Gel-space Ratio Research Articles

Review of researches on strength model of foamed concrete

Compared with ordinary concrete, foamed concrete has the advantages of light weight, high fluidity, good thermal and sound insulation properties. It has been initially applied in the construction industry. As the most basic research content of foamed concrete, the strength prediction model has received widespread attention from scholars. In this paper, the existing foamed concrete strength prediction models were summarized. The development status of the traditional empirical model based on theory or experiment and the artificial intelligence model based on computer technology were analyzed, and the advantages and disadvantages of each model were described comprehensively. The following conclusions can be drawn: ①Porosity is an important factor affecting the density and compressive strength of foamed concrete. Considering pore structure and volume composition will be more convenient and accurate to predict the strength. ② As for gel-space ratio model, considering the contribution of different hydration products produced by the cementitious materials to the strength is helpful to improve the accuracy of the prediction model. ③ The artificial intelligence model overcomes the shortcomings of less factors in the empirical model and large amount of experiments with high cost, which makes the strength prediction more effective and precise. This paper provides a train of thought for further research on the strength prediction model of foamed concrete. The development of foamed concrete strength prediction model can provide theoretical guidance for the design of foamed concrete, and is of great significance for promoting the application of foamed concrete in various occasions.

Influence of negative temperature hardening on hydration, pore structure evolution and strength development of cement paste

The pre-cured 24-h demolded cement pastes were directly cured at −5 °C and +20 °C for different ages to investigate the influence of negative temperature hardening on hydration, pore structure evolution, and strength development. Thermogravimetric (TG) results show that the inhibition effect of negative temperature hardening on the hydration degree and C–S–H gel content is different. As the hydration age increases to 120 days, the hydration degree and gel-space ratio of negative temperature hardening cement paste can reach the positive temperature hardening level of 120 days, while the C–S–H gel content is still lower than the positive temperature hardening level of 120 days. Mercury intrusion porosimetry (MIP) results indicated that negative temperature hardening increases all types of pore volumes, especially the pore volume of <10 nm. Similar to the hydration degree, by giving sufficient negative temperature hardening time (120 days), the pore characteristic parameters of negative temperature hardening cement pastes can approach the level of 120-day positive temperature hardening. Also, the fractal analysis shows that negative temperature hardening increases the proportion of large-diameter pores in the range of <100 nm, and decreases the proportion of large-diameter pores in the range of >3000 nm. In addition, the micro-mechanisms of limiting strength development are different at different negative temperature hardening stages. The early strength development is affected by the coupled effects of hydration and pore structure, while the later strength development is limited due to the lower C–S–H gel content.

Positive impact of polymer impregnated on the enhancement of the physico-mechanical characteristics and thermal resistance of high-volume fly-ash blended cement

To predict the enhanced performance and microstructure design of cement-based composites to achieve sustainability in construction, our study aims to investigate for the first time the positive impact of polymer impregnation on fire resistance and mechanical properties of cement-based composite pastes especially those with high percentages of high-volume fly ash (HVFA) at different water/cement ratios of 0.35, 0.40, and 0.45 and cured up to 90 days. These pastes were dried and impregnated with methyl methacrylate monomer and thermally treated at 70 °C for 3 h using 1% benzoyl peroxide as initiator. The effect of water/cement ratio, curing time, gel space ratio, and bulk density were investigated. Also, XRD, DTG/TGA analysis, and SEM are included. The compressive strength and polymer load were measured after the formation of impregnated- polymer pastes. The results indicated that reducing the w/c ratio leads to the production of hardened cement pastes with dense structures and relatively high compressive strength. For the polymer-impregnated samples, the values of compressive strength were decreased with increasing w/c ratio. The obtained data affirmed that the application of polymer impregnation within a cement matrix can be used to strengthen structures exposed to elevated temperatures. The impregnated cement pastes of ordinary Portland cement (mix OPC1) and blended cement OPC-FA pastes (mix OF1) with water/cement ratios of 0.35 showed higher compressive strength values when thermally treated up to 600 °C after 14 and 28 days of curing.

Impact of limestone fineness on cement hydration at early age

This study compares the influence of two limestone fineness on strength development and cement hydration. The replacement of clinker by 20% of fine limestone (dv,50 = 2 μm and SSA = 4,21 m2/g) gives similar strength development to PC up to 7 days. Fine limestone enhances cement hydration and offsets the dilution effect caused by the decrease of clinker amount. The accelerating effect of the limestone was explained by a higher degree of undersaturation with respect to C3S. The influence of coarse limestone (dv,50 = 130 μm and SSA = 0,46 m2/g) on cement hydration is limited to dilution effect. The combination of fine and coarse limestone was also studied. A linear correlation was obtained between strength and gel space ratio for all investigated systems. Furthermore, the results showed that despite the similarity of specific surface area, limestone is more effective than quartz for the enhancement of cement hydration.

Influence of dispersants and dispersion on properties of nanosilica modified cement-based materials

Properties of cement pastes containing nanosilica (NS) dispersed with seven different dispersants were evaluated. The dispersants used were anionic surfactants, sodium dodecyl sulfate (SDS) and polycarboxylate ether (PCE) based superplasticizer, and two groups of non-ionic surfactants, Tweens (T20, T40) and Tritons (TX114, TX100, TX405). The degree of NS dispersions in de-ionized water and cement pore solutions with ultrasonication and dispersants were quantified based on their average hydrodynamic particle size (Z-avg.) and zeta potential obtained from dynamic light scattering (DLS) measurements. The properties of cement-NS pastes assessed were flowability, heat of hydration, and compressive strength. The results showed that except for PCE, all surfactants reduced the flow of cement paste at a given dosage, and therefore, they were subsequently used along with PCE to achieve adequate flow. The combination of PCE and another surfactant accelerated cement-NS hydration. There were strong correlations between the Z-avg. value of cement-NS suspensions made with non-ionic surfactants and the strength and total heat of hydration of the corresponding cement-NS pastes. Surfactant TX405 provided the cement-NS paste significantly high increment in strength (33%, 41%, and 54% at 1, 3, and 28-day, respectively). SEM and TGA results revealed that enhanced nucleation of CSH fibrils and higher gel-space ratio in TX405-cement-NS paste might be responsible for the strength improvement.

Study on relationships between static mechanical properties and composition of low modulus CA mortar

As an organic–inorganic composite material, the mechanical properties of cement asphalt mortar (CA mortar) are closely related to its composition. In order to studying the relationship between mechanical properties and composition of CA mortar, a mechanical model of CA mortar from four different levels (i.e.,cement hydration products, cement gel skeleton, cement-asphalt composite system and CA mortar) was proposed based on the microscopic experimental results, the gel-space ratio theory and the Hashin composite ball model. The mathematical relationships among the elastic modulus of CA mortar, compressive strength f and composition parameters were derived. The results shown: The volume fraction of cement hydrates is an important parameter affecting the CA mortar elastic modulus ECA and the compressive strength f. When the asphalt is regarded as a pore or a low modulus phase, the static elastic modulus of the CA mortar ECA and the volume fraction of cement hydrates VCH, the compressive strength f and gel-space ratio x all have a power function relationship, and R2 > 0.7. The cement hydrates in the CA mortar are still a continuous phase and constitutes the composite gel skeleton, asphalt interspersing among them, and the contribution of asphalt to the CA mortar strength and elastic modulus is relatively limited.

Investigation of the early-age microstructural development of hydrating cement pastes through electrical resistivity measurements

The non-contact electrical resistivity method was employed to investigate the microstructural development of three grades of cement under a fixed water-to-cement ratio (w/c) and the same curing conditions. The method was verified by measuring the heat of hydration and estimating the degree of hydration of the three pastes. The results showed that the existence of a linear relationship between the compressive strength at 28 d (f 28) and bulk electrical resistivity at 72 h (ρ72); thus, the proposed method can be used to assess the strength of cement. Moreover, the results indicated that the quality of cement can be further distinguished by interpreting the trend in the resistivity curve. The high-strength cement will have lower electrical resistivity values during cement dissolution due to the higher content of ions in the pore solution, while it will have higher resistivity values during cement hardening due to the greater amount of hydrates. The electrical resistivity of the cement solution (ρ0) filtered from the cement paste at an early age was measured to demonstrate the effect of the ionic concentration on the bulk electrical resistivity in the initial hydration period. The measured liquid resistivity of cement can be used as an indication of clinker reactivity; higher-strength cement will always have lower measured values. The rate of electrical resistivity development curve was plotted to elucidate the setting range of cement pastes based on three critical points. The time at the second critical point (C2) coincided with the final setting time determined using a Vicat needle. New general relationships between the gel-space ratio and electrical resistivity were developed. Furthermore, autogenous shrinkage was measured for the three cement pastes, and the behaviour could be anticipated from the electrical resistivity measurements.

Analysis of Hydration and Optimal Strength Combinations of Cement-Limestone-Metakaolin Ternary Composite

Metakaolin (MK) is an aluminosilicate pozzolan material and can contribute to property development of concrete due to the pozzolanic reaction. Limestone (LS) powder presents the dilution effect, nucleation effect, and chemical effect on hydration of cement. When metakaolin and limestone are used together, due to the additional chemical reaction between the aluminum phase in MK and limestone, the synergetic benefit can be achieved. This study presents a hydration model for cement-limestone-metakaolin ternary blends. Individual reactions of cement, metakaolin, and limestone are simulated separately, and the interactions among cement hydration, limestone reaction, and metakaolin reaction are considered through the contents of calcium hydroxide and capillary water. The hydration model considers the pozzolanic reaction of metakaolin, chemical and physics effects of limestone, and synergetic effect between metakaolin and limestone. Furthermore, the gel-space ratio of hydrating concrete is calculated using reaction degrees of binders and concrete mixtures. The strength development of ternary blends is evaluated using the gel-space ratio. Based on parameter analysis, the synergetic effect on strength development is shown and the optimal combinations of cement-limestone-metakaolin ternary blends are determined.

Incorporating Liquid Crystal Display (LCD) Glass Waste as Supplementary Cementing Material (SCM) in Cement Mortars-Rationale Based on Hydration, Durability, and Pore Characteristics.

This paper assesses the feasibility of using liquid crystal display (LCD) waste glass as a supplementary cementing material in cement mortars. Two different sizes of LCD waste glass powder (LGP) particles were used (5 µm and 12 µm) with two substitution levels with cement in mortar (10% and 20%). The resulting mortars were evaluated for strength, hydration, porosity and durability through various experimental techniques. It was found that LGP particles lead to appreciable strength gain at all ages in comparison with control mortar, especially significant strength gain of 18% was observed at 28-day. This is attributed to the greater gel-space ratio as corroborated by the experimental determination of porosity, which is found less for LGP-incorporated mortars as compared to control cement mortar. The smaller particle size of LGPs not only accelerates the pozzolanic reaction in alkaline cementitious matrix, but also fills the smaller pores, thus reducing porosity and contributing to strength gain. Increased hydration was also elucidated qualitatively by backscattered electron imaging. Due to the increased hydration in LGP-incorporated pastes and mortars, the durability (in terms of chloride ion permeability) has also been found improved. Thus, it is established that 10% (by weight) of cement can be replaced with 12 μm LGP, whereas 20% can be replaced with 5 μm LDP for improved strength and durability. Incorporating LCD waste in mortars and concretes as partial replacement of cement can not only help utilize this potentially hazardous waste, but also significantly reduce the associated carbon dioxide emissions, thus promoting sustainable development.

Analysis of hydration and strength optimization of cement-fly ash-limestone ternary blended concrete

Limestone powder improves concrete early-age strength while fly ash improves concrete late-age strength due to its pozzolanic reaction. The optimal mixture of cement, fly ash, and limestone is crucial for material design of ternary blended concrete. This research presents a simulation program for evaluating the hydration and strength optimization of ternary blended concrete. The simulation program begins with a kinetic hydration model which simulates the hydration of cement-fly ash-limestone ternary blends. The hydration model considers the mutual effects among reactions of cement, fly ash, and limestone by means of the contents of calcium hydroxide and capillary water. The individual reaction degrees of components of ternary blends are calculated from the hydration model. Furthermore, the compressive strength growth of hydrating ternary blended concrete is calculated by means of gel-space ratio and Powers’ strength theory. Finally, based on parameter studies, the optimal combinations of cement, fly ash, and limestone at different ages are determined. The proposed numerical procedure is valuable for making composite cements as it pertains to compressive strength and environment regulations.

Modeling of Hydration, Strength Development, and Optimum Combinations of Cement-Slag-Limestone Ternary Concrete

Slag can increase late age strength of concrete, but impairs the concrete early-age strength due to low reactivity. Limestone powder can increase early-age strength, but impairs late-age strength due to dilution effect. The combination of slag and limestone powder can produce a composite concrete with adequate strength at both early ages and late ages. This study shows an integrated hydration-strength-optimization model for cement-slag-limestone ternary blends. First, a blended hydration model is put forward for simulating the hydration of composite binder containing slag and limestone powder. Reaction degrees of individual component of binders are calculated using this hydration model. Second, the gel-space ratio of ternary blended concrete is determined based on reaction degrees of composite binder and mixing proportions. Moreover concrete compressive strength is calculated using gel-space ratio. Third, based on parameters analysis, the isoresponse curves regarding strength of concrete are calculated. The optimum combinations of cement, slag, and limestone powder at different ages are calculated. The proposed numerical procedure is valuable for optimum strength design of cement-slag-limestone ternary concrete.

Analysis of Hydration-Mechanical-Durability Properties of Metakaolin Blended Concrete

Metakaolin (MK) is a highly active pozzolanic material and MK is increasingly used in producing high performance concrete. This study presents an integrated hydration-mechanical-durability model for evaluating various properties of metakaolin blended concrete. First, a blended hydration model is proposed for simulating the hydration of composite binders containing MK that considers the dilution effect and pozzolanic reaction due to metakaolin addition. The interactions between the metakaolin reaction and cement hydration were taken into account by means of the contents of calcium hydroxide and capillary water in hydrating the composite binder. The reaction degrees of cement and metakaolin were calculated using the hydration model. Second, the gel-space ratio of the MK blended concrete was determined based on the reaction degrees of the composite binders and proportions of concrete mix. Moreover, concrete compressive strength was calculated using the gel-space ratio. Third, the volumetric phase fractions were calculated based on the reaction degrees of the binders. Chloride penetrability of metakaolin blended concrete was analyzed through the capillary porosity of concrete. The proposed integrated hydration-mechanical-durability model is valuable for the material design of metakaolin blended concrete.

Modeling of Hydration, Compressive Strength, and Carbonation of Portland-Limestone Cement (PLC) Concrete.

Limestone is widely used in the construction industry to produce Portland limestone cement (PLC) concrete. Systematic evaluations of hydration kinetics, compressive strength development, and carbonation resistance are crucial for the rational use of limestone. This study presents a hydration-based model for evaluating the influences of limestone on the strength and carbonation of concrete. First, the hydration model analyzes the dilution effect and the nucleation effect of limestone during the hydration of cement. The degree of cement hydration is calculated by considering concrete mixing proportions, binder properties, and curing conditions. Second, by using the gel–space ratio, the compressive strength of PLC concrete is evaluated. The interactions among water-to-binder ratio, limestone replacement ratio, and strength development are highlighted. Third, the carbonate material contents and porosity are calculated from the hydration model and are used as input parameters for the carbonation model. By considering concrete microstructures and environmental conditions, the carbon dioxide diffusivity and carbonation depth of PLC concrete are evaluated. The proposed model has been determined to be valid for concrete with various water-to-binder ratios, limestone contents, and curing periods.

Optimization of the MgO[sbnd]SiO2 binding system for fiber-cement production with cellulosic reinforcing elements

A range of MgO and SiO2 blends mixed with water are analyzed to develop clinker-free fiber-cement products reinforced with cellulosic fibers. The target is the development of a binder which is not chemically aggressive to the fibers, but which develops high mechanical strength Mechanical performance of the materials developed is not only influenced by magnesium silicate hydrate (M-S-H) gel content, but is more related to the void content within the paste due to unreacted water, meaning that the gel-space ratio concept is valuable in describing the compressive strengths of these materials. A higher MgO content in the mix formulation leads to M-S-H gels with increased Mg/Si ratio. The Mg/Si ratio also increases over time for each mix, indicated by changes in the gel structure as reaction is not yet complete after 28days. SEM shows a heterogeneous microstructure which also has regions of high Si content. The 60wt%MgO-40wt%SiO2 system is chosen as the optimal formulation since it is the least alkaline binder with high mechanical strength. Bending tests on pastes reinforced with cellulosic pulps prove the efficiency of this binder, which preserves the reinforcing capacity of the fibers much better than Portland cement pastes after 200cycles of accelerated ageing.

A Microstructure Based Strength Model for Slag Blended Concrete with Various Curing Temperatures

Ground granulated blast furnace slag, which is a byproduct obtained during steel manufacture, has been widely used for concrete structures in order to reduce carbon dioxide emissions and improve durability. This paper presents a numerical model to evaluate compressive strength development of slag blended concrete at isothermal curing temperatures and time varying curing temperatures. First, the numerical model starts with a cement-slag blended hydration model which simulates both cement hydration and slag reaction. The accelerations of cement hydration and slag reaction at elevated temperatures are modeled by Arrhenius law. Second, the gel-space ratios of hardening concrete are calculated using reaction degrees of cement and slag. Using a modified Powers’ gel-space ratio strength theory, the strength of slag blended concrete is evaluated considering both strengthening factors and weakening factors involved in strength development process. The proposed model is verified using experimental results of strength development of slag blended concrete with different slag contents and different curing temperatures.

Estimation of compressive strength of cement-treated marine clays with different initial water contents

A formula to estimate unconfined compressive strength of cement-treated marine clays is proposed. The formula has a form similar to the gel–space ratio theory. In the proposed formula, the strength of cement-treated soil is given by volumetric solid content, strength increase coefficient due to cement, exponential parameter N representing the effect of the void structure of soil and cement content in respect to the solid material of soil. The formula was adapted to the results of laboratory strength tests of cement-treated soils made of six dredged marine clays with different levels of initial water content. The strengths estimated by the proposed formula agreed with the measured strengths fairly well, using the parameter N=3.5–4.6. The formula was applied to the strength estimate of foam- and bead-treated soils made from dredged marine clay, using the parameter N=2.1–2.5. The applicability of the proposed formula was examined with the results of strength tests carried out for the design of cement-treated soil for the construction of D runway at Tokyo’s Haneda Airport. Seven samples collected in the construction sites were mixed with different amounts of cement and the different levels of initial water content. On fitting the proposed formula to the results of all the data, the proposed formula estimates the measured strength well. On comparing the estimates with those using a conventional formula based on the water–cement ratio, the proposed formula generated better-fit estimates.

Chloride migration in concrete with superabsorbent polymers

Superabsorbent polymers (SAP) can be used as a means for internal curing of concrete. In the present study, the development of transport properties of concrete with SAP is investigated. The chloride migration coefficient according to NT BUILD 492 is used as a measure of this. Twenty concrete mixtures are tested 7, 14, and 28days after casting. The development of degree of hydration is followed for 20 corresponding paste mixtures.Both when SAP is added with extra water to compensate the SAP water absorption in fresh concrete and without extra water, the internal curing water held by SAP may contribute to increase the degree of hydration. No matter if SAP is added with or without extra water, it appears that the so-called gel space ratio can be used as a key parameter to link age and mixture proportions (water-to-cement ratio and SAP dosage) to the resulting chloride migration coefficient; the higher the volume of gel solid relative to the space available for it, the lower the chloride migration coefficient, because the pore system becomes more tortuous and the porosity becomes less.

Prediction of compressive strength of slag concrete using a blended cement hydration model

Partial replacement of Portland cement by slag can reduce the energy consumption and CO2 emission therefore is beneficial to circular economy and sustainable development. Compressive strength is the most important engineering property of concrete. This paper presents a numerical procedure to predict the development of compressive strength of slag blended concrete. This numerical procedure starts with a kinetic hydration model for cement–slag blends by considering the production of calcium hydroxide in cement hydration and its consumption in slag reactions. Reaction degrees of cement slag are obtained as accompanied results from the hydration model. Gel-space ratio of hardening slag blended concrete is determined using reaction degrees of cement and slag, mixing proportions of concrete, and volume stoichiometries of cement hydration and slag reaction. Furthermore, the development of compressive strength is evaluated through Powers\'gel-space ratio theory considering the contributions of cement hydration and slag reaction. The proposed model is verified through experimental data on concrete with different water-to-binder ratios and slag substitution ratios.

Dependence of compressive strength on phase assemblage in cement pastes: Beyond gel–space ratio — Experimental evidence and micromechanical modeling

Cement paste is a complex material, with a porous microstructure spread over several orders of magnitude, filled with water and gas, and a solid matrix composed of many different minerals — unreacted anhydrous products and crystalline or amorphous hydration products. The relationship between this complex phase assemblage and its mechanical properties is usually described through the ‘gel–space ratio’ descriptor, which links compressive strength to the ratio between the volume of hydration products to the sum of volumes of hydration products and capillary porosity. Surprisingly, little has been done on how the nature of hydrates impacts on the mechanical properties of cement paste. This is partly due to the difficulty to quantify the different hydrates in the system, and partly due to the absence of morphology homogenization models enabling one to relate the mineral assemblage to its mechanical properties. In this work, we generated experimental results in order to explore the ‘mechanical efficiency’ of C–S–H compared to portlandite, by hydrating C3S or C2S. The experimental results show that C–S–H, in the domain explored, is the first-order parameter explaining compressive strength. On this basis, a micromechanical model was developed, in which the smallest scale consists in solid C–S–H and total porosity (self-consistent scheme). Residual anhydrous cement and portlandite are added in a second homogenization step, as inclusions (Mori–Tanaka scheme). Additional experiments were performed to assess the role of hydrated calcium sulfoaluminates by hydrating synthetic clinkers or blends made of C3S, C3A and calcium sulfate. Using the same model, we show that monosulfoaluminate (and all non C–S–H hydrates) can be considered, as for anhydrous cement and portlandite, as inclusions in the C–S–H+total porosity matrix.

Evaluation of compressive strength of hardening silica fume blended concrete

Silica fume (SF) is a byproduct of induction arc furnaces and has long been used as a mineral admixture to produce high-strength and high-performance concrete. Owing to the pozzolanic reaction between calcium hydroxide and SF, compared with Portland cement, the hydration of concrete containing SF is much more complex. In this paper, by considering the production of calcium hydroxide in cement hydration and its consumption in the pozzolanic reaction, a numerical model is proposed to simulate the hydration of concrete containing SF. The degree of hydration of cement and degree of reaction of SF are obtained as accompanied results from the proposed hydration model. Furthermore, on the basis of the volume stoichiometries, mixing proportions and the degree of reactions of cement and SF, the gel–space ratio of hydrating blended concrete is calculated. Finally, the development of compressive strength of SF blended concrete is evaluated through Powers’ strength theory considering the contributions of cement hydration and SF reaction. The proposed model is verified through experimental data on concrete with different water-to-cement ratios and SF substitution ratios.