Influence of various amounts of limestone powder on performance of Portland limestone cement concretes

The benefits of limestone as a partial replacement for Portland Cement (PC) are well established. Economic and environmental advantages by reducing CO₂ emissions are well known. The paper describes the effect of various amounts of limestone on compressive strength, water penetration, sorptivity, electrical resistivity and rapid chloride permeability on concretes produced by using a combination of PC and limestone at 28, 90 and 180 days. The percentages of limestone that replace PC in this research are 0%, 5%, 10%, 15% and 20% by mass. The water/(clinker + limestone) or (w/b) ratios are 0.37, 0.45 and 0.55 having a constant total binder content of 350 kg/m³. Generally, results show that the Portland limestone cement (PLC) concretes having up to 10% limestone provide competitive properties with PC concretes.     

Effectiveness of using CO2 pressure to enhance the carbonation of Portland cement-fly ash-MgO mortars

Acceleration on the carbonation of reactive MgO cement is essential for its widespread application. There is currently a dearth of published reports on the effect and sensitivity of using pressurized CO₂ on the properties and performance of reactive MgO cement blends. This study is motivated by improving the understanding of the effectiveness of accelerating the carbonation process. Pressurized CO₂ (up to 1.0 MPa) was employed to enhance the carbonation of mortar blends consisting of Portland cement, fly ash and reactive MgO. Results revealed that the carbonation front and mechanical properties of the mortars were developed quickly owing to the effectively accelerated carbonation under pressurized CO₂. In comparison to the 0.1 MPa pressure, the relatively higher pressure (0.55 and 1.0 MPa) were much more effective in achieving stronger mechanical properties within 1 day. However, an increasing curing duration from 1d to 14d under the lower CO₂ pressure of 0.1 MPa caused a 1.8–2.9 times increase in compressive strength. This indicates that either increases in pressure or curing duration under pressurized CO₂ enhances the carbonation and mechanical properties of the mortars.     

Accelerated carbonation and performance of concrete made with steel slag as binding materials and aggregates

Steel slag has been used as supplementary cementitious materials or aggregates in concrete. However, the substitution levels of steel slag for Portland cement or natural aggregates were limited due to its low hydraulic property or latent volume instability. In this study, 60% of steel slag powders containing high free-CaO content, 20% of Portland cement and up to 20% of reactive magnesia and lime were mixed to prepare the binding blends. The binding blends were then used to cast concrete, in which up to 100% of natural aggregates (limestone and river sands) were replaced with steel slag aggregates. The concrete was exposed to carbonation curing with a concentration of 99.9% CO₂ and a pressure of 0.10 MPa for different durations (1d, 3d, and 14d). The carbonation front, carbonate products, compressive strength, microstructure, and volume stability of the concrete were investigated. Results show that the compressive strength of the steel slag concrete after CO₂ curing was significantly increased. The compressive strengths of concrete subjected to CO₂ curing for 14d were up to five-fold greater than that of the corresponding concrete under conventional moist curing for 28d. This is attributed to the formation of calcium carbonates, leading to a microstructure densification of the concrete. Replacement of limestone and sand aggregates with steel slag aggregates also increased the compressive strengths of the concrete subjected to CO₂ curing. In addition, the concrete pre-exposed to CO₂ curing produced less expansion than the concrete pre-exposed to moist curing during the subsequent accelerated curing in 60 °C water. This study provides a potential approach to prepare concrete with low-carbon emissions via the accelerated carbonation of steel slag.     

Effects of portland cement replacement with limestone on the properties of hardened concrete

Limestone portland cement has a lower environmental impact during the production phase in comparison with portland cement. However, the environmental advantages initially gained should be correlated to the long-term performance of concrete structures. Hence, the knowledge of the long-term properties, and in particular durability performance, is essential to assess the actual environmental impact of limestone replacement. In the literature, there is disagreement on durability behaviour and the contribution of limestone to the resistance to chloride and carbonation penetration is controversial. In this paper, the effect of the percentage of replacement of portland cement with ground limestone, water/binder ratio and cement content on compressive strength, electrical resistivity, sorptivity and resistance to carbonation and chloride penetration was evaluated. Results showed that both mechanical properties and resistance to penetration of aggressive agents decreased by replacing 15% of portland cement with limestone; a further decrease occurred with 30% limestone.     

Accelerated microwave curing of concrete: A design and performance-related experiments

Microwave (MW)-accelerated curing has emerged as an innovative and popular curing method for concrete materials. This paper reports the results of a study to model the horn antenna used for the MW irradiation of a workpiece with a mobile MW-accelerated concrete curing unit, based on a coupled thermal and electromagnetic analysis. The mathematical models were useful for evaluating the heat generation within a horn antenna and as a basis for constructing a mobile MW-accelerated curing unit with an operating frequency of 2.45 GHz and a MW power level of 800 W. Further, the early-age compressive strength development and volume stability of MW-cured concrete were investigated in terms of its shrinkage and compared to the properties of autoclave-cured concrete. The design results showed that under the concept of the allowable maximum temperature for the concrete workpiece, which was controlled to less than 80 °C, a horn antenna that was 216.70 mm wide, 333.68 mm long, and 273.0 mm high produced a uniform thermal distribution in a concrete workpiece. Moreover, experimental investigations showed that the application period for curing using a mobile MW-curing unit was considerably shorter than that in autoclave curing methods. The appropriate delay time (time after concrete mixing) was 30 min, and MW irradiation for 45 min could improve the maximum 8-h early-age compressive strength of MW-cured concrete, whereas an application time of 15 min produced the 28-day compressive strength. When a concrete workpiece was cured at high temperature using MW energy for more than 15 min at a temperature greater than 80 °C, the effect was a continuous increase in the early-age compressive strength, which was greater than that achieved by autoclave curing. In terms of volumetric stability, MW-curing for 30 and 45 min increased the ultimate shrinkage to a greater extent than that by autoclave curing and vice versa in the case of a MW application time of 15 min.     

Effect of curing methods on strength and durability of concrete under hot weather conditions

This paper reports the results of a research study conducted to evaluate the effect of curing methods on the mechanical properties of ordinary Portland cement (OPC) and Silica Fume Cement (SFC) concretes. Slab and beam specimens were prepared and cured by covering them with wet burlap or applying a curing compound under field conditions. Four types of curing compounds, namely water-, acrylic-, and bitumen-based and coal tar epoxy, were applied on the concrete specimens. The curing compounds were applied immediately after casting or after an initial period of burlap curing. The effect of the selected curing regime on the properties of OPC and SFC concrete specimens was evaluated by measuring compressive strength, water-absorption and chloride permeability. The strength and durability characteristics of both OPC and SFC concrete specimens cured by applying the selected curing compounds were similar or better than that of concrete specimens cured by covering with wet burlap. Though no significant change in strength could be noted due to the curing methodology; however, its effect was noticeable on the durability. The best performance was shown by concrete specimens cured by applying the bitumen-based curing compound followed by those cured by applying coal tar epoxy, acrylic-based or water-based curing compound. The initial period of water curing, prior to the application of the curing compound, was also noted to be beneficial in increasing the durability of concrete.     

Strength and durability aspects of calcined kaolin-blended Portland cement mortar and concrete

Economic and sustainability arguments require carefully assessing the potentialities of indigenous resources for the production of mortar and concrete for the construction industry. In Vietnam, significant efforts should be bestowed on urban development, coastal protection and harbour construction works. In a joint Vietnamese-Dutch co-operation program, the practical use for this purpose of relevant resources in Northern Vietnam is assessed experimentally. This paper concentrates on kaolin, which is widely available in this region. The key issues this paper is dealing with are the effects of partial replacement of Portland cement by calcined kaolin in mortar and concrete on compressive strength as well as on durability characteristics of mortar and concrete mixes pertinent to the coastal environment. Workability measures are also mentioned. Data are therefore presented on compressive strength development over a maximum curing period of 180 days of mixes in which the water to binder ratio was varied between 0.40 and 0.53. Moreover, partial replacement was considered in the range from 0% to 30% by weight. The results of this study render possible the assessment of optimum replacement percentages of Portland cement by calcined kaolin, and the associated strength gain. Additionally, this paper reports on the performance aspects of similarly blended mortar and concrete specimens stored for a period of one year in a low concentration of a sodium sulfate solution. It could be concluded that a strength gain due to blending will be accompanied by improved durability in this environment.     

Effect of cement type and limestone particle size on the durability of steam cured self-consolidating concrete

This paper describes a laboratory program to investigate the influence of cement and limestone filler (LF) particle size on the hardened properties and durability performance of steam cured self-consolidating concrete. In addition, the interplay between cement type and LF particle size was investigated. CSA (Canadian Standards Association) Type GU (General Use) and HE (High Early-strength) cements were used with 5% silica fume (SF) [1]. The water-to-cement ratio was 0.34. LF with two nominal particle sizes of 17 μm and 3 μm, which correspond to Blaine fineness of 475 and 1125 m²/kg, respectively, were used. In addition to fresh concrete properties, hardened properties including compressive strength, elastic modulus, ultrasonic pulse velocity and density were measured at 12 h and 16 h, and at 3, 7 and 28 days. Indicators of durability performance including rapid chloride permeability testing (RCPT), sulfate resistance, linear shrinkage, salt scaling resistance and freeze-thaw resistance were evaluated. The results showed that LF improved the 12 and 16-h strength with no influence on later age strength (i.e., 3–28 days). The linear shrinkage and RCPT decreased with the addition of LF. This reduction was linked to the production of calcium mono-carboaluminate. LF did not impact the sulfate resistance, salt scaling resistance or freeze-thaw resistance of concrete.     

Effect of further water curing on compressive strength and microstructure of CO2-cured concrete

This study aims to investigate the effects of further water curing on the compressive strength and microstructure of CO₂-cured concrete. The results showed that concrete with a residual w/c ratio of 0.25 showed the most rapid strength development rate upon further water curing due to hydration of uncarbonated cement particles. Thermogravimetric, IR-spectrophotometric and scanning electron microscope examinations indicated that further hydration of the cement particles could form C-S-H gel and ettringite crystals. The results showed that the calcite formed during the initial CO₂ curing was consumed during the further hydration of C₃A, and produced calcium monocarbonaluminate hydrate. Also, Ca(OH)₂ was not detected due to its reaction with the formed silica gel. Mercury intrusion porosimetry test results indicated that the porosity and pore size of the CO₂ cured mortar decreased further after water curing.     

Influence of limestone and anhydrite on the hydration of Portland cements

The addition of CaCO₃ and CaSO₄ to Portland cement clinker influences the hydration and the strength development. An increase of the CaSO₄ content accelerates alite reaction during the first days and results in the formation of more ettringite, thus in a higher early compressive strength. The late compressive strength is decreased in Portland cements containing higher quantities of CaSO₄. The reduced late compressive strength seems to be related to an increase of the S/Si and Ca/Si content in the C–S–H.The presence of calcite leads to the formation of hemicarbonate and monocarbonate thus indirectly to more ettringite. Only a relatively small quantity of calcite reacts to form monocarbonate or hemicarbonate in Portland cement. Although hemicarbonate is thermodynamically less stable than monocarbonate, hemicarbonate formation is kinetically favored. Monocarbonate is present only after 1 week and longer independent of the quantity of calcite available and the content of sulphate in the cement.     

Durability performance potential and strength of blended Portland limestone cement concrete

This paper describes a study on the durability potential and strength of composite Portland-limestone cement (PLC) concrete mixtures blended with ground granulated blast furnace slag (GGBS) and/or fly ash (FA). Their performance was compared against ordinary Portland cement, plain PLC and Portland-slag cement concrete mixtures. Using the South African Durability Index approach, results indicate reductions in the penetrability of the composite PLC blends compared to the other mixtures. The durability indicators are chloride conductivity, gas (oxygen) permeability and water sorptivity. Compressive strength of the composite PLC mixtures containing both GGBS and FA showed competitive performance with the comparative mixtures, but FA blended PLC mixtures had diminished compressive strength values. The paper also presents considerations on the practical implications of using blended PLC concrete mixtures.     

Contribution of limestone to the hydration of calcium sulfoaluminate cement

Calcium sulfoaluminate (CSA) cements can be blended with mineral additions such as limestone for properties and cost optimization. This study investigates the contribution of limestone to the hydration of a commercial CSA clinker regarding the hydration kinetics, hydrate assemblage and compressive strength. Nine formulations were defined at M-values of 0, 1.1 and 2.1 (M = molar ratio of anhydrite to ye’elimite) without and with medium and high limestone contents.Calorimetric results indicate that limestone accelerates the hydration reaction especially at M = 1.1, probably due to the filler effect. The phase assemblages were calculated by thermodynamic modeling using Gibbs Energy Minimization Software (GEMS). With increasing limestone content the formation of ettringite and calcium monocarboaluminate is predicted at the expense of calcium monosulfoaluminate. With increasing M-value more ettringite is predicted at the expense of the monocarbonate and less calcite takes part in the hydration reactions.The modeled results compare well with the experimental data after 90 d of hydration, except that calcium hemicarboaluminate was found instead of monocarbonate, which is assumed to be due to kinetics considerations.The lowest compressive strength occurs in ternary formulations, whereas in the absence of calcium sulfate, strength is significantly higher.The results presented here indicate that in CSA cements, limestone accelerates early hydration kinetics, takes part in the hydration reactions at M < 2, and has a positive effect on strength development in systems without anhydrite.     

Some effects of cement and curing upon carbonation and reinforcement corrosion in concrete

Experimental data are presented to illustrate the effects of cement type and curing upon the depth of carbonation and reinforcement corrosion in cover concrete after exposure for 18 months at 20°C and 60% relative humidity. Three curing periods (1, 3 and 28-days) and 17 cements, with various proportions of granulated blastfurnace slag or limestone, were used to make concretes, at 0.59 water/cement ratio, with 28 day strengths in the range 26 to 46 MPa. The depth of carbonation after 18 months was 64% greater than after 6 months and was affected more by cement type than by curing. The depth of carbonation increased when Portland cement clinker was replaced by 19% or more of limestone or granulated blastfurnace slag. The depth of carbonation after 18 months correlated better with the air permeability of cover concrete, initial weight loss (an indicator of moisture diffusion rate in cover concrete) or the cube strength 8 days after the end of curing than it did with 28-day cube strength. The rate of reinforcement corrosion increased steeply when the carbonation front approached the reinforcing steel, and it was still increasing after the carbonation front had completely passed the reinforcement. For a given unneutralised remainder (i.e. cover depth minus the depth of carbonation), curing had little effect upon the rate of corrosion but higher rates were observed when the cement contained granulated blastfurnace slag. The results were broadly consistent with a simple engineering strategy in which the rate of carbonation was related to the air permeability of cover concrete, and the rate of any subsequent reinforcement corrosion was largely dependent upon moisture conditions, without any obvious influence of the cover depth or the permeability of the cover concrete. The results also suggested that estimation of the rate of reinforcement corrosion could be improved by taking account of the cement type and treating the unneutralised remainder as a variable.     

Mineral wool production waste as an additive for Portland cement

This study aims at investigating the possibility of using dust, collected in air filters during the melting of mineral wool raw materials (mineral wool cupola dust) as an additive for Portland cement. It was found that the investigated dust mainly consists of quartz, periclase, albite, dolomite, and the amorphous phase. The main impurities are halite and sylvite. The investigated additive was additionally milled and prepared as a microfiller. The results showed that the cupola dust additive increases the initial hydration of cement, yet prolongs the dormant period. It was estimated that up to 15 wt% of Portland cement can be replaced by the dust additive without impairing the strength properties of samples after 28 days of hardening. However, after 90 days of hydration, the compressive strength of all samples with the investigated additive is lower than in pure OPC samples. This phenomenon is concerned with the formation of a significant amount of Friedel's salt. The content of chlorides in the raw material was reduced from 4.901 to 0.612 wt% by washing with water, when the water-to-solid ratio was equal to 10. The results of the investigation showed that the washed and ground cupola dust had a positive effect on the compressive strength of the cement samples. When 5, 10, and 15 wt% of prepared dust additive were used, the compressive strength of samples after 28 and 90 days of hydration was greater than that of pure Portland cement sample. The findings suggest that the additionally prepared dust additive leads to the formation of a stable structure of the cement stone, accelerates the calcium silicates hydration, and promotes the formation of gismondine.     

Properties of binary and ternary reactive MgO mortar blends subjected to CO2 curing

Research and development of low CO₂ binders for building material applications is warranted in efforts to reduce the negative environmental impacts associated with the cement and concrete industry. The purpose of this study is to investigate the effect of carbonation curing on the mineralogy, morphology, microstructure and evolution of compressive strength of mortars comprised of general use (GU) cement, ground granulated blast furnace slag (GGBFS), and reactive MgO used as cement replacement. This study investigates binary (GU–MgO) and ternary (GU–GGBFS–MgO) blends exposed to atmosphere curing (0.0038%CO₂) and carbonation curing (99.9%CO₂). Carbonation-cured mortars exhibited greater compressive strengths than atmosphere mortars at all ages (7 d, 28 d, and 56 d). Increasing percentages of reactive MgO decreased the compressive strength markedly less for carbonation-cured mortars than atmosphere-cured mortars particularly due to magnesium calcite formations. Magnesium calcite influenced the morphology of carbonates and promoted the carbonate agglomeration resulting in a dense and interconnected microstructure.     

Influence of curing temperatures on the hydration of calcium aluminate cement/Portland cement/calcium sulfate blends

In this paper, the effects of curing temperature on the hydration of calcium aluminate cement (CAC) dominated ternary binders (studied CAC: Portland cement: calcium sulfate mass ratio were 22.5: 51.7: 25.8) were estimated at 0, 10, 20 and 40 °C, respectively. Both α-hemihydrate and natural anhydrite were employed as the main source of sulfate. The impacts of temperature on the phase assemblages, morphology and pore structure of pastes hydrated up to 3 days were determined by using X-ray diffraction (XRD), backscattered electron imaging (BEI) and mercury intrusion porosimetry (MIP). Results reveal that the main hydration products are firmly related to calcium sulphoaluminate based phases. Increasing temperature would result in a faster conversion from ettringite to plate-like monosulfate for both calcium sulfate doped systems. When the temperature increases to 40 °C, an extraordinary formation of strätlingite (C₂ASH₈) and aluminium hydroxide is observed in anhydrite doped pastes. Additionally, increased temperature exerts different effects on the pore structure, i.e. the critical pore diameter shifts to finer one for pastes prepared with α-hemihydrate, but changes to coarser one for those made with anhydrite. From the mechanical point of view, increased temperature accelerates the 1-day strength development prominently, while exerts marginal influence on the development of 3-day strength.     

High-volume natural volcanic pozzolan and limestone powder as partial replacements for portland cement in self-compacting and sustainable concrete

A laboratory study demonstrates that high volume, 45% by mass replacement of portland cement (OPC) with 30% finely-ground basaltic ash from Saudi Arabia (NP) and 15% limestone powder (LS) produces concrete with good workability, high 28-day compressive strength (39 MPa), excellent one year strength (57 MPa), and very high resistance to chloride penetration. Conventional OPC is produced by intergrinding 95% portland clinker and 5% gypsum, and its clinker factor (CF) thus equals 0.95. With 30% NP and 15% LS portland clinker replacement, the CF of the blended ternary PC equals 0.52 so that 48% CO₂ emissions could be avoided, while enhancing strength development and durability in the resulting self-compacting concrete (SCC). Petrographic and scanning electron microscopy (SEM) investigations of the crushed NP and finely-ground NP in the concretes provide new insights into the heterogeneous fine-scale cementitious hydration products associated with basaltic ash-portland cement reactions.     

Influence of curing conditions on properties of high calcium fly ash geopolymer containing Portland cement as additive

This paper investigated the mechanical properties and microstructure of high calcium fly ash geopolymer containing ordinary Portland cement (OPC) as additive with different curing conditions. Fly ash (FA) was replaced with OPC at dosages of 0%, 5%, 10%, and 15% by weight of binders. Setting time and microstructure of geopolymer pastes, and flow, compressive strength, porosity and water absorption of geopolymer mortars were studied. Three curing methods viz., vapour-proof membrane curing, wet curing and temperature curing were used. The results showed that the use of OPC as additive improved the properties of high calcium fly ash geopolymer. The strength increased due to the formation of additional C–S–H and C–A–S–H gel. Curing methods also significantly affected the properties of geopolymers with OPC. Vapour-proof membrane curing and water curing resulted in additional OPC hydration and led to higher compressive strength. The temperature curing resulted in a high early compressive strength development.     

Optimization of calcium chloride content on bioactivity and mechanical properties of white Portland cement

This research investigates the optimization of calcium chloride content on the bioactivity and mechanical properties of white Portland cement. Calcium chloride was used as an addition of White Portland cement at 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10% by weight. Calcium chloride was dissolved in sterile distilled water and blended with White Portland cement using a water to cement ratio of 0.5. Analysis of the bioactivity and pH of white Portland cement pastes with calcium chloride added at various amounts was carried out in simulated body fluid. Setting time, density, compressive strength and volume of permeable voids were also investigated. The characteristics of cement pastes were examined by X-ray diffractometer and scanning electron microscope linked to an energy-dispersive X-ray analyzer. The result indicated that the addition of calcium chloride could accelerate the hydration of white Portland cement, resulting in a decrease in setting time and an increase in early strength of the pastes. The compressive strength of all cement pastes with added calcium chloride was higher than that of the pure cement paste, and the addition of calcium chloride at 8 wt.% led to achieving the highest strength. Furthermore, white Portland cement pastes both with and without calcium chloride showed well-established bioactivity with respect to the formation of a hydroxyapatite layer on the material within 7 days following immersion in simulated body fluid; white Portland cement paste with added 3%CaCl₂ exhibited the best bioactivity.     

石灰石粉对泵送混凝土性能影响的试验研究

本文主要研究了单掺石灰石粉、石灰石粉与粉煤灰复掺对泵送混凝土拌合物工作性和抗压强度的影响,研究表明：掺加石灰石粉可以改善泵送混凝土的工作性能,与单掺石灰石粉相比,石灰石粉与粉煤灰复掺,具有复合叠加效应,不仅可以改善泵送混凝土的工作性能,同时可提高泵送混凝土的抗压强度。