A molecular dynamics and microporomechanics study on the mechanical properties of major constituents of hydrated cement

Cement paste is the matrix material for concrete and cement based composites. This paper presents a molecular dynamics (MD) method for estimating mechanical properties of hydrated cement major constituents: calcium–silicate–hydrate (C–S–H) structurally related tobermorite 14 Å and jennite, and calcium hydroxide (CH). Microporomechanics technique is used to calculate properties of two types of C–S–H, namely, low density (LD) and high density (HD) C–S–H gels. Simulation results reported by the authors were compared with existing computational and experimental values. This research is intended to give a general step to study the complicated cement hydrated products from a multiscale view.     

Study of Portland cement fracture surfaces by scanning electron microscopy techniques

An extensive scanning electron microscopy study was carried out with respect to the fracture surfaces of Portland cement hydrated for various times. It is shown that the two major products of hydration are calcium silicate hydrate spherulites, which consist of radiating fibres and calcium hydroxide platelets. These fibres bond with one another to hold the spherulites together. The volume between the spherulites consists of calcium hydroxide platelets. The fracture is frequently found to be across the weakly bonded basal planes of the calcium hydroxide, and is believed to limit the strength of the Portland cement.     

活化煤矸石对水泥水化的影响

研究了活化煤矸石-氢氧化钙体系的水化热、水化产物成分以及活化煤矸石水泥体系的水化过程、水化产物的微结构,结果表明,在石膏的激发下,活化煤矸石能够发生二次水化,与Ca（OH）2反应形成钙矾石、水化硅酸钙、水化铝酸钙等有利于提高水泥石强度的水化产物;活化煤矸石水泥硬化浆体中Ca（OH）2的含量在水化3d时最多,而后随龄期逐渐减少;阐明了活化煤矸石能够降低水化产物中氢氧化钙的含量、抑制氢氧化钙晶体的生长和聚集,并改善水泥石结构．     

Effect of carboxylic and hydroxycarboxylic acids on cement hydration: experimental and molecular modeling study

Most concrete produced includes chemical admixtures such as air entrainers, set modifiers, water reducers, etc., many of which include organic molecules. Hydroxycarboxylic acids, in particular, retard portland cement hydration. The interaction of such acids with hydrating cement phases is a complex, multi-parameter problem. To elucidate the interaction of hydroxycarboxylic and carboxylic acid retarders on hydration of cement, a combined experimental and molecular-computational approach was used. Glycolic acid, acetic acid, calcium glycolate and calcium acetate were used as model compounds. Molecular dynamics simulations were performed to simulate the interactions of select test compounds with the (001) surface of the portlandite crystal (calcium hydroxide) and the (040) surface of the tricalcium silicate crystal. Hydrogen bond density profiles and binding energies were evaluated. The adsorption isotherm for chelate complexes was determined experimentally by equilibrating aqueous solutions of the agents in the presence of various amounts of solid-phase calcium hydroxide. Finally, isothermal calorimetry experiments were used to quantify effects on hydration rate. The glycolic acid shows significant cement retardation, whereas acetic acid does not retard. Glycolic acid was found to retard hydration via calcium chelation and surface adsorption that involves the adsorption of the calcium chelate complex preferentially on tricalcium silicate. Simulation results reveal that calcium glycolate forms a strong hydrogen bonding network near to calcium hydroxide and hydrated tricalcium silicate surfaces and are responsible for its strong adsorption on these surfaces. While acetic acid forms a strong calcium chelate, it does not associate with calcium hydroxide or unhydrated or hydrated tricalcium silicate surfaces.     

碳酸化钢渣和矿渣作硅酸盐水泥混合材水化性能研究

通过对钢渣碳酸化前后的硅酸盐相提取及水化放热性能和将碳酸化钢渣和矿渣作为混合材的硅酸盐水泥的胶砂强度和水化产物种类的测定,以及对它们微观形貌的观察,研究了碳酸化钢渣对胶凝体系水化性能的影响.结果表明,碳酸化使钢渣中硅酸盐相的含量由47.06％下降至14.38％;碳酸化促进了钢渣的早期水化,抑制其后期水化;在配比相同的条件下,碳酸化钢渣-矿渣-硅酸盐熟料体系试样的3、28d抗压强度较未碳酸化钢渣-矿渣-硅酸盐熟料体系试样的高;碳酸化生成的CaCO3促进了熟料的水化;碳酸化钢渣促进了胶凝体系中AFt的生成,且生成水合碳铝酸钙.     

Supercritical carbonation treatment on extruded fibre–cement reinforced with vegetable fibres

The objective of this research was to evaluate the effects of supercritical carbonation treatment for 2 h on the main hydrated phases of the cement matrix (calcium hydroxide and calcium silicate hydrate) and durability of extruded fibre–cement reinforced with bleached eucalyptus pulp and residual sisal chopped fibres. The thermal analysis, bulk density, porosity, physical characteristics and mechanical performance were evaluated before and after 200 soaking and drying cycles for following the degradation of the material under accelerated ageing conditions. The higher carbonation rate during the early stage of curing period decreased the porosity by sealing the opened pores around vegetable fibres and, consequently, led to lower water absorption and higher bulk density in the composites. The average MOR-values showed a significant increase in the case of the supercritical carbonated extruded fibre–cement in the initial age and after accelerated ageing. Besides, after 200 soaking and drying ageing cycles, the average values of energy of fracture (γ_WoF) of the carbonated composites decrease only 28%, showing evidences of the preservation of microstructural stability and toughness of the fibre–cement composites after supercritical CO₂ treatment.     

In vitro studies of calcium phosphate silicate bone cements

A novel calcium phosphate silicate bone cement (CPSC) was synthesized in a process, in which nanocomposite forms in situ between calcium silicate hydrate (C–S–H) gel and hydroxyapatite (HAP). The cement powder consists of tricalcium silicate (C₃S) and calcium phosphate monobasic (CPM). During cement setting, C₃S hydrates to produce C–S–H and calcium hydroxide (CH); CPM reacts with the CH to precipitate HAP in situ within C–S–H. This process, largely removing CH from the set cement, enhances its biocompatibility and bioactivity. The testing results of cell culture confirmed that the biocompatibility of CPSC was improved as compared to pure C₃S. The results of XRD and SEM characterizations showed that CPSC paste induced formation of HAP layer after immersion in simulated body fluid for 7 days, suggesting that CPSC was bioactive in vitro. CPSC cement, which has good biocompatibility and low/no cytotoxicity, could be a promising candidate as biomedical cement.     

跨尺度模拟硬化水泥净浆力学性能:微观物相的影响

采用纳米压痕实验测量硬化水泥净浆中未水化物、水化产物和孔隙等微观物相的力学性能,并基于背散射电镜（BSE）图像的灰度分析计算各微观物相的含量。在得到各微观物相含量和力学性能的基础上,针对水泥净浆的弹性模量进行均一化建模,并讨论各微观物相及其模量的选取对跨尺度模拟硬化水泥净浆力学性能的影响。通过微米压痕的实测净浆模量验证模型及参数选取的可靠性。提出在硬化水泥净浆力学性能的多尺度模型中,需要选取12 GPa作为孔隙有效模量,并将水化产物划分为低密度的水化硅酸钙凝胶（LD-CSH）和高密度的水化硅酸钙凝胶（HD-CSH）两种物相,而不同种类的未水化物可被视作一种物相。在此基础上,使用Mori-Tanaka模型或自洽模型计算得到的净浆模量与实测净浆模量吻合。     

Measurement of elastic properties of calcium silicate hydrate with atomic force microscopy

Atomic force microscopy (AFM) based indentation is compared to conventional nanoindentation for measuring mechanical properties of cement pastes. In evaluating AFM as a mechanical characterization tool, various analytical and numerical modeling approaches are compared. The disparities between the numerical self-consistent approach and analytical solutions are determined and reported. The measured elastic Young’s modulus determined from AFM indentation tests are compared to elastic Young’s modulus determined from nanoindentation tests of cement paste. These results indicate that the calcium silicate hydrate (C-S-H) phase of hydrated Portland cement has different properties on the different length scales probed by AFM versus nanoindenters. Packing density of C-S-H particles is proposed as an explanation for the disparity in the measured results.     

A model predicting carbonation depth of concrete containing silica fume

Silica fume (SF) has been used since long as a mineral admixture to improve durability and produce high strength and high performance concrete. Due to the pozzolanic reaction between calcium hydroxide and silica fume, compared with ordinary Portland cement, the carbonation of concrete containing silica fume is much more complex. In this paper, based on a multi-component concept, a numerical model is built which can predict the carbonation of concrete containing silica fume. The proposed model starts with the mix proportions of concrete and considers both Portland cement hydration reaction and pozzolanic reaction. The amount of hydration products which are susceptible to carbonate, such as calcium hydroxide (CH) and calcium silicate hydrate (CSH), as well as porosity can be obtained as associated results of the proposed model during the hydration period. The influence of water-binder ratio and silica fume content on carbonation is considered. The predicted results agree well with experimental results.     

Surface treatmet of sillimanite-based microspheres for strength enhancement of high-temperature lightweight cement composites

The compressive strength of high-temperature lightweight cementing materials containing sillimanite-based hollow microspheres as a filler can be improved by treating the surfaces of the microspheres with a calcium hydroxide-saturated solution at temperatures up to 200° C. The precipitation of an epitaxial layer formed by an interaction between the hot calcium hydroxide solution end the surface of the sphere played an essential role in developing favourable bonding characteristics at the interfaces and in promoting the hydration of the cement matrix. Bonding is associated with the formation of an intermediate layer of aluminium-rich calcium silicate hydrate, produced by an interfacial reaction of the cement paste with the epitaxy under the hydrothermal environment at 300° C. A dense intermediate layer consisting of a rim structure of 4m thickness acts in cross-linking and coupling functions that serve to connect the cement matrix and spheres, thereby improving the interfacial bond strength. The presence of the epitaxial layer on the treated sphere surfaces leads to the formation of a well-crystallized tobermorite matrix phase, which is responsible for the development of strength in autoclaved lightweight cements.     

Alkali activation of reactive silicas in cements: in situ 29Si MAS NMR studies of the kinetics of silicate polymerization

In the highly alkaline environment of the cement paste of a concrete, a source of silica can potentially react in two ways. In the pozzolanic reaction, it can combine with free lime to generate additional calcium silicate hydrate binding phase. Alternatively, reaction with alkali to form a gel can occur; this gel may swell and degrade the concrete. ²⁹Si magic angle spinning (MAS) and cross-polarization (CP) MAS nuclear magnetic resonance (NMR) studies have been performed to determine the silicate connectivity in some model cement systems; ²⁹Si enrichment was utilized to enable a series of spectra to be acquired in situ from a single sample.The hydrate from pozzolanic reaction of lime with silica was similar to the hydrate formed around silica in blended pozzolanic cements, with a relatively high crystallinity and long silicate chains. In the absence of lime, silica reacted with an alkaline solution to produce a gel having a high degree of cross linking, and a range of silicate mobilities. Tricalcium silicate hydration was found to be accelerated significantly by high levels of alkali (KOH) in solution; the hydrate formed had shorter silicate chains and was more crystalline than that produced by reaction in pure water. Hydration in alkali solution of a model blended cement, comprising a mixture of tricalcium silicate and silica, gave rise to two products, a long chain calcium silicate hydrate (C-S-H) and an alkali silicate of low rigidity. The alkali silicate phase gradually polymerized; at later ages it underwent a phase change, although no crystalline phase appeared to be formed. Silicate exchange took place between the C-S-H and the alkali silicate phase at a slow rate.     

A solid-state NMR and X-ray powder diffraction investigation of the binding mechanism for self-healing cementitious materials design: The assessment of the reactivity of sodium silicate based systems

The identification of affordable healing agents for cement based materials, able to promote autonomous crack healing, is a challenge to improve the durability of building structures. In this study, a thorough investigation of the reactivity between a hydrated Portland cement and sodium silicate solutions, as healing agents, has been carried out.The goal is to quantitatively assess the chemical reactivity and actual binding capacity of sodium silicate. Mechanical recovery was evaluated by means of a healing agent strength test on hydrated cement treated with sodium silicate. XRPD and Solid-state NMR allowed the definition of reaction times, the involved species, and the nature and stability of the reaction products. Highlights show that sodium silicate reacts not only with Ca(OH)₂ (namely portlandite), but also with calcium aluminate phases (AFt, AFm, TAH) to extract calcium and/or aluminum ions, with the formation of crystalline/semi-crystalline C-S-H/C-A-S-H tobermorite phase.     

Hydration-induced reinforcement of rigid polyurethane–cement foams: mechanical and functional properties

Mechanical and functional properties of a newly proposed hybrid foam based on rigid polyurethane foam and Portland cement for application in the building field are herein reported. The hybrid is characterized by the co-continuity of the two phases, hydrated cement and polyurethane, which cooperate in a synergistic way to the properties of the resulting material. Furthermore, the closed-cell foam structure gives the material properties typical of porous materials: in particular, the hybrid foam evidences thermal insulation, sound absorption and acoustic insulation, high impact energy and low density typical of polymeric foams. At the same time, the hybrid foam exhibits water vapor permeability, improvement of thermal stability, high compressive mechanical behavior, and adhesion to concrete and mortars typical of inorganic binders such as cement. The materials were obtained by mixing cement powder with polyurethane foam precursors, i.e., methylene di(phenyl-isocyanate), polyol polyether and catalysts, and silicone surfactants. Water was used as blowing reagent. The resulting compounds were foamed in flat closed molds. The cement phase was then allowed to hydrate in accelerated conditions, i.e., in water at 60?°C for 72?h. Mechanical, morphological, and functional characterization showed that the hydrated cement particles interacted with each other, forming an inorganic network within the polymeric matrix (co-continuity), thus “hydration-induced reinforcement of polymer–cement” hybrid foam, in contraposition with the term “composite foam.”     

Microanalysis and optimization-based estimation of C-S-H contents of cementitious systems containing fly ash and silica fume

In this study, a new approach to characterize hardened pastes of pure portland cement as well as those containing cement with supplementary cementitious materials (SCM) was adopted using scanning electron microscopy (SEM) and energy dispersive X-ray spectra (EDS) microanalyses. The volume stoichiometry of the hydration reactions was used to estimate the quantities of the primary and secondary calcium silicate hydrate (C-S-H) and the calcium hydroxide produced by these reactions. The 3D plots of Si/Ca, Al/Ca and S/Ca atom ratios given by the microanalyses were compared with the estimated quantities of C-S-H to successfully determine the Ca/Si ratio of eleven different cementitious systems at four different ages using a constrained nonlinear least squares optimization formulation by General Algebraic Modeling System (GAMS). The estimated mass fraction of calcium hydroxide from the above method agreed well with the calcium hydroxide content determined from the thermogravimetric analyses (TGA).     

Alkali-activated complex binders from class C fly ash and Ca-containing admixtures

Processes that maximize utilization of industrial solid wastes are greatly needed. Sodium hydroxide and sodium silicate solution were used to create alkali-activated complex binders (AACBs) from class C fly ash (CFA) and other Ca-containing admixtures including Portland cement (PC), flue gas desulfurization gypsum (FGDG), and water treatment residual (WTR). Specimens made only from CFA (CFA100), or the same fly ash mixed with 40 wt% PC (CFA60–PC40), with 10 wt% FGDG (CFA90–FGDG10), or with 10 wt% WTR (CFA90–WTR10) had better mechanical performance compared to binders using other mix ratios. The maximum compressive strength of specimens reached 80.0 MPa. Geopolymeric gel, sodium polysilicate zeolite, and hydrated products coexist when AACB reactions occur. Ca from CFA, PC, and WTR precipitated as Ca(OH)₂, bonded in geopolymers to obtain charge balance, or reacted with dissolved silicate and aluminate species to form calcium silicate hydrate (C-S-H) gel. However, Ca from FGDG probably reacted with dissolved silicate and aluminate species to form ettringite. Utilization of CFA and Ca-containing admixtures in AACB is feasible. These binders may be widely utilized in various applications such as in building materials and for solidification/stabilization of other wastes, thus making the wastes more environmentally benign.     

Stabilization/solidification of hazardous and radioactive wastes with alkali-activated cements

This paper reviews progresses on the use of alkali-activated cements for stabilization/solidification of hazardous and radioactive wastes. Alkali-activated cements consist of an alkaline activator and cementing components, such as blast furnace slag, coal fly ash, phosphorus slag, steel slag, metakaolin, etc., or a combination of two or more of them. Properly designed alkali-activated cements can exhibit both higher early and later strengths than conventional portland cement. The main hydration product of alkali-activated cements is calcium silicate hydrate (CSH) with low Ca/Si ratios or aluminosilicate gel at room temperature; CSH, tobmorite, xonotlite and/or zeolites under hydrothermal condition, no metastable crystalline compounds such as Ca(OH)(2) and calcium sulphoaluminates exist. Alkali-activated cements also exhibit excellent resistance to corrosive environments. The leachability of contaminants from alkali-activated cement stabilized hazardous and radioactive wastes is lower than that from hardened portland cement stabilized wastes. From all these aspects, it is concluded that alkali-activated cements are better matrix for solidification/stabilization of hazardous and radioactive wastes than Portland cement.     

应用水化硅酸盐纳米粉体制备块体材料

以磨细生石灰和硅质原料的混合物为前驱物,采用动态水热合成方法制备了水化硅酸盐纳米粉体.借助SEM、TEM及氮吸附等测试方法,分析了制得的水化硅酸盐粉体的形貌、粒径和比表面积.将制得的水化硅酸盐纳米粉体压制成块状固体.采用正交设计方法研究了成型压力、水泥掺量、纤维掺量、聚合物掺量等因素对块体材料力学性能的影响.结果表明:由该粉体压制成的块体材料立即具有强度和耐水性,且表观密度低;成型压力是影响其力学性能最显著的因素.     

Low-temperature fabrication of macroporous scaffolds through foaming and hydration of tricalcium silicate paste and their bioactivity

A low-temperature fabrication method for highly porous bioactive scaffolds was developed. The two-step method involved the foaming of tricalcium silicate cement paste and hydration to form calcium silicate hydrate and calcium hydroxide. Scaffolds with a combination of interconnected macro- and micro-sized pores were fabricated by making use of the decomposition of a hydrogen peroxide (H₂O₂) solution that acted as a foaming agent and through the hydration of tricalcium silicate cement. It was found possible to control the porosity and pore sizes by adjusting the concentration of the H₂O₂ solution. The in vitro bioactivity of the highly porous scaffolds was investigated by immersion in simulated body fluid (SBF) for 7 days. Hydroxyapatite (HAp) was formed on the surface of the scaffolds. Their bioactivity could be expected to be as good as that of tricalcium silicate cement, making the material competent for the bone tissue engineering application.     

Comparing the pozzolanic behavior of sugar cane bagasse ash to amorphous and crystalline SiO2

The present paper aims to clarify the pozzolanic behavior of sugar cane bagasse ash-SCBA by comparing it to amorphous and crystalline silica. It is shown that calcium silicate hydrate is formed in lime solution with SCBA but the reaction is slow and does not consume all the material. Comparing its results with the obtained in tests with silica fume and with crushed quartz show a better agreement with the latter. Characterization of cement pastes shows 20% of cement replacement by sugar cane bagasse ash leads to only a minor reduction in the amount of calcium hydroxide formed. This behavior is also closer to the observed in quartz than in silica fume. The results suggest SCBA should be used as a replacement for inert constituents in cement composites rather than pozzolanic addition. Analysis of the microstructure of the cement pastes revealed the presence of calcium hydroxide in samples prepared with partial replacement by silica fume, quartz and sugar cane bagasse ash. The presence of this phase in the sample prepared with silica fume was attributed to agglomeration of the particles that affected the reactivity of this material.