Early carbonation curing of concrete masonry units with Portland limestone cement

The effect of carbonation curing on the mechanical properties and microstructure of concrete masonry units (CMU) with Portland limestone cement (PLC) as binder was examined. Slab samples, representing the web of a CMU, were initially cured at 25 °C and 50% relative humidity for durations up to 18 h. Carbonation was then carried out for 4 h in a chamber at a pressure of 0.1 MPa. Based on Portland limestone cement content, CO₂ uptake of PLC concrete after 18 h of initial curing reached 18%. Carbonated and hydrated concretes showed comparable compressive strength at both early and late ages due to the 18-h initial curing. Carbonation reaction converted early hydration products to a crystalline microstructure and subsequent hydration transformed amorphous carbonates into more crystalline calcite. Portland limestone cement could replace Ordinary Portland Cement (OPC) in making equivalent CMUs which have shown similar carbon sequestration potential.     

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.     

Effect of SO3 content and fineness on the rate of delayed ettringite formation in heat cured Portland cement mortars

This paper presents the findings of a long-term study on the expansion rate and microstructure of heat cured cement mortars. For this purpose, cements with different fineness and SO₃ contents were produced by using the same clinker. Different mortar specimens were prepared and subjected to heat curing. Length changes of specimens were measured within a period of 540 days. The microstructures of young (2 day after heat curing) and old (1.5 years after heat curing) specimens were also investigated by SEM and EDS analysis. The expansion rates and microstructures observed were compared with the control specimens.Results showed that, at the initial stages of testing (2–3 months), expansion rates of heat cured mortars prepared with finer cements were less than those prepared with coarser cements. However, in the long term, the rate of expansion of mortars prepared with finer cements exceeded the coarser ones’ expansion values. This result may be attributed to the different hydration characteristics and pore structure of heat cured mortars including cements of different fineness.     

Comparison of ultrasonic wave reflection method and maturity method in evaluating early-age compressive strength of mortar

This paper focuses on the comparison between the ultrasonic wave reflection method and the widely known maturity method in their ability to evaluate compressive strength development of portland cement mortars. The experiments were conducted under laboratory conditions on cement mortars with different water–cement ratios (0.35, 0.5, 0.6) cured under different isothermal and non-isothermal conditions (15 °C, 25 °C, 35 °C, 15–35 °C). The results show that the application of the maturity method for accurate strength estimation requires the knowledge of the limiting strength of the mixture for the specific curing condition that is considered. It is not sufficient to base this estimation on the values of the limiting strength obtained from the calibration tests. The relationship between reflection loss and compressive strength was found to be independent of curing temperature. This finding is an important step towards establishing the wave reflection method as a non-destructive method for estimating early-age strength in cement-based materials.     

Compressive strength and drying shrinkage of fly ash-bottom ash-silica fume multi-blended cement mortars

This paper studies the physical properties, compressive strength and drying shrinkage of multi-blended cement under different curing methods. Fly ash, ground bottom ash and undensified silica fume were used to replace part of cement up to 50% by weight. Specimens were cured in air at ambient temperature, water at 25, 40 and 60 °C, sealed with plastic sheeting for 28 days. The results show that absorption and volume of permeable pore space (voids) of blended cement mortars at 28 day under all curing methods tend to increase with increasing silica fume replacement. The compressive strength of blended cement with fly ash and bottom ash was lower than that of Portland cement control at all curing condition while blended cement with silica fume shows higher compressive strength. In addition, the compressive strength of specimens cured with water increased with increasing curing temperature. The drying shrinkage of all blended cement mortar cured in air was lower than that of Portland cement control while the drying shrinkage of blended cement mortar containing silica fume, cured with plastic sealed and water at 25 °C was higher than Portland cement control due to pore refinement and high autogenous shrinkage. However, the drying shrinkage of blended cement mortar containing SF cured with water at 60 °C was lower than that of Portland cement control due to lower autogenous shrinkage and the reduced microporosity of C–S–H.     

Modelling of slaked lime–metakaolin mortar engineering characteristics in terms of process variables

The purpose of the present study is to determine the effect of factors such as dosage, curing conditions and use of a superplasticiser admixture on the porosity, mechanical strength and composition of slaked lime (SL)–metakaolin (MK) mortars. Statistical correlations have been established to describe the mechanical properties as well as porosity and composition of the slaked lime–metakaolin mortars.The SL/MK ratio has a moderate effect on mortar flexural and compressive strengths. The SL + MK/sand ratio is the factor with the highest impact on all the properties studied: strength, porosity and mortar composition. As this ratio increases, strength, porosity and amount of hydration and carbonation products formed in the samples also rise. The next factor by order of importance is the presence of a superplasticiser admixture, which affects porosity, strength and the amount of calcite in the sample. The presence of this superplasticiser admixture increases strength, raises the percentage of calcite in the mortars and reduces porosity. It is particularly striking that neither curing nor open air carbonation time (in the range studied) has a significant effect on the composition or porosity of the SL–MK mortars studied, although they do have a moderate effect on mechanical strength.     

Mechanical properties of reactive powder concrete containing high volumes of ground granulated blast furnace slag

The mechanical properties (flexural strength, compressive strength, toughness and fracture energy) of steel microfiber reinforced reactive powder concrete (RPC) were investigated under different curing conditions (standard, autoclave and steam curing). Portland cement was replaced with ground granulated blast furnace slag (GGBFS) at 20%, 40% and 60%. Sintered bauxite, granite and quartz were used as aggregates in different series. The compressive strength of high volume GGBFS RPC was over 250 MPa after autoclaving. When an external pressure was applied during setting and hardening stages, compressive strength reached up to 400 MPa. The amount of silica fume can be decreased with increasing amount of GGBFS. SEM micrographs revealed the tobermorite after autoclave curing.     

Durability of very high volume fly ash cement pastes and mortars in aggressive solutions

The resistance of very high volume fly ash cement pastes and mortars activated by Na₂SO₄ has been monitored following immersion for up to 90 d in 0.1 M HCl, 4.4% Na₂SO₄ and ASTM-compliant sea water. Changes in the compressive strengths of mortars and in crystalline phases, bond environments, and the microstructure of pastes following immersion were monitored. Experiments were repeated with a commercially available sulfate resistant cement. Both cements were found to present adequate resistance to both sea water and the Na₂SO₄ solution. However, both were severely degraded by acid immersion. Differences in potential degradation mechanisms based on the chemistry of the fly ash binder and the reference cement are discussed.     

Structure and strength of NaOH activated concretes containing fly ash or GGBFS as the sole binder

The influence of the concentration of the activating agent (4, 6, or 8 M sodium hydroxide solution), and activator-to-binder ratio (0.40, 0.50, or 0.60) on the compressive strengths, pore structure features, and microstructure of concretes containing Class F fly ash or ground granulated blast furnace slag (GGBFS) as the sole binder is reported. The starting material contents and the curing parameters (temperature and curing duration) are optimized to provide the highest compressive strengths. Statistical analysis of the compressive strength results show that the activator concentration has a larger influence on the compressive strengths of activated concretes made using fly ash and the activator-to-binder ratio influences the compressive strengths of activated GGBFS concretes to a greater degree. Activated fly ash concretes and pastes are found to be more porous and contains a larger fraction of pores greater than 10 μm in size as compared to activated GGBFS mixtures. The differences in the microstructure and the reaction products between activated fly ash and GGBFS pastes are detailed.     

A study on the effect of nano silica on compressive strength of high volume fly ash mortars and concretes

This paper presents the effect of nano silica (NS) on the compressive strength of mortars and concretes containing different high volume fly ash (HVFA) contents ranging from 40% to 70% (by weight) as partial replacement of cement. The compressive strength of mortars is measured at 7 and 28 days and that for concretes is measured at 3, 7, 28, 56 and 90 days. The effects of NS in microstructure development and pozzolanic reaction of pastes containing above HVFA contents are also studied through backscattered electron (BSE) image and X-ray diffraction (XRD) analysis. Results show that among different NS contents ranging from 1% to 6%, cement mortar containing 2% NS exhibited highest 7 and 28 days compressive strength. This NS content (2%) is then added to the HVFA mortars and concretes and the results show that the addition of 2% NS improved the early age (7 days) compressive strength of mortars containing 40% and 50% fly ash by 5% and 7%, respectively. However, this improvement is not observed at high fly ash contents beyond 50%. On the other hand, all HVFA mortars exhibited improvement in 28 days compressive strength due to addition of 2% NS and the most significant improvement is noticed in mortars containing more than 50% fly ash. In HVFA concretes, the improvement of early age (3 days) compressive strength is also noticed due to addition of 2% NS. The BSE and XRD analysis results also support the above findings.     

Reaction products and strength development of wastepaper sludge ash and the influence of alkalis

Wastepaper sludge ash (WSA) from a newsprint paper mill was investigated for its mineralogical composition and its reaction products and strength development after activation with water and sodium and potassium hydroxide solutions. The results showed the WSA to consist of calcite, free lime, gehlenite, tricalcium aluminate, belite, talc, quartz and probably a glassy phase. The principle reaction product was monocarboaluminate (CO₃–AFm) for the water- as well as for the alkali-activated WSA. Formation of monocarboaluminate and strength gain was more rapid for the alkali-activated WSA until 1 day of curing. However, afterwards reactions proceeded much slower when alkali solutions were used, leading to an about twice as high compressive strength for water-activated WSA mortars after 28 days of curing. The observed behavior is tentatively ascribed to a less uniform microstructure of the alkali-activated WSA. Significant differences between NaOH- and the KOH-activated WSA were not observed.     

Resistance of lignite bottom ash geopolymer mortar to sulfate and sulfuric acid attack

This paper presents an investigation of the compressive strength and the durability of lignite bottom ash geopolymer mortars in 3% sulfuric acid and 5% sodium sulfate solutions. Three finenesses of ground bottom ash viz., fine, medium and coarse bottom ash were used to make geopolymer mortars. Sodium silicate, sodium hydroxide and curing temperature of 75 °C for 48 h were used to activate the geopolymerization. The results were compared to those of Portland cement and high volume fly ash mortars. It was found that the fine bottom ash was more reactive and gave geopolymer mortars with higher compressive strengths than those of the coarser fly ashes. All bottom ash geopolymer mortars were less susceptible to the attack by sodium sulfate and sulfuric acid solutions than the traditional Portland cement mortars.     

Supercritical carbonation of calcareous composites: Influence of mix design

This work combined compression moulding with subsequent super-critical carbonation treatment (100 bar, 60 °C, 24 h) to fabricate cement and/or lime based ceramic composites with various aggregates. Composites were examined using mechanical testing, XRD, He pycnometry and thin-section petrography. Composites with lime-only binders were significantly weaker than those with cement-lime binders regardless of the degree of carbonation. Flexural strengths in excess of >10 MPa were routinely achieved in large (>100 mm) specimens. Aggregate type (calcareous vs. siliceous) had a significant effect on the microstructure and properties of the composites. Calcareous aggregates appear to augment the strength enhancement effected during super-critical carbonation by encouraging preferential precipitation of calcite at the binder-aggregate interface.     

Microstructure and performance of energetically modified cement (EMC) with high filler content

Energetically modified cement (EMC) has been produced by high intensive grinding/activation of normal portland cement (NPC) together with 20% and 50% quartz sand. EMC concretes were compared to NPC based concrete using the k-factor concept. The k-factor for concrete with w/c = 0.60–0.45 was 0.7–0.9 for 1 d and 1.1–1.3 for 28 d compressive strength. k > 1 for both capillary suction, porosity, vapor diffusion and chloride permeability. For carbonation resistance k was ≈0.55.Microstructure of EMC paste with 50% quartz sand and w/c = 0.40 showed that the quartz was extensively ground and formed agglomerates with cement having a high inner surface. The degree of hydration of the cement in EMC was as high as 71% after 1 d compared to 45% for untreated blend. Refined pore size distribution of EMC versus the blend means that even for equal hydration at higher ages EMC will perform better.     

A study on the effects of high temperature on mechanical properties of fiber reinforced cementitious composites

The flexural strength and ductility properties of cementitious composites (mortar) under high temperature may be significantly improved by incorporating different types of fibers. In this study, four different types of fibers are added to cement mortars with the aim to investigate their mechanical contributions to mortars under high temperature, comparatively. Polypropylene (PP), carbon (CF), glass (GF) and polyvinyl alcohol (PVA) fibers are chosen for research. These fibers are added into mortars in five different ratios (0.0%, 0.5%, 1.0%, 1.50% and 2.0%) by volume. The mortars are subjected to the following temperatures: 21 °C (normal conditions), 100 °C (oven dry), 450 °C and 650 °C. The mechanical properties investigated are flexural strength, deflection and compressive strength of the cement mortars. In addition, thin sections of mortars are investigated to obtain changes in mortar because of high temperature. It is concluded that all fiber types contribute to the flexural strengths of mortars under high temperature. However, this contribution decreases with an increase in temperature. The samples with PVA show the best flexural performance (75–150%) under high temperature. CF which does not melt under high temperature also gives high flexural strength (11–85%). The compressive strengths of the mortars reduce under high temperature or with fiber addition. The highest increase in flexural strength and the lowest decrease in compressive strength is at 0.5–1.5% for CF if all temperature conditions are taken into consideration. The optimum fiber addition ratios of the samples containing PP and GF are 0.5% by volume. And for PVA, it is between 0.5% and 1.5% by volume.     

Formulation of mortars with nano-SiO2 and nano-TiO2 for degradation of pollutants in buildings

This paper reports on the design of cement mortars that use nano-SiO₂ (nS) and nano-TiO₂ (nT) particles, aiming to improve the durability of traditional building materials while giving new functionalities (aerial decontamination of pollutants). Samples with 0–2 wt.% nS, 0–20 wt.% nT, 0.45–7 wt.% superplasticizer (SP) and 0.45–0.58 water/binder weight ratio were prepared. The formulations of mortars were defined according to rheology and flow table measurements, then showing suitable workability. The temperature of hydration, compressive strength, water absorption, and photocatalytic degradation of pollutants (NO_x and Orange II dye) were also evaluated. In general, the rheological behavior and the temperature of hydration changed in distinct levels, depending on the dosage and type of nanoadditives, but nT influenced more significantly the results. However, such differences were not identified on the compressive strength and water absorption. In addition, NO_x photocatalytic degradation up to 1 h under solar light ranged from 65% to 80%, while Orange II degradation after 9 h under visible light changed from 18% to 50%.     

Grey correlation analysis between strength of slag cement and particle fractions of slag powder

The particle size distributions of slag powder were investigated by Laser Scatter equipment. The influence of particle fractions of slag powder on the compressive strength of slag cement composed of 50% slag powder and 50% Portland cement was also studied by the method of grey correlation analysis. The results indicated that the volume fraction of particles 5–10 μm had a maximum positive effect on the mortar compressive strength of slag cement at 7 d and the volume fraction of particles 10–20 μm had a maximum positive effect on the mortar compressive strength at 28 d, whereas the volume fraction of particles larger than 20 μm had a negative effect on the mortar compressive strength at 7 and 28 d.     

Properties of low temperature belite cements made from aluminosilicate wastes by hydrothermal method

The aim of this study is the synthesis at low temperature (1000 °C) of reactive belite cement, rich in reactive C₂S phases (α′_L and/or β-C₂S), starting from aluminosilicate wastes (oil well drilling mud and hydraulic dam sludge) and hydraulic lime dust recovered from bagging workshops. A hydrothermal treatment of the raw cement mixture was performed in an alkaline solution of KOH (0.6 M), with heating at 100 °C for 4 h under atmospheric pressure and continuous agitation. The burning of the hydrothermal mixture at 1000 °C produced reactive belite cements containing between 79% and 86% of C₂S (α′_L and/or β polymorphs), the rest being C₁₂A₇ (5–8%), C₄AF (7–11%) and free lime (2%). The cements were characterized by X-ray Fluorescence (XRF), X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), microprobe, and laser granulometry. The hardening evolution of belite cement pastes and mortars was followed by thermo-gravimetric analysis (TGA), setting time and compressive strength. The results showed a rapid production of hydrates (C–S–H/C–A–S–H, calcium aluminate hydrates and portlandite), with a fast setting time (reaction of C₁₂A₇) and compressive strength evolution that led to these cements being classified in the 32.5 category according to EN 197-1.     

Accelerated carbonation effect on behaviour of ternary Portland cements

The carbonation of hydrated ternary Portland cement (TPC), containing thermally activated paper sludge and fly ash, was studied in a semi-dynamic atmosphere of 100% CO₂, and 65% relative humidity at 20 ± 1 °C, for a period of 30 days. The changes of the mineralogical phases and porosity before and during the carbonation were characterized by X-ray diffraction and mercury intrusion porosimetry. The kinetics of the process was evaluated from the total CaCO₃ content by means of thermogravimetric analysis. The pH of the simulated pore solution was analyzed before and after different periods of time. An equivalent study was carried out on ordinary Portland cement (OPC), whose results were compared to those of TPC one. The carbonation attack was 2.2 times faster in the case of the TPC. The results were discussed on the basis of the different porosity, portlandite content and pH of the pore-solution.     

Experimental investigation of the size effects of SiO2 nano-particles on the mechanical properties of binary blended concrete

In the current study, the size effects of SiO₂ nano-particles on compressive, flexural and tensile strength of binary blended concrete were investigated. SiO₂ nano-particles with two different sizes of 15 and 80 nm have been used as a partial cement replacement by 0.5, 1.0, 1.5 and 2.0 wt.%. It was concluded that concrete specimens containing SiO₂ particles with average diameter of 15 nm were harder than those containing 80 nm of SiO₂ particles at the initial days of curing. But this condition was altered at 90 days of curing. Also from the viewpoint of free energy, it can be concluded that the C–S–H gel formation around the particles with average diameter of 15 nm was more at the primary days of curing. This can be as a result of more nucleation sites that causes acceleration in early age strength. On the other hand, the growth probability of C–S–H gel around the 80 nm particles was more at 90 days of moist curing. This is due to the fact that the nucleus of strengthening gel could simply reach to the critical volume of nucleation that causes increase in the strength.