EFFECTS ON RHEOLOGY AND HYDRATION OF THE ADDITION OF CELLULOSE NANOCRYSTALS (CNC) IN PORTLAND CEMENT

2019-06-10T17:14:45Z (GMT) by Francisco J Montes Sr.
Cellulose Nanocrystals have been used in a wide range of applications including cement composites as a strength enhancer. This work analyses the use of CNC from several sources and production methods, and their effects on rheology and hydration of pastes made using different cement types with different compositions. Cement Types I/II and V were used to prepare pastes with different water to cement ratios (w/c) and measure the changes in rheology upon CNC addition. The presence of tricalcium aluminate (cement chemistry denotes as C3A) made a difference in the magnitude of CNC effects. At dosages under 0.5vol% to dry cement, CNC reduced the yield stress up to 50% the control value. Pastes with more C¡A reduced yield stress over a wider range of CNC dosages. CNC also increased yield stress of pastes with dosages above 0.5%, twice the control value for pastes with high C3A content at 1.5% CNC and up to 20 times for pastes without C3A at the same dosage.
All the CNCs used were characterized in length, aspect ratio, and zeta potential to identify a definitive factor that governs the effect in the rheology of cement pastes. However, no definitive evidence was found that any of these characteristics dominated the measured effects.
The CNC dosage at which the maximum yield stress reduction occurred increased with the amount of water used in the paste preparation, which provides evidence of the dominance of the water to cement ratio in the rheological impact of CNC.
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Isothermal calorimetry showed that CNC cause concerning retardation effects in cement hydration. CNC slurries were then tested for sugars and other carbohydrates that could cause the aforementioned effect, then slurries were filtered, and impurities were detected in the filtrate, these impurities were quantified and characterized, however, the retardation appeared to be unaffected by the amount of the species detected, suggesting that the crystal chemistry, which is a consequence of the production method, is responsible of this retardation.
This work explores the benefits and drawbacks of the use of CNC in cement composites by individually approaching rheology and heat of hydration on a range of physical and chemical tests to build a better understanding of the observed effects.
Understanding the effect of CNCs on cement paste rheology can provide insights for future work of CNCs applications in cement composites.