Carbonation process of alkali-activated slag mortars

This study analyzes the behaviour of waterglass- or NaOH-activated slag mortars after carbonation. The effect of a superplasticizer based on vinyl copolymer and shrinkage reducing polypropylenglycol derivative admixtures on that process was also examined. The same tests were run on cement mortars for reference purposes. The mortars were carbonated in a chamber ensuring CO₂ saturation for four and eight months, after which ages the samples were tested for mechanical strength; mercury porosimetry and mineralogical (XRD, FTIR) and microstructural characterization (SEM/EDX) were also conducted. The results obtained indicate that alkali-activated slag mortars were more intensely and deeply carbonated than Portland cement mortars. Carbonation took place directly on the gel, causing decalcification. When waterglass was the alkaline activator used, carbonation caused a loss of cohesion in the matrix and an important increase in porosity and decrease in mechanical strength. When a NaOH solution was used as the alkali activator, carbonation enhanced mortar compaction and increased mechanical strength. Finally, in waterglass-activated slag mortars, the inclusion of organic admixtures had no effect either on their behaviour after carbonation or the nature of the reaction products.     

Effects of mix composition and water–cement ratio on the sulfate resistance of blended cements

This paper presents an experimental investigation on the sulfate resistance of blended cements containing various amounts of natural pozzolan and/or Class-F fly ash. The performance of blended cements was monitored by exposing the prepared mortar specimens to a 5% Na₂SO₄ solution for 78 weeks. For comparison, an ordinary Portland cement (produced with the same clinker as blended cements) and a sulfate resistant Portland cement (produced from a different clinker) were also used. In addition to the cement chemistry, water–cement (w/c) ratio of mortars was another parameter selected that will presumably affect the performance of mortars. The experimental results of expansion measurements showed that the effect of w/c ratio was more pronounced for the low sulfate resistant cements with higher C₃A amounts, while the blended cements were less affected by an increase in the w/c ratio.     

Carbonation of calcium sulfoaluminate mortars

This study investigated potential physical and chemical parameters that could govern the carbonation rate of calcium sulfoaluminate (CSA) mortars and endeavored to elucidate the microstructural and chemical factors that govern CSA cement's carbonation rate. Experiments included: water absorption, oxygen diffusion, mercury intrusion porosimetry, quantitative X-ray diffraction, thermogravimetric analysis, accelerated carbonation, compression and flexure tests. Additionally, the carbonation process was investigated using thermodynamic modeling. The results show that CSA mortars carbonate much faster than Portland cement mortars and at approximately the same rate as calcium aluminate cement mortars. Additionally, CSA mortars carbonate slower with decreasing w/c, and the anhydrite content of the CSA mortars strongly affects the ye'elimite reaction kinetics which plays an important role in imparting carbonation resistance in CSA mortars. Finally, calcium sulfate additions to CSA clinker to produce CSA cement dilutes the clinker content and reduces the amount of CO₂ that the CSA cement can ultimately bind.     

Durability of mixtures containing calcium nitrite based corrosion inhibitor

The influence of calcium nitrite based corrosion inhibitor on the corrosion of reinforcing steel embedded in 14 different mortars is experimentally investigated in this work. Two Portland cements, NPC and SR (type I and V according to ASTM Standards) and 12 blended cements were used. The pozzolanic materials used were three lignite fly ashes, silica fume and one natural pozzolan (Milos' Earth). All blended cements were produced in the laboratory by grinding Portland clinker, gypsum and the appropriate pozzolanic material. One commercially available blended cement (CEM II/A-M 32.5N) was also used in this research. Mortar specimens (cylinders 100 × 40 mm) were prepared, according to DIN 1164 with and without calcium nitrite and used for measurements of carbonation and chloride-induced corrosion for a time period of 2 years. Chloride resistance was monitored according to ASTM C876 on specimens immersed in a 5% NaCl solution after an initial curing of 28 days. The carbonation depth was measured on cylinders cured in a severe environment using a spray indicator enabling the estimation of different pH values.Results show that calcium nitrite has a beneficial effect in shifting the corrosion potential towards electropositive direction especially in the case of NPC and SR cements. The corrosion potential of blended mixtures was also shifted towards electropositive direction, but since the pozzolanic materials had a beneficial effect by themselves, the reduction was comparative smaller. The beneficial effect of calcium nitrite was also confirmed by the gravimetric weight loss measurements performed after 2 years of immersion in the 5% NaCl solution.Carbonation depth of all mixtures was reduced or remained the same when calcium nitrite was used. Chloride permeability was not seriously influenced by the addition of calcium nitrite, as it is indicated by the total chloride measurements performed after 2 years of immersion in the 5% NaCl solution.     

Blended cements containing high volume of natural zeolites: Properties, hydration and paste microstructure

In this study, properties and hydration characteristics as well as paste microstructure of blended cements containing 55% by weight zeolitic tuff composed mainly of clinoptilolite mineral were investigated. Free Ca(OH)₂ content, crystalline hydration products and decomposition of zeolite crystal structure, pore size distribution and microstructural architecture of hydrated cement pastes were examined. Superplasticizer requirement and compressive strength development of blended cement mortars were also determined. The blended cements containing high volume of natural zeolites were characterized with the following properties; (i) no free Ca(OH)₂ in hardened pastes at the end of 28 days of hydration, (ii) less proportion of the pores larger than 50 nm when compared to portland cement paste, (iii) complete decomposition of crystal structure of zeolite at the end of 28 days of hydration, (iv) presence of tetra calcium aluminate hydrate as a crystalline product of pozzolanic reaction, (v) more compatibility with the melamine-based superplasticizer when compared to the naphthalene based product, and (vi) similar 28 days compressive strength of mortars to that of reference portland cement.     

Corrosion behaviour of steel in high alumina cement mortar cured at 5,25 and 55 °C: chemical and physical factors

The corrosion behaviour of embedded steel was related to the composition of the pore phase in equilibrium with the hydrated phases and the porosity of the high alumina cement mortars subsequent to curing at 5,25 and 55 °C. The corrosion of reinforcements was evaluated by electrochemical techniques. The effect on corrosion of 3% by weight of cement of NaCl, added during the mixing process, and of the accelerated carbonation of mortars in CO₂ atmosphere were also determined. The pH value and the chemical composition of pore fluid of plain high alumina cement (HAC) mortar cured at all three temperatures suggested that the embedded steel was in a passivated state. The resistance of HAC to carbonation and its greater potential for chloride binding by chloroaluminate formation are believed to make HAC inherently more protective to steel, relative to normal Portland cement, during ingress of chloride from external sources. High corrosion rates reported in literature for steel embedded in HAC may be attributable to bad practice, not to lack of passivity.     

Influence of cellulose ether on hydration and carbonation kinetics of mortars

The hydration process of Portland cement and retarding effect of cellulose ether (CE) on hydration and carbonation were studied. The degree of CE-substitution is a major parameter which plays an important role in terms of retardation of both hydration and carbonation. For the hydration process, this CE-effect was highlighted through the results of an experimental campaign based on thermogravimetric analysis (TGA) performed on mortar samples conserved in an ambient air in which the atmospheric CO₂ was absorbed by whitewash solution. This type of conservation is chosen in order to make precise the measurement of dehydration rate by TGA tests. While for the carbonation mechanism, the CE-effect was identified by the measurement of carbonation depth with phenolphthalein spraying.This paper aims to determinate a coefficient of retardation of hydration according to the CE-rate used in the manufacturing of mortars. This coefficient may be taken into account in the calculation of the reaction rate of anhydrous constituents of cement in order to determine a precise hydration degree of mortars. Consequently, this delay in cement hydration delays the carbonation processes because of the lack of hydrates to react with CO₂.     

Effect of magnesium sulfate on the durability of limestone mortars based on quaternary blended cements

Currently, the use of blended cements incorporating various supplementary cementing materials, preserved in aggressive environments has become common. This paper describes the investigation results conducted on the evaluation of the resistance to magnesium sulfate solution (MgSO₄) of limestone mortars containing simultaneously; limestone filler, blast furnace slag and natural pozzolan. In this study, the deterioration of limestone mortars due to sulfate attack was evaluated by measuring changes in weight, length and compressive strength at the ages of 30, 60, 90, 120 and 180 days of immersion in exposure environments. The X-ray diffraction was also used in order to determine the different mineral phases. It is noteworthy that, the pH variation of the conservation solutions has been monitored during tests. The exposure solution was renewed monthly until the end of tests. The results showed that, the resistance to sulfate attack of mortars made with quaternary binders was better than that of mortars based on ordinary Portland cement.     

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.     

Strength development of ternary blended cement with limestone filler and blast-furnace slag

The benefits of limestone filler (LF) and granulated blast-furnace slag (BFS) as partial replacement of portland cement are well established. However, both supplementary materials have certain shortfalls. LF addition to portland cement causes an increase of hydration at early ages inducing a high early strength, but it can reduce the later strength due to the dilution effect. On the other hand, BFS contributes to hydration after seven days improving the strength at medium and later ages.Mortar prisms in which portland cement was replaced by up to 20% LF and 35% BFS were tested at 1, 3, 7, 28 and 90 days. Results show that the contribution of LF to hydration degree of portland cement at 1 and 3 days increases the early strength of blended cements containing about 5–15% LF and 0–20% BFS. The later hydration of BFS is very effective in producing ternary blended cements with similar or higher compressive strength than portland cement at 28 and 90 days. Additionally, a statistical analysis is presented for the optimal strength estimation considering different proportions of LF and BFS at a given age. The use of ternary blended cements (PC–LF–BFS) provides economic and environmental advantages by reducing portland cement production and CO₂ emission, whilst also improving the early and the later compressive strength.     

Sulphate resistance of type V cements with limestone filler and natural pozzolana

Sulphate performance of concrete depends primarily on permeability. Under severe conditions of sulphate exposure, low-permeability concrete is prescribed and it must also be made with high sulphate resisting cement. For portland cement, the sulphate resistance depends on the C₃A content and the amount of CH produced at early stages of hydration. Some parameters that modify the quantity of early CH in the hardened cement paste are investigated in this paper. Two type V cements with quite different C₃S content and blended cements containing natural pozzolana or limestone filler were used. Expansion, flexural and compressive strength of mortar, immersed until 1 yr in sodium sulphate solution, with pH-controlled are presented. Results show that the sulphate performance of portland cement with high C₃S content is very poor compared with low C₃S portland cement. Addition of natural pozzolana provides the maximum sulphate resistance while the addition of 20% limestone filler declining sulphate performance of low C₃A cements. This behaviour can be attributed to the reaction between sulphate ions with CH into the paste that produces an alteration of the predominant mechanism of sulphate attack.     

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.     

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.     

The sulphate resistance of cements containing red brick dust and ground basaltic pumice with sub-microscopic evidence of intra-pore gypsum and ettringite as strengtheners

This paper presents a laboratory study on the deterioration of blended cement combinations of plain Portland cement (PPC) with red brick dust (RBD) and ground basaltic pumice (GBP). One type of clinker, same Blaine values and two different proportions of additive by mass of clinker, were employed. In addition to these blends, Portland cements without additives were prepared as control specimens.The compressive strength and the sulphate resistance of cements have been experimentally studied in this paper. A series of laboratory tests were undertaken on all specimens. A large quantity of sheet-like C-S-H was found in the mortars incorporating RBD and GBP. The results indicated that the increase in the additive content caused a significant increase in the sulphate resistance of the mortars. Hence, the studied RBD and GBP can be recommended for use as admixtures in cement production. The development of the particular microstructure including the secondary minerals in the plain and blended cements were studied via SEM analysis. SEM images revealed the presence of ettringite and Portlandite minerals, where the former was most probably responsible for the increase (together with the gypsum roses) as well as a decrease of strength based on its formation at different sites and crystal form. Portlandite was responsible for an increase in the specimen strength.     

Relation between carbonation resistance,mix design and exposure of mortar and concrete

When cement with mineral additions is employed, the carbonation resistance of mortar and concrete may be decreased. In this study, mortars containing mineral additions are exposed both to accelerated carbonation (1% and 4% CO₂) and to natural carbonation. Additionally, concrete mixtures produced with different cements, water-to-cement ratios and paste volumes are exposed to natural carbonation. The comparison of the carbonation coefficients determined in the different exposure conditions indicates that mortar and concrete containing slag and microsilica underperform in the accelerated carbonation test compared to field conditions. The carbonation resistance in mortar and concrete is mainly governed by the CO₂ buffer capacity per volume of cement paste. It can be expressed by the ratio between water added during production and the amount of reactive CaO present in the binder (w/CaO_reactive) resulting in a novel parameter to assess carbonation resistance of mortar and concrete containing mineral additions.     

Effect of curing time on granulated blast-furnace slag cement mortars carbonation

Currently, ground granulated blast-furnace slag cements use in cement-based materials is being increasing because perform well in marine and other aggressive environments. However, mortars and concretes made of this type of cement exhibit high carbonation rates, particularly in badly cured cement-based materials and when high blast-furnace slag contents are used. Concrete reinforcement remains passive but can be corroded if the pore solution pH drops as a result of the carbonation process promoting the reinforced concrete structure failure during its service life. Results show the very sensitive response to wet-curing time of slag mortars with regard to the natural carbonation resistance. Then, a minimum period of 3–7 days of wet curing is required in order to guarantee the usual projected service life in reinforced concrete structures. In this work, estimation models of carbonation depth and carbon dioxide diffusion coefficient in ground granulated blast-furnace slag mortars as a function of the curing period and the amount of ground granulated blast-furnace slag are proposed. This information will be useful to material and civil engineers in designing cement-based materials and planning the required curing time depending on their ground granulated blast-furnace slag content.     

Performance of Portland limestone cements in mortar prisms immersed in sulfate solutions at 5 °C

The results of a test programme to investigate the sulfate resistance of mortars, immersed up to 12 months at 5 °C in magnesium sulfate and sodium sulfate solutions, is described. The mortars were prepared from four cements; a Portland cement, a sulfate-resisting Portland cement and two Portland limestone cements containing 15% by mass of an oolitic limestone and a carboniferous limestone. The mortar specimens were subject to BS 5328 Class 4A and 4B sulfate exposure conditions. These are the highest classes for concretes prepared using sulfate-resisting Portland cement (SRPC) before surface protection is required and are two and three classes higher than those recommended for concretes prepared using Portland cement (PC) and Portland limestone cement (PLC), respectively. Two free water-cement ratios were used, 0.5 and 0.75. Performance was monitored by visual assessment, expansion and changes in flexural and compressive strengths.At a free water-cement ratio of 0.75, the PC mortars and PLC mortars exhibited visually very severe attack with the former showing expansion and reductions in strength, and the latter mainly reductions in strength. At a free water-cement ratio of 0.50 both the PC mortars and PLC mortars showed slight/moderate to severe visual attack, the degree of deterioration appearing slightly greater in the PLC mortars, more especially those made with oolitic limestone. The PLC mortars also exhibited reductions in compressive failure load. The SRPC mortars exhibited little visual deterioration, no expansion, a small increase in flexural strength and no significant reductions in compressive strength. At a free water-cement ratio of 0.75 substantial amounts of thaumasite, together with ettringite was present in the surface layers of the deteriorated PLC mortars whilst ettringite was present in the surface layers of the deteriorated PC mortars. It is concluded that mortars made with a PC with a C₃A content of about 10% by mass were broadly similar in their vulnerability to sulfate attack at 5 °C as PLC mortars containing 15% limestone by mass, although the mode of attack was different.     

Mechano-chemical modification of cement with high volumes of blast furnace slag

The application of chemical admixtures significantly improves the performance of cement-based materials. Some admixtures can also be used to modify the cement grinding process and induce changes in the structure of cement minerals due to mechano-chemical activation. A reactive silica-based complex admixture was developed for the modification of cement grinding. This paper examines the effect of grinding on the strength of a modified cement containing granulated blast furnace slag in high volumes. According to the test results, mortars based on the modified cement possess a compressive strength of up to 91.7 MPa, a 62% increase over the reference.     

Investigations into the effect of addition of flyash and burnt clay pozzolana on certain engineering properties of cement composites

In cementitious binders when used for masonry mortars the requirements for properties of importance during early and later ages are different. Whereas during the early periods the attributes required are water retention, workability, plasticity, adhesion, etc. to allow the mortar to possess good working properties such as the ease of spreading, proper filling of joints and also to provide a water resistant crack free smooth surface, but at later ages strength becomes the main criterion to sustain the imposed load of the structure. Portland cement based mortars though harden rapidly and attain high strength but possesses a relatively poor early age properties. In composite mortars there is a common practice to incorporate lime along with portland cement, whose presence improves upon the early age rehological properties. Once the setting and hardening take place and the role of these early age properties is completed, lime has little role to play, as it harden through the lethargic process of carbonation i.e. by the chemical action of lime with atmospheric carbon dioxide forming insoluble carbonate. The process of carbonation is very slow and takes place from surface inwards. Modified composite mortars have been developed by the replacement of certain part of lime with pozzolana such as burnt clay or flyash and has been found to be of advantage. Laboratory investigations on a series of such mixtures have revealed the possession of good early age properties and at the same time better strength at later ages. Some of the results are reported in this paper.