Early-age Compressive Strengths Research Articles

Early age strength of ambient-cured geopolymer mortars from waste concrete and bricks with different alkaline activators.

Background: The dataset in this work emanates from preliminary studies comparing early-age compressive strengths of geopolymer mortars produced from construction and demolition wastes (CDW) commonly found in Qatar using different alkaline activators. Methods: Waste concrete, waste bricks and steel slag - an industrial waste produced in large quantities in the country - were used as aluminosilicate sources. Waste concrete was used as fine aggregate (75 μm to 4 mm), while solid or hollow red clay bricks were used together with steel slag as aluminosilicate powders. Solid red clay brick (75 μm to 1.4 mm) was also used as fine aggregate in some mixes. Different alkaline activators including solid powder or ground pellet forms of Ca(OH)2, CaO, and Ca(OH)2-NaOH, NaOH-CaCO3 and Na2SiO3-Na2CO3-Ca(OH)2 mixtures were employed by just adding water. A few mixes included both solid powder Ca(OH)2 and viscous solutions of NaOH and NaOH-Na2SiO3 as alkaline activators. The geopolymer mortars also included small amounts of some other additives such as gypsum, microsilica and aluminium sulfate to enhance the geopolymerization and hydration process. Random proportions of the materials were considered in the range-finding experiments, and the mortars produced were tested for compressive strength. Results: The data show the 7-day compressive strengths and densities of the 40 mixtures considered with mostly ambient temperature (20°C) curing. It also shows such data for mixtures in which variables such as curing at 40°C, mixing with hot water at 50 - 60°C temperature, grading of waste concrete aggregates, and collective grinding of the powdered materials were considered. Conclusions: The dataset shows possible early-age compressive strengths of different geopolymer mortar mixture designs and the materials and mixture design methods that can be used to achieve desired early-age strengths from waste concrete and bricks.

Influence of calcium sulfate dihydrate as chemical activator on compressive strength of hardened fly ash-cement pastes

The present study focuses on evaluating influence of calcium sulfate dihydrate (CaSO4.2H2O) as a chemical activator on compressive strength of hardened pastes containing fly ash and cement as cementitious materials. A high amount of Class-F fly ash used to replace Portland cement ranged from 85 to 93% by mass of cementitious materials. For all the pastes, a low water-to-cementitious materials ratio of 0.20 was prepared. The amounts of CaSO4.2H2O added to the mixtures were 0.0, 1.5, 2.0, and 2.5% by mass of cementitious materials. Experimental results showed that at the age of 3 days, the compressive strength of hardened paste specimens using CaSO4.2H2O was higher 1.21–2.15 times than that of the control paste specimen without CaSO4.2H2O. At the age of 7 days, the compressive strength of hardened paste specimens using CaSO4.2H2O still continued to be higher 1.03–2.06 times than that of the control paste specimen without CaSO4.2H2O. At the ages of 14 and 28 days, the compressive strength of hardened paste specimens using CaSO4.2H2O was approximately equal to that of the control paste specimen without CaSO4.2H2O. Consequently, the addition of CaSO4.2H2O improved the early-age compressive strengths of the fly ash-cement pastes, and the CaSO4.2H2O content of 2.0% by mass of the cementitious materials was effective in increasing the compressive strength of such pastes.

Investigating mechanical properties and microstructure of reactive powder concrete blended with sulfoaluminate cement

Super absorbent polymers, expansion agents, and shrinkage reduction agents have been used to improve the self-shrinkage of ultra-high-performance concrete (UHPC), but they may degrade the mechanical properties of concrete. Sulfoaluminate cement (SAC) has been proven to improve the compressive strength and self-shrinkage of UHPC. However, few studies have focused on the role of SAC in the tensile strength and pore structure of reactive powder concrete (RPC), a type of UHPC. In our study, we used compressive strength and splitting tensile strength tests, isothermal calorimetry, X-ray diffraction (XRD), thermogravimetric analysis (TG-DTA), mercury intrusion porosimetry (MIP), and scanning electron microscopy (SEM) to determine the mechanical properties and microstructure of RPC modified by SAC. The results showed that the addition of SAC effectively enhanced the strength of RPC, especially the early strength. Specifically, using an 8 wt% SAC caused an 871.4 % increase in compressive strength and 600% improvement in tensile strength after 8 h of curing. The tensile strength exhibited a strong positive linear correlation with the compressive strength. The rapid hydration of SAC promoted the generation of ettringite in RPC, which significantly increased the cumulative heat release and improved the strength of RPC at an early age; however, its effect was not significant on the later strength. The key reasons for the improvement in the mechanical properties of RPC by SAC modification were pore refinement and aggregate-paste interface improvement. Thus, incorporating SAC into RPC significantly enhance its early-age compressive and tensile strengths, which contributes to accelerate the formwork turnover of RPC components and operational efficiency of the stacking area.

Influence of compressive strength and maturity conditions on shrinkage of ordinary concrete

The paper presents results of investigations of compressive strength and shrinkage of concrete samples made on the basis of the Portland cement CEM I 32.5R, after 2, 7, 14, 28, 90, and 365 days of maturation in four different maturation conditions. It was shown that after 28 days the samples cured according to the standard in the cuvettes with water achieved the highest compressive strength, although the early-age compressive strengths after 7 and 14 days were lower than those for the samples cured in building film and in dry conditions. A determined correlation between the compressive strength and shrinkage of the concrete proves that wet curing also allows a total elimination of the shrinkage in the first 28 days. Along with the growth of the compressive strength, the drying shrinkage reduces. Obtained results confirmed that the best way of concrete curing, among the analyzed methods, from the point of view of both compressive strength and volume changes is the wet curing.

Compressive strength development and durability properties of high volume slag and slag-fly ash blended concretes containing nano-CaCO3

This paper presents the effect of nano-CaCO3 (NC) on the compressive strengths and durability properties of high volume slag (HVS) and high volume slag-fly ash (HVS-FA) blended concretes. The study examined the improvement in early and later age compressive strengths and durability properties such as sorptivity, volume of permeable voids, rapid chloride penetration and drying shrinkage of HVS concrete containing 69% blast furnace slag (BFS) and HVS-FA concrete containing combined BFS and fly ash (FA) content of 69% due to the addition of 1% NC. Results show that the addition of 1% NC improved the compressive strengths of HVS and HVS-FA concretes significantly by 43% and 28%, respectively at 3 days compared to the control HVS and HVS-FA concretes without NC and exceeded the compressive strengths of control OPC concrete at later ages. It is also found that 1% NC inclusion reduced the water sorptivity of HVS and HVS-FA concretes reasonably after 28 days of curing and reduction is greater after 90 days of curing exhibited comparable water sorptivity to OPC concrete. Significant improvement is also observed in reducing the volume of permeable voids and controlling the drying shrinkage strain at early age as well as later ages of both HVS and HVS-FA concretes due to 1% NC inclusion. Outstanding resistance against chloride ion penetration is also observed in HVS and HVS-FA concretes due to addition of 1% NC to the very low level of chloride ion penetration according to ASTM standard. SEM and EDS analysis revealed a denser microstructure of paste and interfacial transition zone (ITZ) around aggregates.

Setting behaviours and early-age microstructures of alkali-activated ground granulated blast furnace slag (GGBS) from different regions in China

Fourteen raw slag samples were collected from different steel companies from China, using which setting, mechanical and early-age microstructural behaviours of alkali-activated slag (AAS) cements were investigated. To assess the reaction process between ground granulated blast furnace slag (GGBS) and activator, isothermal calorimetry tests were conducted on three selected AAS pastes for 50 h, while TG, FTIR, and Raman tests were conducted to characterise the early microstructural changes. Main findings include that the setting behaviour of AAS pastes depends primarily on the fineness and chemical compositions of GGBS, and statistical analysis results indicate that three of the constituents namely, SiO2, MgO and K2O, have a strong impact compared to the rest constituents. It is also found that commonly-used parameters to describe characteristics of GGBS are not sufficient to predict setting behaviours of AAS paste made by different GGBSs. Furthermore, the 28-d compressive strength of AAS mortars cannot be considered as a reliable indicator to assess its reaction potential at early age, at least, has no relationship to its setting behaviour. The early age microstructural analysis indicates that one possible reason behind the huge different setting and early age compressive strengths of AAS pastes is the variation in the releasing speed of Ca ions.

Utilization of crumb rubber and FBC-based ternary binder in shotcrete lining

This paper describes the early-age properties of shotcrete modified by i) up to 12.5% by volume crumb rubber as a replacement for aggregates and ii) substitution of ordinary Portland cement (OPC) by a fluidized bed combustion-based ternary binder (FBC-TB) up to 80% of mass. For the tests conducted, an increase in the crumb rubber volumetric fraction of the mixture always led to a decrease in uniaxial compressive strength. Replacing OPC with FBC-TB up to 20% enhanced the development of the early-age compressive strengths; however, a larger substitution resulted in the reduction in the concrete’s final compressive strength. In a full-size experimental mock-up test, the highest-performing mixture—in terms of acceptable mechanical properties and a reasonable amount of reused by-products—was subjected to full deployment in a concrete batching plant. Two batches (6 m3 in total) of concrete with crumb rubber and FBC-TB replacement were sprayed onto a full-scale model of a tunnel for the mock-up test. Evaluation of shotcrete performance included an analysis of the development of compressive strength and a comparison of results with criteria from the relevant code of practice, the New Austrian Tunnelling Method. Based on laboratory experiments, optimization, and in-situ measurements, the mock-up tests proved shotcrete containing crumb rubber and FBC-TC is applicable for full-scale use.

Compressive strength and hydration characteristics of high-volume fly ash concrete prepared from fly ash

This paper investigated the feasibility of replacing cement with fly ash (FA) mass ratios ranging from 40 to 70% to produce high-volume FA (HVFA) concrete. A suite of mechanical tests were conducted over a 180-day time frame. The hydration properties were systematically evaluated using isothermal calorimetry test, X-ray diffraction analysis, thermogravimetric analysis and selective dissolution method. Specially, a thermodynamic modeling was also used to predict phase assemblage and compare the experimental results in terms of the HVFA blended cement system. Experimental results showed that HVFA concrete exhibited lower early-age compressive strengths. However, beyond 28 days, the compressive strength of HVFA concrete developed faster than that of control concrete and the HVFA concrete with FA replacement of 40% showed comparable 180-day compressive strength to the control concrete. Hydration tests showed that the dilution and the pozzolanic effects of FA on cement hydration can be the critical reasons for the compressive strength development of HVFA concrete at early and later ages, respectively. Additionally, a good agreement between the experimental and modeling results can also be observed. These findings help to provide a sustainable and environmentally friendly solution to local FA disposal.

Potential use of Jordanian volcanic tuffs as supplementary cementitious materials

Ten Volcanic tuffs collected from several locations in the northern region of Jordan were investigated for potential use as supplementary cementitious materials. The volcanic tuffs were subjected to screening tests which evaluated their chemical composition, morphology and pozzolanic activity. The more promising tuffs were evaluated further as partial (10–40 wt.%) replacement for Portland cement. Their effects on the fresh mix workability, and hardened material strength and susceptibility to alkali-silica reactions were examined. Volcanic tuffs with higher SiO2 content produced higher early-age compressive strengths, and those with higher CaO contents produced improved long-term strength development qualities. Partial replacement of Portland cement with the selected volcanic tuffs reduced the susceptibility to deleterious alkali-silica reactions.

Influence of silica flour–silica fume combination on the properties of high performance cementitious mixtures at ambient temperature curing

The compressive strength, chloride ion permeability, bulk density, volume of permeable voids, and drying shrinkage of high performance cementitious mixtures (HPCM) containing both silica fume (SFU) and silica flour (SFL) were studied under ambient-temperature curing. SFU was used as cementitious material in addition to cement; SFL was used as filler material replacing part of the fine aggregate. The use of SFL in combination with SFU increased the early-age compressive strength of HPCM, but decreased the later-age compressive strengths. Based on the results of both thermo-gravimetric analysis and loss on ignition tests, this phenomenon was attributed to the improved hydration of cementitious materials by SFL at early ages, but inhibited continued hydration at later ages. The addition of SFL combined with SFU resulted in further reduction of chloride ion permeability and the volume of permeable voids. The addition of SFL slightly increased the drying shrinkage. Among SFLs with different fineness, the finest SFL which densified the microstructure of HPCM most resulted in the highest drying shrinkage. For HPCM cured under ambient-temperature the use of SFU alone will suffice when used at sufficient dosages; and the addition of SFL along with SFU presents advantages mostly in improving the early-age compressive strengths.

Ternary blends containing slag and interground/blended limestone: Hydration, strength, and pore structure

The influence of high volume cement replacement using a combination of slag and limestone, on the hydration, reaction products and pore structure, and strength of cementitious systems is reported in this paper. Total replacement levels vary from 20% to 50% by volume. Slag is blended with: (i) Portland-limestone cement (PLC) that contains limestone interground with cement, or (ii) OPC and limestone of four different sizes in such a way that the resulting particle size distribution of the composite matches that of the corresponding PLC-based mixture. The hydration response of cement and cement–slag mixtures are found to be modified in the presence of limestone. It is observed from calorimetric and thermogravimetric analysis that a favorable slag–limestone synergy exists, that enables high volume replacement of cement without concomitant loss in properties. The early-age compressive strengths are beneficially impacted by the presence of limestone whereas the clinker factor does not play a significant role in later-age strengths in both the blended and interground systems. The study paves the way for development of multiple-material binders containing higher levels of cement replacement that demonstrate early and later age properties that are comparable to or better than that of traditional straight cement systems.

Use of gypsum and CKD to enhance early age strength of High Volume Fly Ash (HVFA) pastes

Use of higher proportions of fly ash as a cement replacement in concrete has obvious environmental and performance benefits but high volumes of fly ash are not commonly used due to perceived lower early age strengths. In this investigation, addition of cement kiln dust (CKD) and gypsum to activate the fly ash was studied and the proportions used in the paste mixes were designed to optimize the mixture ingredients to achieve the highest early age compressive strength. Change of mineral phase composition and micro structure of the composites was analyzed. It was found that CKD was much more effective in activating the fly ash than gypsum. Appreciable early age compressive strengths were achieved for fly ash contents up to 60% of the binder and these observations were supported by analysis of the mineral phases.

Comparative Analysis of the Influence of Sodium and Potassium Silicate Solutions on the Kinetics and Products of Slag Activation

Abstract This paper primarily explores the influence of the alkali cation (Na or K) on the reaction kinetics, product formation, gel structure, and mechanical properties of alkali activated slag systems. For the same activator Ms, i.e., molar SiO2–M2O ratio (M = Na or K), a shorter induction period, a larger acceleration peak, and consequently, a higher amount of total heat release under isothermal conditions is observed for the K-silicate activated slag pastes. The early-age compressive strengths in these systems roughly relate to the heat release response. The later-age (7 days and beyond) compressive strengths are observed to be higher for the Na-silicate activated systems, which is corroborated by: (1) higher amounts of C–(A)–S–H gel in this system indicated by a thermal analysis-based approximate quantification method, and (2) higher combined intensities of Q1 and Q2 structures that point to increased degrees of reaction, and lower amounts of unreacted slag obtained from 29Si magic-angle-spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. The NMR spectra also show evidences of Al-substituted C–S–H gel, with a higher amount of substitution when Na-silicates are used.

Structural-grade concrete containing FBC and PCC residues. Part II: Partial-cement concrete

Part I of this study focused on the fresh and hardened properties of different non-cement concretes containing pulverised coal combustion (PCC) fly ash and fluidised bed combustion (FBC) spent bed in order to ascertain their capabilities and drawbacks in the production of structural-grade concrete. While these mixtures produced sufficient structural strength, their long setting times and excessive soaked sulfate-induced expansions posed major limitations on their acceptability as viable construction materials. In order to mitigate these concerns, Part II of the study, presented here, evaluates the characteristics of a number of partial-cement concretes in comparison with those of non-cement and a reference (control) Portland cement concrete. The test results revealed that the two abovementioned concerns were eliminated with proper mixture proportioning of partial-cement concretes. It was found that while early-age compressive strengths of partial-cement concretes were lower than those of the reference concrete, late strengths exceeded those of the reference concrete. Their plastic and drying shrinkages were also lower than those of the reference concrete. The studied partial-cement concretes also displayed resistance to abrasion superior to that of the reference concrete under dry conditions, whereas the opposite results were obtained under wet conditions.

Effects of activator characteristics on the reaction product formation in slag binders activated using alkali silicate powder and NaOH

The influence of different levels of alkalinity, expressed using the Na 2 O-to-source material ratio ( n ) and activator SiO 2 -to-Na 2 O ratio ( M s ), on the compressive strength development of, and reaction product formation in sodium silicate and NaOH powder activated slag binder systems is discussed. Higher n value mixtures are found to exhibit higher early and later age compressive strengths. An increase in M s results in reduced early age and slightly increased later age strengths. Compositional coefficients, which are functions of n and M s are proposed, that relate to the early and later age strengths of the activated slag binders as well as to the shift in the FTIR spectra. The reaction product formation in these systems as a function of the total alkalinity is explained using the shifts of the dominant peak in the FTIR spectra. Fundamental changes in reaction products of powder activated binders as a function of alkalinity is observed. The deductions from the peak shifts are substantiated using the FTIR spectra of the pastes before and after salicylic acid–methanol (SAM) attack.

Influences of Initial Curing Conditions on the Microstructure of Fly Ash Cement System

In order to study the influences of initial curing conditions on fly ash (FA) cement concrete durability, fly ash cement samples with 30% replacement ratio were fabricated and cured in water at 10°C, 20°C, 30°Cand 40°C for 3d, 7d, 14d and 28d respectively. Hydration degrees of fly ash at early age were measured using the selective dissolve method. Correspondingly the pore structure and morphology of FA-cement mortar and compared cement mortar were studied by using MIP and SEM methods. Then early age compressive strengths of FA-cement concrete and compared normal cement concrete were tested. Experimental results show that initial curing temperatures and ages are important factors to fly ash early age hydration degree, FA-cement system microstructure, morphology and early age compressive strength etc. High curing temperatures and longer curing time can lead higher fly ash hydration degree, and then higher compressive strength of FA-cement concrete, and make the micro-structures of fly ash-cement system denser.

Energy-Efficient Accelerated Curing of Concrete for Plant-Produced Prestressed Concrete

T he cost of energy is at an all-time high and continues to rise. Therefore, any industry which employes an energy-intensive process needs to develop information to optimize that process. The prestressed concrete industry is no exception, with its necessity to attain high early-age compressive strengths of approximately 3500 to 5500 psi (24 to 38 MPa), usually at an age of 14 hours or less. The technical literature does not contain information to guide a producer of precast prestressed concrete products to optimization of curing at elevated temperatures. Studies l 2 in the 1960's concentrated on developing the highest early-age compressive strength possible in a given length of time, with little regard to the expenditure of energy which then was inexpensive and in seemingly endless supply. This laboratory study was designed primarily to provide the producer of precast prestressed concrete products with curing techniques which are capable of developing high early-age compressive strengths while significantly reducing the levels of heat energy consumption. Prior to undertaking this laboratory study, an extensive state-of-the-art re-