Hydration Characteristics Research Articles

Performance of preserved alpha prime belite phase within belite-ye’elimite-ferrite clinker

Belite-ye’elimite-ferrite (BYF) cement has garnered significant interest as an alternative to ordinary Portland cement (OPC) due to its lower limestone content, early-age strength development, and improved durability. However, the main challenge to the commercial production of BYF cement is the requirement for large quantities of costly aluminous raw materials. The introduction of alpha prime belite (α'C2S) to BYF clinker would minimize production costs while maintaining mechanical properties. Herein, α'C2S containing BYF clinkers were synthesised and analysed to determine their properties and hydration characteristics. The clinker samples were synthesised using a previously reported clinkering technique wherein a sulfur-containing atmosphere was used, with a raw mix design including additions of a combination of boron, potassium, and sodium-containing doping agents. Preliminary data showed adequate amounts of α'C2S were preserved reaching more than 40 wt.% of total clinker composition. Moreover, the heat release curves revealed enhanced hydration kinetics for the modified clinkers compared to the unmodified ones. While there was a noticeable improvement in ettringite precipitation upon hydration, the precipitation of strätlingite remained the same after doping. The experimental findings showed high potential for the existence of low ye’elimite BYF clinker formulations as part of the wider portfolio of OPC clinker alternatives.

All-solid-waste cementitious materials for grouting: Effects of alkali content and elemental ratios on performance and sustainability

The use of all-solid waste cementitious materials (ACM) in coal mine grouting backfill offers substantial green, low-carbon, and energy-saving benefits. This study systematically examines the effects of combining carbide slag (CS), fly ash (FA), and ground granulated blast furnace slag (GGBS) on the strength, workability, hydration characteristics, and microstructure of ACM. The utilization of sulfur-containing CS was achieved. The detrimental effects of delayed FA reaction and prolonged setting time can be mitigated by the introduction of high alkalinity or an increase in the Ca/Si ratio. The compressive strength may also be enhanced. The addition of excessive alkalinity (10 %) will result in a prolongation of the setting time. The primary hydration products include calcium aluminum silicate hydrate and magnesium-aluminum layered double hydroxide. Low Ca/Si ratios favor alkali metal ion charge balance, facilitating the transformation of silicate gels from single to double chains, while excessively high ratios reduce polymerization. Gmelinite forms when the (Ca+Na)/(Al+Si) ratio exceeds 1.6, and high NaOH concentrations inhibit ettringite formation. Validation shows that a GGBS:FA:CS= 3:1:3 mix with 4 % alkali binder (Group A2) meets mine grouting backfill criteria, with 80 % lower carbon emissions, 68 % lower energy intensity, and 50 % lower cost than cement. This research offers a viable pathway for the comprehensive utilization of multi-solid waste in mining applications.

Research on resistivity characteristics of grain-coating and pore-filling hydrates in different sediment packings based on voxelated 3D models

Natural gas hydrate is a clean energy resource with abundant reserves. Before conducting large-scale exploitation, understanding its physical properties, such as electrical properties, is crucial for gas reserve evaluation and production process monitoring. Due to the complexity of the pore space geometry and different hydrate growth morphology, electrical resistivity may vary for different samples at the same hydrate saturation. This study considered three packings of unconsolidated marine sediment grains (simple cubic, body-centered cubic, and face-centered cubic) and two end distribution patterns of hydrates in pores (grain-coating and pore-filling). The original 3D geometries of hydrate-bearing models were replaced with voxelated meshes to achieve better stability and versatility. The resistivities of models were calculated using the finite element method and compared with the results from Archie's law. The simulation results show that the resistivity curve's shape is jointly affected by the hydrate distribution pattern and the initial pore space geometry. A rational function can be used to fit the saturation exponent curves to give critical brine saturations. The grain-coating hydrate prefers blocking the throats and has a critical brine saturation above 0. In contrast, the pore-filling hydrate has resistivities lower than the Maxwell-Garnett equation's lower boundary due to a bypassing effect, which partially compensates for the conductivity loss of the pore fluid and is more remarkable for models distributed by bigger hydrate particles, and has a critical brine saturation equal to 0. The findings are helpful in understanding the non-Archie phenomenon of hydrate-bearing sediments.

GelMA hydrogels reinforced by PCL@GelMA nanofibers and bioactive glass induce bone regeneration in critical size cranial defects

BackgroundThe process of bone healing is complex and involves the participation of osteogenic stem cells, extracellular matrix, and angiogenesis. The advancement of bone regeneration materials provides a promising opportunity to tackle bone defects. This study introduces a composite hydrogel that can be injected and cured using UV light.ResultsHydrogels comprise bioactive glass (BG) and PCL@GelMA coaxial nanofibers. The addition of BG and PCL@GelMA coaxial nanofibers improves the hydrogel’s mechanical capabilities (353.22 ± 36.13 kPa) and stability while decreasing its swelling (258.78 ± 17.56%) and hydration (72.07 ± 1.44%) characteristics. This hydrogel composite demonstrates exceptional biocompatibility and angiogenesis, enhances osteogenic development in bone marrow mesenchymal stem cells (BMSCs), and dramatically increases the expression of critical osteogenic markers such as ALP, RUNX2, and OPN. The composite hydrogel significantly improves bone regeneration (25.08 ± 1.08%) in non-healing calvaria defects and promotes the increased expression of both osteogenic marker (OPN) and angiogenic marker (CD31) in vivo. The expression of OPN and CD31 in the composite hydrogel was up to 5 and 1.87 times higher than that of the control group at 12 weeks.ConclusionWe successfully prepared a novel injectable composite hydrogel, and the design of the composite hydrogels shows significant potential for enhancing biocompatibility, angiogenesis, and improving osteogenic and angiogenic marker expression, and has a beneficial effect on producing an optimal microenvironment that promotes bone repair.

Study on the hydration characteristics and mechanical properties of recycled powder-slag powder-cement system

This study systematically investigated the influence of different blending ratios of recycled powder and slag powder on hydration heat evolution and mechanical properties. Various modern microscopic testing techniques were employed to quantitatively characterize hydration products and microstructures, revealing their hydration mechanisms. The results indicate that both recycled powder and slag powder can accelerate the early hydration rate. When recycled powder and slag powder are co-blended, their early hydration rate and cumulative heat release surpass those of specimens with single additions of recycled powder. Moreover, the mechanical properties in the later stage are superior to those of specimens with single additions of recycled powder. At 60d, the PRS70–1–2 in the multi doped recycled powder and slag powder system even exceeds that of the uni doped SL system, but is much higher than that of the uni doped RP system，the compressive strength of the multi doped recycled powder and slag powder system increased by 25.5 %, 37.1 % and 36.9 %, respectively, compared with that of the uni doped RP system. Specimens with single additions of recycled powder and slag powder have a relatively high proportion of harmful and very harmful pores. At 28d, the porosity of the multi doped recycled powder and slag powder system decreased by 19.2 %, 24.2 % and 14.8 %, respectively, compared with that of the uni doped RP system. However, when recycled powder and slag powder are co-blended, the proportion of harmful and very harmful pores decreases, while the proportion of less harmful and harmless pores increases. This leads to an overall reduction in porosity and an improvement in mechanical properties.

Effect of Fe3+ doping on the electronic structure and surface hydration properties of quartz

In this study, the effects of Fe3+ doping on the crystal structure and surface properties of quartz were analyzed using density functional theory (DFT) calculations and molecular dynamics (MD) simulations. The crystal structure, surface model, and adsorption model of the water molecule layer for Fe3+-doped α-quartz were established. The DFT calculations reveal that Fe3+ doping affects the α-quartz lattice parameters and surface active sites. The adsorption energy near the sites occupied by Fe3+ is −66.20 kJ/mol, which is considerably lower than that of the undoped system (−42.35 kJ/mol). This makes it easier for water molecules to be adsorbed on the Fe3+-doped α-quartz (001) surface. The frontier orbital energy, electron density difference, charge density, and Mulliken bond population analyses indicate that the presence of Fe3+ improves the ability of the (001) surface of α-quartz to attract and bind water molecules. This improvement is mainly due to the formation of hydrogen bonds between the surface hydroxyl groups and water molecules on the surface of Fe3+-doped quartz and the equilibrium of the electrostatic interaction between the cation and water molecules. MD simulations show that a hydration layer with a thickness of approximately 8 Å is formed on the surface of Fe3+-doped α-quartz. Compared with the undoped system, the hydration film on the surface of the doped system is thicker, indicating that the incorporation of Fe3+ impurities in the lattice enhances the surface hydration characteristics of quartz. These results offer theoretical guidance for achieving efficient separation and thorough purification of quartz.

Soluble and insoluble dietary fiber at different ratios: Hydration characteristics, rheological properties, and ameliorative effects on constipation

The proportion of soluble components in dietary fiber is an important factor affecting its physicochemical properties and physiological functions. The influence mechanisms of insoluble (IDF) and soluble dietary fiber (SDF) at different ratios on constipation were investigated. Results showed that SDF had higher active groups and water swelling capacity than IDF. The viscosity of chyme with SDF alone was the highest in oral and gastric phases. The gastric emptying rate and small intestine propulsion capacity increased significantly, especially when IDF/SDF was 1:1. IDF and a lower proportion of SDF (< 50 %) promoted gut microbiota diversity and short-chain fatty acids production. The contents of 5-hydroxytryptamine, acetylcholinesterase and gastrin reached the maximum value when the IDF ratio was 50 %. In conclusion, IDF could act synergistically with SDF to promote defecation and relieve constipation, and the effect was the best when the ratio of IDF to SDF was 1:1.

Study on growth characteristics of TBAB clathrate hydrate from flowing TBAB solution in a rectangular channel

Study on growth characteristics of TBAB clathrate hydrate from flowing TBAB solution in a rectangular channel

Methylglyoxal alters collagen fibril nanostiffness and surface potential

Collagen fibrils are fundamental to the mechanical strength and function of biological tissues. However, they are susceptible to changes from non-enzymatic glycation, resulting in the formation of advanced glycation end-products (AGEs) that are not reversible. AGEs accumulate with aging and disease and can adversely impact tissue mechanics and cell-ECM interactions. AGE-crosslinks have been related, on the one hand, to dysregulation of collagen fibril stiffness and damage and, on the other hand, to altered collagen net surface charge as well as impaired cell recognition sites. While prior studies using Kelvin probe force microscopy (KPFM) have shown the effect glycation has on collagen fibril surface potential (i.e., net charge), the combined effect on individual and isolated collagen fibril mechanics, hydration, and surface potential has not been documented. Here, we explore how methylglyoxal (MGO) treatment affects the mechanics and surface potential of individual and isolated collagen fibrils by utilizing atomic force microscopy (AFM) nanoindentation and KPFM. Our results reveal that MGO treatment significantly increases nanostiffness, alters surface potential, and modifies hydration characteristics at the collagen fibril level. These findings underscore the critical impact of AGEs on collagen fibril physicochemical properties, offering insights into pathophysiological mechanical and biochemical alterations with implications for cell mechanotransduction during aging and in diabetes. Statement of SignificanceCollagen fibrils are susceptible to glycation, the irreversible reaction of amino acids with sugars. Glycation affects the mechanical properties and surface chemistry of collagen fibrils with adverse alterations in biological tissue mechanics and cell-ECM interactions. Current research on glycation, at the level of individual collagen fibrils, is sparse and has focused either on collagen fibril mechanics, with contradicting evidence, or surface potential. Here, we utilized a multimodal approach combining Kelvin probe force (KPFM) and atomic force microscopy (AFM) to examine how methylglyoxal glycation induces structural, mechanical, and surface potential changes on the same individual and isolated collagen fibrils. This approach helps inform structure-function relationships at the level of individual collagen fibrils.

Amino Acid Supported Conductive Nanocomposite for Developing Flexible Electrode Material for Energy Storage

Introduction: This study focused on synthesizing biocompatible, flexible and wearable electrode materials for energy storage applications. The unique zwitterionic structure of L-proline provides numerous interesting properties to the nanocomposite such as high ionic interactions through the various ion migration channels, and strong hydration characteristics. These features are key to thehigh performance of energy deposition systems.Methods: Binary nanocomposites containing L-proline (Pro) amino acid and polypyrrole (Ppy) were produced on rGO modified carbon textile (rGO-CC) to develop electroactive materials. Two step hydrothermal method was used to produce flexible electrodes. DRIFT spectroscopy andAFM analysis were performed to clarify the structural and the morphological characterization. Electrochemical behavior was evaluated utilizing CV, GCD and EIS methods.Results: ProPpy@rGO-CC electrode materials exhibit high electrochemical performances in aqueous electrolytes (0.1 M NaCl). The prepared electrode shows high specific capacitance of 500.4 Fg-1 (at 25 mVs−1) at the ambient conditions. Additionally, after 5,000 charge/discharge cycles the specific capacitance retains a high level of 95% confirming the good cycle stability. The energy and the power densities were found to be 278 Wh kg−1 and 12.5 kW kg−1, respectively.Conclusion: The results indicate that the ProPpy@rGO-CC electrode is a promising candidate for next-generation high-performance energy deposition systems. The unique structural features of L-proline contribute to the formation of a large number of electroactive sites and short diffusion pathways.

Characterization of Natural Gas Hydrate Constrained by Well and Seismic Data in Qiongdongnan Basin

This study investigates the natural gas hydrates within the Qiongdongnan Basin by integrating well-log and seismic data. Through pre-stack inversion and rock physics analysis, key parameters such as P-wave and S-wave impedances were utilized to distinguish hydrate-bearing formations from other geological bodies. A low-frequency model was constructed using the Inverse Distance Weighting (IDW) algorithm to improve the precision of parameter inversion. This study employs a multi-constraint inversion strategy, incorporating hard constraints from multiple wells and soft constraints from geological frameworks, ensuring reliable inversion results. Findings indicate that hydrate reservoirs are characterized by increased wave velocity and density due to hydrate accumulation, providing insights into the spatial distribution and characteristics of hydrates. This research enhances the understanding of hydrate reservoirs and offers valuable data for exploration in the Qiongdongnan Basin.

Enhanced Study of CO2 Hydrate Formation in Marine Oil–Gas Based on Additive Effect

During marine oil–gas extraction, significant amounts of carbon dioxide (CO2) gas are often produced. Effectively separating these associated CO2 gases during extraction has become a critical technical challenge. Therefore, this paper aims to enhance the efficiency of CO2 hydrate-based capture technology and conduct relevant research. The goal is to increase the driving force for hydrate formation by combining the traditional thermodynamic additive TBAB with pressure modulation and to improve the hydrate formation rate through the use of multiple kinetic promoters. This paper presents the initial investigation into the effect of the thermodynamic accelerator tetrabutylammonium bromide (TBAB) on the characteristics of CO2 hydrate formation. The promotion effects of TBAB solutions with varying mass concentrations (3%, 5%, 7.5%, 10%) and reaction pressures (3 MPa, 4 MPa) were subjected to a systematic analysis, and the optimal conditions were identified as 4 MPa and a 5 wt% TBAB concentration. Subsequently, the impact of combining TBAB with kinetic promoters (SDS, nano Al2O3, L-methionine, L-leucine) on CO2 hydrate generation characteristics was further investigated. In this paper, the effect of a single promoter on the generation characteristics of CO2 hydrate was investigated, and the efficient carbon trapping ability of the complex promoter was verified, which provides theoretical support for the application of CO2 trapping technology using the hydrate method.

Impact of TiO2 powder type on hydration and photocatalytic NOx degradation in cement paste

This study was aimed at developing novel functional construction materials with NOx removal capabilities using TiO2 powder. To this end, the effects of the chemical composition of TiO2 powder on the hydration process and the mechanical properties of cement were investigated. The hydration characteristics of the cement were assessed by analyzing the setting time and heat of hydration, while the mechanical properties were evaluated by measuring compressive strength at different curing ages. Additionally, the NOx degradation performance was examined. The results demonstrate that TiO2 powder accelerates the hydration of cement, enhancing the compressive strength during the early stages of hydration. Furthermore, the NOx degradation performance of mortar specimens was enhanced with the incorporation of TiO2 powder. However, the extent of these improvements varied significantly depending on the chemical composition of the TiO2 powder, with the composition ratio of anatase and rutile being particularly influential. When utilizing TiO2 powder to enhance the performance of construction materials or impart new functional properties, it is advisable to consider the composition of the TiO2 powder to ensure alignment with the intended performance improvement objectives.

Preparation of steel slag-based cementitious materials compounded with GBFS and phosphogypsum: mix design, hydration, and microstructure characteristic

Preparation of steel slag-based cementitious materials compounded with GBFS and phosphogypsum: mix design, hydration, and microstructure characteristic

Mechanical properties and hydration mechanism of nano-silica modified alkali-activated thermally activated recycled cement

The recycling and reuse of waste concrete are crucial for reducing environmental pollution, minimizing resource waste, and lowering carbon emissions. However, current efforts to improve the performance of recycled cement (RC) using the synergistic effects of alkali activation and thermal activation have been suboptimal. Therefore, this study explores the influence of nano-silica (NS) incorporation on the mechanical properties and hydration mechanism of alkali-activated and thermally activated RC. The research focuses on cement mortar with a thermal activation temperature of 750 °C, an RC replacement rate of 30 %, and an alkali content of 6 %. Various analytical techniques, including thermogravimetric analysis (TG-DTG), X-ray diffraction (XRD), and scanning electron microscopy (SEM), were used to evaluate the impact of different NS dosages (1 %, 2 %, 3 %, and 5 %) on the mechanical properties and hydration characteristics of nano-modified alkali-activated and thermally activated RC mortar. The study reveals the mechanism by which NS enhances the microstructure of alkali-activated and thermally activated RC. The experimental results show that as the NS content increases, the compressive and flexural strengths of RC initially increase and then decrease. The optimal strength was achieved with a 2 % NS content, with a 31.5 % increase in compressive strength compared to RC without NS. At this dosage, RC exhibited a higher hydration rate during the acceleration period of hydration. Additionally, the initial hydration heat release rate of RC increased with rising NS content. While NS incorporation did not alter the phase composition of RC, it activated the pozzolanic effect of RC, causing free active substances such as CaO and gypsum in the components to dissolve and react to form Aft at the start of the hydration reaction. The unsaturated bonds (≡Si-O- and ≡ Si-) on the surface of NS absorbed Ca2⁺ and OH⁻ from the RC, promoting the hydration of C2S and C3S. However, excessive NS content (greater than 3 %) led to a decline in RC strength and the formation of more cracks. This is primarily due to the excess NS particles not contributing to an increase in effective nucleation sites, causing agglomeration, an increase in large pores, and a reduction in transition and gel pores, which negatively affected the microstructure and thus reduced the mechanical properties of the RC. Our findings enhance the understanding of the role of nanomaterials in alkali-activated RC and provide valuable insights for further research into high-performance recycled cement materials.

Exploring microwave activation as a novel method for activating coal gangue: Focus on microwave activation mechanisms and hydration characteristics of cementitious supplementary materials

To support the decarbonization of the cement industry, coal gangue (CG) has potential as a supplementary cementitious material. The calcination process significantly impacts the phase transformation of clay minerals and the pozzolanic activity of CG. This study involved calcination of CG at 600 ℃ and 800 ℃ using both microwave and thermal activation methods. Employ XRD, XPS, FTIR, TG, and SEM to examine the impact of calcination mechanism on the crystal structure, element binding energy, chemical bonds, microstructure, and mineral composition of CG. The results indicate that the transformation of kaolinite in CG to metakaolin is completed around 600 °C. Microwave activation increased the reactive aluminate content in CG, enhancing pozzolanic activity and promoting secondary hydration with cement. This process generated additional C-(A)-S-H gels and AFm phases, which filled matrix pores, increased system density, and improved mechanical properties. The optimal activation method for CG was identified as microwave activation at 600 °C.

Effect of Hydrate Saturation on Triaxial Shear Mechanical Properties of Methane Hydrate-Bearing Sediment

Abstract As a clean energy source, natural gas hydrate resources are essential to the development of energy in the twenty-first century. However, the degree of consolidation of the hydrate reservoir is low, and complex accidents such as wellbore instability, reservoir collapse, and gas production leakage will be caused if improperly exploited. The laboratory employs a self-developed triaxial shear test equipment that operates under high pressure and low temperature conditions. This equipment is specifically designed to determine the mechanical parameters and strength characteristics of methane hydrate bearing sediment (MHBS) samples. It is used to create water-saturated hydrate deposits with varying levels of saturation. The synthesis of MHBS samples with varying saturations is achieved by precisely regulating the amount of water. The saturation of MHBS is then determined and confirmed by the utilization of the “water content method” and the “gas collection method.” Triaxial shear tests are conducted on MHBS to examine the impact of hydrate saturation on the mechanical parameters and strength behaviours of MHBS. The experimental findings demonstrate that the presence of hydrate crystals enhances the robustness of the depositing medium. The stress-strain curves of MHBS have a hyperbolic shape, indicating strain hardening features. The stress-strain curves may be categorized into three distinct stages: the initial elastic deformation, followed by a slow shift to the initial yield deformation, and ultimately leading to strain hardening. As axial strain rises, the behaviour of the MHBS transitions progressively from elastic deformation to plastic deformation. The shear strength and initial yield strength of MHBS enhance with higher hydrate saturation. The reason for this is that as the hydrate saturation rises, the influence of hydrate crystals on the enhancement of sediment strength becomes more pronounced. However, the initial yield strain of each MHBS sample is between 0.5% and 1.5%, and the difference is not significant. When the hydrate saturation rises gradually, the cohesion of MHBS enhances. Nevertheless, the hydrate saturation has a very minor impact on the internal friction angle.

Effects of C-S-H Seed Prepared by Wet Grinding on the Properties of Cement Containing Large Amounts of Silica Fume

This study aimed to utilize the hydration characteristics of cement through wet grinding techniques to efficiently and conveniently prepare a stable C-S-H seed suspension, providing key parameters and a scientific basis for their large-scale production, which ensures the stability of the C-S-H suspension during production, transportation, and application. This preparation aimed to mitigate the adverse effects of high-volume silica fume on the early mechanical properties of high-performance cement concrete. The properties of C-S-H seed were characterized in detail by SEM, XRD, and TD. In the concrete performance test, silica fume was used to replace part of the cement, and different contents of C-S-H seed were added to test its effect on the compressive strength of concrete, with XRD and SEM used to analyze the performance differences. The results show that the particle size and hydration degree of cement no longer developed after 90 min of wet grinding. Polycarboxylate ether (PCE) superplasticizer can increase the fluidity of the crystal C-S-H seed suspension when the content exceeds 1.5%. When the content of PCE exceeded 2%, the C-S-H seed suspension precipitated. Adding 5% C-S-H seed can increase the compressive strength of cement concrete by 10% under the condition of reducing the amount of cement and increasing the amount of silica fume. And Ca(OH)2 (CH) was produced by cement hydration consumed by silica fumes to generate C-S-H gel, by which the concrete became denser with more strength. However, when the amount of C-S-H seed exceeded 7%, the compressive strength of the concrete decreased.

Alkali‐activated rice husk ash‐foamed composites: Correlation between pore structure, hydration, and hardening properties

AbstractSolid waste recycling plays a crucial role in environmental protection and energy conservation. This study explores the impact of the modulus of the alkaline activator on the dry density, water absorption rate, thermal conductivity, electromagnetic wave absorption, and compressive and flexural strength of rice husk ash (RHA)‐foamed composites. Additionally, the foamed composite's micropore structure and hydration characteristics were characterized. For the first time, the study reveals the correlation between the pore structure, hydration products, and hardening properties of alkali‐activated RHA‐foamed composites. The study found that decreasing the modulus of the alkaline activator increased the amount of OH− ions available in the gel, and the number of micropores (r ≤ .1 µm) in the foamed composites increased from 7.1903% to 21.3156%. This resulted in the refinement of pore sizes and optimization of heat transfer paths. Meanwhile, increasing the C–S–H gel composition in the hydrated products improved its compressive and flexural strength. When the alkali modulus reaches 1, 28 days foamed composites display compressive strength of 8.33 MPa and thermal conductivity of .1404 W/(m·K). Furthermore, the superior pore structure improved the electromagnetic wave absorption of the foamed composites, yielding a reflection loss value of −12.02 dB.

Hydration and Mechanical Properties of High-Volume Fly Ash Cement under Different Curing Temperatures.

This study aimed to investigate the effects of different curing temperatures on the hydration and mechanical properties of high-volume fly ash (HVFA) concrete. The key variables were curing temperature (13 °C, 23 °C, 43 °C) and fly ash (FA) content (0%, 35%, 55%). The hydration characteristics of HVFA cement were examined by evaluating the setting time and heat of hydration under different curing temperatures. The mechanical properties of HVFA concrete were analyzed by preparing concrete specimens at various curing temperatures and measuring the compressive strength at 7, 28, 56, and 91 days. The results indicated that concrete with high FA content was more sensitive to curing temperature compared to ordinary Portland cement.