International Concrete Abstracts Portal

Due to the high carbon emissions that result from cement production, it is desirable to limit the cement content of concrete to make it a more sustainable material. This is possible through substantial replacement of cement with supplementary materials, such as fly ash. The positive effects of this approach are twofold. First, reducing the cement content of concrete will reduce its carbon footprint. Second, fly ash is a coal combustion by-product, so essentially a waste material, which must be stored in landfills and enclosures if unused. Therefore, the productive use of fly ash by incorporating it into building materials at high volumes can help alleviate a waste storage issue. This paper is a summary of studies performed at the Center for Advanced Cement-Based Materials - Northwestern University, in collaboration with Iowa State University, relating to the activation of fly ash through nanomodification. Through seeding effects and increased reactivity, nanoparticles can accelerate cement hydration and subsequently the production of calcium hydroxide (CH), which can help activate the pozzolanic reaction of fly ash particles. Two types of nanoparticles are discussed in this summary paper: silica (SiO2) and calcium carbonate (CaCO3). The study on CaCO3 nanoparticles addresses the issue of dispersion, which is critical for nanomaterials, and the resultant effects on the hardening and early-age properties of fly ash-cement pastes. And the study on nano SiO2 focuses on determining the mechanisms underlying the effect of the pozzolanic nanoparticle on the early-age and long-term compressive strength gain of fly ash-cement mortars.

Waste glass is considered for use in a geopolymer binder systems based on the high Si content, amorphous framework structure, adequate hardness and widespread availability. A lack of Al within the system, however, must be taken into account, as Si/Al and Na/Al ratios have been shown to affect properties such as setting time, compressive strength and microstructure. Metakaolin and fly ash were added to a glass-based system, lowering the Si/Al and bringing Na/Al closer to unity. Mortars made using 100% glass as well as 25 and 50% of fly ash or metakaolin by mass were activated with 10M NaOH and cured at 80°C for 24 hours. Microstructural characterization of fracture surfaces and thin sections as well as compressive strength and degree of reaction data was collected. The 100% glass mixture (Si/Al – 8.39, Na/Al – 1.61) and 25% metakaolin (Si/Al – 4.96, Na/Al – 0.97) mixtures showed a dense, continuous microstructure. The 25% MK mix resulted in a 1-day f’c of above 5000 psi (35 MPa), while the 50% metakaolin mixture (Si/Al = 3.45, Na/Al – 0.69) developed little strength and had a low-density microstructure, possibly due to the high water demand. Mixtures containing fly ash resulted in reasonable compressive strengths and moderately dense microstructures.

Geopolymers are made from natural or waste aluminosilicate powders that come from a variety of sources and have highly variable compositions. These powders are mixed with caustic solutions, which must be selected carefully to optimize strength and durability. Geopolymer cement can be designed by tailoring caustic solution composition to the reactive phase composition of the solid component of the mixture; however, assessing which phases are reactive is challenging for complex and heterogeneous solids such as fly ash. Previous research has suggested that scanning electron microscopy and multispectral image analysis (SEM‐MSIA) can be used to identify and quantify the glassy phases in fly ash and allows for the determination of how these phases dissolve over time in caustic solutions. In this study, a Class F fly ash was analyzed for phase content using x‐ray diffraction and Rietveld analysis (RQXRD) and SEM‐MSIA, which identified multiple glassy phases in the fly ash. Next, the fly ash was suspended in 8 M NaOH and tested at various time intervals with SEM‐MSIA to track changes in the amounts of each individual glassy phase initially identified in the fly ash. The results showed that for this fly ash, all of the glassy phases identified were reactive in the alkaline solution and decreased in amount after being subjected to the sodium hydroxide solution. Two distinct reaction products were identified for this fly ash as well.

Compared to the hydration process of traditional Portland cements, phosphate-based cements rely on an acid/base reaction process to quickly achieve strong, lightweight and durable binders with lower embodied energy. Since the binding action relies on the chemical composition of the initial components, the rheological and mechanical properties of the resulting ceramic concretes can also be influenced by other mix components including fly ash, fillers and aggregates. This paper reports on an ongoing study examining properties of concretes produced with magnesium potassium phosphate cement binders that incorporate fly ash contents of up to 80% of the total binder mass. Highly flowable mixes were developed with setting times that could be controlled through use of commonly available admixtures. The highest compressive strength of the binders and mortars were achieved when the fly ash content was 50% of the total binder mass. The produced binders and sand mortars had densities of 1800 kg/m3 [3034 lb/yd3] and 2100 kg/m3 [3540 lb/yd3] and compressive strengths of 35 MPa [5.0 ksi] and 60 MPa [8.7 ksi] after 28 days of simple ambient curing. Decreases in both strength and density were observed as the fly ash content was increased further, but remained within practical ranges for common construction applications with high fly ash contents.

Over the last few decades there has been an increasing interest in low-carbon-foot-print binders that can replace hydraulic cement in concrete mixtures. Given that hydraulic cement is so ingrained in the building materials industry, alternative binders –in addition to offering a low carbon footprint– must exhibit similar or improved consistency and properties in a cost effective manner. A candidate that meets these criteria is activated high-calcium fly ash (AHCFA), which as the name implies uses high-calcium fly ash (HCFA) as main reactive powder along with an activator to increase its hydraulic activity and a setting retarder to regulate the rate of reaction. It is an attempt to get a better understanding of the factors that have greater influence in the reactivity of HCFA. The physical, chemical and crystallographic characterization as well as glass fragility analyses of several HCFA samples was paralleled with the compressive strength of their corresponding AHCFA mortars. Correlations between HCFA characteristics and the compressive strength of the resulting AHCFA mortars were sought. The results suggest that the reactivity of HCFA can be evaluated in terms of glass fragility using non-bridging oxygens per tetrahedron (NBO/T) and alumina saturation index (ASI) as main parameters.