Compressive Strength Development Research Articles

Systematic Mix Design Study on Geopolymers—Prediction of Compressive Strength

Due to the demand for decarbonization of the construction sector, research on alkali-activated binders and material-minimized carbon-reinforced structures has gained momentum in recent years. Most of the research into alkali-activated binders is focused on developing market-ready alternatives, mainly using a trial-and-error approach. In this study, an attempt is made to identify and quantify the factors influencing compressive strength development. Due to their worldwide availability, investigations are being carried out into binders based on calcined clays and natural pozzolans. The goal is to develop a method to produce tailor-made AAB for continuous manufacturing methods to combine carbon reinforcement and alkali-activated materials. For this purpose, an experimental matrix with 20 variation parameters was set up, in which the activator solution and the precursor composition varied. The design of the experiments was used to minimize the number of experiments. It was shown that no single factor is responsible for the development of compressive strength but instead involves several interacting factors. It was possible to find empirical formulas for predicting the compressive strength after 2, 7, and 28 days.

Experimental research on mechanical properties and elastic modulus of glass fiber reinforced concrete

By adding ordinary glass fiber and alkali resistant glass fiber into concrete, the development of axial compressive strength, axial tensile strength and elastic modulus of concrete are studied. The results show that with the increase of glass fiber, the axial compressive strength increases first and then decreases; the axial tensile strength increases; the elastic modulus decreases. The results show that glass fiber can improve the strength of concrete, and reduce the elastic modulus.

Experimental Study into the Mechanical Properties of Plastic Concrete: Compressive Strength Development over Time, Tensile Strength and Elastic Modulus

Plastic Concrete is a low-strength (fcm,28d≤ 1.0 MPa), low-stiffness, impervious concrete used for cut-off wall construction in earthen dams. To date, there has been no systematic study on the effect of mix design on the long-term time development of the mechanical properties (compressive strength, tensile strength, and elastic modulus) of Plastic Concrete. Therefore currently no Plastic Concrete specific constitutive law for cut-off wall design exists. The present study closes this gap. Ten Plastic Concrete mixes with two bentonite-cement ratios and three types of sodium bentonite were produced. Fresh concrete workability tests were performed for all mixes. Compressive strength tests were performed at ages 28 d, 56 d, 91 d and four years. Splitting tensile strength and elastic modulus tests were performed at 28 days. The workability results show a good linear correlation between slump and flow table tests. Plastic Concrete’s compressive strength development over time is notably slower than that of ordinary concrete, and a new time development model following fib MC 2020 is established using a fitted sC coefficient. The splitting tensile strength shows an overall good, linear correlation with compressive strength, with an approximate ratio of 0.135. Furthermore, the elastic modulus EC,S according to EN 12390-13 concrete tests show significantly higher elastic modulus than the available data, and a model approximation for the elastic modulus EC,S as a function of compressive strength is also provided. Overall, this study provides the first Plastic Concrete specific mechanical property models, confirming the critical role mix design plays on Plastic Concrete’s short and long-term mechanical properties. Thus, the developed models enable a more realistic Plastic Concrete cut-off wall design and provide an important basis for future research.

Influence of partial substitution of metakaolin by palm oil fuel ash and alumina waste ash on compressive strength and microstructure in metakaolin-based geopolymer mortar

Geopolymer is recognized as an alternate binder that compounds silica-rich and alumina-rich materials as raw materials and is produced by alkaline activation. In general, biomass ashes in particular for palm oil fuel ash (POFA) is an ideal starting material on the grounds of its high silica concentration, while industrial by-products, especially alumina waste (AW) is also employed as starting material due to its high alumina content. The objective of this research is to address the effects on the compressive strength and also the microstructure of metakaolin-based geopolymer mortar studied through the adaptability of the substitution of metakaolin (MK) by POFA and AW. In order to investigate the effects of POFA and AW substitution on the polymerization reaction and its outcomes, metakaolin-based geopolymers were produced. The most recent data suggests that POFA and AW probably serve as feedstocks for geopolymers with a little strength reduction. The partial substitution of MK by POFA reduces the polymerization reaction rate while maintaining the same water/binder ratio. However, because of the long-lasting polymerization reaction, it is still obtainable to reach a significant degree of polymerization reaction and also provides a comparable 28-day curing age compressive strength for 20 % replacement of MK by POFA. Considering 10 % substitution of POFA by AW, it results in 22.08 % improvement in the compressive strength after 28 curing days. On top of that, the compressive strength development of the geopolymer sample was investigated using scanning electron microscopy and energy dispersive x-ray spectroscopy (SEM/EDS), x-ray diffraction (XRD), and Fourier-transform infrared spectroscopy (FTIR). It was discovered that the density and the compressive strength were induced by the particle morphologies, the surface area of POFA and AW, and the chemical composition as a result of the geopolymerization.

Deterioration and mitigation of efflorescence of alkali-activated phosphorus slag

To achieve the efficient application of alkali-activated phosphorus slag (AAPS), this study revealed the efflorescence mechanisms and found a feasible way to mitigate the efflorescence deterioration. The efflorescence formations were mainly constituted of Na2CO3·H2O, Na3H(CO3)2·2H2O and NaHCO3. The order of the efflorescence ability of Na species was Naf (free Na in unreacted alkali activator) > Naw (weakly bonded Na in calcium-sodium aluminosilicate hydrate (CNASH)) > Nas (strongly bonded Na in CNASH). CNASH with lower Na/Si ratios and higher polymerization degrees was harder to form efflorescence. The accelerated efflorescence not only caused the dealkalization, dealumination and polymerization shrinkage of CNASH, but also deteriorated the homogeneity of CNASH distribution in pastes, due to the uneven carbonization of the upper and lower parts of pastes. The growth of efflorescence formation and the polymerization shrinkage of CNASH caused the upper parts of pastes to crack, which deteriorated the integrity of pastes. The accelerated efflorescence deteriorated the homogeneity and integrity of pastes. An effective strategy to mitigate the efflorescence and crack of pastes resulted from the addition of aggregates, which assisted in the development of compressive strength. AAPS mortars with low Na2O doses or moderate silicate moduli had better resistance to efflorescence.

Investigation of physicochemical processes taking place in chemically activated very high volume fly ash mixtures

In order to reduce the negative impact of cement production on the natural environment, and in particular to reduce CO2 emission, new types of binders are sought. It is beneficial to use supplementary cementitious materials, such as fly ash. A very high volume of fly ash and a reduced amount of cement in the binder can, however, result in deterioration of some properties of the final composite. Such systems require activation. The aim of this work was to investigate the effects of different activators: spent aluminosilicate catalyst (very active pozzolanic material), Na2SO4 and Ca(OH)2 on the hydration/activation processes and development of 28-day compressive strength of the blends containing a very high volume of fly ash. Activators were introduced into the system in various combinations: alone or in mixtures. The following research methods were used: calorimetry, thermal analysis, X-ray diffraction and IR spectroscopy. It was shown that each type of activation accelerates cement hydration, improves pozzolanic activity and increases compressive strength compared to the results of non-activated reference sample. However, the activation kinetics and mechanism of interaction are different for different activators. The most promising results were obtained for the combination of three activators: spent aluminosilicate catalyst, Na2SO4 and Ca(OH)2.

Exploring Design Optimization of Self-Compacting Mortars with Response Surface Methodology

The ever-evolving construction sector demands technological developments to provide consumers with products that meet stringent technical, environmental, and economic requirements. Self-compacting cementitious mixtures have garnered significance in the construction market due to their enhanced compaction, workability, fluidity, and mechanical properties. This study aimed to harness the potential of statistical response surface methodology (RSM) to optimize the fresh properties and strength development of self-compacting mortars. A self-compacting mortar repository was used to build meaningful and robust models describing D-Flow and T-Funnel results, as well as the compressive strength development after 24 h (CS24h) and 28 days (CS28d) of curing. The quantitative input factors considered were A (water/cement), B (superplasticizer/powder), C (water/powder), and D (sand/mortar), and the output variables were Y1 (D-Flow), Y2 (T-Funnel), Y3 (CS24h), and Y4 (CS28d). The results found adjusted response models, with significant R2 values of 87.4% for the D-Flow, 93.3% for the T-Funnel, and 79.1% for the CS24h. However, for the CS28d model, a low R2 of 39.9% was found. Variable A had the greatest influence on the response models. The best correlations found were between inputs A and C and outputs Y1 and Y2, as well as input factors A and D for responses Y3 and Y4. The resulting model was enhanced, thereby resulting in a global desirability of approximately 60%, which showcases the potential for the further refinement and optimization of RSM models applied to self-compacting mortars.

Modifications to reaction mechanisms, phase assemblages and mechanical properties of alkali-activated slags induced by gypsum addition

Alkali-activated slags (AASs) are often reported to exhibit fast settings, undermining their utilisation as sustainable binders from a technological point of view. Although gypsum was shown in previous studies to change the setting time of AASs, understanding the mechanism behind the changes in the reactivity and microstructure of the AASs in the presence of gypsum remains unclear which limits its utilisation in AASs. For the activator conditions used in this research (i.e., Na2O/raw material = 4.5 % and SiO2/Na2O = 1.0), gypsum was observed to dissolve quickly in the system, promoting the precipitation of portlandite and thenardite phases which led to a reduction in the pH of the pore solution causing a slow dissolution of slags. Furthermore, gypsum addition increased the setting time and delayed the compressive strength development of the paste, accompanied by a reduction in the total measured heat. The description of the changes in phase assemblages and kinetic of the system provided here is fundamental for predicting the behaviour of gypsum in AASs and promoting the industrial wide-uptake of AAS technology.

Field study of the dielectric constant of concrete: A parameter less sensitive to environmental variations than electrical resistivity

In this work, a field study was conducted to evaluate the curing effectiveness of concrete slabs cured by air and compound. Two non-destructive testing methods including ground penetrating radar (GPR) and electrical resistivity measurement were used. The dielectric constant value developments of air and compound cured concrete slabs were determined by GPR measurement. The compressive strength development was tested by concrete cylinders. The hydration degrees of air and compound cured slabs were determined by Ca(OH)2 (CH) content through thermogravimetric analysis (TGA) by a modified calculation method. The microstructure development of air and compound cured concrete slabs were characterized by mercury intrusion porosimetry measurement (MIP). It has found that the dielectric constant value of compound cured concrete slab was always higher than air cured concrete slab. The second reflected pulse peak amplitude of GPR waveform was less influenced by weather conditions, making it a reliable indicator of the dielectric constant value. On the other hand, the dielectric constant value determined by the first reflected pulse peak amplitude was greatly influenced by the weather conditions. The dielectric constant value determined by the two-way travel time method exhibited strong correlations with compressive strength, CH content and porosity. However, the electrical resistivity development of air and compound cured concrete slabs were not consistent with compressive strength, CH content and microstructure development. Our findings suggested that the GPR measurement can be practically used to evaluate the curing effectiveness of concrete and can be a good indicator of “hydration potential” from a development perspective.

Seawater-Mixed Lightweight Aggregate Concretes with Dune Sand, Waste Glass and Nanosilica: Experimental and Life Cycle Analysis

The use of alternative and locally available materials is encouraged in the construction industry to improve its sustainability. Desert regions with shortages in freshwater and river sand as fine aggregates in concrete have to search for alternative materials such as seawater, dune sand, and waste glass powder to produce lightweight concretes. The potential negative effects of adding these alternative materials can be reduced by adding nanosilica to the cementitious system at very low quantities. This study evaluates the feasibility of using these alternative materials and nanosilica (NS) in producing lightweight aggregate concretes (LWACs). A systematic study was carried out to understand the synergistic effect of nanosilica and seawater in improving the hydration characteristics of the developed cementitious systems. Also, the effect of these alternative materials on the fresh properties of the cementitious system was assessed by slump flow tests. The evolution of compressive strength at early ages was investigated after 2, 7, and 28 days of moist curing and an improvement in the strength development in concretes with seawater was observed. Furthermore, the integrity of the developed LWACs was analyzed using oven-dry density, thermal conductivity, water porosity and shrinkage measurements. Moreover, the capillary porosity and sorptivity measurements revealed the denser microstructure in the nano-modified seawater lightweight concretes. In the end, the life-cycle assessment study calculated the benefit of alternative materials in terms of carbon footprint and water consumption. As an outcome, a sustainable solution for producing LWACs containing seawater, dune sand or glass powder was proposed.

Effects of recycled mortar powder on the properties and microstructure of magnesium potassium phosphate cement

Magnesium potassium phosphate cement (MKPC) has become a popular research topic owing to its rapid hardening and high early strength. However, its high carbon emissions and cost limit its large-scale applications. To reduce MKPC’s carbon emissions and preparation costs and to improve the resource reutilization of construction waste, we investigated the effects of the particle size and dosage of recycled mortar powder (RMP) on the setting time, mechanical properties, physical phase, and microstructure of MKPC. The results indicated that when the particle size of RMP increased from 30 to 66 μm, the setting time of MKPC increased from 10.5 to 13.1 min, and the compressive strength (at 28 d) decreased from 52.4 to 38.0 MPa. Therefore, increasing the RMP particle size can prolong the setting of MKPC and reduce its compressive strength. Additionally, increasing the dosage of RMP reduced the setting time of MKPC and promoted the development of compressive strength in later stages. When the RMP dosage increased from 0% to 40%, the setting time of MKPC decreased by 26.1%, and the compressive strength increased by 50.2%. According to the results of pore structure and microstructural analysis, the incorporation of RMP improves the microstructure of MKPC, significantly enhancing its mechanical performance. Therefore, RMP can be used as an economical and ecofriendly cement blending material to reduce the preparation cost of MKPC and the carbon emissions of the construction industry and improve the resource reutilization of construction waste.

Co-disposal of municipal solid waste incineration bottom ash (MSWIBA) and steel slag (SS) to improve the geopolymer materials properties

In previous studies, municipal solid waste incineration bottom ash (MSWIBA) exhibited low compressive strength when made into geopolymer materials due to the lack of active Ca. The introduction of steel slag (SS) not only supplements MSWIBA with active Ca, but also enables further treatment of SS, an underutilized solid waste. In this study, mechanical properties, XRD, TGA, FTIR and MIP are the means to evaluate this binary geopolymer. The heavy metal leaching concentration of this geopolymer was used as a basis for assessing its environmental impact. The results show that the introduction of SS helps to improve the compressive strength of geopolymers. The introduction of SS supplements the active Ca and promotes the production of C-(A)-S-H gels. Increasing the alkali doping on this basis contributes to the dissolution of active substances in MSWIBA and SS and promotes the generation of silica-aluminate gels, which likewise contributes to the development of compressive strength of geopolymers. The activation of MSWIBA by alkali can be used as an aluminum removal process, which can reduce the volume of harmful pores in the geopolymer. The solidification efficiency of heavy metals after the introduction of SS can be>90%.

Alkali Activation of Solid Waste from Cable Wire Manufacturing Industry for Building and Construction

Iron-laden tailings have received a lot of attention in recent years, especially for their potential use in civil engineering projects. Alkaline activation was studied as a method to use iron sludge from cable wire manufacturing industry (CWI). Two reagents, namely sodium silicate and sodium hydroxide concentrations of 1 M, 5 M, and 10 M were studied for alkaline activation. To make a paste, the solutions were combined with CWI waste. Using 50x50x50 mm3 molds, the paste was cast into triplicates and allowed to set. The molds were demolished once the paste had solidified, and the specimens were cured at increased temperatures and at room temperature. The specimens were cured for days, 7 days, and 14 days at ambient temperature. The specimens were subjected to geochemical and geotechnical testing after the curing period to determine the mechanism that contributed to unconfined compressive strength (UCS) development. The findings demonstrated that when the two reagents were compared, sodium hydroxide proved to be a better activator, resulting in a greater unconfined compressive strength, particularly when the specimen was cured at a high temperature. When cured at both elevated and ambient temperatures, the sodium silicate did not perform well as an activator, as the specimen were not durable.

Compressive strength and hydrate assemblages of wollastonite-blended magnesium potassium phosphate cements exposed to different pH conditions

Magnesium potassium phosphate (MKP) cements maybe used in various service conditions. The long-term behavior, compressive strength and hydrates formed under different environments was investigated using diluted nitric acid, water and simplified Portland cement pore solution. The main hydrate K-struvite in MKP cements is destabilized to bobierrite if exposed to solutions with pH ≤ 7, and to brucite and hazenite at pH > 7. The formation of additional K-struvite with time leads to cracks and strength loss. Addition of wollastonite to MKP cements slows down compressive strength development, but leads to comparable strengths at later ages, and prevents strength loss with time. Under acidic conditions the formations of calcium hydroxyapatite (or calcium pyrophosphate) and likely M-S-H are observed, while under basic conditions wollastonite accelerates K-struvite destabilization to brucite. The presence of wollastonite in MKP cements prevents cracking and improves the resistances to aggressive acid and alkaline solutions.

Effects of carbon-sequestering coral aggregate on pore structures and compressive strength of concrete

CO2 sequestration/storage shows considerable impacts on the pore structures and compressive strength of concrete. This paper presents a study in which coral aggregates were presoaked in Ca(OH)2 slurries with different solid-to-liquid ratios (i.e. 0.2, 0.4, and 0.6 g/mL) followed by accelerated carbonation. The effects of CO2 sequestration on the particle size distribution, cylinder compressive strength, water absorption, and apparent density of coral aggregate were investigated. The evolution of pore structures in coral aggregate concrete after CO2 sequestration was also studied. Additionally, the effect of CO2 sequestration on the development of compressive strength of coral aggregate concrete was explored. The results showed that CO2 sequestration affected the properties of coral aggregate. Moreover, the porosity of CaCO3 formed by CO2 sequestration was the highest in the concrete. With the increase of solid-to-liquid ratio, the porosity of cement pastes and the CaCO3 increased, and more big pores existed in the cement pastes and CaCO3. Furthermore, the compressive strength of coral aggregate concrete when the solid-to-liquid ratio was 0.2 g/mL increased compared with that before CO2 sequestration, but the compressive strength reduced when the ratio increased to 0.6 g/mL.

The mechanical properties and sustainability of phosphogypsum-slag binder activated by nano-ettringite

The cementitious material based on phosphogypsum (PG) and ground granulated blast furnace slag (GBFS) demonstrates good economy and sustainability, whereas its drawback of ultra-slow strength development seems unacceptable. In this study, an attempt to drive the hydration of PG-GBFS and further facilitate the strength development by introducing nano-ettringite (NE) was carried out. The impact of 1– 5 % NE on the compressive strength, hydration process, dissolution behavior, and microstructure evolution of PG-GBFS were investigated. The results showed that the incorporation of NE significantly increased the compressive strength of PG-GBFS. At 7 d, the strength grew from 0 MPa to a range of 7.6– 20.2 MPa, and at 28 d, it was enhanced from 22.9 MPa to a range of 45.6– 79.0 MPa. The reason was that the introduction of NE induced the formation of AFt, thereby accelerating the hydration process and promoting the development of the skeletal network, resulting in higher early strength. Besides, NE facilitated the formation of C-S(A)-H gel, which further refined the pore structure and led to continuous growth in later strength. Additionally, PG-GFBS with 5 % NE exhibited significantly lower total costs (35.0 % of NaOH-activated slag and 51.7 % of water glass-activated slag) and lower carbon emissions (30.8 % of NaOH-activated slag and 49.8 % of water glass-activated slag) at the same 28 d compressive strength, indicating its strong competitiveness in both sustainability and economy.

Alterations in the reaction properties of Portland cement against coupled deteriorations of magnesium and sulfate upon carbonation curing

The present study explored the alterations in the reaction properties of Portland cement (PC) against coupled deteriorations of magnesium and sulfate upon carbonation curing. The physical and chemical features of the samples containing MgSO4 at levels of 0, 1, 5, or 10 wt% were evaluated under the 10% carbonation curing condition for 27 days. The test results provided in this study signified that the incorporated MgSO4 reduced the reaction of clinkers, thereby reducing the compressive strength development of the samples, still the strength was vastly enhanced by the carbonation curing. The carbonation curing resulted in the presence of carbonated minerals and Ca-modified silica gel, regardless of MgSO4 content. However, a sufficient supply of sulfate ions induced the excessive formation of gypsum and lowered the CO2 uptake levels after carbonation curing. These outcomes are anticipated to serve as fundamental information with regard to the integrity of carbonation curing of PC with CO2 gas contaminated with magnesium and sulfate which would closely be influential to the safety of the final products.

The Characteristics of Self-Hydration and Carbonation Reaction of Coal Ash from Circulating Fluidized-Bed Boiler by Absorption of CO2.

The by-products of the circulating fluidized-bed boiler combustion (CFBC) of coal exhibit self-hardening properties due to the calcium silicates generated by the reaction between SiO2 and CaO, and the ettringite generated by the reaction of gypsum and quicklime with activated alumina. These reactions exhibit tendencies similar to that of the hydration of ordinary Portland cement (OPC). In this study, the self-hydration and carbonation reaction mechanisms of CFBC by-products were analyzed. These CFBC by-products comprise a number of compounds, including Fe2O3, free CaO, and CaSO4, in large quantities. The hydration product calcium aluminate (and/or ferrite) of calcium aluminate ferrite and sulfate was confirmed through instrumental analysis. The CFBC by-products attain hardening properties because of the carbonation reaction between calcium aluminate ferrite and CO2. This can be identified as a self-hardening process because it does not require a supply of special ions from the outside. Through this study, it was confirmed that CFBC by-products generate CaCO3 through carbonation, thereby densifying the pores of the hardened body and contributing to the development of compressive strength.

Effect of pretreatment procedure on the properties of cementitious materials with limestone cured by CO2

Carbon sequestration in cementitious materials is an effective way to promote sustainable development of the cement industry. Traditionally, specimens are cast and then cured by CO2. Cement particles are first pretreated with CO2 and cast into specimens to study their performance properties are innovatively investigated in this paper. In this study, the carbonation depth, CO2 uptake amount, compressive strength, reaction heat, XRD and SEM/BSE of cementitious materials with and without pretreatment procedure were investigated. The results showed that the pretreatment procedure enhanced the CO2 uptake amount of paste samples and contributed significantly to the carbonation depth, but was not conducive to the development of compressive strength. As seen in the SEM/BSE images, a rim of dense nano-sized CaCO3 was produced around the clinker particle after pretreatment, which was identified as mainly calcite in combination with XRD analysis. The considerable reduction in cumulated heat release of the pretreated cement implied that the rim of dense CaCO3 hinder the subsequent hydration of unreacted clinker particle to some extent. The sample prepared with pretreated cement caused a rim of crack around the clinker particle after 1 day of CO2 curing and 27 days of subsequent water curing. 5%, 10% and 15% limestone powder were added in order to improve the degree of carbonation and hydration of cementitious materials. With the increase of limestone powder addition, the carbonation depth and CO2 uptake amount increased gradually. The compressive strength reached the maximum when its content was 10%.

Self-compacting concrete produced with recycled concrete aggregate coated by a polymer-based agent: A case study

Understanding the effects of recycled concrete aggregate (RCA) and polymer impregnation on the properties of self-compacting concrete (SCC) is crucial for the construction industry's efforts to reduce environmental impact and utilise recycled materials. This study aims to examine the impact of RCA inclusion and polymer impregnation on the properties of SCC, in response to the urgent demand for sustainable construction practices. Two different methods of polymer impregnation were evaluated to assess their impact on the workability and hardened characteristics of SCC. The SCC mixtures were prepared using river sand (natural fine aggregate (NFA)), natural crushed limestone (natural coarse aggregate (NCA)) and a polymer impregnation level of 0.05 wt% of the water content, with a water-to-cement ratio of 0.34. RCA was used as a complete replacement for NFA and NCA, adhering to the American Society for Testing and Materials (ASTM) C33 standards for recycled fine concrete aggregate (RFCA) and recycled coarse concrete aggregate (RCCA)The workability of the SCC mixtures was evaluated based on the criteria set by the European Federation of National Associations Representing for Concrete (EFNARC). Among the hardened properties, the compressive strength of the hardened properties was tested at four different time intervals (1, 3, 7 and 28 days), and the integrity of the SCC specimens was assessed using ultrasonic pulse velocity. The polymer impregnation process resulted in a slower compressive strength development rate compared to that observed in the control concrete. However, combining polymer impregnation type 1 with either RFCA and NCA or RCCA and NFA significantly increased the 28-day compressive strength by 5.02% and 1.80%, respectively. These findings provide valuable insights into the behaviour of SCC incorporating RCA and polymer impregnation. Optimising the selection and combination of materials can enhance the performance and sustainability of SCC in construction applications.