28-day Compressive Strength Research Articles

Rapid early age strength development of in-line activated geopolymer for concrete 3D printing

The in-line activation using print head mixing technology involves two parts pumping and mixing at the print head. This technology can be applied in geopolymer concrete, where the base mix (precursors, sand and water) and alkaline activators can be pumped as two separate parts and mixed at the print head for activation. While such methods provide prolonged open time, the early age strength development is low since water from the base mix dilutes the activator solution during print head mixing. Besides, choosing an activator solution with high alkalinity to combat the dilution effect reduces the immediate static yield strength development after print head mixing, which is essential for rapid building. Therefore, this study investigates the hydrated lime as an additive in the base mix to overcome the challenge to attain high 1-day compressive strength along with immediate static yield strength development. The effect of hydrated lime dosage on the pumpability (i.e. evolution of viscosity with time), static yield strength development after print head mixing (buildability) and hardened properties were assessed. The optimum hydrated lime dosage of 1 wt% of the precursors exhibited 1-day compressive strength of 20 MPa and static yield strength of 38.8 kPa after 5 min from print head mixing with minimal changes in pumpability of the base mix for up to 6 h. Moreover, the effect of rapid static yield strength development of the printed layers on the interlayer bond strength was assessed with varying cycle times between 5 s and 40 min. It was found that the interlayer bond strength reduced by 63% when the cycle time was increased from 5 s to 40 min, however, the reduction can be decreased to 15% by surface wetting of the previous layers.

Preparation and properties of Portland cement from secondary recycle waste of rare earth tailing

Secondary recycle waste of rare earth tailing (RET) was used as raw material to prepare Portland cement. The phase composition and microstructure of clinker, mechanical strength and hydration characteristic of cement were investigated. The results indicated that the RET recycle waste increased the practical content of early-strength and high-durability mineral phases (C3S and C4AF) in clinker, which respectively rose from 45.3% to 67.5% and 6.1% to 10.6%, and significantly reduced the ratio of f-CaO from 0.4% to 0.23%, finally causing the intensity reduction of first exothermic peak of hydration, prolonging the appearing time of the second and third exothermic peak, and rising initial and final setting time. This RET recycle waste-fabricated cement generated high strength, and the improving rate of 1-day and 3-day compressive strength achieved to 81.2% and 60.3%, due to dense microstructure caused by fast hydration of rich C3S.

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.

The impact of red clay brick and perlite wastes on the reusability of katoite-based binder after exposure to elevated temperatures

This study investigated the effect of red-clay brick and perlite wastes (RCBW and PW) on the performance and rehydration of katoite-based binder after exposure to elevated temperatures. The KT-binder was produced by combining dolomite waste with sodium aluminate. The exposure of KT binder to temperatures up to 1000 °C (CA-1000) caused a significant compressive strength loss associated with the decomposition of KTand other phases into mayenite, sodium aluminate, and lime. The CA-1000 acts as a one-part alkali-activated binder, as it can re-interact with water to yield a layered double hydroxide-based binder with 1-day compressive strength of 9.7 MPa and volume expansion of 160%. The individual inclusion of 50 wt% RCBW and PW improved the thermal resistance of the CA-binder but decreased its ability to rehydrate, as they hindered the reformation of sodium aluminate and mayenite through the formation of lower hydraulic calcium/sodium aluminosilicates.

Mechanical properties, carbon footprint and cost of ultra-high performance concrete containing ground granulated blast furnace slag

This paper deals with the possibility of formulating low-carbon UHPC by replacing cement with ground granulated blast furnace slag (GGBS). Slags of two fineness levels (420 m2/kg (SL1) and 700 m2/kg (SL2), respectively) are incorporated as volume replacements for cement at 30% and 50%. The possibility of completely replacing silica fume (SF) with superfine slag (SL2) in UHPC is also investigated.The results show that reducing the SF allows for the production of UHPC with satisfactory mechanical properties. The incorporation of 30% of GGBS (SL1 or SL2) accelerates the hydration process due to the reduced SP content and the nucleation site effect, which improves long-term compressive strength. In the presence of a high level of ordinary slag (50% of SL1), the dilution effect dominates and a decrease in compressive strength can be observed at all ages. Due to the physical effect of superfine slag, 30% of SL2 yields a high 28-day compressive strength (151.2 vs. 136.5 MPa for the SL2-based reference mix) despite the absence of SF particles. The relatively low bound water content, as determined by thermogravimetric analysis (TGA), indicates the presence of a large amount of unreacted cement, which contributes to the improved mechanical performance of UHPC mixes. A 28-day splitting tensile strength greater than 7 MPa is measured across all UHPC produced, with a maximum value of 9.2 MPa for concrete containing 30% of SL2 and without SF. A 50% SL2 UHPC with a very low carbon dioxide index (approx. 4.15 kg/m3/MPa) and a compressive strength of 139 MPa, similar to the SF-based reference mix, have indeed been achieved. However, this mixture is found to be the most expensive (at 496 €/m3), due to the high cost of superfine slag, in comparison with cement.

Experimental study of lightweight concrete prepared from super absorbent polymers (SAPs) and glass fibers (GF)

ABSTRACT This study investigates the effect of super absorbent polymers (SAPs) and glass fiber (GF) additions on the properties of lightweight concrete. The SAPs act as a pore-forming agent, with the addition of various GF contents (0.1–1.0% by weight of dry ingredients). The results indicate that the use of SAPs at 7% by weight of OPC produces a product with similar properties to conventional lightweight concretes which meet the ASTM C332–17 standard. The addition of GF to the mixture tends to reduce the dry density and compressive strength, but increases the porosity and flexural strength. The results indicate that the use of SAPs at 7% by weight of OPC produces a product with similar properties to conventional lightweight concretes which meet the ASTM C332–17 standard. The 28-day compressive strengths decreased from 7.04 ± 0.07 for control sample to 4.59 ± 0.11 MPa, while the flexural strengths increased from 3.89 ± 0.04 for control sample to 6.72 ± 0.06 MPa. High-porosity lightweight concrete prepared from SAPs and GF shows reduced thermal conductivity and an increased sorption rate. The initial rate of water absorption gradually increases from 2.48 × 10−1 mm/sec0.5 in the sample without GF up to 5.45 × 10−1 mm/sec0.5 in the sample containing 1.0 wt.% GF. Comparison with the literature indicates that the use of SAPs as pore-forming agents in conjunction with GF has the potential to produce fiber-reinforced lightweight concrete with improved properties.

A facile approach to disperse metakaolin for promoting compressive strength of cement composites

Metakaolin (MK), a highly reactive supplementary cementitious material, is extensively employed in cement composites to improve their compressive strength. Nevertheless, MK suffers from extremely poor dispersion, resulting in agglomerating in the cement-based materials, thus failing to exhibit its positive effects fully. To overcome this drawback, for the first time, this work elaborately developed a facile graphene quantum dots (GQDs)-assisted ultrasonic dispersion approach to improve the dispersity of MK effectively. Furthermore, it investigated effect of as-obtained MK (GMK) on compressive strength of cement composites. More specifically, GMK owned a smaller size and a larger specific surface area and presented a higher dispersity than UMK (namely, MK was identically treated by ultrasonic dispersion only without GQDs) and original MK. And in light of the characteristics of three types of MKs, the dispersing mechanism of MK caused by GQDs-assisted ultrasonic treatment was rationally ascribed to intercalation, exfoliation and surfactant effects of GQDs. Crucially, GMK purely by the incorporation of 1.0 wt.% (by weight of cement) demonstrated a significant enhancement of 31.03% for the 28-day compressive strength of mortar; While UMK had little influence on the compressive strength of mortar under the identical conditions. And based on the comprehensive analyses of hydration heat, phase composition and microstructure of different cement pastes, the most significant positive effect of highly dispersed GMK purely by a dosage of 1.0 wt.% on the compressive strength of cement composites was accredited to the fact that GMK was able to accelerate hydration process, boost the formation of C−(A)−S−H and refine the microstructure. This study firstly provided a facile, efficient and low-cost method to fabricate high-dispersity GMK, and verified its remarkable promotion effect on compressive strength of cement composites. These observations would improve the benefits and reduce the cost of MK-like materials applied to cement-based materials.

Forecasting the strength of micro/nano silica in cementitious matrix by machine learning approaches

This work provides an intelligent machine learning-based technique for anticipating cement paste compressive strength incorporating nanocomposite. In this approach, artificial neural network (ANN), gene expression programming (GEP), random forest (RF), and ensemble approaches are used to combine multiple weak learners capable of identifying the mapping between input and output data into a robust learner. The accuracy of the strong learner as a whole will be increased by incorporating the weak learner with the least prediction error with enhanced correlation. The components of the cement paste mixture (such as water-to-cement ratio, nano-silica (%), micro-silica (%), curing time (days), and the compressive strength value are used as input data and output data. Thus, 205 datasets of cement compressive strength tests are collected to train and assess the learners. The reliability of the models was evaluated using cross-validation with K-folding and statistical error analysis (i.e., MAE, MAE, and RMSE). Therefore, interpretable methods using AdaBoost provide a significant correlation with a lower error rate of R2 = 0.96. The two that perform the best are ANN with ensemble Adaboost and RF with an R-square of 0.97. Additionally, the cross-validation findings demonstrate that the answer was correct and had minimal mistakes. The ensemble model, therefore, displays a strong connection. In addition, new expressions for the predictions of the 28-day compressive strength of nanocomposite containing silica content are developed based on the database. Sensitivity analysis reveals that curing time and silica content has a major influence on prediction. In addition, a model capable of predicting the values of the desired outputs when the necessary input parameters are provided is trained to generate the graphical user interface (GUI). It simplifies the procedure and provides a useful instrument for employing the model's civil engineering abilities.

Durability characteristics of slag based geopolymer concrete modified with crumb rubber

Today, due to the increasing demand in the building sector, the interest of researchers in sustainable building materials is increasing. Geopolymer concretes especially come to the forefront due to energy saving and recycling of waste materials. In this paper, slag based geopolymer concretes were produced by substituting crumb rubber (CR) in different proportions instead of fine aggregate. In the samples using 12 M potassium hydroxide (KOH) as alkali activator, river aggregate was used as aggregate. The 28-day compressive strengths of the samples were between 25.95 MPa and 38.17 MPa, between 9.8 MPa and 11.95 MPa at the end of 300F-T cycles, and between 40.65 MPa and 35.70 MPa in sulfate solutions at the end of 12 weeks. Sulfate (5% Na2SO4 and 5% MgSO4) and freeze–thaw (F-T) (100, 200 and 300 cycles) resistances of the samples cured in the laboratory were determined. As a result of durability tests, the compressive strength, weight changes, sorptivity, water absorption, visual and microstructure analysis of the samples were examined. While the compressive strength of the samples kept in sulfate solutions increased, the compressive strength of the samples exposed to F-T cycles decreased. Sodium sulfate solution caused more strength increase in the samples compared to magnesium sulfate solution. After 300F-T, the strength losses in the samples were over 60%. In general, although the CR slightly reduced the mechanical properties of the control samples, it reduced the permeability of the samples, which increased their durability.

Soft computing techniques for predicting the properties of raw rice husk concrete bricks using regression-based machine learning approaches

In this study, the replacement of raw rice husk, fly ash, and hydrated lime for fine aggregate and cement was evaluated in making raw rice husk-concrete brick. This study optimizes compressive strength, water absorption, and dry density of concrete brick containing recycled aggregates via Response Surface Methodology. The optimized model's accuracy is validated through Artificial Neural Network and Multiple Linear Regression. The Artificial Neural Network model captured the 100 data's variability from RSM optimization as indicated by the high R threshold- (R > 0.9997), (R > 0.99993), (R > 0.99997). Multiple Linear Regression model captured the data's variability the decent R2 threshold confirming- (R2 > 0.9855), (R2 > 0.9768), (R2 > 0.9155). The raw rice husk-concrete brick 28-day compressive strength, water absorption, and density prediction were more accurate when using Response Surface Methodology and Artificial Neural Network compared to Multiple Linear Regression. Lower MAE and RMSE, coupled with higher R2 values, unequivocally indicate the model's superior performance. Additionally, employing sensitivity analysis, the influence of the six input parameters on outcomes was assessed. Machine learning aids efficient prediction of concrete's mechanical properties, conserving time, labor, and resources in civil engineering.

An Experimental Study on Compressive Strength of the Concrete Containing Type II Cement and Tire-Rubber Particles at Various Ages

Abstract Compressive strength of concrete is one of the most important properties of concrete for designers and quality control engineers and the engineering community is interested in predicting 28-day compressive strength of concrete because of the strength of concrete at younger ages such as 7 days. Portland cement type II is one of the most widely used cements, which has the highest consumption because of the weather conditions and the construction process of structures in Iran. The relation for computing the compressive strength at the age of t based on the 28-day strength has been presented only for cement types I and III and there is no relation for type II cement in ACI codes of practice. In this study, the compressive strength of concrete at the age of t based on the 28-day strength has been experimentally investigated for the concretes containing cement type II and tire rubber particles. To this end, three cement grades of 350, 400, and 450 kg/m3 as well as water-to-cement ratios (w/c) of 0.5 and 0.65 were used. Moreover, cement was replaced with tire-rubber particles at contents of 5 to 20 % by volume. The values for α and β coefficients to be applied in the concrete strength relation based on the age of 28 days according to ACI209 for the concretes containing type II cement and tire-rubber particles were obtained equal to 3.13 and 0.91 as well as 3.32 and 0.9, respectively. Finally, the best mix design was found to do with the ordinary concrete with cement grade of 400 kg/m3, w/c of 0.5 and tire-rubber particles of 5 % that leads to the highest compressive strength by which the environmental issues could be markedly mitigated.

Hydration characteristics, structure evolution of marine cement composites with high durability modified by colloidal nano-silica for low carbon footprints

The porous structure of cement composites provides channels for the invasion of chloride ions in marine environment, which limits their service life and low carbon footprints. Colloidal nano-silica (CNS) with the particle size of 10–30 nm theoretically can refine the pore structure, block the chloride ion transport channel. Thus, it is beneficial to improve the service life of cement composites and reduce the consumption of cement and carbon dioxide emission. In this paper, the CNS was introduced to partially replace high sulfate resistance Portland cement (HSRPC) preparing HSRPC-CNS composite. The hydration characteristics, strength property, porous structure and chloride ion resistance were investigated. The results revealed that the addition of CNS was effective in improving the properties of HSRPC-CNS composite, and the most suitable content of CNS was 3 wt%. In this case, the hydration process of HSRPC-CNS composite was accelerated, and the 28-days compressive strength increased by 12.26%. Additionally, as the CNS content increases to 3 wt%, the porosity and chloride diffusion coefficient decreased by 32.39% and 41.99%, respectively. Therefore, CNS can be used as a sustainable nano additive that improved compactness, alleviated chloride ion permeability and reduced carbon dioxide emissions in HSRPC-based composites.

Utilization of coal bottom ash from thermal power plants as a cement replacement for building: A promising sustainable practice

Coal bottom ash (CBA) is claimed to carry some pozzolanic qualities that can be stimulated by pre-treatment. This study investigates the feasibility of partially replacing ordinary Portland cement (OPC) with ground CBA to produce CBA-cement paste. A thorough experimental program was designed to explore the effect of the CBA source and particle size, liquid-to-binder ratio (l/b), and superplasticizer (SP) on the 28-day compressive strength of CBA-cement paste. The CBA source with a high SiO2/Al2O3 ratio (approximately 3.6) and CaO content (5.3%) yielded higher compressive strengths. Grinding breaks down the large raw CBA particles and enhances their reactivity with water and pozzolanic character. Specimens with 50% ground CBA attained a 28-day compressive strength of up to 51 MPa at l/b of 0.225. The low pozzolanic nature combined with the porous texture of CBA necessitates the incorporation of a superplasticizer to enhance workability at low l/b. A 28-day strength of 61 MPa was achieved at 30% cement replacement with 150 μm CBA at l/b of 0.25 using 1% SP. In view of this, partial replacement of cement with ground CBA has proven to be a viable and sustainable construction option.

From dolomite waste to katoite-based binder: Synthesis, performance and characterization

Calcium aluminate cement, in which katoite (KT) is the main binder, has garnered significant attention for its resistivity against aggressive environments. Despite its impressive durability, the production of this cement is costly and generates a substantial CO2 emission. Therefore, the objective of this study was to develop an eco-friendly KT-based binder using exchange reaction between sodium aluminate (NaAlO2) solution and dolomite waste powder. The performance of KT-binder was evaluated by measuring compressive strength, drying shrinkage, bulk density, and porosity. The phase composition and microstructural changes were also investigated by XRD, TG/DTG, FT-IR, and SEM/EDS techniques. The resulting hardened materials exhibited 28-day compressive strengths ranging from 5 MPa to 50 MPa and contained primarily katoite Ca3Al2(OH)12, along with pirssonite Na2CO3·CaCO3·2H2O, thermonatrite Na2CO3·H2O, and aluminum hydroxide Al(OH)3 phases. EDS mapping indicated that only a small amount of Mg2+ was present in the katoite phase, suggesting that the alkalinity and atomic radius difference between Ca2+ and Mg2+ were the main factors. Altering the content of concrete waste (CoW) in the cementitious system within the range from 10 to 50 wt% leads to an improvement in compressive strength and a reduction in both pore volume and drying shrinkage of NaAlO2-DW mixture. Conversely, perlite waste (PW) shows an opposite trend despite its high amorphicity. The presence of highly water-soluble thermonatrite, which is a byproduct of the cationic exchange reaction, is responsible for the low water resistivity of the prepared binder. As a result, more research is needed to tackle this problem.

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.

Feasibility of waste foundry sand in high-strength self-compacting concrete and the effects of elevated temperatures

The disposal and recycling of waste foundry sand (WFS) as industrial solid waste can effectively solve potential environmental pollution problems. This paper conducted an experimental investigation on the effects of elevated temperatures on mechanical properties and microstructural characteristics of high-strength self-compacting concrete (HSSCC) incorporating WFS for substituting river sand with different contents (i.e., 0%, 25%, 50%, 75% and 100% by weight). The workability of fresh concrete including slump flow, J-ring, T500, and V-tunnel box tests were first measured. A series of tests including ultrasound pulse velocity (UPV), residual compressive strength and the behavior of capillary water absorption were conducted before and after different temperatures (100 °C, 200 °C, 300 °C, 400 °C) following an air-cooling time. Further, the thermal analysis and microstructural observation were conducted for the evaluation of high-temperature damage evolution by using thermogravimetry (TG), differential scanning calorimetry (DSC), and scanning electron microscope (SEM). The test results showed that the specimens obtained excellent self-compaction performance at 50% WFS replacement rate. The coupling effect of WFS substitution rate and temperature level has a large effect on the mechanical properties and thermal resistance. Compared with the control group, the 28-day compressive strength of HSSCC-WFS50 was increased by 8.2%. In addition, thermal analysis tests (TG/DTG/DSC) and SEM also elucidated the changes of HSSCC-WFS products and microstructures at high temperature from a fine microscopic perspective to reveal the deterioration mechanism of HSSCC-WFS durability after high temperature damage.

Effect of zirconium oxide nanofiber on the strength and chloride ion penetration coefficient of concrete

In this study, zirconium oxide nanofiber with a mean diameter of 100 nm was added to concrete at various concentrations as a cement replacement. Various tests, including compressive strength, splitting tensile strength, flexural strength, and electrical resistance tests, as well as a rapid chloride penetration test, were performed on specimens containing zirconium oxide nanofibers for the concrete assessment, and the results were compared to those obtained from control specimens that did not contain nanofibers. The results showed that adding zirconium oxide nanofibers at 135 gr/m3 of concrete yielded a 28-day compressive strength equal to 44.62 MPa, which exhibits a 20.40% increase in strength with respect to the specimen that lacked nanofibers. The flexural strength and splitting tensile strength tests at 28 days of age and in the presence of 135 gr/m3 mentioned nanofibers were increased by 22.28 and 33.47%, respectively, in comparison to the control specimens. Moreover, revealed that at 28 days of age, in the specimens containing 270 gr/m3 zirconium oxide nanofibers, the migration coefficient of chloride ion was reduced by 29.86%, and its electrical resistance was increased by 68.33%. These findings highlight the potential of nanofibers as a promising solution for enhancing the strength and performance of concrete structures.

Effect of clinoptilolite on properties and fractal characteristic of pore structure of polypropylene fiber-reinforced cement-based composite

To further analyze the effect mechanism of clinoptilolite on the macro-mechanics and microstructure of polypropylene fiber-reinforced cement-based composite, the samples with 0, 5%, 10%, 15%, 20% and 25% clinoptilolite contents were investigated by compression test, flexural test, X-ray diffraction (XRD), scanning electron microscope (SEM) and mercury intrusion porosimetry (MIP), and the pore structure characteristics were quantitatively analyzed by fractal theory. Results showed that: With the increase of clinoptilolite content, the compressive strength, flexural strength and porosity of samples first increased and then decreased. The addition of clinoptilolite reduced the 7-day strength but increased the 28-day compressive strength, 60-day compressive strength, 28-day flexural strength and 60-day flexural strength of the samples by about 6%, 14.3%, 13.9% and 19.1%. It was recommended that the clinoptilolite content should not exceed 15%. Due to the filling effect and pozzolanic effect of clinoptilolite, adding clinoptilolite into the matrix could promote a hydration reaction to produce more C-S-H gels and reduce the proportion of more harmful pores and harmful pores, effectively improving the internal pore structure of the matrix. Meantime, it was found that the evolution law of pore structure based on fractal dimension calculated by the thermodynamic fractal model was consistent with the variation law of the pore size distribution curves and pore volume measured by MIP. Furthermore, a polynomial relationship model between clinoptilolite content, the quantitative fractal characteristic of pore structure and the macroscopic mechanical behaviors was established, which indicated that the pore fractal dimension could quantitatively describe the effect of clinoptilolite content on the pore microstructure.

Study on the Effect of Three Types of Calcium Sulfate on the Early Hydration and Workability of Self-Compacting Repair Mortar.

Despite having a high early mechanical strength and using sulfoaluminate cement as the primary cementitious material, self-compacting repair mortar (SCRM) suffers from rapid hydration rates leading to construction time constraints. This study examined how several forms of calcium sulfate, including hemihydrate gypsum, anhydrite, and dihydrate gypsum, affected SCRM's workability, hydration process, and microstructure. The outcomes demonstrated that adding hemihydrate gypsum sped up SCRM's early hydration rate and boosted its expansion rate. For a cement with 8% hemihydrate gypsum, 6 h after adding the water, the flexural strength and compressive strength increased by 39.02% and 34.08%, respectively. The hydration rate of SCRM can be efficiently delayed by dihydrate gypsum, although the result is subpar. The material exhibited the lowest fluidity loss in 20 min, the setting time was extended, and the 28-day flexural and compressive strengths were raised by 26.56% and 28.08%, respectively, after adding 8 percent anhydrite.

Eco-friendly lightweight composite prepared with a geopolymer and wheat straw

Agricultural waste shows promise for use in lightweight building materials. The combination of a geopolymer and wheat straw (WS) is adopted to produce a kind of eco-friendly lightweight composite. Using WS pretreated with sodium hydroxide (NaOH) and sodium metasilicate (Na2SiO3) solutions, the influence of WS on the properties of the geopolymer–WS composite (GWSC) is investigated. The pretreated WS avoids the retarding effect of the untreated WS on the hydration of the geopolymer and even presents an accelerating effect. Increasing the content of pretreated WS generally results in a reduction in apparent density and strength. A GWSC with an apparent density of 1000–1500 kg/m3 can be prepared, corresponding to a 28-day compressive strength between 5 and 25 MPa. The water absorption linearly increases with the increase in the pretreated WS content, while an appropriate amount of the pretreated WS can enhance the water resistance of the GWSC. A WS/fly ash ratio lower than 8% can produce a GWSC with qualified thermal insulation performance. Alkaline treatment, particularly sodium hydroxide treatment, significantly improves the bonding strength between the fiber and the geopolymer matrix. In addition, pretreated WS saturated with sodium hydroxide and sodium metasilicate solutions is beneficial to the fluidity of the geopolymer–WS mixture and could act as an internal curing agent for enhancing the hydration of the geopolymer.