Abstract

Abstract

The ettringite (AFt) formation and damage process are investigated for cement paste subjected to external sulfate attack with and without heating treatment during curing. Slice and disc specimens were exposed to 10 g/L Na2SO4 + pH of 13. The evolutions of AFt formation, pore structure, and expansion on slice specimens were characterized through 27Al NMR, MIP, and micrometer. The surface cracks on disc specimens were observed through an optical microscope. The experimental results show that: (1) heating treatment dissolves part of AFt and accelerates the ettringite formation and material expansion; and (2) heating treatment initiates the microcracks, thus promoting the material cracking.

Abstract

Cementitious building materials like Ultra High Performance Concrete (UHPC), Self-Compacting Concrete (SCC) or concrete with low clinker content possess complex rheological properties. Due to high packing densities and the use of various additives and chemical admixtures, a huge range of non-Newtonian flow characteristics from shear-thinning to shear-thickening, visco-elastic material behaviour and structural build-up can appear. For these concretes, transient computational modelling using Computational Fluid Dynamics (CFD) requires a meaningful choice of rheological parameters and the associated boundary conditions. The authors present the rheological analysis of five cementitious pastes with low ( = 0.45) to high ( = 0.58) solid volume fraction . Rheological parameters from rheometric flow protocols are compared with empirical stoppage test results for short and steady (slump flow) and transient flow (L-Box) conditions. Following, numerical simulation with the measured rheological parameters as input parameters is compared to the experimental flow results. CFD analysis using OpenFOAM is performed for the flow in empirical stoppage tests. We found that with increasing non-Newtonian behaviour, deviations between real and simulated flow appear due to insufficient transient flow descriptions and unknown secondary effects. The results provide new insight into computational modelling of complex cementitious building materials and serve as basis for further advanced CFD based modelling and characterization of time-dependent non-Newtonian concrete flow.

Abstract

Layered double hydroxide (LDH) has a positive effect on the adsorption and immobilization of heavy metals. The compressive strength and the heavy metal ions Pb2+ immobilization of CaAl-LDH modified cement paste were studied in this paper. Through research, it was found that compared with the cement paste sample with a Pb2+ content of 1% after curing for 7d and 28d, the addition of CaAlLDH to the cement paste sample increased the compressive strength by 6.1% and 1.6%, respectively, demonstrating the enhancing effect of CaAl-LDH on the strength of cement paste. Compared with the sample containing only Pb2+, the addition of CaAl-LDH to the cement increased the immobilization rate by 0.3% after 12 hours of leaching time and showed varying degrees of improvement at different leaching times. The immobilization mechanism of heavy metals in LDH blended cement paste was revealed by microstructure characterization. It was founded that Pb2+ can replace Ca2+ in AFt, AFm, and LDH, and can co-precipitate with OH-.