Chemical Mechanism Controlling Accelerated Carbonation of Alkali-Activated Slags

Abstract

Alkali-activated slag (AAS) is a promising cementitious material that can be used as a low-carbon alternative to ordinary Portland cement (OPC) in the construction industry. AAS is made by reacting ground granulated blast furnace slag, a by-product from iron production, with an alkaline activator, typically in solution form, to form a solid cement-like material. The mechanical and engineering properties of AAS have been previously studied and found to be comparable to OPC, but there are still open questions about the long-term durability of AAS and its interactions with environmental conditions that can degrade its integrity. One important degradation mechanism in OPC and AAS is carbonation, in which the material reacts with CO2 in the environment. In particular, it has been found that slag with a high magnesium content forms an AAS that resists accelerated carbonation (concentrations of CO2 greater than that of the atmosphere), which could have applications in niche fields like geological carbon sequestration infrastructure. The goal of this thesis is to elucidate the mechanisms by which magnesium acts to help AAS resist accelerated carbonation. This new insight will aid the development of AAS formulations that are highly resistant to CO2-induced degradation at high partial pressures. In the first part of this thesis, AAS was investigated under a range of accelerated carbonation conditions to determine how varying chemical and environmental factors affect the extend of decalcification of the sodium-containing calcium-alumino-silicate-hydrate (C-(N)-A-S-H) gel, the strength-giving phase of AAS, and the products formed upon carbonation. Using X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR), it was found that silicate-activated samples with high activator concentration formed weakly crystalline calcium carbonate, while lower concentration silicate activator and hydroxide activator led to greater crystallinity of the calcium carbonate. XRD results also showed that magnesium was incorporated into the calcite phase for the high-Mg sample. Thermogravimetric analysis (TGA) of carbonated AAS revealed that low-Mg AAS had a greater extent of decalcification of C-(N)-A-S-H, and thus more susceptibility to carbonation degradation. These results provide evidence for an updated mechanism for the carbonation resistance of AAS in which both magnesium and silica are incorporated into and stabilize the ACC that initially forms upon carbonation. The ACC contributes to carbonation resistance by ultimately decreasing the driving force for decalcification of the C-(N)-A-S-H gel. In the middle of this thesis, the local atomic structure of AAS before and after carbonation was investigated with spatial resolution using the relatively new synchrotron-basted technique, pair distribution function computed tomography (PDF-CT). With a resolution of 10 μm, PDT-CT was able to partially resolve the C-(N)-A-S-H gel from the unreacted slag in order to characterize the phases separately in a way not possible with bulk characterization techniques. New evidence for variability in local atomic structure and chemical composition for the C-(N)-A-S-H gel was discovered. For AAS after carbonation, the spatial distribution of the phases was determined using PDF peak intensities, and the local atomic structure of the individual phases was characterized. The local atomic structure of partially decalcified C-(N)-A-S-H gel was different for AAS with different magnesium concentrations, with the lower magnesium content being more decalcified. Analysis of the PDF peak decay in the calcium carbonate phase revealed smaller nano-crystalline sizes in the calcium carbonate in AAS with greater magnesium content. These results confirm that magnesium stabilizes a more amorphous calcium carbonate phase, leading to greater carbonation resistance in AAS. In the final part of the thesis, the chemical composition of phases in AAS before and after carbonation were mapped using nano X-ray fluorescence (XRF). This technique provided complimentary data to PDF-CT with nanoscale resolution to examine phases that were too finely intermixed for PDF-CT. Nano-XRF was able to resolve C-(N)-A-S-H gel from unreacted slag in AAS prior to carbonation. Carbonated AAS samples revealed high calcium and low calcium regions corresponding to calcium carbonate and decalcified gel, respectively. The low-Mg AAS showed that the C-(N)-A-S-H gel was decalcified to a significantly greater extent than the high-Mg AAS due to the lower calcium content in the decalcified C-(N)-A-S-H gel regions. The calcium carbonate regions also showed silicon present, giving evidence of silica incorporation that was implied by the XRD results from the first part of this thesis. Thus, silica plays an important role along with magnesium in stabilizing ACC and providing carbonation resistance to AAS.