Microestrutura E Propriedades Físicas E Mecânicas De Cimento Portland Tipo Ii Álcali-Ativado / Microstructure and Physical and Mechanical Properties of Alkali-Activated Type Ii Portland Cement

Abstract

A oportunidade gerada pela álcali-ativação de escórias granuladas de alto forno abriu perspectiva para ex-ploração desta tecnologia no tocante a outros materiais baseados no sistema SiO2-Al2O3-CaO. Apesar disso, poucos estudos evidenciam os efeitos da álcali-ativação nos cimentos Portland que contém adições de escó-ria ou de pozolana, e que elucidem o que ocorre, em termos de microestrutura e propriedades, se diferentes tipos e teores de álcalis forem utilizados. Este artigo apresenta um trabalho de investigação dos efeitos da álcali-ativação nos tempos de pega e na resistência à compressão de um tipo comercial de cimento Portland composto com escória de alto forno, e na microestrutura desenvolvida. Os resultados revelaram que a pega e o endurecimento podem ser acelerados até o ponto de serem instantâneos, e que resistências à compressão da ordem de 70% da resistência máxima prevista pelo fabricante, aos 28 dias de idade, podem ser alcança-das em menos de 24 horas. Contudo, as análises microestruturais por MEV, DRX e DSC, indicaram que a incorporação dos álcalis interfere significativamente na cinética das reações de hidratação, principalmente na formação do hidróxido de cálcio e das fases AFm e AFt, levando-as à alterações morfológicas e compo-sicionais anômalas, que podem ser instáveis e gerar o comprometimento da resistência mecânica, em idades mais tarde.

Alkali-activation of metakaolin and blast furnace Portland cement blends

“…However, it is possible that part of the portlandite generated by the hydration of Portland cement has not been consumed by pozzolanic reactions and has undergone carbonation. Hence, it is likely that the drop in the rate of development of compressive strength in later ages is related to a greater number of AFm and AFt phases formed, to the detriment of C-S-H [4,21,22] .…”

“…In aluminous cements (CACs), the formation of this phase, identified as CAH10, is common, especially when the Al2O3/CaO ratio in cement is high. These hexagonal aluminates are also unstable and are transformed into C2AH6 during hydration [4,21] .…”

“…The results also reveal the absence of portlandite and the formation of ettringite and calcium aluminates, as well as strätlingite and remaining C3S, in addition to C2S, confirming the SEM analysis. It is likely that the CAH10 diffraction peaks have been confused with those generated by the other aluminate phases [4] .…”

“…Characteristics such as resistance to acids and sulphates, low heat release during hydration and low permeability are the main reported benefits [1,2] . However, increasing the content of pozzolans, decreases the rate of strength development, a fact that makes pozzolanic cements little applicable in situations where high initial strengths are necessary [3,4] . As they are relatively inert in the presence of water, when incorporated into cement, pozzolans depend on the release of calcium ions during hydration to begin to react.…”

“…Alkali-activation of granulated blast furnace slag is a technology used for almost a century. In 1925, Odler [4] realized that alkaline sulfates present in water in contact with these slags promoted the hardening reaction. In the 1940s, researchers concluded that alkalis were responsible for awakening the latent properties of slag, for promoting pH levels capable of solubilizing the vitrified compounds present in granulated slag, promoting the formation of hydrated phases of high mechanical strength and stability in highly aggressive environments to Portland cement [7,8] .…”