International Concrete Abstracts Portal

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.

This study evaluates the potential to use shrinkage-reducing admixture (SRA) and pre-saturated lightweight sand (LWS) to shorten the external moist-curing requirement of ultra-high-performance concrete (UHPC), which is critical in some applications where continuous moist-curing is challenging. Key characteristics of UHPC prepared with and without SRA and LWS and under 3 days, 7 days, and continuous moist curing were investigated. Results indicate that the combined incorporation of 1% SRA and 17% LWS can shorten the required moist-curing duration because such a mixture under 3 days of moist curing exhibited low total shrinkage of 360 με and compressive strength of 135 MPa (19,580 psi) at 56 days, and flexural strength of 18 MPa (2610 psi) at 28 days. This mixture subjected to 3 days of moist curing had a similar hydration degree and 25% lower capillary porosity in paste compared to the Reference UHPC prepared without any SRA and LWS and under continuous moist curing. The incorporation of 17% LWS promoted cement hydration and silica fume pozzolanic reaction to a degree similar to extending the moist-curing duration from 3 to 28 days and offsetting the impact of SRA on reducing cement hydration. The lower capillary porosity in the paste compensated for the porosity induced by porous LWS to secure an acceptable level of total porosity of UHPC.

This paper describes an approach to predict the mechanical and fracture behavior of cement-based systems by combining thermodynamic and finite element analysis models. First, the reaction products in a hydrated cementitious paste are predicted using a thermodynamic model. Second, a pore partitioning model is used to segment the total porosity into porosity associated with gel pores and capillary pores. A property-porosity relationship is used to predict the elastic modulus, tensile strength, and fracture energy of the hardened cement paste. The paste’s modulus, fracture energy, and tensile strength, along with information on the aggregate properties and interfacial transition zone properties, are used as inputs to a finite element analysis model to predict the flexural strength and fracture response of mortars.

The apparent density of lightweight aggregate (LWA)-modified ultra-high-performance concrete composite is 2080 kg/m3, and the compressive strength is not less than 110 MPa at 28 days. Lightweight ultra-high-performance concrete (LUHPC) not only has light weight and high strength, but also reduces the consumption of raw materials and the section size of the structure, thus reducing the cost. The macroscopic properties are closely related to the pore structure characteristics, but the structural nature of LUHPC under different curing regimes and the LWA on their pore structure remain unclear. To comprehensively understand the pore structure of LUHPC and then control its properties, capillary absorption method, low-field nuclear magnetic resonance (LF-NMR), computed tomography (CT), and nitrogen adsorption (BET) technologies were used to characterize the pore structure characteristics of LUHPC. The experimental results show that there are many nanoscale pores (mainly harmful and more-harmful pores) in LUHPC. With the increase of water absorption of the added LWA, the porosity of LUHPC and the proportion of less-harmful pores increase, thus changing the pore structure of LUHPC. With the increase of temperature and pressure, the internal curing effect of LWA is accelerated. Heat treatment promotes the formation of dense additional hydrates such as tobermorite and xonotlite, and the average chain length of the hydrates and the pozzolanic reaction between supplementary cementitious material and Ca(OH)2. Steam curing increases the total porosity and coarsens the pore size while accelerating the hydration of cementitious paste. Autoclaved curing can stimulate the pozzolanic activity of inert SiO2, promote the formation of secondary hydration products, and fill the pores in the matrix. The evolution of the pore structure of LUHPC plays a key role in improving its performance due to the curing regimes and presence of LWA.

This study evaluates the acid resistance of cement mortar using granulated ferronickel slag (FNS) as fine aggregate and fly ash or ground FNS (GFNS) as a supplementary cementitious material (SCM). The deterioration was evaluated by visual inspection, and changes of mass and strength after immersion in 1% sulfuric acid solution for up to 180 days. Acid resistance was marginally reduced when 50% volume of sand was replaced by FNS. While the control specimens suffered significant spalling and strength loss, the use of fly ash or GFNS considerably reduced the deterioration. This is attributed to the formation of a protective zone and densification of microstructure by the pozzolanic reaction, as confirmed by strength activity index, permeable voids, thermogravimetric analysis, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. Overall, the specimens with FNS aggregate and GFNS or fly ash showed less deterioration than the control specimens after prolonged acid exposure.