A model to predict the amount of calcium hydroxide in concrete containing mineral admixtures

This study examines in detail the degree of reactivity of admixtures, such as fly ash and blast furnace slag, and their effect on the levels of calcium hydroxide in cement paste. Experimental results indicate that reactivity between calcium hydroxide and mineral admixture is dependent on the amount of calcium hydroxide and the degree of hydration of mineral admixtures.From these results, a model was formulated to predict the reaction between calcium hydroxide and mineral admixtures, and its validity verified by comparing calculated data with the data from the tests with cement mortar specimens. The calculated values of calcium hydroxide agree well with the test results. The parameters of the prediction model are dependent on the physical and chemical characteristics of mineral admixtures.     

Effect of steam curing on class C high-volume fly ash concrete mixtures

The effect of steam curing on concrete incorporating ASTM Class C fly ash (FA), which is widely available in Turkey, was investigated. Cement was replaced with up to 70% fly ash, and concrete mixtures with 360 kg/m³ cementitious content and a constant water/binder ratio of 0.4 were made. Compressive strength of concrete, volume stability of mortar bar specimens, and setting times of pastes were investigated. Test results indicate that, under standard curing conditions, only 1-day strength of fly ash concrete was low. At later ages, the strength values of even 50% and 60% fly ash concretes were satisfactory. Steam curing accelerated the 1-day strength but the long-term strength was greatly reduced. Setting time of fly ash-cement pastes and volume stability of mortars with 50% or less fly ash content were found to be satisfactory for standard specimens. In addition, for steam curing, this properties were acceptable for all replacement ratios.     

Prediction model of compressive strength development of fly-ash concrete

Based on experimental results concerning the compressive strength development of concrete containing fly ash, the authors derived an estimation equation for compressive strength development. The equation can express coefficient , which indicates the activity of fly ash as a binder, in the form of a function of age, fly-ash content, and Blaine specific surface area of fly ash.

This equation is capable of explaining the increases in the early strength due to fly ash in place of part of fine aggregate, the decreases in the early strength due to fly ash in place of part of cement, the increases in the long-term strength due to pozzolanic reaction, the relationship between the fly-ash replacement ratio and the ratio of strength increase/decrease, and the effect of fly ash's Blaine specific surface area on the strength.     

Comparison of sintering and compressive strength tendencies of a model coal mineral mixture heat-treated in inert and oxidizing atmospheres

The chemical interactions responsible for sintering in a coal mineral mixture were investigated in air and in N₂. A mineral mixture was made up by mixing kaolin, pyrite, quartz, calcite, hydromagnesite, FeCO₃ and anatase in a fixed ratio. The mineral mixture was pelletized and heat-treated up to 1100 °C in order to evaluate sintering by recording the compressive strength values and visual assessment with scanning electron microscopy (SEM). Chemical interactions responsible for the trends in the compressive strength results were investigated with simultaneous thermogravimetric and differential thermal analysis (TG/DTA), as well as X-ray diffraction. The results indicated that the formation of anhydrite (CaSO₄) was responsible for increased mechanical strength in the mineral mixture pellets heated in air at temperatures higher that 400 °C. CaSO₄ formed from the reaction of the decomposition products of pyrite and calcite (SO_x and CaO). The TG/DTA results also indicated that the reaction with pyrite in air caused the decomposition of calcite in the mixture at a lower temperature than was observed for calcite only. The pellets heated in N₂ did not increase in mechanical strength during heat-treatment due to the lack of CaSO₄ formation in the inert atmosphere. However, SEM analysis indicated that sintering did occur at the higher temperatures in N₂. A decrease was observed in the compressive strength values obtained in air at temperatures from 900 °C to 1100 °C. Reasons for the decreased compressive strengths may include increased porosity, decomposition of CaSO₄, and changes in the characteristics of the aluminosilicate phases.     

An experimental study on strength development of concrete containing fly ash and optimum usage of fly ash in concrete

This paper presents a laboratory study on the strength development of concrete containing fly ash and optimum use of fly ash in concrete. Fly ash was added according to the partial replacement method in mixtures. A total of 28 mixtures with different mix designs were prepared. 4 of them were prepared as control mixtures with 250, 300, 350, and 400 kg/m³ cement content in order to calculate the Bolomey and Feret coefficients (K_B, K_F). Four groups of mixtures were prepared, each group containing six mix designs and using the cement content of one of the control mixture as the base for the mix design. In each group 20% of the cement content of the control mixture was removed, resulting in starting mixtures with 200, 240, 280, and 320 kg/m³ cement content. Fly ash in the amount of approximately 15%, 25%, 33%, 42%, 50%, and 58% of the rest of the cement content was added as partial cement replacement. All specimens were moist cured for 28 and 180 days before compressive strength testing. The efficiency and the maximum content of fly ash that gives the maximum compressive strength were obtained by using Bolomey and Feret strength equations. Hence, the maximum amount of usable fly ash amount with the optimum efficiency was determined.This study showed that strength increases with increasing amount of fly ash up to an optimum value, beyond which strength starts to decrease with further addition of fly ash. The optimum value of fly ash for the four test groups is about 40% of cement. Fly ash/cement ratio is an important factor determining the efficiency of fly ash.     

Prediction of compressive strength of fly ash concrete by new apparent activation energy function

A new prediction model using apparent activation energy is proposed to estimate the variation of compressive strength of fly ash concrete with aging. After analyzing the experimental result with the model, fly ash replacement content and water-binder ratio influence on apparent activation energy was investigated.According to the analysis, the model provides a good estimation of compressive strength development of fly ash concrete with aging. As the fly ash replacement content increases, limiting relative compressive strength and initial apparent activation energy increase. Concrete with water-binder ratio smaller than 0.40 gives nearly constant limiting relative compressive strength and initial apparent activation energy when analyzed with various water-binder ratios. However, concrete with water-binder ratio larger than 0.40 increases limiting relative compressive strength and initial apparent activation energy.     

Effect of supplementary cementitious materials on the compressive strength and durability of short-term cured concrete

This research focuses on studying the effect different supplementary cementitious materials (silica fume, fly ash, slag, and their combinations) on strength and durability of concrete cured for a short period of time—14 days. This work primarily deals with the characteristics of these materials, including strength, durability, and resistance to wet and dry and freeze and thaw environments. Over 16 mixes were made and compared to the control mix. Each of these mixes was either differing in the percentages of the additives or was combinations of two or more additives. All specimens were moist cured for 14 days before testing or subjected to environmental exposure. The freeze-thaw and wet-dry specimens were also compared to the control mix.Results show that at 14 days of curing, the use of supplementary cementitious materials reduced both strength and freeze-thaw durability of concrete. The combination of 10% silica fume, 25% slag, and 15% fly ash produced high strength and high resistance to freeze-thaw and wet-dry exposures as compared to other mixes. This study showed that it is imperative to cure the concrete for an extended period of time, especially those with fly ash and slag, to obtain good strength and durability. Literature review on the use of different supplementary cementitious materials in concrete to enhance strength and durability was also reported.     

The effect of porosity on the strength of foamed concrete

A study has been undertaken to investigate the effects of replacing large volumes of cement on the properties of foamed concrete (up to 75% by weight) with both classified and unclassified fly ash. This is the third paper in a series; it investigates the relationship between porosity and compressive strength and presents mathematical models that have been developed to describe this relationship. The compressive strength of the foamed concrete was shown to be a function of porosity and age, and a multiplicative model (such as the equation derived by Balshin) was found to best fit the results at all ages up to 1 year. In addition, it was concluded that the equation derived by Hoff could effectively be used to predict the compressive strength of foamed concrete mixtures containing high percentages of ash.     

Effects of supplementary cementing materials on the properties of cement and concrete

The effect of bentonite, colemanite ore waste (CW), coal fly ash (FA) and coal bottom ash (BA) on the properties of cement and concrete has been investigated through a number of tests. The properties examined include setting time, bending strength, volume expansion, compressive strength and water consistency of the mortar. The result showed that setting time of the cements was generally accelerated when bentonite replaced a part of the cement. Bentonite exhibited a significant retarding effect when used in combination with CW in Portland cement at lower replacement level and showed an accelerating effect at higher replacement level. Although the inclusion of bentonite at replacement levels of 5-10% resulted in an increase in compressive strength at early ages, it decreased the compressive strength when used in combination with other materials. The results obtained were compared with Turkish standards and, in general, were found to be acceptable.     

Influence of heat curing on the pore structure and compressive strength of self-compacting concrete (SCC)

The mechanical properties of self-compacting concrete (SCC) are well understood. But there are no scientific investigations available on the influence of a heat treatment on the properties of SCC. To evaluate the influence on the compressive strength, SCCs as powder type, combination type and viscosity-agent type in the strength classes between C20/25 and C70/85 were designed and exposed to heat treatment with different maximum temperatures. It has been found that there is an influence of the composition of the concrete, especially the (w/c)_eq ratio, on the compressive strength after heat treatment. The reason for the substantial loss of strength in some cases compared to the strength of the concrete, which was stored under standard conditions, is a change of the pore size distribution. An empirical formula is presented to calculate the influence of the heat treatment on the compressive strength of the SCCs.     

The role of alkali content of Portland cement on the expansion of concrete prisms containing reactive aggregates and supplementary cementing materials

This paper presents results covering the effects of alkali content of Portland cement (PC) on expansion of concrete containing reactive aggregates and supplementary cementing materials (SCM). The results showed that the alkali content of PC has a significant effect on expansion of concrete prisms with no SCM. When SCM is used, the expansion was found to be related to both the chemical composition of the SCM and, to a lesser extent, the alkali content of the PC. The concrete expansions were explained, at least partly, on the basis of the alkalinity of a pore solution extracted from hardened cement paste samples containing the same cementing blends. An empirical relation was developed correlating the chemical composition (Ca, Si and total Na₂O_e) of the cementing blend (PC + SCM) and the alkalinity of the pore solution. Results from accelerated mortar bar test (ASTM C 1260) and a modified version thereof are also presented.     

The experimental investigation of concrete carbonation depth

Phenolphthalein indicator has traditionally been used to determine the depth of carbonation in concrete. This investigation uses the thermalgravimetric analysis (TGA) method, which tests the concentration distribution of Ca(OH)₂ and CaCO₃, while the X-ray diffraction analysis (XRDA) tests the intensity distribution of Ca(OH)₂ and CaCO₃. The Fourier transformation infrared spectroscopy (FTIR) test method detects the presence of C-O in concrete samples as a basis for determining the presence of CaCO₃. Concrete specimens were prepared and subjected to accelerated carbonation under conditions of 23 °C temperature, 70% RH and 20% concentration of CO₂. The test results of TGA and XRDA indicate that there exist a sharp carbonation front. Three zones of carbonation were identified according to the degree of carbonation and pH in the pore solutions. The TGA, XRDA and FTIR results showed the depth of carbonation front is twice of that determined from phenolphthalein indicator.     

Sorptivity and strength of air-cured and water-cured PC-PFA-MK concrete and the influence of binder composition on carbonation depth

The paper reports the influence of the composition of Portland cement-pulverised fuel ash-metakaolin (PC-PFA-MK) binders on sorptivity and strength development of PC-PFA-MK concrete cured both in air and in water and on carbonation depth, and relates this to measured changes in sorptivity of the concrete. Concrete mixtures covering four different total cement replacement levels (10%, 20%, 30% and 40%) for PC-PFA-MK concrete with various MK/PFA proportions, water and air cured for up to 18 months, were investigated. The change in compressive strength and sorptivity with age at all cement replacement levels under both water and air curing are compared with those of the control PC concrete. The results presented in this paper form part of an investigation into the optimisation of a ternary blended cementitious system based on ordinary PC, PFA and MK for the development of high-performance concrete.     

Influence of environmental temperatures on the concrete compressive strength: Simulation of hot and cold weather conditions

The objective of this work is to study the influence of mixing hour on the properties of concrete, such as workability and compressive strength, under hot and cold weather conditions, with a view to industrial application. The variable focused on was the concrete mixing hour, and five different mixing hours were used for each type of weather condition. Three batches of concrete were prepared for each mixing hour, and the compressive strength of 15 cylindrical concrete specimens was measured after 7 and 28 days. In addition, the hydration kinetics of each batch of concrete was studied on the basis of the climatic conditions and the mixing hour. The results for compressive strength show that the concrete's best mechanical performance occurred when there was the least difference between ambient temperature and concrete temperature, that is, during the later hours of the day in hot weather conditions.     

Masonry repair lime-based mortars: factors affecting the mechanical behavior

The increasing use of lime-based mortars for the restoration of historic buildings and structures justifies the research on these materials. The focus of this paper is the effect of technological variables on pore structure and mechanical properties of lime-based mortars. The influence of curing time, binder-aggregate (B/Ag) ratio, aggregate attributes and porosity is discussed. Mortars prepared with aerial lime, varying aggregate types and B/Ag ratios ranging from 1:1 to 1:5 by volume were tested. Compressive and flexural strength measurements, as well as X-ray diffraction (XRD) and thermal studies, were performed after 3, 7, 28, 91, 182 and 365 days. A strong increase in strength of mortar mixtures after 365 curing days (as compared to 28 curing days) is found. In spite of the fact that larger amounts of binder increase the total porosity, the strength of these mixtures is also increased. A good interlocked structure is obtained as binder contents increase. Also, higher porosities allow better portlandite carbonation. A relationship between mechanical properties and pore structure was established. However, in case of binder excess, the increase in voids leads to a strength reduction. The use of calcareous aggregates improves strength more as compared to the use of siliceous aggregates. Factors as grain size distribution and grain shape of the aggregates have also been considered.     

Microanalysis of alkali-activated fly ash-CH pastes

Samples of a Class F fly ash and calcium hydroxide (CH) hydrated in pH 13.2 sodium hydroxide solution were analyzed using backscattered electron, scanning Auger, and X-ray microanalysis. The Class F fly ash, composed mainly of aluminosilicate glass and silica, was reacted for 8, 14, and 78 days at various temperatures. These samples represent both long-term and early-age stages of hydration. Results show that a hydrate product with calcium to silica ratio near 1.4 and katoite are formed. X-ray and scanning Auger microanalysis show evidence of the formation of hydrate product on the surface of both fly ash and CH particles at early ages. This finding suggests a new mechanism to explain prior data that shows that the hydration rates increase with increasing CH-ash content in the starting mixture.     

Supplementary cementing materials in concrete: Part II: A fundamental estimation of the efficiency factor

For comparing the relative performance of various supplementary cementing materials (SCMs: silica fume, fly ash, slag, natural pozzolans, etc.) as regards Portland cement, the practical concept of an efficiency factor may be applied. The efficiency factor (or k value) is defined as the part of the SCM in an SCM-concrete that can be considered as equivalent to Portland cement. In the present work, an alternative procedure for experimental determination of the k value is proposed, using the concept of the pozzolanic activity index. For the first time, also, the k value for equivalent strength was correlated with the active silica content of the SCM through analytical expressions. Artificial pozzolanic materials of various compositions and some natural pozzolans were studied. It was found and verified by experimental comparison that these expressions are valid only for artificial SCMs (fly ash, slag), whereas in the case of natural SCMs the k value is overestimated. Thus, knowing primarily the active silica content of the SCM, a first approximation of the k value can be obtained and, further, the strength of a concrete incorporating artificial SCM can be predicted.     

Preliminary study on the water permeability and microstructure of concrete incorporating nano-SiO2

Water permeability resistant behavior and microstructure of concrete with nano-SiO₂ were experimentally studied. A water permeability test shows that, for concretes of similar 28-day strength, the incorporation of nano-SiO₂ can improve the resistance of water penetration of concrete. An ESEM test reveals that the microstructure of concrete with nano-SiO₂ is more uniform and compact than that of normal concrete. Mechanism about the effect of nano-SiO₂ on concrete is described.     

Determination of the elastic constants of portlandite by Brillouin spectroscopy

The single crystal elastic constants C_ij and the shear and adiabatic bulk modulus of a natural portlandite (Ca(OH)₂) crystal were determined by Brillouin spectroscopy at ambient conditions. The elastic constants, expressed in GPa, are: C₁₁ = 102.0(± 2.0), C₁₂ = 32.1(± 1.0), C₁₃ = 8.4(± 0.4), C₁₄ = 4.5(± 0.2), C₃₃ = 33.6(± 0.7), C₄₄ = 12.0(± 0.3), C₆₆ = (C₁₁-C₁₂)/2 = 35.0(± 1.1), where the numbers in parentheses are 1σ standard deviations. The Reuss bounds of the adiabatic bulk and shear moduli are K_0S = 26.0(± 0.3) GPa and G₀ = 17.5(± 0.4) GPa, respectively, while the Voigt bounds of these moduli are K_0S = 37.3(± 0.4) GPa and G₀ = 24.4(± 0.3) GPa. The Reuss and Voigt bounds for the aggregate Young's modulus are 42.8(± 1.0) GPa and 60.0(± 0.8) GPa respectively, while the aggregate Poisson's ratio is equal to 0.23(± 0.01). Portlandite exhibits both large compressional elastic anisotropy with C₁₁/C₃₃ = 3.03(± 0.09) equivalent to that of the isostructural hydroxide brucite (Mg(OH)₂), and large shear anisotropy with C₆₆/C₄₄ = 2.92(± 0.12) which is 11% larger than brucite. The comparison between the bulk modulus of portlandite and that of lime (CaO) confirms a systematic linear relationship between the bulk moduli of brucite-type simple hydroxides and the corresponding NaCl-type oxides.