International Concrete Abstracts Portal

This study evaluated the effect of class F fly ash (5, 10, 15, and 20%) and silica fume (20%) as partial cement replacements on bacterial crack healing. Concrete cylinders were prepared, cracked into one-inch disks, and submerged in fresh water. Healing progress was monitored over 18 weeks using microscopy and quantified through a healing index. Results showed that bacterial activity substantially improved healing compared to natural hydration in control specimens. Fly ash replacement did not prevent healing, and several disks across all percentages achieved complete crack closure. However, higher fly ash levels shortened the duration of bacterial activity, indicating sensitivity to calcium availability. At 20% fly ash, healing progressed more slowly but remained active at 18 weeks. In contrast, specimens containing 20% silica exhibited significantly lower healing efficiency, with few disks achieving full closure and overall lower healing indices. These results confirm that bacteria-based self-healing concrete remains effective with fly ash but is constrained by high silica fume content due to very low to zero calcium content in silica fume. The findings indicated that lower calcium content in supplementary cementitious material (SCM) replacement, either due to higher fly ash content with lower calcium compared to OPC or with silica fume with almost zero calcium content, with bacteria, may have a significant effect on the healing progress.

This research aims to prepare porous ceramsite with low thermal conductivity. The porous ceramsite was also used as fine aggregate to substitute the river sand in pumice concrete. Its impact on improving the thermal insulation performance of pumice concrete was thoroughly investigated. The experimental method included high-temperature calcination, transient planar heat source analysis, as well as the use of X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), and Mercury-Intrusion Porosimetry (MIP) techniques. The investigation revealed that the best calcination parameters were a preheating temperature of 400°C, a preheating duration of 25 minutes, a calcination temperature of 125°C, and a calcination duration of 25 minutes. Under these conditions, the crushing index of the porous ceramsite was determined to be 29.1%, with a thermal conductivity of 0.138 W/(m·K). It is worth noting that an increase in calcination temperature promotes the hole content in ceramsite, leading to a 52.19% increase in macropore volume and a corresponding decrease in thermal conductivity. Furthermore, as the replacement rate of ceramic aggregate increases, the thermal conductivity of pumice concrete gradually decreases, with values ranging from 18% to 34.8%. This reduction occurs because the replacement elevates the volume of coarse capillary pores and non-capillary pores in pumice concrete, increasing by 13.9 to 91.3% and 63.1 to 128.5%, respectively. Additionally, a prediction model for the thermal conductivity of pumice concrete has been established using the Mori-Tanaka homogenization method. The model's verification accuracy falls within an error range of 5%, demonstrating its effectiveness in accurately predicting the thermal conductivity of pumice concrete.

ASTM C31/C31M describes the procedure of making concrete specimens in the field. Its origin can be traced to 1920, proposing rodding or stroking each 100 mm thick layer 25 to 30 times. Concrete technology has evolved tremendously over the last century, but specimens are still prepared following this 100-year-old methodology. This paper investigates the density and compressive strength of concrete cylinders for different consolidation procedures. Mixture design variations include paste volume, water-cement ratio (w/c), aggregate grain size distribution, fly ash, and water-reducing agent. An increase in compressive strength of approximately 5 MPa can be obtained if 100 x 200 mm cylinders are rodded in four layers, 25 rods each, if the slump is not over 100 mm. For all other mixtures, the current rodding procedure of two layers, 25 rods each, is recommended. For mixtures with higher slump, two layers with less rodding per layer deliver similar strength values, but the variability is high.

To prepare the SiO2 aerogel gypsum-based lightweight thermal insulation wall materials with better water resistance, α-hemihydrate gypsum (HG) was used as the main cementitious material. By adding Portland cement (PC), fly ash (FA), and hydrated lime (HL), HG was modified. Using these materials, the HG-PC system and HG-PC-FA-HL system were constructed, respectively. The effects of inorganic admixture content on the performance of both systems were analyzed. Results show that the mechanical properties and water resistance are improved after adding a certain proportion of mineral admixtures to HG. The mechanical properties and water resistance of the HG-PC-FA-HL system are better than the HG-PC system. At the content of 9 wt% FA, 20 wt% PC, and 4 wt% HL, the 28-day strength reaches 41.07 MPa (5955 psi), the water absorption after soaking for 48 h is 12.7 %, and the softening coefficient is 0.72.

The concentration of chloride ions involves both chemical binding and physical adsorption. This study investigated how limestone powder and supplementary cementitious materials (SCMs) synergistically affect chloride concentration in cement paste, using analyses of corrosion products, pore structure, and chloride concentration coefficient. Cement pastes with 0 to 50% limestone powder and fly ash or slag were tested. Results showed that the synergy between limestone powder and fly ash or slag promoted carboaluminate formation, which completely converted to Friedel’s salt in chloride environments. This enhanced chemical binding and increased physical adsorption of chloride ions, while reducing porosity and the most probable pore diameter. When limestone powder was 5 to 25% with fly ash less than 10%, or both limestone powder and slag were 20 to 30%, the chloride concentration coefficient reached its peak. Thus, proper limestone powder content improves chloride resistance by enhancing both chemical and physical chloride binding.