UC Berkeley

Carbon dioxide emission from ordinary Portland cement manufacturing is one of the major sustainability issues facing the concrete industry. In fact, the annual worldwide CO2 emission from cement manufacturing is nearly 7% of the global emissions. Roughly 60% of these emissions come from the calcination of limestone, the main raw material for making Portland-cement clinker. The remaining CO2 emission is as a result of fuel combustion required to generate the heat necessary for the reactions forming clinker. Although considerable gains in energy efficiency have been achieved during the production of cement for the last two decades, calcination of limestone is the major concern as a source of CO2 emissions. Utilization of high-volume of by-products or natural pozzolanic material, such as basaltic ash pozzolan or fly ash as a replacement of Portland cement clinker, is a possible approach to reduce the clinker factor of Portland cement. In addition, self-consolidating concrete mixtures are being increasingly used for the construction of highly reinforced complex concrete elements and for massive concrete structures such as thick foundation due to its technical advantages such as shortened placement time, labor savings, improved compaction, and better encapsulation of rebar. Self-consolidating concrete requires utilization of high dosage of a plasticizing agent or viscosity-modifying chemical admixtures. The purpose of this study is to develop highly flowable self-consolidating concrete mixtures made of high proportions of cement replacement materials such as basaltic ash pozzolan, fly ash and pulverized limestone instead of high dosage of a plasticizer or viscosity-modifying admixtures, and characterize the effects of Portland cement replacement on the strength and durability. The two replacement materials used are high-volume finely-ground basaltic ash, a Saudi Arabian aluminum-silica rich basaltic glass and high-volume Class-F fly ash, from Jim Bridger Power Plant, Wyoming US. As an extension of the study, limestone powder was also used to replace Portland cement, alongside finely-ground basaltic ash and Class-F fly ash, forming ternary blends. Along with compressive strength tests, non-steady state chloride migration, water absorption and gas permeability tests were performed, as durability indicators, on self-consolidating concrete (SCC) specimens. The results were compared to two reference concretes; 100% ordinary Portland cement and 85% ordinary Portland cement – 15% limestone powder by weight. The high-volume of basaltic pozzolan and fly ash concrete mixtures showed strength and durability results comparable to those of the reference concretes at later ages; identifying that both can effectively be used to produce low-cost and environment–friendly self-consolidating concrete without utilizing viscosity-modifying admixture.

Even though the slump flow diameter of SCC specimens was held in the similar range by utilizing varied amount of water reducer admixture, they were not identical. To enable a precise comparison among the specimens, the mortar specimens were produced that had same cement–replacement ratios with the ones in SCC specimens utilizing basaltic ash pozzolan (NP), Class-F fly ash (FA) and limestone powder without using water reducer admixture. Overall the binary and ternary FA samples had higher strength than NP mortar samples up to 1 year. This can be attributed to the higher pozzolanic reactivity of FA compared to NP which is supported by X-ray diffraction, isothermal calorimetry and thermogravimetric analysis. The normal consistency and setting time of the mixtures were determined. It showed that cement replacement with limestone powder in the ternary blended cements containing either basaltic ash pozzolan or Class-F fly ash along with ordinary Portland cement lowered the initial and final time of setting relative to the binary blended cements containing similar ratio of cement replacement. Also, the water demand of mixtures incorporate with basaltic ash pozzolan was greater than the one with Class-F fly ash.

The influence of the basaltic ash pozzolan, Class-F fly ash and limestone powder in the binary and ternary Portland cement blends is discussed, while following the physicochemical changes such as crystalline transition, hydration kinetics, and mechanical property that are a direct result of the addition of supplementary cementitious material or filler. Selected cement pastes were characterized by X-ray diffraction (XRD), petrographic microscopy and scanning electron microscopy with energy dispersive spectroscopy, isothermal calorimetry and thermogravimetric analysis (TGA). Integrating these techniques helps to understand the fresh and hardened properties of concretes and brings new insight into the effect of basaltic ash pozzolan, Class-F fly ash and limestone powder on the hydration of Portland cement. Isothermal calorimetry analysis presents that the addition of limestone powder, for instance, increased the rate of hydration reaction relative to the control specimen. This suggests that as a result of the further participation of aluminate phases in hydration reaction, the hydration products were improved. This outcome was confirmed with the analysis of XRD results by the finding carboaluminates in the limestone powder containing blended cements. It is important to note that the enhancement of hydration reaction was not adequate to compensate for the dilution effect due to addition of limestone powder. While the replacement of ordinary Portland cement with Class-F fly ash retarded the rate of hydration reaction relative to the one with basaltic ash pozzolan at first, the reactivity of Class-F fly ash improved after 2 days of hydration and surpassed the cumulative heat of hydration of basaltic ash pozzolan. This result is supported by TGA analysis demonstrating that the mixtures containing Class-F fly ash had more hydrate water with respect to the one of with basaltic ash pozzolan. XRD analysis showed that the addition of limestone powder in the ternary cement containing either basaltic ash pozzolan or fly ash led to stabilize the transformation of ettringite to monosulfate and introduce the carboaluminates in the hydration products. TGA analysis indicated that the degree of pozzolanic reaction of fly ash was higher than the one with basaltic ash in the binary and ternary blended mixtures.

For a comprehensive analysis and quantification of emissions and global warming potential (GWP) from concrete production, life-cycle assessment was used on the concrete mixture containing Class-F fly ash. It is found that high volume, up to 55% by weight replacement of ordinary Portland cement with Class-F fly ash, or Class-F fly ash and limestone powder produces highly workable concrete that has high 28-day and 365-day strength, and extremely high to very high resistance to chloride penetration along with low GWP for concrete production.