Fly Ash Blended Cement Paste Research Articles

Viscosity and water demand of limestone- and fly ash-blended cement pastes in the presence of superplasticisers

The rheological behaviour of fresh cement has a direct effect on the microstructural development of mortar and concrete. Inasmuch as the presence of mineral additions impact cement paste rheology and consequently its permanent microstructure and strength, a full understanding of blended cement behaviour should be pursued. The present study addresses the joint effect of mineral additions (limestone and fly ash) and superplasticisers admixtures on the viscosity and water demand of cement pastes.Cement pastes were prepared with 10, 30 or 50 wt% limestone or fly ash as mineral admixtures. Melamine-, naphthalene- and polycarboxylate-based superplasticisers were used. Paste rheology was studied in terms of variations in yield stress and viscosity with the solids content and amount of mineral additions added. The strength and microstructure of the blended cement pastes were determined at viscosity values of 1.5 Pa·s. in the presence of superplasticisers.The findings showed that the Krieger–Dougherty equation could be used to determine the effect of solids content on the apparent viscosity of limestone- and fly ash-blended cement suspensions, as well as the effect of superplasticisers. Adding less than 30% limestone to cement had no effect on paste rheology: i.e., the w/c ratios for minimum and optimal workability were similar to the ratios for ordinary cement. However, adding fly ash did lower the minimum water demand, and the optimal amount of water needed for suitable fluidity. The inclusion of 10% of either addition raised paste strength, while higher proportions 30 or 50%) had the opposite effect. The use of mineral additions reduced the effectiveness of cement superplasticisers.

Time-Dependent Rheological Changes of Fly Ash Blended Cement Paste

Abstract Cement-based materials are known to be time-dependent, especially at early ages. The time-dependent flow-behavior change controls the workability and compactability of fresh concrete. This time-dependent nature of cement-based materials is associated with the cement hydration and the consequent microstructure development. Time-dependent rheological changes of fly ash blended cement paste are investigated in this study with the flow spread, needle penetration, and rheometer tests. Hydration of the fly ash blended cement paste is monitored continuously with the ultrasonic propagation method. The results show that the thixotropy of fresh cement paste is increased by lowering the water-to-binder ratios and is mitigated by replacing Portland cement with fly ash or adding superplasticizers. The yield stress changes of the cement paste are mainly affected by the curing time rather than the water-to-binder ratio and the fly ash proportions. On the other hand, the viscosity changes slightly with time. Viscosity of the fresh paste is the controlling factor on the flow spread, and the yield stress determines the needle penetration depth. Models show that the flow spread of fresh paste is related to its viscosity with a power function and the needle penetration depth is inversely related to the yield stress.

Experimental Investigation on Reaction Rate and Self-healing Ability in Fly Ash Blended Cement Mixtures

This paper investigates the hydration rate in fly ash blended cement paste and self-healing ability in mortar incorporating fly ash for long-term period. The hydration rate of fly ash and consumption of calcium hydroxide in fly ash paste containing calcium hydroxide reagents were examined at different ages and curing temperatures. Five types of fly ash blended cement paste, each of which with 10% by mass fly ash replacement ratio, were prepared for the acceleration test at 80&degC. Four fly ash cement pastes, with two types of fly ash and with fly ash replacement ratios by mass 10% and 30%, were tested to measure the rate of reaction of fly ash in the mixtures. Ten mixtures were tested to evaluate the self-healing ability of mortar incorporating fly ash, considering different types of cement and fly ash. Compressive strength, bending strength, accelerated carbonation depth, after applying freeze/thaw cycling until 60% and 80% relative dynamic modulus of elasticity, and porosity were examined. The experimental results revealed that incorporating fly ash in cement paste would affect the hydration rate of fly ash and consumption of calcium hydroxide. Curing temperature can affect hydration rate, acceleration rate and velocity of reaction rate in fly ash cement paste. Moreover, it is confirmed that self-healing ability, carbonation rate coefficient and the pore volume modification in mortar incorporating fly ash rely on the curing conditions. Finally, it is suggested that the practical fly ash replacement ratio in concrete could be 10% to 15% and 11% to 20% for water to cement ratios 0.50 to 0.55 and 0.55 to 0.60, respectively.

Adsorption of superplasticizers in fly ash blended cement pastes and its rheological effects

The adsorption of superplasticizers in fly ash blended cement paste and its rheological effects were investigated. It is shown that the absorption of superplasticizer on portland cement particles is very different from that on fly ash particles. The fly ash particles have smooth surfaces and are negatively charged, so its adsorption capacity is weaker than the portland cement particles. The amount of adsorbed SP in the fly ash blended cement paste depends highly on the replacement proportion of portland cement with fly ash, and to a much less extent on the nature of the fly ash. However, the amount of adsorbed superplasticizer does not correspond well the ζ-potential of the solid particles, due the strong adsorbing capacities of the Portland cement particles. When fly ash replaces portland cement in the paste, the rheological behavior is radically changed, which is closely related to the fineness and density of the ash. The packing and agglomeration of the solid particles are the controlling factors on the rheological parameters of the fresh paste, instead of the amount and type of adsorbed superplasticizer.

Zeolite Formation in Class F Fly Ash Blended Cement Pastes

Zeolite formation in Class F fly ash blended cement pastes is under investigation. A Na–P type zeolite and Zeolite Y were synthesized from Class F fly ash and NaOH solution after 2 days of aging at room temperature and 6 days of curing at 80°C. However, no zeolites formed when KOH was used. In additional experiments, a Na–P type zeolite, Zeolite Y, and chabazite developed in cement pastes blended with Class F fly ash and NaOH solution which had been aged 2 days at room temperature and then cured 6 days at temperatures ranging from room temperature to 90°C. Seeding the pastes with natural zeolites was also investigated.