ABSTRACT
Selecting of pozzolanic mineral additions to compose Portland cement requires previous determination of their reactivity. This is defined directly by the modified Chapelle test (NBR 15.895), or indirectly, by compressive strength test of lime mortar (NBR 5.751). The calcium hydroxide content consumed by pozzolan and formed hydrated compounds, are parameters in the evaluation and discussion of the standard methods used to characterize the pozzolanic activity. Determination of the lime content consumed requires, in any case, alkaline availability in the system during the test period established by the standard methods. The early consumption of lime invalidates the determination of fixed calcium hydroxide content, interferes in the compressive strength of the mortar, and finally, underestimates the reactivity of the pozzolan. This paper evaluates the pozzolanic activity of calcium hydroxide + mineral additions system, from standard methods in Brazil, and further analysis based on compression strength with lime mortar, X-ray diffraction (XRD) and thermogravimetry (TG / DTG) applied in paste of mixing ratios identical to those contained in the mortar. The combined analysis of the pozzolanic activity index (IAP) with XRD and TG / DTG aims to analyze the consumption of portlandite in the systems and demonstrate that some pozzolans have low performance when evaluated by the compressive strength. This performance is found even if pozzolans are able to consume high calcium hydroxide content to form hydrated compounds. Therefore, the indirect method, according to NBR 5.751, is not suitable to rank high reactivity pozzolans such as silica fume and metakaolin.
Keywords:
mineral additions; pozzolanic activity; compressive strength; modified Chapelle test
