International Concrete Abstracts Portal

Many of the papers presented in this volume were included in the two-part session Nanotechnology for Improved Concrete Performance, sponsored by ACI Committee 241, Nanotechnology of Concrete at the ACI Convention in Philadelphia, PA, on October 26, 2016. In line with the practice and requirements of the American Concrete Institute, peer review, followed by appropriate response and revision by authors, has been implemented.

In cold regions, freezing temperatures limit the construction season to few months, usually between May and September. The use of nanoparticles, which have high specific surface and vigorous reactivity, may potentially enhance the performance of concrete placed at low temperatures. Therefore, this study focused on developing concrete mixtures incorporating nano-silica which were mixed, placed and cured at -5°C (23°F) without any insulation or protection targeting field applications in late fall and early spring periods. Eight mixtures incorporating general use (GU) cement, fly ash (up to 25%), and nano-silica (up to 4%) were tested for this purpose, with water-to-binder ratios of 0.32 and 0.4. All mixtures contained a combination of calcium nitrate and calcium nitrite as an antifreeze admixture. Testing involved concrete setting time (placement), 7 and 28 days compressive strengths (hardened properties) and resistance to freezing-thawing cycles (durability). Moreover, mercury intrusion porosimetry, thermal analysis and scanning electron microscopy were performed to corroborate the trends from the macro-scale tests. It was found that nano-silica significantly improved the overall performance of concrete placed and cured at -5°C (23°F), which implicates its promising use for construction applications under low temperatures.

Due to the ubiquity of concrete in the urban environment and the upscaling of nanomaterial production, the incorporation of nanoparticles into cementitious materials has gained increased attention. This study compares the performance of various titania (TiO₂) and silica (SiO₂) nanoparticles-modified coatings, including their photocatalytic performance and the quality of their adhesion to the cementitious substrates. The photocatalytic performance with respect to air purification and self-cleaning are evaluated by nitrogen oxide (NO_x) and methylene blue (MB) dye photodegradation, respectively. The results show that the Portland cement (OPC)-based cementitious materials exhibit greater photocatalytic efficiency than calcium aluminate cement (CAC)-based ones. It is proposed that the superior performance is due to a greater proportion of finer porosity and the presence of high surface area calcium silicate hydrates (C-S-H) in OPC-based cementitious materials. Interactions between coatings and cementitious substrates are examined through wettability and adhesion. The results show that the inclusion of silica layer can affect the interaction of coated cementitious surface with water, as well as the bond strength between coating and cementitious substrate.

In this work, effects of nanosilica (NS), nanolimestone (NL), and nanoclay (NC) additions on hydration and strength of cement pastes were studied. The pastes were made with Type I ordinary Portland cement (OPC), 0 and 30% Class F fly ash (FA), and 0 or 1% nanomaterials. All pastes had a water-to-binder ratio of 0.5. Chemical shrinkage was monitored as an indication of cement hydration process. X-ray diffraction (XRD) was conducted to identify crystalline hydration products. Thermogravimetric analysis (TGA) was used to quantify calcium hydroxide (CH) and chemically bound water. The results indicate that the rate of chemical shrinkage curve can be divided into five stages, similar to that observed from the rate of cement hydration curve measured from a calorimetry test. All nanomaterials increased the rate of chemical shrinkage associated with C3S and C2S reactions; but different types of nanomaterials had different effects on the rate of chemical shrinkage associated with secondary C3A reaction. All nanomaterials improved strength of OPC paste at ages up to 28 days; but the improvement was not clear for OPCFA pastes. Through reaction with OPC and FA, NL stabilized voluminous ettringite and produced hemicarbonate (Hc) instead of less voluminous monosulfate (Ms).

In this study, nanosilica was applied to the surface of polypropylene (PP) fibers to introduce self-healing abilities when incorporated into cement-composites. When the fiber is at the site of a crack, the nanosilica can form additional hydration products through pozzolanic reaction to effectively seal the crack. Nanosilica was synthesized onto the fibers through a sol-gel process. Then the fibers were dried at room temperature or 50°C (122°F) to remove the excess solution and adhere the nanosilica particles onto the fiber surface. The existence of nanosilica was confirmed by observing the mass change before and after the sol-gel process, water absorption, soluble matter loss and microscopy. The self-healing performance of cement-composites reinforced with treated and untreated macro and micro PP fibers at dosages of 1.8kg/m3 (3.0lb/yd3) and 0.9kg/m3 (1.5lb/yd3), respectively, were evaluated through flexural strength testing according to ASTM C348. To evaluate strength recovery, samples were loaded to 60% of the peak load to induce cracking. The cracked specimens were cured for 28 days under laboratory conditions to undergo self-healing. A significant recovery in flexural strength (112.8%) was observed by using nanosilica treated micro PP fibers dried at room temperature.