizing strategy for mineral materials.Besides, by understanding the catalytic role of unhydrated C 3 S in the sublayer, we can further estimate the reactivity of other specimens.For instance, due to the absence of unhydrated C 3 S in the sublayer of C28d-SW, and its similar surface composition-mainly composed by CaCO 3to C14d, C1d-W, and C1d-MSW, the reactivity of the C28d-SW surface should be relatively low for the HDTMS functionalization.
The surface reactivity, namely, the surface content of HDTMS, determines the inherent wettability, that is, the wettability of the atomically smooth surface with identical surface composition, which together with the surface morphology conjointly affect the apparent surface wettability.[67] Under Wenzel state, the water droplet fully wets the rough surface, increasing the contacting area compared with smooth surface.Thus, the surface roughness would amplify the intrinsic surface wettability, making the hydrophilic surface more hydrophilic.For a smooth silanized surface, the intrinsic WCA is reported to be ≈75 • (still partially hydrophilic), [68] hence only the C1d-SW-HDTMS surface with lower WCA could be under Wenzel state.As illustrated in Figure 6C, such relatively low WCA should be collectively determined by the lower surface HDTMS content and the morphology with dispersed clusters.All the other HDTMS functionalized surfaces should be under Cassie-Baxter state that the water droplets contact with a composite surface comprising the sub-strate solid and air, in which the superhydrophobic air fraction increase the apparent surface hydrophobicity.The calcite particles on C14d-HDTMS and C1d-MSW-HDTMS surfaces share similar polyhedron morphology, while for the latter, the larger and closer-packed polyhedron induce lower area fraction of trapped "air pocket", which then, combined with lower surface HDMTS content, should result in a lower WCA compared with that of C14d-HDTMS.For C1d-W-HDTMS surface, the loosely packed micropillars can lead to area fraction of water-air interface even higher than that of C14d-HDTMS and hence the highest WCA.Further, besides the static superhydrophobicity, the C1d-W-HDTMS surface also presents dynamic water repellency (Figure 6D).For C14d-HDTMS and C1d-MSW-HDTMS, the local geometric φ of the polyhedron particles is higher than the intrinsic contact angle θ, thus the direction of capillary force F should be downward, leading to inclination of water penetration. [65,68]As to C1d-W-HDTMS, the micropillars are of sharp, straight or even concave edge, which could greatly remediate the downward F, or even reverse the direction of F to upward when φ is lower than θ, thus endow the surface with the endurability for the repetitive impacts of water droplet.As such, by using a commercial hydrophobizing agent with simple immersion procedure, dynamical water repellency is achieved on the surface with sophisticated microstructures, which is facilely and spontaneously engineered by the dissolution-precipitation chemistry.
As a result, we highlight the significance of the surface microstructure in affecting surface functionality that following the same functionalization procedure, different microstructural surfaces show varying degrees of reluctance or inclination for reactions.Our findings provide insights that for the functionalizing of mineral surface, dissolution should be a key factor to consider because it can couple with surface reaction and profoundly affect reactivity.Our study also demonstrates the versatility of the dissolution-precipitation chemistry in surface engineering, enabling tunable secondary functionalization.

CONCLUSIONS
In summary, for the first time, we have successfully achieved surface engineering of diverse sophisticated and controllable microstructures on mineral surfaces in fluids through spontaneous dissolution-precipitation chemistry.The complete mechanism underlying this process has been unraveled using C 3 S-containing hydrated cement as a model system.We highlight the following findings: (1) The initial surface dissolution and fluid chemistry play key roles in determining the microstructural outcomes.(2) In seawater, surface dissolution leads to kinetically preferentially precipitation of brucite, followed by aragonite.A remarkable long-range ordered film of vertically aligned brucite flakes spontaneously forms on the mineral surface via interfacial nucleation, confined space-induced orientation, and directional growth of brucite flakes, with the thickness of these flakes and the film tunable by controlling supersaturation.Subsequently, fusiform aragonite precipitates on the brucite flake edge via epitaxy.When the dissolution is negligible, hexagonal prisms of aragonite epitaxially precipitate on calcite polyhedron ascribed to the parent surface.(3) Fluid chemistry is found to influence surface microstructure by tuning precipitation kinetics.In the absence of Mg 2+ in the fluid, the product surface consists only of calcite, with the grain morphology and packing density varying from closely packed micro-polyhedron to loosely packed micro-pillars, depending on supersaturation levels.(4) The as-engineered surface microstructures significantly impact surface functionality.By applying the same hydrophobizing treatment, these diverse microstructures results in distict reactivity, influencing surface wettability, ranging from hydrophilic to dynamically water-repellent.This study introduces a novel approach to engineering surface microstructures by leveraging the natural dissolutionprecipitation properties of minerals, and this technique can be readily extended to open new possibilities for the surface engineering of various other minerals or dissolvable materials.Besides, by using cement as model system, we provide insights into surface penetrability and protection efficiency in ubiquitous cement-based constructions, which have signficiant implications for carbon sustainability.More broadly, this work offers valuable insights into developing long-range ordered, heterogenous, and programmable microstructures, with applications in the fields of controllable synthesis, geochemistry and biomineralization.

Cement sample preparation and reaction in fluid
Cement paste with a water-to-cement ratio of 0.4 was casted into mold (20 × 20 × 20 mm), demolded after 24 h, stored in standard curing chamber (20 ± 2 • C, relative humidity > 95%) for additional 0 day, 13 days or 27 days.For reaction, 5 pieces of cement specimens was immersed in 500 mL of DI water, artificial seawater, or monovalent saline water.The aqueous solution was under stirring and routinely replaced every two days for a total conditioning duration of 13 days.After demolding, cure or reaction, the cement specimen was immersed in absolute ethanol to cease the hydration reaction and placed in a vacuum oven at 40 • C to remove moisture.

Surface functionalization
A typical hydrophobic saline agent, trimethoxyhexadecylsilane (HDTMS), was used to functionalize the conditioned cement specimen surface based on previously reported procedure. [69]HDTMS (3 vol%) was added into the mixture of DI water and ethanol (10-87 volume ratio) under stirring.Then the cement specimen was added into the solution to be functionalized for 2 h, extensively rinsed by ethanol, and dried in vacuum oven at 40 • C for 2 days.

Water absorption test
Water absorption tests were performed on the international ASTM standard (C1794) via partial submersion.The initial weights of the cement specimens and areas of the absorbing surfaces were measured first.Then, the lateral surfaces perpendicular to the surface under inspection were sealed by a commercial modified acrylate adhesive, to avoid the bypassing transport of water.The cement surface under inspection was placed face down into a tank containing a specific level of water.The inspected surface is immersed in the water with a gap left between the surface and the tank bottom by using the pin spacer.The specimen weight with adsorption time of 5 min, 30 min, 1 h, 2 h, 4 h, 8 h, 24 h were measured.

Surface characterization
The preparation procedure for specimen is to cleave a slice from each sample, perpendicular (for depth profiling) or parallel (for plane-view morphological characterization or composition characterization) to the surface for investigation.
A scanning electron microscope (SEM, Quanta 3D FEG) was used for morphology characterization, and the coupled energy dispersive X-ray spectroscopy (EDS) was used for the depth profiling of element

Fluid characterization
The ion content in the reaction fluid was monitored by an inductively coupled plasma optical emission spectroscopy (ICP-OES, SPECTROBLUE).The pH of the reaction fluid was monitored by a pH meter (Mettler-Toledo, FE28).

Transmission electron microscope characterization
The product layer on C1d-SW surface, as the white crust, was carefully cleaved and ground.The obtained powder is dispersed in ethanol and dropped on copper grid for characterization.Talos F200X scanning transmission electron microscope (STEM) with Schottky X-FEG electron source was operated to acquire high resolution TEM and high-angle annular dark-field (HAADF)-STEM images, and to perform selected area electron diffraction (SAED).

F
I G U R E 1 (A) Schematic of the dissolution-precipitation reaction on a mineral surface.Ions dissolve from the parent surface, creating an interfacial layer supersaturated with new product phases, leading to precipitation.During precipitation, simultaneous dissolution and ion transfer occur between the interface and bulk, resulting in a dynamically coupled microstructure of the progressively precipitated product and the interfacial chemistry revolution.(B) Coral polyps construct a skeleton via a similar ion secretion-precipitation process on the surface of calicodermis.The methodology by analyzing the elemental band of skeleton to track the geochemistry revolution inspires the depth profiling of the product microstructure for retracing the dissolution-precipitation history on mineral surface.XRD spectra of cement surface cured for 1 day, 14 days, and 28 days (C1d, C14d, C28d) (C) before and (D) after reaction in seawater (C1d-SW, C14d-SW, C28d-SW).The inset in (D) shows standard powder XRD pattern of brucite (JCPDS No. 86-0441).(E) XRD spectra of C1d surface after the reaction in water (C1d-W), monovalent salt water (C1d-MSW), and seawater (C1d-SW).

F I G U R E 2
Microstructure of the C1d-SW surface.(A) On the parent cement surface, dissolution induces the precipitation of a brucite film composed by vertically aligned brucite flakes with flat top surface, while a couple of radically assembled brucite flowers also form.Fusiform aragonite aggregates precipitate on top of brucite subsequently.(B) Details of face-to-face packed and vertically aligned flakes in the brucite film.(C) Elemental mapping (EDS) of brucite flowers marks the Ca concentrated cores.(D) High-angle annular dark-field (HAADF) TEM image of a few layers of brucite flakes with selected area electron diffraction (SEAD) pattern.(E) Schematic of the plane orientation in the brucite flake.(F) Top view of C1d-SW surface.(G) Detailed morphology of brucite flake edges exposed on the top surface.(H,I) Selective nucleation of aragonite grains on the brucite flake edges.faces may not restrict to {110} planes.The brucite flakes in C1d-SW surface are of lower thickness (≈49 nm) and pack more closely compared with those in C14d-SW surface (thickness of ≈88 nm).
(1) How brucite develops crystal habit as flakes?(2) Why the brucite flakes vertically aligned and assembled orderly with {100} planes preferentially oriented to the parent surface?(3) Does the epitaxial F I G U R E 3 Spontaneous growth of the heterogenous aragonite-brucite microstructure.(A) Unit-cell structure of brucite.The dashed lines indicate the non-specific interactions between hydroxyl groups from two opposing (001) planes.(B) (001) plane with the growth directions of {110} and {100} planes marked, in both of which the layers of planes are connected by covalent bonds.Schematic illustration of (C) the formation of brucite flake in diluted solution with {001} planes as the planar surface, and (D) the simultaneous growth of multiple brucite nuclei in the supersaturated interfacial layer on the parent cement surface.The confined space leads to hindered growth in the lateral direction, the vertical orientation, and the directed growth of (110) plane to form vertically assembled film.Film thickening is enabled by the ion transportation to the growth front.(E) Unit-cell structure of aragonite.(F,G) Schematic of the atomic arrangement of aragonite (010) plane on brucite (110) plane with [001] Brucite //[100] Aragonite showing the epitaxial relationship.growth of aragonite on brucite exist and what is the epitaxial relationship?

F I G U R E 4
Microstructure analysis of the C28d-SW surface and the epitaxial growth.(A) SEM images of the product layer.(B) Bright field microscopy image (top) and the confocal Raman microscopy (bottom) derived false color images of the boxed area demonstrate the distribution of aragonite and calcite.(C) Representative Raman spectra acquired from the boxed area in c show the characteristic peaks of calcite and aragonite.(D) Unit-cell structure of calcite.(E) Coincident pseudohexagonal supercell between aragonite (001) plane and calcite (001) plane with [100] Aragonite //[110] Brucite .(F) Schematic of the epitaxial growth of prismatic aragonite grain on calcite polyhedral.

F I G U R E 5
Microstructures on C1d surfaces engineered by fluidic monovalent seawater (C1d-MSW) and water (C1d-W).SEM images of (A) C1d-MSW and (B) C1d-W surface.(C) The pre-existed HCO − 3 ions in MSW quickly capture the dissolved Ca 2+ ions at the interface, to precipitate compact calcite layer on the parent cement surface, performing as the barrier for subsequent dissolution.(D) In water, the supply of HCO − 3 ions from the slow solvation of CO 2 in air retards the precipitation of calcite.The loosely packed calcite layer enables the continuous dissolution and the growth of the micropillars.
. An X-ray diffractometer (XRD, Bruker D8 Discover) with Cu Ka radiation (40 kV, 35 mA) in the 2 • range from 20 • to 70 • was applied to characterize surface crystallographic composition.The chemical composition was surveyed by X-ray photoelectron spectroscopy (XPS, ThermoFischer, ESCALAB 250Xi) with monochromated Al Kα radiation at 1486.6 eV.The Confocal Raman Microscopy (CRM, Horiba France SAS, XPlora Plus) was applied to acquire the bright field microscopy image and the coupled Raman spectra.The 532 nm laser excitation was used in combination with a 50× microscope objective.LabSpec6 Software was used to derive the false color images showing polymorph distribution.