28-day Compressive Strength Research Articles

Lime-activated one-part geopolymer mortars from construction, demolition and industrial wastes

This work focused on the production of one-part geopolymer mortars from construction and demolition wastes (CDW) blended with steel slag. Previous related studies on geopolymer production from CDW utilized conventional two-part geopolymers comprised of highly alkaline activator solutions and CDW materials. Thus, the study's significance consists in producing high-strength (≥35 MPa) ambient-cured mortars from CDW with predominantly concrete waste by replacing conventional highly alkaline activator solutions with an environmentally-friendly alkaline activator, Ca(OH)2 powder. Four mortar mixtures were produced with CDW contents ranging from 50 to 65 % dry weight, varying the brick waste content from 3 to 18 %. The effect of elevated temperature (40 °C) curing was also considered. The results showed that 55 % CDW content had optimum performance across all parameters studied such as compressive and flexural strengths, setting time, as well as changes in nuclear magnetic resonance (NMR)-determined pore structure (porosity and mean pore size) and x-ray diffraction (XRD)-determined degree of crystallinity over time. It had 28-day compressive and flexural strengths of 42 and 5.8 MPa, respectively, and initial and final setting times of 25 and 50 min. The importance of sufficient brick waste content in the geopolymer mixtures for effective mechanical performance is highlighted. The inclusion of concrete waste in powder form reduced compressive strength under ambient curing but improved performance at 40 °C curing. It is concluded that sustainable structural mortars can be produced by ‘just adding water’ to an optimized CDW mixture with predominantly concrete waste blended with brick waste and slag and activated by powdered Ca(OH)2.

Limestone powder-based alkali-activated materials: Influence of activator type

Limestone powder (LP)-based cementitious materials with four types of alkali activators: NaAlO2, NaOH+Al(OH)3, NaOH, and Na2SiO3, were investigated at ambient temperature. The NaAlO2 and NaOH+Al(OH)3 groups produced crystalline monocarboaluminate (Mc, 3CaO∙Al2O3∙CaCO3∙11H2O), which was associated with the second exothermic peak. TEM observations revealed that AH3 (Al2O3·3H2O) generated in the NaAlO2 group was a microcrystalline phase. This microcrystal enabled cementing LP and lamellar-shaped Mc for forming compacted microstructure. However, unreacted gibbsite (Al(OH)3) in the NaOH+Al(OH)3 group did not have such effects. Thus, the 28-day compressive strength of the NaAlO2 group (20.90 MPa) considerably surpassed that of the NaOH+Al(OH)3 group (0.81 MPa). No strength appeared in the NaOH group due to the limited cementitious properties of the newly-formed gaylussite (CaNa2(CO3)2·5H2O) and pirssonite (CaNa2(CO3)2·2H2O). The Na2SiO3 group tended to generate calcium silicate hydrate (CSH) gels, resulting in a 28-day compressive strength of 8.20 MPa. NaAlO2 was more efficient in activating LP than other activators.

Suitability of Crushed Sandcrete Block (CSB) as a Partial Replacement for Fine Aggregate in Concrete Structures

In concrete structures, debris generated from construction, renovation, and demolition of buildings is often referred to as construction and demolition waste. These waste materials may be very important to recycle for several reasons for instance, recycling materials such as glass, metal, concrete, and wood waste can lessen our reliance on virgin resources, preserve natural resources, and lessen the negative environmental effects of resource exploitation. The suitability of crushed sandcrete block (CSB) for use as a partial replacement for fine aggregate in concrete was examined. The physical and mechanical properties of the crushed sandcrete block and river sand were determined and compared. The specific gravity of the crushed sandcrete block was found to be 2.58 while that for river sand was 2.66. The concrete with compressive strength of 25N/mm2 at 28 days’ hydration period was calculated for the normal mixture as the control. The percentage mix of the fine aggregate was substituted with CSB in different mix proportions of 0:1(0%), 1:3(25%), 1:2(33.3%), 1:1(50%), 2:1(66.7%), 3:1(75%) and 1:0(100%) of crushed sandcrete block and river sand by weight as fine aggregate. The concrete cubes were cured, and compressive strength tests were carried out at 7, 14, and 28 days. It was observed that the 28-day density and compressive strength for concrete cubes with crushed sandcrete block alone as fine aggregate (i.e. 100%) were found to be 2420kg/m³ and 22N/mm² respectively compared to the 0:1(0%) proportion which was found to be 2485kg/m³ and 26N/mm² respectively. It was observed that the density and compressive strength reduced with the increasing addition of CSB in all the proportions.

Physical and high temperature properties of basalt fiber-reinforced geopolymer foam with hollow microspheres

For ultra-light geopolymer foam, it is challenging to maintain excellent mechanical properties and foaming stability while continuously reducing the density. In this study, basalt fibers (BF) and hollow microspheres (HM) were used to prepare a basalt fiber-reinforced metakaolin (MK)-based geopolymer foam (FRGF) and the workability, microstructure, physical properties, high-temperature performance, and shrinkage characteristics were systematically investigated. Results indicate that incorporating 50% HM into FRGF significantly reduces the fluidity by 31.2% and reaction rate of the geopolymer paste, slows down the decomposition rate of hydrogen peroxide, and enhances the stability of the foaming process. Additionally, the incorporation of HM increases the total porosity of the FRGF from 75.4% to 86.3%, lowers the bulk density from 0.42 g/cm3 to 0.34 g/cm3, and decreases the thermal conductivity to 0.0681 W/(m⋅K). Concurrently, the introduction of BF significantly enhances the mechanical properties of FRGF, with a 28-day compressive strength reaching 2.49 MPa. After treatment at 1000 ℃, the FRGF transformed from an amorphous gel structure to a crystalline structure. The sintering between BF and the matrix notably enhances the performance of interfacial transition zone (ITZ), further increasing the energy absorption capability and decreasing the linear shrinkage of FRGF at high temperatures (1000 °C) by 55.2%. In conclusion, 50% HM and 0.6% BF demonstrated superior mechanical properties, thermal insulation, and high-temperature performance. It offers a strategy to resolve the contradiction between the density, mechanical performance, and stability of geopolymer foams and has potential applications in the field of high-temperature insulation.

Coordinated disposal of FGD gypsum and power plant concentrated brine via preparation of α-hemihydrate gypsum

α-hemihydrate (α-HH), known for its exceptional mechanical properties, is a widely used building material. Despite its advantages, the traditional method of producing α-HH through regular pressure brine poses significant drawbacks. These include substantial industrial salt demands, high production costs, and water pollution. A more environmentally friendly α-HH preparation process has been developed to counteract these issues, combining FGD gypsum and concentrated brine. This study delves into the effects of the principal ions present in concentrated brine on the phase transition process and the microstructure of the final product. Our findings reveal that the reaction rate accelerates with increased concentrations of Ca2+, Mg2+, K+, and Na+ in concentrated brine. However, excessive Na+ concentration can alter the phase transition process, converting the product to β-HH. Additionally, an overly high concentration of KCl may prompt a part of the K+ to engage in the reaction, leading to the formation of isomeric potassium gypsum. This process can disrupt the original structure of hemihydrate gypsum. The addition of maleic acid to the salt solution was found to foster anhydrous gypsum formation. In contrast, succinic acid can decrease the crystals' aspect ratio, resulting in the growth of short columns, thereby enhancing the product's strength performance. Our research also identified that the reaction rate elevates as the pH value declines. However, an acidic environment diminishes the size of the crystal and counteracts the crystal modifier's effects. Lastly, incorporating CaCl2 and MgCl2 into concentrated brine with a low Na+ concentration can convert FGD gypsum into high-strength α-HH. This transformation results in a product boasting a 1-day compressive strength of 40.5 MPa.

Formulation of PG-FA-L composite modifier for repairing expansive soil based on the statistical mixed design method

The generation of large amounts of solid waste has led to exploration of solid waste-modified expansive soils; however, the effect of a single solid waste-modified expansive soil is not ideal. This study proposes a composite modification of expansive soils using a PG-FA-L system. Statistical analysis showed that the properties of the cured soil were significantly improved. PG and FA increased soil strength after a certain threshold, and L increased it at all stages. The presence of PG accelerated the volcanic ash reaction. Both PG and FA have a small effect on the swelling of the soil, whereas lime improves it significantly, but has a negative effect after a certain threshold. The 28-day unconfined compressive strength and deformation characteristics were used to derive the relevant regions for roadbed fill requirements and determine the optimum dosage.

Expanded titanium-bearing blast furnace slag phase change aggregate: Preparation, performance and phase change energy storage mortar application

Expanded titanium-bearing blast furnace slag (ETS), containing rich connected pores, largely accumulated, due to low hydration activity and particle strength. In this study, the pore system of ETS was fully utilized to load paraffin for fabrication of phase change aggregate (PCA), and then the PCA was used to prepare phase change energy storage mortar (PCEM) and mechanical properties, thermal performances and energy-saving efficiency were subsequently evaluated. The results indicated that the 28-day and 56-day compressive strength of mortar containing 100 % PCA reached to 7.8 MPa and 8.6 MPa. The energy saving efficiency of building reached to 8.1 %, the peak indoor temperature of building decreased by 1.2 °C and 2.3 °C in spring and summer, respectively. These facts give confidence of ETS consumption in building energy-saving materials.

Synthesis and Performance Evaluation of Nano-Calcium Carbonate-Modified Geopolymers Incorporating Fly Ash and Manganese Slag: A Comprehensive Investigative Study

Grounded in the auspicious horizons of geological polymers as alternative replacements for Portland cement and aligned with the national endeavor of constructing an ecological civilization and harnessing solid waste as a resource, this study delves into the integration of nanostructured calcium carbonate (CaCO3) into geological polymers derived from fly ash and manganese slag. Employing a comprehensive methodology involving modalities, such as X-ray diffraction, scanning electron microscopy, and attenuated total reflectance Fourier-transform infrared spectroscopy, the influence of nano-CaCO3 on the compressive strength, pore architecture, and polymerization degree of geological polymers is meticulously unveiled. The outcomes reveal that nano-CaCO3 adeptly infiltrates the intricate microporous architecture of geological polymers, thereby providing a compact and intrinsically reinforcing matrix, ultimately endowing a marked increase in compressive strength. The assimilation of nano-CaCO3 correlates conspicuously with an increase in monomeric calcium concentrations, thereby catalyzing and expediting the formation of polymeric assemblages within the system, which in turn accelerates the progression of geological polymerization. This catalytic effect augments the intricate three-dimensional lattice-like gel structures, consequently orchestrating a substantial amelioration in mechanical attributes. When the dosage of nano-CaCO3 was 3.5%, sodium silicate was 10%, and NaOH was 12%, the integrated performance of fly ash–Mn slag geopolymer was optimal. Specifically, the 28-day compressive strength reached 25.6 MPa, and the compressive strength of the weathering performance test increased by 8.31%. The polymer achieved 96.77% curing of Mn, and it was non-radioactive. Thus, the prepared geopolymers are safe and reliable and support the subsequent development of nanomaterial activators.

Facile preparation of vaterite dominated incineration fly ash (IFA) by slurry carbonation and its performance as supplementary cementitious material

Municipal solid waste incineration fly ash (IFA) can be treated by slurry carbonation and adopted as a supplementary cementitious material (SCM) to reduce the carbon emission of the cement industry. However, effects of control parameters of carbonation on the mineralogy and morphology of the treated IFA require further investigation. In this work, the control parameters were sequentially optimized, aiming at producing more chemically active vaterite during carbonation. The vaterite dominated IFA was subsequently used as an SCM and its performance was studied and compared with limestone (pure calcite). Results indicated that precipitation of vaterite was favored under lower temperature and higher water-ash ratio conditions. CO2 flow rate has relatively smaller impact on the mineralogy of the treated IFA, but a higher rate led to agglomeration of vaterite particles. Under the optimized condition, vaterite dominated IFA containing 60% vaterite and 10% calcite was obtained. For the blended cement, its early hydration was suppressed by calcium sulfite (CaSO3) in the IFA, but the additional sulfate (CaSO4) accelerated the ettringite formation. The vaterite calcium carbonate in the treated IFA stabilized ettringite and reacted with aluminate phases to form hemicarbonate, refining the pore structure. However, hemicarbonate formation was limited by availability of aluminum that was consumed to form ettringite. Compared to limestone, the vaterite dominated IFA behaved better performance as an SCM, and the Porland cement can be substituted by 20% without compromising the 3-day and 28-day compressive strength. Toxicity leaching tests indicated only Pb exceeded regulatory limits in raw IFA, while Pb leaching in mortar samples was greatly declined below 1% of the limit.

Experimental study on macro–micro-structural properties and strength prediction model of wastewater concrete

This study is intended to investigate the mechanical, durability, and drying shrinkage properties of concrete mixtures incorporating wastewater from ready-mixed concrete plants. The microstructural attributes were investigated by X-ray diffraction (XRD), scanning electron microscope (SEM) imaging, energy dispersive spectrometry (EDS), and mercury intrusion porosimetry (MIP). Meanwhile, rough set theory (RST) was used to analyze the influence of mean pore radius, porosity, fractal dimensions, and pore size distribution (PSD) on the compressive strength of wastewater concrete. The results showed that the seven-day cube compressive strengths for C20, C40, and C60 concrete specimens increased when wastewater was utilized, with the highest rise in the strength properties being observed as 16.84% for C40 mixture. Nevertheless, the 28-day cube compressive strengths of C20, C40, and C60 concretes were reduced by 4.85%, 9.33%, and 18.63%, respectively. The durability results showed that wastewater mixing reduced the frost resistance of C40 and C60 concrete, but improved for C20 concrete. The microstructural investigation revealed that mixing concrete with wastewater densifies the early-age concrete microstructure and refines the pore size. Nevertheless, the compactness of concrete microstructure at the later stage was reduced, and the pores in concrete were enlarged due to the poor cohesiveness of wastewater particles. The porosity and PSD have the greatest influence on the compressive strength of wastewater concrete. A strength and pore structure model of wastewater concrete was also developed and validated by considering PSD and porosity as bivariate parameters. Highlights The addition of wastewater can improve the early compressive strength of concrete. The addition of wastewater reduces the carbonization resistance of concrete. A high-precision model of wastewater concrete strength was established.

Effect of super absorbent polymer (SAP) on volume stability and application performance of cement mortar with accelerator

Shotcrete has high cementitious material content, small aggregate particle size, high setting accelerator content, and is prone to shrinkage and deformation, and there are few studies on the volume stability of shotcrete at present. Super absorbent polymer (SAP) is widely used as an internal curing agent, it has low cost and small dosage usage, but it has not been applied to shotcrete to improve volume stability. In this study, super absorbent polymers with different dosages, particle sizes, and extra water were used. The application performance, volume stability and moisture change of different cement mortars were studied under the condition of 4 % of alkali accelerator or 7 % of alkali-free accelerator. The results show that with the increase of super absorbent polymer dosage, the setting time of cement mortar mixed with accelerator is shortened, the 1-day compressive strength is reduced, and the 28-days compressive strength is on average increased. Among them, 0.4SAPAS (addition of 0.4 % SAP) can reduce the initial and final setting time by 101 s and 154 s. Further, the autogenous and drying shrinkage are reduced, and the internal humidity change and mass loss are slowed down. At the age of 180 days, the autogenous and drying shrinkage of 0.4SAPAS were 51 % and 47 % lower than those of AS-7. The XRD, SEM and hydration temperature test showed that the use of super absorbent polymer slightly delayed the hydration process at early age, the hydration temperature peaks of AS-7,0.2SAPAS and 0.6SAPAS are 38.5 °C, 36.7 °C, and 35.1 °C. But at 28 days, the surrounding cement particles were further hydrated due to the release of water from super absorbent polymer.

Strength, mineralogical, microstructural and CO2 emission assessment of waste mortars comprising excavated soil, scallop shells and blast furnace slag

The construction sector is actively seeking alternatives to Portland cements, with a view to decarbonising cementitious construction materials, such as mortars and concretes. Using pozzolanic industrial mineral wastes (e.g. fly ash and slags) as supplementary cementitious materials (SCMs) is an effective strategy. However, further investigation is required to identify more SCMs that have longevity in supply, with a view to benefiting the circular economy. Excavated soil and scallop shell wastes are becoming increasingly abundant due to urbanisation and aquaculture sectoral expansion. Managing these wastes through their valorisation as construction materials is more sustainable and therefore highly desirable over disposing them to landfill. This study investigates the use of excavated soil and scallop shell wastes as raw materials for manufacturing low-carbon cementitious mortars. Mechanochemical and thermal techniques were investigated as alternatives to alkali reagents for activating strength-gaining properties of the soil waste. Mixtures comprising mechanochemical+thermally activated soil waste with 10% blast furnace slag produced the highest strengths (2.43 MPa) after 28-days and generated ∼50% less CO2 emissions compared with CEM-I-treated mixtures. Furthermore, the compressive strengths performance recorded for mechanochemical+thermally activated soil waste with 10% scallop shells (1.07 MPa) were only 8% less compared with strengths for equivalent mixtures containing 10% type 1 Portland cement. Based on results from regression analyses, Si/Al was the most effective elemental ratio for predicting 7- and 28-day compressive strength of mortars based on filter cake that had been mechanochemical and/or thermally treated through linear functions. Exponential functions were more appropriate for predicting strength based on other elemental ratios, which considered one or a combination of Ca, Na, K and Fe.

Influence of aluminum powder content and powder-to-sand ratio on the physical and mechanical properties of aerated lightweight mortar

After the 2015 Nepal earthquake, numerous human casualties resulted from the collapse of substantial brick walls. Concurrently, the proliferation of air pollution attributed to brick kilns has become a pressing concern in urban areas of Nepal. The dual challenge of fortifying building structures and safeguarding the urban environment necessitates innovative solutions. This paper outlines the development of aerated lightweight mortar, incorporating diverse proportions of aluminium powder and various powder-to-sand combinations, aiming to achieve a density below 1000 kg/m3. Three fundamental mixtures, characterized by water-powder ratios (W/P) of 63.3 %, 57.9 %, and 35.3 %, and total powder to sand ratios (P/S) of 0.344, 0.520, and 1.275 (by weight) were employed. The aluminium powder content ranged from 0 % to 1.2 % (by weight of cement). Standard-sized cubes and cylinders were prepared to evaluate the impact of aluminium powder on density, strength, and water absorption. From the test results, the most suitable mixture for aeration proved to be the mortar with a W/P of 35.3 % and P/S of 1.275. This formulation demonstrated a significant 50 % density reduction (<1000 kg/m3) with 0.6 % aluminium powder, accompanied by a 58 % decrease in 28-day compressive strength, a 52 % drop in modulus of elasticity, and a 44 % reduction in splitting tensile strength. The study emphasized the critical role of both aluminium powder content and powder-to-sand ratio in mortar aeration. The developed aerated lightweight mortar not only enhances seismic resilience by reducing building weight but also serves as an eco-friendly alternative to traditional burnt clay bricks, mitigating environmental impact.

Use of municipal solid waste incineration fly ash as a supplementary cementitious material: CO2 mineralization coupled with mechanochemical pretreatment

The use of municipal solid waste incineration fly ash, commonly referred to as “fly ash”, as a supplementary cementitious material (SCM), has been explored to mitigate the CO2 emissions resulting from cement production. Nevertheless, the incorporation of fly ash as an SCM in mortar has been shown to weaken its compressive strength and increase the risk of heavy metal leaching. In light of these challenges, this study aims to comprehensively evaluate the influence of CO2 pressure, temperature, and residual water/binder ratio on the CO2 uptake and compressive strength of mortar when combined with fly ash. Additionally, this study systematically examines the feasibility of mechanochemical pretreatment, which enhances the homogenization of fly ash and augments the density of the mortar's microstructure. The results indicate that the use of mechanochemical pretreatment leads to a notable 43.6% increase in 28-day compressive strength and diminishes the leaching of As, Ba, Ni, Pb, Se, and Zn by 17.9-77.8%. Finally, a reaction kinetics model is proposed to elucidate the CO2 sequestration process under varying conditions. These findings offer valuable guidance for incorporating fly ash as an SCM and CO2 sequestrator in mortar.

Synergetic Effect Of Ternary Mixture Of Nano Silicon Carbide-Polyvinyl Alcohol-Lithium Sulfate On Early Strength Of Cement

T o solve the problems of low compressive strength and unstable sealing capability of traditional hole sealing materials in the early stage of underground coal mines, a new cement-based sealing material was developed in this study by mixing ordinary Portland cement with nanomaterial, water-soluble polymer and lithium salt as additives. The early strength and the microstructure evolution of the as-prepared cement material were investigated with uniaxial compression test, scanning electron microscope (SEM), thermogravimetric analysis and X-ray diffraction (XRD). The results show that the ternary composite material (nano silicon carbide, polyvinyl alcohol and lithium sulfate) can effectively improve the early strength of cement. The 1-day compressive strength increased from 8.56 to 14.21 MPa, pointing out an improvement rate of 66.00%. It can be seen from the SEM images that a large number of fibrous C-S-H crystals and rod-shaped AFt crystals formed in the ternary composite cement sample. The nanosized silicon carbide (SiC) can accelerate the generation of hydration products through forming nucleation sites. As a dispersing agent polyvinyl alcohol (PVA) can effectively prevent the agglomeration of SiC nanoparticles and the formation of cement particle flocculation structure. On the other hand, the lithium cations and the sulfate anions of lithium sulfate could penetrate the hydration film structure and react with the hydration product Ca(OH)2, respectively, which promotes the hydration reaction. The promotion effect of the ternary composite material on the formation of hydration products results in a clearly improved early strength of the new cement-based sealing material.

Development of artificial leak-free phase change material (PCM) aggregates using emulsion technique, cementless binder, and cold-bonded pelletization

The present study developed new leak-free artificial PCM aggregates using an emulsion technique, cementless binder, and cold-bonded pelletization (hereafter, PCM-surfactant-aggregate) and significantly increased the PCM quantity in aggregates without PCM leakage, which was demonstrated from differential scanning calorimetric (DSC) and thermogravimetry (TG) results. The PCM-water-emulsion was produced by using a non-ionic surfactant, Pluronic P123, and used as mixing water to produce cementless artificial aggregates using Ca(OH)2-Na2CO3-activated fly ash and cold-bonded pelletization. The produced artificial PCM-surfactant-aggregates showed no material separation during preparation and no visual PCM leakage on the surface and were lightweight aggregates in terms of compressive strength (ASTM C330). In the concrete testing results, the 28-day compressive strength was 18.1 MPa when all fine and coarse aggregates were replaced with artificial PCM-surfactant-aggregates and the thermal conductivity decreased with increasing aggregate content. The testing on the heated concrete specimens having PCM-surfactant-aggregates indicated no PCM leakage, no dimensional change, and no strength reduction due to the PCM phase transition from solid to liquid. Overall, the concrete containing PCM-surfactant-aggregates showed an increase in the lowest temperature and a decrease in the highest temperature in the temperature profile of the indoor space in the dynamic heat flow test.

Effect of Polycarboxylate Admixture on the Performance of Fluorogypsum-Based Self-Leveling Material

The study explores the influence of polycarboxylate admixture (PCE) on fluorogypsum-based self-leveling material (FSLM) performance. Wev conducted an array of tests to assess workability and mechanical properties, and utilized XRD, TG-DSC, SEM, and MIP techniques for microscopic analysis. The fresh state results showed that PCE enables FSLM to achieve good workability at lower water demand. It was found that PCE partially inhibited plaster hydration from anhydrite to dihydrate, as observed by X-ray diffraction analysis and thermogravimetric analysis, and with the increasing in PCE, the mass loss of samples reached 15.66% at 28 days. The mechanical properties and the microstructure studies proved that the optimal PCE doping level is 0.14%. At this doping level, there is an enhancement in the denseness of the hardened structure, a reduction in porosity—especially when the pores are more than 200 nm, optimization of pore size distribution, an increase in crystal aspect ratio, an enhancement in effective intergranular overlap, and a significant improvement in the 28-day flexural and compressive strength to 7.2 MPa and 36 MPa, respectively. The FSLM prepared under these conditions demonstrates good performance and meets the primary performance index requirements of the Chinese Industry Standard JC/T 1023-2021 (gypsum-based self-leveling floor compound), thereby promoting the comprehensive utilization of fluorogypsum.

Effects of the early curing regime on the properties and pore structure of concrete in an environment with high altitudes and low atmospheric pressures

High altitudes have harsh, complex climates and changing environments. These conditions considerably challenge the service life of concrete structures. The engineering quality of concrete structures at high altitudes can be improved by scientific curing regimes; similarly, carbon emissions can be reduced throughout their life cycle. In this paper, the effect of the early curing environment on the macroscopic properties, microstructure, and hydration products of concrete realistically exposed to harsh environments (10–30 °C, RH 20–80 %, 70 kPa) of high altitude was studied. The mechanism of the effect of the early curing environment on the microstructure of the concrete at high altitudes was determined. The concrete performance at high altitudes was investigated regarding the enhancement mechanism of steam curing. At high altitudes, the deterioration of concrete properties was primarily caused by an increase in the percentage of pores sized between 20 and 200 nm and the total porosity. Appropriate steam curing can be employed to enhance concrete hydration in these areas. This process refines the pores in hardened cement paste at the micro-nano scale (20 nm–200 nm and 50 μm–100 μm), ultimately reducing total porosity. Additionally, it alleviates the “aging phenomenon” of the C–S–H gel, which is induced by the low atmospheric pressure environment. The comprehensive performance of concrete in high-altitude areas can be significantly improved by steam curing at 35 °C. Compared with the uncured and sprinkling curing cases, steam curing at 35 °C for 3 days increased the 28-day compressive strength of concrete exposed to the natural environment at high altitudes by 42.8 % and 23.5 %, respectively, reduced the 56-day drying shrinkage strain by 38.7 % and 38.5 %, respectively, and could significantly reduce the risk of concrete cracking at high altitudes.

Effects of NaHCO3 as the activator and the in-situ carbonate seeding on the properties of high-dosage Ca(OH)2 + slag (HCHS) cement

This study investigated the effects of the addition of NaHCO3 (5, 10, and 15% molar amount of Ca(OH)2) on the setting time, compressive strength, drying shrinkage, and hydration process of high-dosage Ca(OH)2 + Slag (HCHS) cement. The results indicated that the NaHCO3 addition could significantly increase the 1-day compressive strength of HCHS cement from almost 0 to 10 MPa and decrease the drying shrinkage of HCHS cement up to 46.1% compared to it without NaHCO3. But the addition of NaHCO3 above 5% would result in accelerated setting, increased porosity, and decreased strength in the later stages. NaHCO3 played an in-situ carbonate seeding effect, forming CaCO3 crystals through the initial reaction of Ca(OH)2, and the activator that increased solution alkalinity promoted the formation of hydration products. These could contribute to the HCHS cement with high early compressive strength, good volume stability and certain porosity, suitable for further carbon capture.

Design, preparation and hydration of ecological ultra-high performance concrete (eco-UHPC) with raw waste rock chips

High carbon emissions and high costs have limited the applications of UHPC. The use of environmentally friendly aggregates and fillers can help to address this issue. In this study, an ecological UHPC (eco-UHPC) was designed by using raw waste rock chips (RWRC) with waste rock chips (WRC) and waste rock powder (WRP) to replace quartz sand (QS) and part of the cement to obtain the UHPC with low carbon emissions and low cost. The compressive strength of the eco-UHPC was tested and the microstructure and hydration product were characterized by SEM, XRD and TGA. Calorimetry and MIP tests were carried out to determine the exothermic behavior and pore structure of eco-UHPC. The results showed that the 28-day compressive strength of eco-UHPC with WRC as fine aggregate was 4.3 % higher than that of the sample with QS. For the UHPC sample with WRC as the only aggregate, the addition of 10 wt% WRP slightly increased the compressive strength from 7 d to 56 d by promoting the formation of hydration products and refining the pore structure of UHPC, but the opposite was true for the sample with high content of WRP. In addition, the results of the ecological evaluation showed that the advantages of eco-UHPC produced by RWRC are environmental friendliness and low cost.