Influence of the use of accelerating admixture based on calcium nitrate in the performance of fly ash blended Portland cement mortars.

ABSTRACT

Civil construction tends to require the optimization of processes with reduced times. Therefore, it is necessary to search for alternatives that provide the required properties to the concrete, in shorter periods of time. The objective of this work is to evaluate the influence of a calcium nitrate based accelerator admixture on the properties of Portland cement mortars with different fly ash (FA) contents, in order to analyze the effect in the incorporation of the admixture, mainly if the accelerator is able to compensate the low strength contribution of the FA at early ages. Tests were carried out using isothermal calorimetry, water absorption by capillarity, consistency index and compressive strength in mixtures with admixture contents of 0%, 0,33%, 0,66% and 1,00%. Regarding setting time results, as excepted the admixture accelerated the final setting time. By the other hand, in the samples with 40% and 60% of FA, an increase of the final setting time was verified. For the 1-day compressive strengths, the admixture content that have generated the highest values was 1.00% for mortars with 0% and 40% FA. At higher ages, the admixtures led to reduce the compressive strength values of the samples without FA. On the other hand, in the samples with 40% and 60% of FA with the use of the accelerator higher compressive strength were identified. It was verified that the mortars with admixture and without addition of FA showed a reduction in the absorption index by capillarity. For the samples with 40% and 60% of FA, the lower absorption index was obtained in the samples without admixture. Therefore, it is possible to conclude that the admixture evaluated in this research accelerated the hydration reactions, generating an increase in the compressive strength at early ages, but with an insufficient increase to compensate the lower reactivity of the FA. Taking into account the different properties evaluated, it was not possible to establish an optimum admixture content for each FA content. It is necessary to evaluate which property is of greater interest to the user.

Keywords
Calcium nitrate; accelerating admixture; cement; fly ash; hydration