The tricalcium aluminate (C3A) and sulfate content in cement influence the hydration chemistry, setting time and rheology of cement paste, mortar and concrete. Here, in situ experiments are performed to better understand the effect of gypsum on the early hydration of cubic (cub-)C3A and Na-doped orthorhombic (orth-)C3A. The isothermal calorimetry data show that the solid-phase assemblage produced by the hydration of C3A is greatly modified as a function of its crystal structure type and gypsum content, the latter of which induces non-linear changes in the heat release rate. These data are consistent with the in situ X-ray diffraction results, which show that a higher gypsum content accelerates the consumption of orth-C3A and the subsequent precipitation of ettringite, which is contrary to the cub-C3A system where gypsum retarded the hydration rate. These in situ results provide new insight into the relationship between the chemistry and early-age properties of cub- and orth-C3A hydration and corroborate the reported ex situ findings of these systems.
The main objective of this research is to propose a fast and effective method for evaluating cement reactivity to sulfate attack. Resistance to sulfate attack was measured by determining the combined sulfate in cement paste samples with exposure to Na 2 SO 4 solution (2810 and 45426ppm of SO 4 2-) and MgSO 4 solution (2810 and 54424ppm of SO 4 2-), at a high temperature (65°C). The samples of cement paste were triturated (powdered) in the proposed method. Using this procedure, the attack was accelerated and the variables that are a function of the porosity and permeability of the paste were eliminated. This method has the advantage of eliminating the diffusion interference as well as accounting for the chemical reaction. Despite the similarities of the chemical composition of the cements studied herein, the results of combined SO 3 showed that the Class G cement was somewhat less susceptible to the sulfate attack because it has a larger amount of Fe 2 O 3. The ettringite formation was observed in the attack for both sulfate salts, while the gypsum formation was observed in the attack for MgSO 4 (results of TG and XRD). The blended cement presented the same results as the others; however, when the Ca(OH) 2 was totally consumed, the decalcification of the C-S-H, by XRD was observed.
ResumoA substituição parcial de cimento Portland por resíduo de bauxita (RB) resulta em produtos com, no mínimo, propriedades similares às dos produtos fabricados com o ligante puro. No entanto, as interações físico-químicas entre os diferentes tipos de cimento e o RB durante a reação de hidratação ainda é pouco explorada em literatura. A elevada quantidade de sódio e a presença de Al 2 O 3 , SiO 2 e Fe 2 O 3 são fatores que afetam a formação dos produtos hidratados e dependem do tipo de cimento utilizado na mistura. Sendo assim, este trabalho foi realizado com o objetivo de avaliar o impacto da presença do resíduo de bauxita em associação ao cimento Portland simples nas primeiras idades de hidratação. Calorimetria de condução isotérmica, difração de raios X, análise termogravimétrica e espectroscopia de infravermelho foram as técnicas escolhidas para o monitoramento. Os resultados ilustraram que a presença do resíduo coletado na planta da Alcoa (Poços de Caldas) aumenta o tempo de indução, o consumo de portlandita, e as quantidades de etringita e monossulfoaluminato formadas. Ao mesmo tempo, houve formação de silicoaluminato de sódio e bicarbonato de sódio, devido às reações com silicatos, aluminatos e com o carbonato do cimento. Palavras-chave: resíduo de bauxita, cimento, hidratação, termogravimetria, DRX, infravermelho. Abstract The partial substitution of Portland cement by bauxite residue (BR) results in products with
RESUMO: Este estudo apresenta uma revisão da literatura sobre o tema impacto de variáveis atmosféricas em materiais da construção civil. Ao longo do trabalho é apresentada a definição das variáveis atmosféricas e a relação destas com a degradação de materiais cerâmicos, metálicos, poliméricos e madeiras. Por exemplo, a temperatura do ar pode causar dilatação e retraimento em materiais cerâmicos causando o colapso desse material. Essa variável atmosférica também é responsável pela alteração estrutural de polímeros. Já os materiais metálicos sofrem corrosão pelo efeito da umidade do ar; nas madeiras, esta variável favorece o desenvolvimento de agentes biológicos degradadores. Por fim, destaca-se que este estudo serve de referência para graduandos em engenharia civil e arquitetura bem como para aqueles interessados no tema. ABSTRACT: This study presents a review of literature on the subject impact of atmospheric variables in the construction materials. Throughout the paper, we present the definition of atmospheric variables and their relationship to the degradation of ceramic, metallic and polymeric materials and wood. For example, the air temperature can cause dilatation and retraction of ceramic materials causing the collapse of the material. This atmospheric variable is also responsible for structural polymers change. Metallic materials corrode by the humidity effect. While, in the woods, this variable favors the development of degrading biological agents. Finally, it is emphasized that this study serves as a reference for graduate students in civil engineering and architecture as well as for those interested in the topic.
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