Unreacted Cement Research Articles

Flexural failure of ultra-high performance concrete subjected to the alternating cryogenic and elevated temperature via acoustic emission characterization

This paper aims to reveal the mechanical evolution and potential mechanism of Ultra-high Performance Concrete (UHPC) exposed to a complex temperature-variation environment typical of Liquefied Natural Gas storage tank. Herein, the effect of steel fiber geometries (straight and hooked-end) on flexural failure of UHPCs in the range of −170 °C ∼ 200 °C was studied, and dynamic fracture evolution during the loading process was tracked using acoustic emission (AE) test. Results indicated that the flexural strength of concrete specimen enhanced at −170 °C but diminished at 200 °C in the first cycle. For multiple cycles, its improved strength subsisted until the third cycle, after which gradually decreased. Moreover, the hooked-end fiber generally had superior strengthening and toughening effects than straight fiber, except for cryogenic temperature and third cycles tests. At cryogenic temperature, the freezing of pore moisture inside specimen enhanced the matrix strength and the interfacial bonding between the matrix and fibers. At elevated temperature, the thawing, diffusion and evaporation of moisture in concrete could induce matrix dehydration and weaken the bonding effect between interfaces. In varying exposure environments, the moisture migration would also undergo secondary hydration reactions with unreacted cement particles. Finally, using AE parameters to effectively record damage progression in UHPCs exposed to extreme temperatures was also highlighted.

Acidification in cement paste in which clinker is partially replaced with limestone or silica

This paper examines the acid resistance of cement pastes where a portion of the cement clinker is replaced with limestone (LS or calcium carbonate, CaCO3) or ground silica (GS). Specifically, the work is intended to better understand the acid resistance of ASTM C595 IL cement as compared with ASTM C150 cement. The performance of OPC, OPC + GS, and OPC + LS systems were tested in sulfuric acid baths where the pH was held constant at 2.0 and 3.0 using an automated setup that uses titration to add acid. The degradation of the cement paste was measured as a function of time. Thermogravimetric analysis (TGA) was used to quantify changes in the calcium hydroxide (Ca(OH)2) and calcium carbonate (CaCO3) contents of the paste. In addition, the flexural strength of the cement paste specimens was measured. Results indicate that the dissolved sulfate and calcium concentrations due to acidification were not noticeably different for the OPC + GS and OPC + LS mixtures exposed to the same pH. However, as expected, differences were observed between the samples immersed in the solution of pH∼2 and pH∼3 sulfuric acid with the lower pH corresponding to more severe deterioration. TGA results showed that Ca(OH)2 is more susceptible to acid attack than limestone as evidenced by the larger Ca(OH)2 and sulfuric acid consumption in samples immersed at pH∼2. The additional acid consumption that is beyond the consumption of Ca(OH)2 can be explained by the acid attack of other hydration products such as CSH and unreacted cement phases. This results in a significant B3B flexural strength loss for the samples immersed in a pH∼2 as compared to those in the pH∼3 solution. The results demonstrated that the performance of ASTM C595 IL cements was promising and comparable with ASTM C150 cements.

Mechanical properties, carbon footprint and cost of ultra-high performance concrete containing ground granulated blast furnace slag

This paper deals with the possibility of formulating low-carbon UHPC by replacing cement with ground granulated blast furnace slag (GGBS). Slags of two fineness levels (420 m2/kg (SL1) and 700 m2/kg (SL2), respectively) are incorporated as volume replacements for cement at 30% and 50%. The possibility of completely replacing silica fume (SF) with superfine slag (SL2) in UHPC is also investigated.The results show that reducing the SF allows for the production of UHPC with satisfactory mechanical properties. The incorporation of 30% of GGBS (SL1 or SL2) accelerates the hydration process due to the reduced SP content and the nucleation site effect, which improves long-term compressive strength. In the presence of a high level of ordinary slag (50% of SL1), the dilution effect dominates and a decrease in compressive strength can be observed at all ages. Due to the physical effect of superfine slag, 30% of SL2 yields a high 28-day compressive strength (151.2 vs. 136.5 MPa for the SL2-based reference mix) despite the absence of SF particles. The relatively low bound water content, as determined by thermogravimetric analysis (TGA), indicates the presence of a large amount of unreacted cement, which contributes to the improved mechanical performance of UHPC mixes. A 28-day splitting tensile strength greater than 7 MPa is measured across all UHPC produced, with a maximum value of 9.2 MPa for concrete containing 30% of SL2 and without SF. A 50% SL2 UHPC with a very low carbon dioxide index (approx. 4.15 kg/m3/MPa) and a compressive strength of 139 MPa, similar to the SF-based reference mix, have indeed been achieved. However, this mixture is found to be the most expensive (at 496 €/m3), due to the high cost of superfine slag, in comparison with cement.

Roles of subsequent curing on the pH evolution and further hydration for CO2 cured cement pastes

While CO2 curing has been found to improve the strength and resistance to external ion penetration of concrete, the corrosion susceptibility of rebar in precast reinforced concrete can be affected by the change in pH of the concrete. This study aims to improve the knowledge on the pH evolution and further hydration of carbonated cement pastes at different depths upon subsequent standard curing (hydration). For this purpose, the influence of water-cement (w/c) ratio (0.3 and 0.4) and standard curing time on the pH changes at different layers of cement paste (<5 mm, 5–10 mm and 10–20 mm from the exposed surface) was systematically investigated. The results showed that after 7 d CO2 curing, the pH value of cement pastes with 0.3 w/c ratio remained above 12.0, while the pH value of the outer layer in the carbonated cement pastes with 0.4 w/c ratio decreased to 11.6. This reflects that the higher w/c ratio tend to improve the carbonation reaction of cement pastes. However, with 1 d of subsequent standard curing, the pH of both specimens increased significantly. Based on the TGA, XRD and SEM results, it is inferred that the further hydration of unreacted cement particles in the specimens with 0.4 w/c ratio was inhibited, attributed to the reduced contact and reaction between unreacted cement particles and water caused by the large amount of encapsulating carbonation products and the dense surface by CO2 curing. In general, it can be said that the pH reduction by CO2 curing has little or no adverse effect on the durability of reinforced concrete due to the pH recovery of the cement matrix upon subsequent hydration.

Study on Carbonation Resistance of Polymer-Modified Sulphoaluminate Cement-Based Materials

The use of tricyclic copolymer latex (AMPS) can effectively improve the carbonation resistance of sulphoaluminate cement. This paper investigated polymer AMPS and polycarboxylic acid to modify sulphoaluminate cement materials by exploring the carbonation level of sulphoaluminate cement paste and mortar and the strength before and after carbonation. Then, the optimal dosage of polymer and polycarboxylic acid was obtained so that the carbonation resistance of sulphoaluminate cement reached the best state. The compressive strength was significantly improved by adding AMPS for sulphoaluminate cement paste and mortar. After carbonation, the strength decreased and combined with the carbonation level; it was concluded that the carbonation resistance of sulphoaluminate cement materials was the best when the optimal dosage of AMPS and polycarboxylic acid was 5% and 1.8%, respectively. Due to the addition of AMPS, the hydrated calcium aluminosilicate (C-A-S-H) and hydrated calcium silicate (C-S-H) gels, generated by the hydration of sulphoaluminate cement and the surface of unreacted cement particles, are wrapped by AMPS particles. The water is discharged through cement hydration. The polymer particles on the surface of the hydration product merge into a continuous film, which binds the cement hydration product together to form an overall network structure, penetrating the entire cement hydration phase and forming a polymer cement mortar with excellent structural sealing performance. To prevent the entry of CO2 and achieve the effect of anti-carbonation, adding polycarboxylic acid mainly improves the sample's internal density to achieve the anti-carbonation purpose.

Mechanical Properties of Ultra-High Performance Concrete with Coal Gasification Coarse Slag as River Sand Replacement.

Coal gasification coarse slag (CGCS) is a by-product of coal gasification. Despite its abundance, CGCS is mostly used in boiler blending, stacking, and landfill. Large-scale industrial applications of CGCS can be environment-friendly and cost saving. In this study, the application of CGCS as a substitute for river sand (RS) with different replacement ratios in ultra-high performance concrete (UHPC) was investigated. The effects of CGCS replacement ratios on the fluidity and mechanical properties of specimens were examined, and the effect mechanisms were explored on the basis of hydration products and the multi-scale (millimetre-scale and micrometre-scale) microstructure analysis obtained through X-ray diffraction (XRD), scanning electron microscopy, and X-ray energy-dispersive spectroscopy. With an increase in the CGCS replacement ratio, the water-binder ratio (w/b), flexural strength, and compressive strength decreased. Specimens containing CGCS of ≤25% can satisfy the strength requirement of non-structural UHPC, with flexure strength of 29 MPa and compressive strength of 111 MPa at day 28. According to the XRD results and multi-scale microstructure analysis, amorphous glass beads in CGCS positively influenced ettringite generation due to the pozzolanic activity. Porous carbon particles in CGCS showed strong interfacial bonding with cement slurry due to internal hydration; this bonding was conducive to improving the mechanical strength. However, CGCS hindered hydration in the later curing stage, leading to an increase in the unreacted cement and agglomeration of fly ash; in addition, at a CGCS replacement ratio of up to 50%, an apparent interfacial transition zone structure was observed, which was the main contributor to mechanical strength deterioration.

Surface area and size distribution of cement particles in hydrating paste as indicators for the conceptualization of a cement paste representative volume element

The conceptualization of a representative volume element (RVE) of hardened cement paste for numerical homogenization of mechanical problems rests on identifying the largest discernible microstructural feature, i.e. unreacted cement grains. While the particle size distribution (PSD) of anhydrous cement is a well-controlled production parameter, the size evolution of a representative cement grain throughout hydration remained unresolved. This study analyzes digitized 3D cement paste microstructures obtained from X-ray micro-computed tomography, coupled with CEMHYD3D hydration model, and segmented by image-processing tools, to obtain the full PSD and specific surface area evolutions of unreacted grains throughout hydration. Results provided indicate a representative grain size in the range of 30−40μm regardless of hydration elapsed, implying a cement paste RVE should amount to 150−200μm to realistically represent cement grains. The PSD shape remained self-similar and two distinctive hydration regimes were identified, differing in dissolution rate and specific surface area decrease, correlating with calcium sulfate reactivity peak. Both measures provide easily accessible microstructural features that may be used for constructing artificial RVEs of hardened cement paste in micromechanical models and related simulations, resting on experimental data.

Optimising confocal Raman microscopy for spectral mapping of cement-based materials

Raman spectroscopy combined with confocal imaging, i.e. confocal Raman microscopy (CRM) is a relatively new technique with huge potential for high-resolution chemical mapping of phase composition and spatial distribution in cement-based materials. However, the effects of sample preparation and various operating parameters on mapping quality has not been systematically studied. This paper optimises CRM for spectral mapping of carbonated and non-carbonated cement-based materials. The effects of sample preparation and scanning parameters on the detection of four main phases (calcite, portlandite, ettringite and unreacted cement) were investigated. Results show that although freshly cut cementitious samples can be analysed as-is, the Raman signals improve with short gentle drying and surface grinding/polishing prior to analysis. Increasing laser power, exposure time and scan accumulation, and short laser wavelength yields higher signal-to-noise (SNR) ratio in the obtained spectrum. The use of a 4.15 mW laser power, 2 s exposure time and scan accumulation of 2 with 532 nm laser represents a good operating condition for Raman analysis of cement-based materials. This produces SNR > 10 for all investigated phases at short testing time and low risk of laser-induced damage. Microcracking caused by localised heating during closely-spaced mapping can be limited by impregnating the sample with epoxy to protect the microstructure. We show for the first time that CRM can be used to quantify the volume fraction of calcium carbonate and portlandite at high resolution when combined with SEM. The advantages and limitations of CRM for mapping cement-based materials are discussed.

Real-time monitoring of carbonation of hardened cement pastes using Raman microscopy.

This study investigated the feasibility of Raman microscopy for monitoring early surface carbonation of hardened cement pastes in real time for up to 7 days. Samples were exposed to natural carbonation (440 ppm CO2) and accelerated carbonation (4% CO2), and the evolution of calcium carbonate (CaCO3) polymorphs, portlandite, ettringite, C‐S‐H gel and unreacted cement particles was followed. Results showed that calcite is the main polymorph formed under both natural and accelerated carbonation. Under accelerated carbonation, the formation of calcite on the sample surface completed within 1 day whereas under natural carbonation, the formation of calcite is expected to continue beyond 7 days. The contents of portlandite and ettringite decreased rapidly under accelerated carbonation but much more gradually under natural carbonation. However, calcium silicate minerals in unreacted cement particles remained unchanged throughout the carbonation processes. Overall, this study demonstrated that Raman microscopy is a valuable tool for non‐destructive real‐time imaging of surface carbonation in cement‐based materials.

Hydrostatic compression and pressure phase transition of major Portland cement constituents – Insights via molecular dynamics modeling

The complex composite material cement paste (CP) is under high pressures in underwater applications and when impact loading occurs. The mechanical behavior of cement paste to hydrostatic compression results from mechanical deformations of each phase, including unhydrated and hydrated minerals. Molecular Dynamics was used to study the atomistic deformation of individual unhydrated cement phases with increasing hydrostatic pressures. The pressure-specific volume Birch-Murnaghan equation of state (EoS) and the bulk modulus at zero pressure were determined for each phase. Results show that the bulk modulus and compressibility are pressure dependent. For tricalcium silicate (C3S), dicalcium silicate (C2S), and tricalcium aluminate (C3A), the bulk modulus increases, while the volume compression decreases with increasing pressure. The C3S and C3A phases are stable during hydrostatic compression and exhibit isotropic behavior. The C2S phase is not stable and shows anisotropic behavior. These results explain the effect of unreacted cement clinkers on cement paste mechanical behavior under high pressure based on the response of individual phases.

Influence of effective stress and transport on mechanical and chemical alteration processes at the Cement-Caprock interface

Fractures along interfaces between host rock and wellbore cement have long been identified as potential CO2 leakage pathways from subsurface CO2 storage sites. As a consequence, cement alteration due to exposure to CO2 has been studied extensively to assess wellbore integrity. Previous studies have focused on the changes to either chemical or mechanical properties of cement upon exposure to CO2-enriched brine, but not on the effects of loading conditions. This paper aims to correct this deficit by considering the combined effects of the fracture pathway and changing effective stress on chemical and mechanical degradation at conditions relevant to geologic carbon storage. Flow-through experiments on fractured cores composed of cement and tight sandstone caprock halves were conducted to study the alteration of cement due to exposure to CO2-enriched brine at 3, 7, 9, and 12 MPa effective stress. We characterized relevant reactions via solution chemistry; fracture permeability via changes to differential pressure; mechanical changes via micro-hardness testing, and pore structure changes via x-ray tomography. This study showed that the nature and the rates of the chemical reactions between cement and CO2 were not affected by the effective stress. The differences in the permeability responses of the fractures were attributed to interactions among the geometry of the flow path, the porosity increase of the reacted cement, and the mechanical deformation of reacted asperities. The suite of observed chemical reactions contributed to change in cement mechanical properties. Compared to the unreacted cement, the average hardness of the amorphous silica and depleted layers was decreased while the hardness of the calcite layer was increased. Tomographic imaging showed that preferential flow paths formed in some of the core-flood experiments, which had a significant impact on the permeability response of the fractured samples. We interpreted the observed permeability responses in terms of competition between dissolution of cement phases (leading to enhanced permeability) and mechanical deformation of reacted regions (leading to reduced permeability).

The Synergistic Effect of Secondary Raw Materials and Nano Additive on the Properties of Cement Matrix

The purpose of the research presented in this paper was to analyse the synergistic effect of nano SiO2 (NS), fluidized catalytic cracking catalyst waste (FCCCw) and calcinated at 700°C temperature paper sludge waste (PSw) on the properties the cement matrix. Nano materials are known to reduce voids between cement particles, accelerate cement hydration, and increase the strength of cement matrix. A certain amount of FCCCw and PSw in the cement mix also accelerates cement hydration and increase the compressive strength of the system, but it is important to analyse the synergistic effect of these materials on the properties of cement matrix. The results showed that both waste materials are active pozzolanic additives. The optimal quantities in terms of positive effect on the cement matrix are 10% of FCCCw and 2.5% of PSw. The paper presents the results of XRD analysis, density, ultrasonic pulse velocity and compressive strength tests with hardened cement paste. A 22% increase in compressive strength compared to the control specimen was observed in specimens where 10% of cement was replaced by FCCCw and where NS was additionally added. The replacement of cement by 2.5% of PSw increases the compressive strength about 7%. Moreover, the compressive strength of specimens containing 10% of FCCCw, 2.5% of PSw and NS increases even though the content of cement is reduced. Hardened cement paste specimens containing FCCCw, PSw, and NS have better density and ultrasonic pulse velocity values. XRD analysis showed that a combination of FCCCw, PSw, and NS added to the cement mix increases the amount of calcium hydrosilicates formed, reduces the amount of free portlandite and unreacted cement minerals (allite and belite).

Tracking the hydration of antifreeze treated cement paste at subfreezing temperatures using the TDR technique

Prior work to characterize the hydration products in a cement paste prepared with a sodium nitrite-based antifreeze admixture that effectively promoted hydration at subfreezing temperatures did not reveal anything unusual in the chemical composition, suggesting that the nature of the antifreeze action was physical (i.e., preventing the freezing of water) rather than chemical. To confirm this, the time domain reflectometry (TDR) technique was used to track the evolution of the volumetric water content together with hydrated and unhydrated cement in control and antifreeze-added cement pastes that were cured at −10 °C for three weeks. The results show clear evidence of the existence of liquid water in the antifreeze treated samples, and that the amount of water and unreacted cement decrease with time, confirming the presence of ongoing hydration. Modelling results also suggest that the liquid water available for the hydration reactions is in the form of adsorbed water at subfreezing temperatures and as free water above freezing.

Heat-Induced Acceleration of Pozzolanic Reaction Under Restrained Conditions and Consequent Structural Modification.

This study investigated the heat-induced acceleration of cement hydration and pozzolanic reaction focusing on mechanical performance and structural modification at the meso- and micro-scale. The pozzolanic reaction was implemented by substituting 20 wt.% of cement with silica fume, considered the typical dosage of silica fume in ultra-high performance concrete. By actively consuming a limited amount of water and outer-formed portlandite on the unreacted cement grains, it was confirmed that high-temperature curing greatly enhances the pozzolanic reaction when compared with cement hydration under the same environment. The rate of strength development from the dual reactions of cement hydration and pozzolanic reaction was increased. After the high-temperature curing, further strength development was negligible because of the limited space availability and preconsumption of water under a low water-to-cement environment. Since the pozzolanic reaction does not directly require the anhydrous cement, the reaction can be more easily accelerated under restrained conditions because it does not heavily rely on the diffusion of the limited amount of water. Therefore, it significantly increases the mean chain length of the C–S–H, the size of C–S–H globules with a higher surface fractal dimension. This finding will be helpful in understanding the complicated hydration mechanism of high-strength concrete or ultra-high performance concrete, which has a very low water-to-cement ratio.

Prediction of Self-Healing Potential of Cementitious Materials Incorporating Crystalline Admixture by Isothermal Calorimetry

Crack formation is an inherent property of concrete structures; however, these materials also have the ability to heal cracks autogenously. External water penetrates the inside of concrete through the cracks, and unreacted cement particles present on the crack surface are rehydrated. Cracks are healed by hydration products owing to further hydration. Ground-granulated blast-furnace slag (GGBFS) and fly ash (FA) have slower reactions with water than cement. Because of this late reactivity, there is a high possibility of being present in an unreacted state inside the matrix. This study investigated the self-healing potential of supplementary cementitious materials (SCMs) such as GGBFS, FA, and calcium sulfur aluminate (CSA) expansion agents. For this purpose, isothermal calorimetry and water flow tests were performed. Experimental results showed that the self-healing potential of GGBFS and CSA expansion agent was higher than that of OPC and FA is decreased.

Investigation of Reactive Powder Concrete (RPC)

Results of investigations of reactive powder concrete (RPC) are presented in the paper. Optimization of the concrete composition was performed to achieve the highest degree of grains packing based on the optimal graining curve according to Funk for dmax = 1000 μm and dmin = 0.1 μm. Cement, silica fume, quartz and sand powder were considered in the composition. Steel fibers addition of 25% by mass was applied. A very low water–to–binder ratio, amounting to 0.2, was reached applying novel generation of superplasticizers based on polycarboxylates. The RPC mixture remained fluid during 1 hour. The diameter of slump flow according to PN-EN standard amounted to 250 mm after 60 minutes. The hardened concrete RPC displayed high strength and durability. Compressive strength reached 145 MPa after 2 days and about 200 MPa after 28 days; the bending strength exceeded 50 MPa after 28 days. After 56 freezing/defrosting cycles in the deicing salt solution, the concrete has shown minimal salt scaling of only 0.0007 kg/m2. Therefore, frost resistance of the concrete studied can be rated as very good according to PN-EN standard. The SEM pictures proved the amorphous phase of hydrated calcium silicates (C-S-H) is the dominant phase within the RPC microstructure. Usually, the C-S-H phase tightly covers the quartz grains and is in close contact with the unreacted cement grains. Crystallites of the monosulphate (AFm) were also found. The concrete microstructure was compact; pores of a few micrometers were rarely observed. The RPC porosity was measured using the mercury porosimetry. Porosity reduction by almost twice (from 10.9% down to 4.4%) was found after the RPC curing from 2 to 28 days. In the same period, a fraction of small mezopores (diameter below 20 nm) increased from 39.8% to 77.1%. Based on the research results data, presented the RPC concrete can be regarded as an interesting alternative to other construction materials of enhanced explosion resistance. Key words: Reactive Powder Concrete, strength, durability, explosion resistance

A coupled thermo-hygro-chemical model for characterising autogenous healing in ordinary cementitious materials

Experimental work has demonstrated that cracks can be healed in ordinary cementitious materials in the presence of water. The primary healing mechanisms are hydration of the unreacted nuclei of cement particles and the long-term formation of calcite. A mathematical model for simulating early-age autogenous healing of ordinary cement-based materials is proposed, which employs a coupled thermo-hygro-chemical (THC) framework and which uses a reactive water transport component to predict the movement of healing materials. A single concentration variable is employed for the healing component of the model that is derived directly from the quantity of unreacted cement and computed using a generalised cement hydration model component. The hydration component is directly linked to an expression for capillary porosity and for the porosity of the material within a healed crack. The results from a series of model simulations are in good general agreement with experimental data from tests on autogenous healing.

Effect of CO2-induced reactions on the mechanical behaviour of fractured wellbore cement

Geomechanical damage, such as fracturing of wellbore cement, can severely impact well integrity in CO2 storage fields. Chemical reactions between the cement and CO2-bearing fluids may subsequently alter the cement’s mechanical properties, either enhancing or inhibiting damage accumulation during ongoing changes in wellbore temperature and stress-state. To evaluate the potential for such effects, we performed triaxial compression tests on Class G Portland cement, conducted at down-hole temperature (80 °C) and effective confining pressures ranging from 1 to 25 MPa. After deformation, samples displaying failure on localised shear fractures were reacted with CO2–H2O, and then subjected to a second triaxial test to assess changes in mechanical properties. Using results from the first phase of deformation, baseline yield and failure criteria were constructed for virgin cement. These delineate stress conditions where unreacted cement is most prone to dilatational (permeability-enhancing) failure. Once shear-fractures formed, later reaction with CO2 did not produce further geomechanical weakening. Instead, after six weeks of batch reaction, we observed up to 83% recovery of peak-strength and increased frictional strength (15%–40%) in the post-failure regime, due to carbonate precipitation in the fractures. As such, our results suggest more or less complete mechanical healing on timescales of the order of months.

Residual Cementing Property in Recycled Fines and Coarse Aggregates: Occurrence and Quantification

AbstractThe adhered cement mortar in coarse aggregates and fines from demolished concrete has the potential to induce a residual cementing property upon recycling. A procedure for quantifying the contribution of recycled fines to the strength gain within the new mortar matrix is proposed. The strength gain is found to be more significant in recycled aggregates and fines from brick aggregate concrete than in those from stone aggregate concrete. Isothermal calorimetry measurements indicate the existence of large heat flows immediately after wetting and a prolonged initial reaction period during the hydration of recycled fines because of the presence of unreacted cement compound fractions and depleted amounts of gypsum. This phenomenon has been further confirmed, particularly in recycled fines from brick aggregate concrete, through electron microscopy observations of the formation of new gel structures caused by rehydration. The chemical compositions determined using two independent methods indicated possibl...

Nano-mechanical behavior of a green ultra-high performance concrete

As a revolutionary construction material, ultra-high performance concrete (UHPC) has been extensively studied in the last two decades and one research interest has been focused on preparing UHPC with low-priced raw materials or industrial by-products. In this study, a green UHPC which used high volumes of fly ash and river sand as part of raw materials was prepared, and the objective is to investigate the nano-mechanical behavior of the UHPC using nanoindentation. The results show that the hydration products, which account for about half of the paste by volume, are mainly high-stiffness hydrate phases, and significant quantities of unreacted cement and fly ash have higher mechanical properties than the hydration products and can function as micro-aggregates to strengthen the UHPC paste. Moreover, the mechanical properties of the paste near aggregate or fiber surfaces are similar to those of the bulk paste, which indicates that the UHPC has a strong and efficient bond at the interfacial zone. The experimental findings at the nano-scale could help to understand the macro-performance of the green UHPC.