Warda Ashraf

Carbonation behavior of hydraulic and non-hydraulic calcium silicates: potential of utilizing low-lime calcium silicates in cement-based materials

Abstract This paper presents a study on the carbonation behaviors of hydraulic and non-hydraulic calcium silicate phases, including tricalcium silicate (3CaO·SiO2 or C3S), γ-dicalcium silicate (γ-2CaO·SiO2 or γ-C2S), β-dicalcium silicate (β-2CaO·SiO2 or β-C2S), rankinite (3CaO·2SiO2 or C3S2), and wollastonite (CaO·SiO2 or CS). These calcium silicate phases were subjected to carbonation reaction at different CO2 concentration and temperatures. Thermogravimetric analysis (TGA) tests were performed to quantify the amounts of carbonates formed during the carbonation reactions, which were eventually used to monitor the degree of reactions of the calcium silicate phases. Both hydraulic and non-hydraulic calcium silicates demonstrated higher reaction rate in case of carbonation reaction than that of the hydration reaction. Under specific carbonation scenario, non-hydraulic low-lime calcium silicates such as γ-C2S, C3S2 and CS were found to achieve a reaction rate close to that of C3S. Fourier transformed infrared (FTIR) spectroscopy and scanning electron microscope (SEM) tests were performed to characterize the carbonation reaction products of the calcium silicate phases. The FTIR spectra during the early stage of carbonation reaction showed formation of calcium silicate hydrate (C–S–H) from C3S, γ-C2S, β-C2S, and C3S2 phases with a similar degree of polymerization as that of the C–S–H that forms during the hydration reaction of C3S. However, upon further exposure to CO2, these C–S–H phases decompose and eventually converted to calcium-modified (Ca-modified) silica gel phase with higher degree of silicate polymerization. Contradictorily, CS phase started forming Ca-modified silica gel phase even at the early stage of carbonation reaction. This paper also revealed the stoichiometry of the Ca-modified silica gel that formed during the carbonation reaction of the calcium silicate phases using the SEM/energy dispersive spectroscopy (EDS) and TGA results.

Nanomechanical Characterization of the Carbonated Wollastonite System

This paper focuses on the nano-mechanical characterization of carbonated calcium silicate mineral (wollastonite (CaSiO3)) using nanoindentation technique. While exposed to carbon dioxide (CO2), the calcium component of wollastonite undergoes carbonation reaction which results in formation of two main products: calcium carbonate (CaCO3) and silica (SiO2). The mechanical properties of these partially reacted wollastonite systems were evaluated using the nanoindentation technique from which the reduced elastic modulus (Er) of silicate phase found to be around 38 GPa. For calcium carbonate phase this value was around 60 GPa.

Effects of High Temperature on Carbonated Calcium Silicate Cement (CSC) and Ordinary Portland Cement (OPC) Paste

This paper presents a comparative study on the effects of high temperature (~500°C) on carbonated calcium silicate-based cement (CSC), hydrated ordinary Portland cement (OPC), and hydrated OPC with 20% fly ash (FA) paste samples. The CSC is primarily composed of calcium–silicate minerals with different calcium to silica atomic ratios, such as wollastonite, rankinite, and pseudowollastonite. The major difference between CSCand OPC-based systems is that the CSC system generates strength from the carbonation reaction, while strength development of OPC-based systems depends on the hydration reaction. The microstructure of the carbonated CSC paste consist mainly of calcium carbonate, polymerized silica gel, and unreacted cement grains. The loss of stiffness due to the high-temperature exposure was found to be substantially lower for CSC samples compared with that of the hydrated OPC and OPC + FA paste samples. This observation was also consistent with the observed mass losses of these paste samples upon their exposure to high temperatures during the thermogravimetric analysis (TGA). The higher resistance of CSC paste samples to high temperature is attributed to the presence of microscopic phases which have higher decomposition temperatures than the phases present in the OPC. For the same reason, the increases in total porosities (measured using helium pycnometer) of the hydrated paste samples after exposure to 510°C were significantly higher than those of the CSC paste samples.

Investigation of Anti‐Icing Chemicals and Their Interactions with Pavement Concretes

The interactions of concrete specimens (both plain and with fly ash addition) with six different deicers was investigated by exposing them to solutions of sodium chloride (NaCl), magnesium chloride (MgCl2), calcium chloride (CaCl2), and the combinations of: sodium chloride with magnesium chloride (NaCl + MgCl2), sodium chloride with calcium chloride (NaCl + CaCl2), sodium chloride with agricultural by product – Ice Ban® (NaCl + Ice Ban®). In addition, control group of specimens was exposed to the deionized water. The exposures consisted of wet/dry (W/D) and freeze/thaw (F/T) cycles as well as a continuous storage in lime water at 23°C. The effects of various exposure conditions were evaluated based on the changes in the following: relative dynamic modulus of elasticity (RDME), ultrasonic pulse velocity (UPV), mass of specimens, length of specimens, mass of scaled material and compressive strength. In addition, absorption and chloride penetration measurements were performed for specimens exposed to various deicers at room temperature of 23°C. Finally, the qualitative visual evaluation of the appearance of the samples was also performed along with documentation of microstructural changes using the scanning electron microscopy (SEM). Among the deicer/anti‐icers tested, calcium chloride and magnesium chloride solutions caused comparatively higher degree of deterioration than other solutions. Although the ultimate extent of visual degradation of the specimens exposed to both of these deicers was very comparable, the onset of the degradation process in specimens exposed to magnesium chloride was significantly delayed when compared to the onset of deterioration of specimens exposed to calcium chloride. The best performance (least amount of damage) was observed for specimens exposed to sodium chloride solutions followed by the specimens exposed to the combination of sodium chloride with magnesium chloride and sodium chloride with calcium chloride. The test results indicate that F/T exposure conditions are much more severe than W/D regimes, even though the concentrations of deicers/anti‐icers used for F/T cycles were about 50% lower than those used for W/D cycles. Moreover, the addition of fly ash has a positive influence on performance of the concrete regardless of the type of the exposure regime.