High-performance concrete (HPC) intended for severe environments requires exceptional strength and durability. This study investigates the synergistic potential of silica fume (SF) and nano-silica (NS) in a ternary cementitious system (cement-SF-NS) to overcome the limitations of using single supplementary cementitious materials. The objectives were to determine the optimal SF/NS combination and to elucidate their synergistic effects on the performance and microstructure. An experimental matrix was designed, incorporating SF (0-20%) and NS (0-2%). The methods comprised evaluating workability, mechanical strength (from 3 to 91 days), chloride penetration resistance (via the RCM test), and microstructural evolution (assessed using isothermal calorimetry, XRD, SEM, and MIP). The results revealed that the ternary system containing 15% SF and 1.5% NS yielded optimal performance. This mix achieved 28-day compressive and flexural strengths of 85.6 MPa and 9.5 MPa, representing improvements of 88.2% and 76.5%, respectively, over the plain cement control. Furthermore, this combination reduced the chloride migration coefficient by 59.1%. Microstructural analyses confirmed a synergistic mechanism whereby NS acted as a potent nucleation catalyst, accelerating early-age hydration, while SF provided sustained pozzolanic reactivity. Together, they promoted the formation of denser C-S-H gel and refined the pore structure across both micro-and nano-scales. This work provides a scientifically grounded, optimal ternary design that simultaneously enhances workability, strength, and durability. It thus offers a robust solution for advanced concrete in aggressive service environments.
As a reinforcing material for cement-based composites, the smooth surface and lack of chemical activity of polyoxymethylene (POM) fiber result in poor adhesion to the matrix, thereby limiting the application effect. In this paper, an attempt to improve the bonding performance between the POM fibers and the matrix has been made by increasing the fiber surface roughness and surface activity with nano-SiO2 (NS) modification. The effects of NS (0 similar to 10%) modified POM fibers on the flexural and compressive strengths, bending toughness and tensile strength of mortars were studied. The deposition effect of NS on the surface of the fiber and the microstructure of the bonding interface between the fiber and the cement matrix were characterized by SEM and EDS. Results show that NS was successfully deposited on the surface of POM fiber, which greatly improved the mechanical properties of POM fibers reinforced cementing materials. To be specific, the enhancement effect on flexural strength was most obvious when the mass fraction of NS was 5% of the POM fibers to be modified, which were 16.9% and 16.4% at 7 and 28 d higher than that of the control sample, respectively; the bending toughness coefficient reached the maximum value when the mass fraction of NS was 2.5%, which was 57.9% at 28 d higher than that of the control sample; and the tensile strength reached its maximum when the mass fraction of NS was 7.5%, which was 57.2% at 28 d higher than that of the control sample.
Although magnesium ammonium phosphate cement exhibits promising thermal resistance, the effects of mix proportions on its phase evolution and performance at high temperatures remain unclear. This study investigated the effects of magnesia-to-phosphate, water-to-cement, and boric acid to-magnesia ratios on the performance of magnesium ammonium phosphate cement (MAPC) after exposure to temperatures up to 1000 degrees C. Some engineering properties and residual strength were tested and phase evolution was revealed using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Results indicated that boric acid-to-magnesia ratio (B/M) most strongly controlled high-temperature degradation. Although a lower B/M ratio reduced strength at room temperature (20 degrees C), it enhanced exposure to high temperatures residual strength by mitigating boric acid dehydration and adverse phase transformations. Notably, MAPC specimens with 5-10% boric acid achieved an optimal residual strength of 19.9 MPa at 1000 degrees C. In addition, increasing the magnesia-to-phosphate ratio (M/P) and decreasing the water-to-cement ratio (W/C) improved high-temperature strength and reduced porosity by ensuring sufficient residual MgO. At 1000 degrees C, increasing the M/P ratio from 1.5 to 3.0 elevated the residual compressive strength from 4.8 MPa to 7.8 MPa, while concurrently reducing the porosity from 42.80% to 41.38%. Reducing the W/C ratio from 0.20 to 0.15 bolstered the residual compressive strength from 7.8 MPa to 9.2 MPa, while simultaneously decreasing the porosity from 41.38% to 30.20%. MgNH4PO4 center dot 6H2O transformed into Mg3(PO4)2 after exposure to 800 degrees C. However, with 20% boric acid, the reaction between Mg3(PO4)2, MgO, and B2O3 formed plate-like Mg3(BO3)(PO4) clusters at 1000 degrees C. The associated mass loss and inter-layer microcracks were identified as the primary causes of strength reduction. This study systematically investigates the influence of M/P, W/C, and B/M ratios on the high-temperature performance of MAPC, with a focus on the mechanisms underlying the substantial impact of the B/M ratio.
The decarbonation efforts of the cement industry in the past decades have led to a huge number of innovations of alternative binders with mixed, and for many times, confusing names. This communication provides an overview on the differences of reaction mechanisms of recently fast-developing and widely studied binders, to clarify the nature of today's so-called geopolymers, and more broadly, chemico-activated cementitious materials. Two successful cases of production and utilization of chemicoactivated aluminosilicate cementitious materials are discussed, with one showing the application of alkali-activated slag-fly ash concrete as sewage pipes since 1995 in China, and the other showing the large-scale production and applications of alkali-sulfate hybrid activated cement in road construction and mining back-filling more recently.
Fuel consumption in the calciner represents 60% of the total fuel used in cement clinker production. This study investigates the feasibility of utilizing hydrogen as a sustainable alternative to fossil fuels in the calciner. A numerical model was developed to simulate a hydrogen-powered calciner, and a hydrogen suspension furnace test platform was established to investigate the coupling of H2 with pulverized coal and rice husk as a representative biomass alternative fuel. Results indicate that hydrogen significantly reduces NOx and CO emissions, which are associated with the incomplete combustion of alternative fuels. The study also discusses the mechanism by which H2 contributes to the reduction of CO, NOx, and CO2, highlighting the key role of H and OH radicals. It confirmed the feasibility of hydrogen calcination for cement by full combustion of different kinds of fuels. Additionally, a technology pathway is proposed from an engineering perspective, suggesting that using green hydrogen coupled with alternative fuels could reduce fuel carbon emissions by 80%, offering a potential route for industrial-scale hydrogen-based cement calcination.
