Vivek Bindiganavile

As an alternative to Portland cement systems, geopolymers have been found to display superior acid resistance. However, at present, there exists no strategy to regulate the suitable design of mixtures. Particularly, the mechanisms underlying the effect of principal oxide ratios on the performance of N-A-S-H geopolymers in acid-rich environments are missing. Nor is any information available on the optimal range for SiO₂/Al₂O₃, Na₂O/Al₂O₃, and H₂O/Na₂O ratios under acid attack. This study investigates N-A-S-H geopolymers incorporating varying compositional oxide ratios to assess their resistance to sulphuric acid attack. The results show that the optimal range for acid-resistant durability is a narrow band within the optimal range for workability and strength. A SiO₂/Al₂O₃ ratio of 3.4 balanced the enhanced degree of geopolymerization with an increase in the amount of permeable voids. At the same time, the Na₂O/Al₂O₃ and H₂O/Na₂O ratios should be maintained within 0.8~0.9 and 8~10, respectively. Quantitatively, for the mixture designed within these optimal oxide ranges, the associated strength loss after 84 weeks of acid exposure was only about 10~20%, whereas other mix proportions may lead to a maximum strength loss of up to ~58%. Anything higher will offset the polycondensation and instead raise the volume of permeable voids. A sensitivity analysis suggests that the acid resistance depends chiefly on the Na₂O/Al₂O₃ and H₂O/Na₂O ratios. The proposed multi-factor models predict the acid-induced neutralization efficiently, and the associated output displays a correlation with the loss in compressive strength.

This study models concrete as a multi-phase system that comprises the mortar, coarse aggregates, and interfacial transition zones (ITZs). The diffusivity and the volumetric fraction of these phases are considered to propose a three-phase diffusion–reaction model to simulate the external sulphate attack. Furthermore, the parametric analysis alongside the sensitivity analysis is carried out to quantify the effect of these phases on the expansive cracking in concrete when exposed to a sulphate-rich environment. The results show that mortar dominates the sulphate ingress and the ensuing distress, while the ITZ is found to be least significant. Due to its significantly low permeability, the coarse aggregate may act as a “deceleration strip” or a “dam”, which in turn obstructs the sulphate penetration. More importantly, this effect is further noted to evolve with a decrease in the diffusivity and a rise in the volumetric fraction of coarse aggregates. As for ITZ, its volume fraction may play a more dominant role than its diffusivity on sulphate attack in concrete.

The interaction between compositional ratios, namely, SiO₂/Al₂O₃, Na₂O/Al₂O₃, H₂O/Na₂O and the liquid-to-solid ratio, triggers mutual sacrifice between workability, setting time and strength for N-A-S-H geopolymers. The present study characterizes the mechanism underlying the effect of these compositional ratios and, in turn, develops guidelines for mixture design that requires a simultaneous and satisfactory delivery of these engineering properties. The experimental results show that an increase in either the SiO₂/Al₂O₃, Na₂O/Al₂O₃ or H₂O/Na₂O ratio raises the liquid-to-solid ratio, which in turn improves the workability of fresh mixtures. A continuous increase beyond 2.8 for the SiO₂/Al₂O₃ ratio boosts its strength, but also significantly extends its final set. Lowering the Na₂O/Al₂O₃ ratio from 1.3 to 0.75 raises the compressive strength significantly, while the shortest final set was seen at the median value, 1.0. A H₂O/Na₂O ratio of 9~10 yields the highest strength and the fastest final set simultaneously, due to the maximized degree of geopolymerization. Moreover, the accompanying sensitivity analysis indicates that the workability depends chiefly upon the H₂O/Na₂O ratio, the final setting time on the SiO₂/Al₂O₃ ratio and, that the compressive strength relies on both of them. Also, this study proposes an optimal range of 2.8~3.6 for SiO₂/Al₂O₃, 0.75~1.0 for Na₂O/Al₂O₃ and 9~10 for H₂O/Na₂O to guarantee high strength, together with high flow and within the allowable final setting time. Furthermore, multi-factor predictive models are established with acceptable accuracy for practitioners to regulate oxide compositions in N-A-S-H geopolymers, which will guide future mixture design.

Cellular concrete is a lightweight solid cementitious foam that is used mainly for thermal insulation and sound absorption. Either partially or wholly, when the Portland cement in the binder is replaced with ash, the environmental impact of the material is reduced. However, the resulting changes in the cell structure and their subsequent impact upon the physical properties of the material have not been well examined. In this study, cellular concrete was prepared from alkali activated fly ash binders and separately, from Portland cement binders blended with wood ash. The cell structure was photographed using laser confocal microscopy and the images were analyzed to quantify the cellular network in terms of the porosity and the fractal dimension of the porous phase. Alongside, the cellular concrete specimens were tested for their thermal constants and sound absorption. The results show that there was an increase in the fractal dimension and an associated drop in thermal conductivity for an increase in the porosity.

Coal bottom ash (CBA) is one of the by-products that can be employed as fine aggregate to replace natural sand in concrete. Owing to the very low water demand, roller-compacted concrete (RCC) has the potential to use CBA as fine aggregate at a high proportion. However, little research about RCC using CBA entirely as fine aggregate has been conducted. In this study, the uniaxial compressive strength, deformation, stress–strain curves, and splitting tensile strength of CBA-containing RCC (CBA RCC) were studied to bridge this gap. The compressive strength, elasticity modulus, and splitting tensile strength of all mixtures decreased with increasing CBA content. The relationship between compressive strength and splitting tensile strength of CBA RCC was proposed, which is very close to that recommended by the CEB-FIP code. The uniaxial compressive constitutive model based on the continuum damage theory can well illustrate the stress–strain relationship of CBA RCC. The growth process of damage variable demonstrates the hybrid effect of coarse aggregate, cement, and compacting load on delaying damage under uniaxial compression. The theoretical formula can also accurately illustrate the stress–strain curves of RCC presented in the literature studies.

This paper describes two visual approaches, namely an area ratio method and a grayscale variation method, to evaluate the effective diffusion depth inside cement-based systems when subjected to external sulfate attack. The former is based on the calculation of the amount of pixels within two areas, namely the area of cross section and the undiffused area, while the latter is per the normalized color variation. Three types of binder, all conforming to the Canadian Standards Association (CSA), were employed to produce three batches of cylindrical specimens, which were immersed in a sulfate-rich environment for up to 12 wk. The effective diffusion depths, obtained by the two visual approaches described here, were compared with the sulfate content found experimentally through chemical titration. The results show that the proposed visual methods can capture the effective diffusion depths inside cement-based systems after exposure to sulfate attack. The maximum value ranged from 7.0 to 11.0 mm over 12 wk of exposure. The proposed visual techniques enable a quick assessment to decide whether further repair is warranted.

This study quantifies the pore wall spacing and shape factor of lightweight cement-based foam pore structure and investigates its correlation with density, pozzolan, compressive strength, and thermal conductivity. The lightweight cement-based foam mixtures were prepared for the densities of 800–400 kg/m³ and by replacing cement with fly ash, silica fume, and metakaolin up to 20 % by mass. The nondestructive technique of X-ray microtomography was used for quantifying the pore structure parameters. In addition, the transient plane heat source technique was used to evaluate the thermal conductivity of these mixtures. The results show that the optimal pore wall spacing increases from 0.28 to 0.55 mm as the density rises from 400 to 800 kg/m³. It was observed that the median pore wall spacing (SP50) values increases as the cast density get higher. While the reduction in thermal conductivity was noticed for lower SP50 values. Furthermore, incorporating pozzolanic admixtures at higher cement replacement ratios affects the pore wall spacing. Moreover, spherically shaped pores were found in all mixtures.