Microstructure characterizations, thermal properties, yield stress, plastic viscosity and compression strength of cement paste modified with nanosilica

Abstract

In this study, the effect of nano-silica (NS) as an additive to Ordinary Portland Cement was evaluated and quantified. Scanning electron microscopy (SEM), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy analysis was used to identify the cement and NS contents. Experimental tests and modeling were conducted to quantify and predict the rheological properties of the cement in the liquid phase such as yield stress, maximum shear strength, plastic viscosity, and mechanical behavior such as compressive strength of cement after hardening. The cement modified with NS was tested at water-to-cement ratios (w/c) of 0.35 and 0.45 and temperatures ranging from 25 to 75°C. X-ray diffraction (XRD) and TGA were used to analyze the cement, nano-silica, and cement modified with nano-silica. The behavior of cement paste in the liquid phase (slurry) and hardened phase modified with different percentages of nano-silica up to 1% (by dry weight of cement) was investigated. The compressive strength of cement paste modified with nano-silica was tested from a young age up to 28 days of curing. Non-linear regression (NLR) based model was used to assess the effect of nano-silica on the rheological properties and compressive strength of cement. Replacing the cement with nano-silica substantially reduced the volume of Ca(OH)2. TGA tests showed that the 1% nano-silica additive leads to low cement weight loss up to 800°C due to the de-carbonation of CaCO3 in the hydrated compound and due to interacting the NS with the cement. The addition of NS increased the ultimate shear strength (τmax) and the yield stress (τo) by 15% to 53% and 23% to 186%, respectively based on the NS content, w/c, and temperature. An additional 1% of NS the compressive strength increased of the cement hardened by 15.1% to 72% based on the curing period, and w/c. Based on the model parameters and the experimental performance, the nano-silica is the most effective parameter in improving the properties of cement in both liquid and hardened phases.