Synthesis and Application of Crystalline Calcium Silicate Hydrate Phases

Silicate weathering is important in many natural and engineered contexts including in the performance of cement. Cement is the most common man-made material in the world and is responsible for over 5% of annual CO2 emissions. Here, the reaction of a model calcium silicate, pseudowollastonite (CaSiO3) with aqueous solutions containing CO2 was evaluated in depth to understand how this class of reactions might be deployed in two contexts. The first application is subsurface engineering activity where permeability control is desirable for sustainable deployment of geologic carbon storage, geothermal energy, or energy storage. The second application is in the synthesis of pre-cast concrete that is low carbon and high performance.

The reaction of CaSiO3 with CO2(aq) nominally produces calcium carbonate (CaCO3) and amorphous silica (SiO2) but here, experiments suggest that the crystal structure of the parent silicate and the solution pH determine the way in which the silicate reacts with CO2 and the resulting structures of the reaction products. Batch experiments were carried out using two polymorphs of CaSiO3, wollastonite (chain-structured) and pseudowollastonite (ring-structured), at elevated temperatures and CO2(aq) concentrations. Reaction of CO2(aq) with wollastonite produced CaCO3 and SiO2, whereas reaction of CO2(aq) with pseudowollastonite produced numerous plate-like crystalline calcium silicate hydrate (CCSH) phases, along with CaCO3 and SiO2. Analyses of the resulting CCSH phases suggest that they are similar to those responsible for providing the strength and durability of Roman cements in terms of morphology and composition.

The first application of CCSH-based cement is presented in the context of controlling fluid transport in the subsurface, which is relevant to geologic carbon sequestration, oil and gas well closure, and enhanced geothermal energy production. The CCSH phases, in porous media, were compared to calcium carbonate precipitates, which are typically thought to form in the naturally carbon-rich environments in the subsurface. A suite of analytical methods including electron microscopy, synchrotron-based X-ray diffraction and fluorescence, and permeability measurements, among others, show that the CCSH phases, which formed in the presence of dissolved CO2(aq) and NaOH at pH 6.65, decreased permeability in sand columns by 2.83 orders of magnitude in 495 hours of reaction. Under the same conditions with no NaOH (pH 3.94), calcium carbonate was the predominant precipitate and led to a decrease in permeability of only 1.16 orders of magnitude. Acetic acid injected into the columns revealed that the CCSH phases were more resistant to dissolution at low pH than calcium carbonate, which could result in longer-lasting seals for undesirable fluid migration in the subsurface.

The second application addresses CCSH performance in the context of precast cement and compares characteristics of strength, durability, resistance to acid-dissolution, and environmental impact to those of Portland and carbonate-based cements. Precast mortar specimens were prepared via a novel curing technique that involved first allowing the specimens to harden in a CO2 atmosphere and then submerging them in a heated carbonate-rich solution buffered with NaOH in order for CCSH phases to precipitate. A Taguchi design of experiments was implemented to optimize the curing conditions, which yielded mortar with comparable strength as the alternatives (13.9 MPa at 7 days of curing), while possessing lower diffusivity to dissolved ions, and more resistance to acid-attack. Relative to Portland cement, a lifecycle analysis shows that CCSH cement could be produced with 85% lower CO2 emissions.