Effects of nanoparticles on the properties of cement-based materials

The addition of small amounts of nanoparticles can alter hydration and microstructure evolution of cement-based materials. The present study aimed at investigating the effects of nanoparticles on the properties of cement-based materials. The nanomaterials studied include nano-limestone, nano-silica, and nano-clay particles, and the cementitious materials studied are combinations of ordinary Portland cement (OPC), fly ash (FA) and metakaolin (MK). Isothermal calorimetry and chemical shrinkage measurements were performed to follow the kinetics of hydration. Set times of the pastes were measured according to ASTM C191. Rheological behavior of cement pastes was characterized using a rotational rheometer. Thermogravimetric analysis (TGA), X-ray diffraction (XRD), and Nitrogen adsorption (NAD) were conducted to analyze the microstructural characteristics of the studied cement pastes. Stepwise drying-shrinkage tests were performed on thin disc paste samples (with a diameter of 25 mm and thickness of 0.80 mm). The disc samples were dried from the saturated condition (100% RH) to 30% RH and then re-saturated to 100% RH. Properties of fresh concrete mixtures, such as air content, slump flow, and J-ring flow, were tested. Properties of hardened concretes, including compressive strength, rapid chloride permeability, freezing-thawing resistance, and free drying shrinkage, were examined. The experimental results indicate that the addition of nanoparticles, regardless the types, accelerated cement early hydration (the first 24 h), i.e., the maximum heat flow increased while the time to reach silicate and aluminate reaction peaks decreased, and reduced initial and final set times of cement pastes. Addition of these nanomaterials generally increased yield stress and viscosity of the cement paste, especially after 60 min when cement hydration began to accelerate. Nano-clay greatly affected the rheological behavior of cement pastes. Significantly higher shear stresses were required to initiate the flow. Thermogravimetric analysis (TGA) results indicate the continuous hydration enhancing effects of nanomaterials up to 28 days. When 1% nanomaterials were added to the cement or cement-fly ash paste, the amounts of both calcium hydroxide and total chemically bound water contents increased at 3 days, indicating that the nanomaterials accelerated cement hydration. At 7 and 28 days, nanomaterial addition increased the amount of total chemically bound water content while tended to decrease the calcium hydroxide content, suggesting that calcium hydroxide reacted to formed hydration product. The reaction of calcium hydroxide to form C-S-H when nano-silica was added was more profound in the cement-fly ash pastes. XRD and TGA results indicated that a new hydration product, calcium hemicarboaluminate hydrate (Hc), was formed in the nano-limestone modified cement and cement-fly ash pastes. For the cement pastes, nanomaterial addition increased chemical shrinkage values, which confirmed the hydration acceleration effects of the nanomaterials. However, for the cement-fly ash blended paste, only nano-silica addition increased chemical shrinkage of the paste at all ages tested. At 28 days, the nano-limestone and nano-clay additions actually reduced chemical shrinkage value of the paste. This may imply that in these pastes, the permeation of water was inhibited, suggesting more research on the permeability and transport properties. Regardless the types of nanomaterials, 1% nanomaterial addition improve the compressive strength of both cement and cement-fly ash pastes at all ages studied (up to 28 days). Among three nanomaterials studied, NS appeared to be the most effective one in the strength development, possibly due to its smallest particle size and highest reactivity. The results indicate that additions of the nanomaterials increased the specific surface area and the amount of gel pores (2-10 nm) in the early age (7 days) cement pastes. At 28 days, the addition of nano-silica continued to increase the specific surface area and the volume of gel pores, especially in the fly ash blended paste. However, the addition of nano-limestone reduced the specific surface area and the total amount of pores (2-100 nm) at 28 days, especially in the Portland cement paste. An additional signature pore peak located around 18 nm was observed in the nano-silica modified and 28-days fly ash blended pastes. The addition of nano-silica generally increased the total shrinkage of cement pastes. However, the addition of nano-limestone sometimes reduced drying shrinkage, especially for the later age cement pastes. The shrinkage in each relative humidity range was found to be dependent on the volume of pores evaporate in the corresponding RH range and the resistance of the paste to deformation, especially at 7 days. Nanomaterial additions and extended curing increased reversible drying shrinkage, which suggests that the volume of gel (2-10 nm) pores appeared more closely related to reversible shrinkages. The 7-day OPCFA pastes were more prone to irreversible drying shrinkage, especially with the addition of nanomaterials. Nano-limestone was applied to modify the fresh and hardened properties of self-consolidating concrete (SCC) and semi-flowable self-consolidating concrete (SFSCC). The results indicated that the addition nano-limestone further increased concrete strength and freezing-thawing resistance and reduced concrete permeability