UC Berkeley

After fresh water, concrete is the most consumed material in the world; however, despite its extreme importance to the construction industry, it has drawbacks that must be effectively addressed. The increasing concern with the environmental footprint of concrete along with its life-span service provides strong motivation to develop concrete materials with greater durability, strength and stiffness but without increasing the effective cost of the concrete mix proportion. The aim of this dissertation is to provide new insights on the different physical/chemical interactions of the calcium silicate hydrate (C-S-H), the most important hydrate phase in concrete, with different soluble organic compounds used to improve the particles dispersion or to increase mechanical properties of C-S-H. These new results can contribute to enhance the concrete performance and are a step forward to develop more sustainable and durable concrete materials.

The composition of all C-S-H used in this research had a Ca/Si ratio of 1.3. This composition guaranteed that the all the calcium was incorporated into C-S-H structure creating a monophasic material (i.e. without the presence of Ca(OH)2) and monomeric silicate structures that contain abundant reactive sites denominated as non-bonded oxygen. These reactive sites are thought to be potential locations where chemical bonds can be formed between the inorganic and organic phases.

The first thrust of this study was focused on the understanding between the interaction between C-S-H and polycarboxylate-based superplasticizers (PCEs), one of the most advanced dispersant polymers used in concrete. Major developments in concrete technology have been achieved with the use of PCEs to improve the concrete rheology without increasing the water content of the concrete mixture. Currently, it is possible to control the fluidity of the fresh concrete and obtain stronger and more durable structures. Therefore, there is a strong incentive to understand the interactions between PCEs and cement hydrates at the atomic scale to design new customized functional PCEs according to the ever-increasing requirements of the concrete industry. In this work, the bonding types generated between a PCE with silyl functionalities (PCE-Sil) and a synthetic calcium silicate hydrate (C-S-H) were analyzed using XRD, 29Si NMR and synchrotron-based techniques, such as NEXAFS and EXAFS. The results indicated that the carboxylic groups present in PCE-Sil interact by a ligand-type bond with calcium, which modified not only the symmetry and coordination number of the calcium located at the surface of C-S-H but also the neighboring silicon atoms of the C-S-H. In addition, the silyl functionalities of the PCE-Sil generated covalent bonds through siloxane bridges between the silanol groups of PCE-Sil and the non-bonding oxygen located at the dimeric sites in C-S-H, forming new bridging silicon sites and subsequently increasing the silicate polymerization. These results give relevant information for tailoring new functionalities of PCE in order to develop molecules with adaptation capabilities that overcome the complex variation in the concrete chemistry. Moreover, the interactions described above are valuable data to validate advanced atomistic modeling.

The second thrust of this dissertation focused on modifying the C-S-H mechanical properties by specific interaction with soluble organic compounds. The critical role of C-S-H to the concrete behavior, such as strength development, volume change over time, and durability makes C-S-H a target to enhance the properties of concrete. This can be a step-forward to produce a more sustainable and durable concrete material through a bottom-up approach. Considering the structural similarities between clays and C-S-H and the outstanding improvements of the clay/polymer properties, the modification of the C-S-H nanostructure by the interaction with two specific organic compound, a silane-based 3-aminopropyltriethoxylsilane [H2N(CH2)3Si(OC2H5)3] (APTES) and aromatic amine aniline monomer [C6H7N] (Anil), are evaluated in detail.

To obtain a detailed understanding of the strengthening mechanism of C-S-H, a combination of different techniques were used. High-pressure X-Ray diffraction synchrotron-based source (HP-XRD) was used to determine the deformations along the lattice directions and calculate the mechanical modulus. 29Si nuclear magnetic resonance (29SI NMR) provided relevant information on the polymerization degree of the silicon atoms present in C-S-H and on how their chemical environment was modified by the interaction with the different organic compounds. Other techniques such as gas permeation chromatography (GPC), 1H nuclear magnetic resonance, and thermogravimetry are used for a comprehensive characterization of the materials.

The C-S-H/APTES composites, often referred as nano-hybrid composites due to its stronger interphase (inorganic/organic) bonding, exhibited an increment of about 38% in the bulk modulus with respect to the reference sample. The stiffer behavior in all lattice direction and the consequent increment of the bulk modulus are associated to the formation of new Q2b structures through covalent bond formation, the secondary hydrogen bonds between APTES and the reactive sites of C-S-H, as well as the APTES intercalation in specific 2-D fashion allocations generate.

The final phase of this research evaluated the properties of the C-S-H/Anil nanocomposite, which obtained an increment of 20% in its bulk modulus with respect to the reference. The increment of the bulk modulus is due to the stiffer behavior displayed by all lattice directions mainly by the physical interactions with aniline monomers. Therefore, the constrained allocation configuration adopted by the aniline monomer within the missing Q2b silicon sites of C-S-H as well as the intercalation of the aniline monomers in a single layer configuration within C-S-H basal space, play the main role on the strengthening mechanisms for the bulk modulus increment in C-S-H/Anil sample. Finally, the formation of hydrogen bonds is the only chemical interaction detected between the aniline phenyl structure and the Q1 sites of C-S-H. The C-S-H and aniline interaction can be denominated as a guest effect, since the formation of strong chemical bonds are not present between the two phases.

The results reported in this dissertation are valuable inputs that introduce new insights related to the interaction at atomic scale of C-S-H with soluble polymers. The deeper understanding of features such as polymer adsorption and the improvements of the mechanical properties of the nanocomposites formed, are the baseline to conduct the development of advanced construction materials with tailored properties that allow more durable structures with lower environmental impact.