Research on Material Processes

Carbon Neutral Cementation by Direct Carbonation

Global carbon dioxide emissions, 8-10% of which originate from the cement industry, are a growing problem. Ordinary portland cement and calcium hydroxide both have the potential to carbonate, thus consuming some of this carbon dioxide. This research investigates the carbonation of both of these materials in liquid and supercritical carbon dioxide. High levels of carbonation (>80%) are achieved within two hours in both forms of carbon dioxide, and carbonation in the presence of silica sand results in the formation of stable pellets. Though cementation quality remains to be tested, carbon neutral cementation by such a means would not only mitigate emissions from the construction industry, but could be used to consume flue gas from coal fired power plants or other major emissions sources. Results provide a basis for further investigation into the production of carbon neutral building elements, and hold the potential to revolutionize the green construction industry.

Fly Ash as a Reactive Filler in Ordinary Portland Cement

Production of ordinary portland cement (OPC) is responsible for roughly 8-10% of carbon dioxide emissions worldwide. One strategy to mitigate this source of emissions is to replace a portion of cement in concrete with a supplementary cementing material (SCM). Fly ash, an SCM made up of low-to-moderately reactive calcium aluminosilicate glass, holds great potential for use in this application. Fly ash is already used in most commercial concretes, but this is hindered by its high variability, requiring extensive testing and keeping its use limited to low replacement levels. This research uses an extensive battery of experimental characterization techniques to develop a comprehensive parameter, the network ratio, as a fly ash descriptor. Network ratio is calculated from fly ash composition, and is shown to accurately reflect material structure, reactivity, and property development in cementitious blends. Results provide a basis for systematic classification of fly ashes based on usability, and further work regarding reactivity-performance improvements of initially low reactivity fly ashes.

Bypassing Conversion Phenomena in Calcium Aluminate Cements

Calcium aluminate cements (CACs) present several advantages over ordinary portland cement (OPC), including their rapid early strength gain and abrasion resistance. However, conversion over time of the initial hydration products in CAC to hydrogarnet can lead to loss of strength and potentially disastrous failures of the material. This conversion occurs due to the relative difference in thermodynamic stability of these phases. This research utilizes the introduction of a new phase, nitrate AFm, to bypass this conversion. Nitrate AFm is formed during early hydration, and is thermodynamically stable enough to prevent the negative effects of the aforementioned phase conversion. AFm phases also can accommodate other anionic species aside from nitrate, broadening the impact of this finding. Results provide a basis for design of better CACs by promoting the formation of nitrate AFm, or other AFm phases, and for further work to extend the use of nitrate AFm in CAC to chloride capture coatings for corrosion protection.

Filler Effect in Calcium Aluminate Cements

Production of cement accounts for a sizable portion of global carbon dioxide emissions. Calcium aluminate cements (CACs), used in specialty applications, are no exception. These cements differ from ordinary portland cement (OPC), made up of primarily calcium silicates, in that they are, unsurprisingly, composed primarily of calcium aluminates. These cements may be replaced in part with mineral fillers such as limestone and quartz, as was the case with OPC. This research follows the systematic methods set forth in a previous study on filler effect in OPC, and reveals several notable differences. In CAC, limestone is not a preferred filler, and provides benefits equivalent to quartz for a given filler surface area. Additionally, water-to-cement ratio does influence hydration kinetics. As with OPC, surface area provided controls increases in reactivity. Results extend the findings of previous work to CAC, and point to the chemical differences between CAC and OPC as the point of origin of limestone’s preferred effect in the latter.

Microfluidic Size Classification of Cement Particles

As noted in many other studies (including that on Filler Effect), particle size plays a major role in reactivity of cementitious materials. In the case of cement, size also may relate to composition, yet it is not currently possible to obtain cement particles in a well defined size range for targeted investigations. This is because no current methods have high enough throughput or selective enough size separation methods. This research uses inertial microfluidics to obtain tightly bounded size fractions of cement particles. The method operates upon the principle that particles will find equilibrium positions as they flow along a thin channel, and the distance from the edges of these positions depends on particle size. At the outlet, particles of a given size will be in the same streamline, and so will all flow to the same collection bin. Results show successful separation of size classified cement particles, and provide a basis for further work to investigate the influence of irregular particle shapes and high throughput methods.

Prehydration of Ordinary Portland Cement

Upon premature exposure to water, ordinary portland cement prehydrates, where a layer of hydration product forms upon and coats cement grains. As illustrated in the figure, the prehydration layer heavily reduces the reactivity of cement. This reduction  has implications on mechanical properties of cement, especially compressive strength. Prehydrated systems show lower degrees of reaction along with lower compressive strength when compared to pristine systems. This disparity is even more pronounced when prehydration occurs with liquid water (as opposed to water vapor). This research quantifies prehydration, using an index derived from the mass loss of a prehydrated cement upon heating to 980° C. Results provide a guideline for usability of prehydrated cements, and outline mitigation strategies using fine limestone as a mineral filler to counteract reductions in reactivity and strength.

Corrosion Inhibiting Chemical Admixtures

Chemical admixtures are commonly used in concrete to accelerate cement hydration, improve workability, and control time of set (i.e. solidification). Most prominent among these is calcium chloride, an admixture that provides exemplary performance in accelerating both time of set and rate of strength gain at early ages. However, chloride ions promote corrosion (i.e. rusting and eventual failure) of steel rebar in reinforced concrete, making this admixture fundamentally incompatible for such applications. It is not an option to substitute rebar, which provides reinforced concrete with its tensile strength, and so alternate admixtures are in order. Calcium nitrate is a set accelerating admixture, which also acts to prevent corrosion. This research puts forward an evaluation of calcium nitrate, in comparison with calcium chloride. Results show that in mature cement paste specimens, calcium nitrate provides comparable strength to calcium chloride, cementing its spot as a viable alternative accelerating admixture.

Filler Effect in Ordinary Portland Cement

Production of ordinary portland cement (OPC) is responsible for roughly 8-10% of carbon dioxide emissions worldwide. One strategy to mitigate this source of emissions is to replace a portion of cement in concrete with a supplementary cementing material (SCM). The simplest such solution is to use abundant and inert (i.e. largely unreactive) mineral fillers such as limestone and quartz. However, cement pastes using too much filler often suffer from lowered reactivity and performance. To accurately assess the benefit of using fillers, one must account for dilution (i.e. using less cement), filler surface area, and interfacial and chemical properties in filler systems. In many cases, filler addition actually accelerates cement hydration reactions despite negatively impacting performance, termed the “Filler Effect”. This research couples experimental and simulation results to rigorously evaluate limestone and quartz as fillers. It is found that limestone is a preferred filler to quartz, and that dilution, manifested as changes in water-to-cement ratio, has no effect on reaction kinetics. Results provide a relative metric for judging filler quality, simple methods to predict both reactivity and performance of cement-filler systems, and commercially relevant comparisons between blended vs. interground fillers with regard to effectiveness. A basis is established for further work regarding systematic quantification of the strengths and weaknesses of mineral fillers, which provides a path toward cement-filler concretes with higher replacement levels and lower associated carbon dioxide emissions.