International Concrete Abstracts Portal

One of the primary approaches to producing more sustainable concretes consists of replacing 50 % or more of the portland cement in a conventional concrete with fly ash, producing a so-called high volume fly ash (HVFA) concrete. While these mixtures typically perform admirably in the long term, they sometimes suffer from early-age performance issues including binder/admixture incompatibilities, delayed setting times, low early-age strengths, and a heightened sensitivity to curing conditions. Recent investigations have indicated that the replacement of a portion of the fly ash in these concrete mixtures by a suitably fine limestone powder can mitigate these early-age problems. The current study investigates the production of concrete mixtures where either 40 % or 60 % of the portland cement is replaced by fly ash (Class C or Class F) and limestone powder, on a volumetric basis. The mixtures are characterized based on measurement of their fresh properties, heat release, setting times, strength development, rapid chloride penetrability metrics and surface resistivity. The limestone powder not only accelerates the early age reactions of the cement and fly ash, but also provides significant benefits at ages of 28 d and beyond for both mechanical and transport properties.

The fly ash based hydraulic binder (FAHB) described in this paper is comprised of ASTM Class C fly ash, ASTM class F fly ash and two proprietary non-caustic liquid activators. FAHB, a zero carbon footprint binder, is successfully used in making Green Concrete (eGC.) The eGC neither needs wet curing like Portland cement Concrete (PCC) nor needs elevated curing temperature like geopolymer cement concrete. The water demand of eGC is much lower than the water demand of PCC. Though, eGC follows Abrahms’ water-to-cement ratio (W/C) law, it has different sets of curves to estimate the W/C from the required 28-day compressive strength of concrete (f’cr). A little different approach is needed for proportioning the eGC using FAHB. This paper presents the model for estimating the water demand of eGC, the approximate relation between the W/C and f’cr and the step- by- step guideline for mix proportioning of eGC using FAHB, a carbon neutral binder system consisting of no Portlannd cement. The mixture proportioning method proposed in this paper will help concrete engineers and ready mixed concrete producers in designing cost effective and durable eGC. This method permits users to design eGC for wide range of workability and compressive strength.

Rice husk ash can be used as a supplementary cementing material. It is a waste material from the burning of rice husks for energy. Rice husk ash contains a mesoporous morphology of silica and this morphology has high hydrophilic properties. When rice husk ash is used in concrete mixtures, the flowability of the mixtures decreases. This makes the acceptance of rice husk ash in the concrete industry more challenging. Reduced workability hinders the potential use of rice husk ash in the field. Some research has investigated the potential use of using mechanical grinding (e.g., ball mill) to reduce particle size of rice husk ash with the hopes of improving the workability of concrete containing RHA, but this method requires significant energy and results in high wear of the equipment. The work investigates the use of a chemical treatment process of rice husk ash. This chemical treatment process reduces the particle size and breaks down the mesoporous morphology, thereby improving the fresh characteristics of concrete containing RHA and decreasing its water requirements. Because changes in setting behavior and reduced early-age strength development are other concerns when using some supplementary cementing materials, this work also investigates the setting, early-age compressive strength development, and porosity of cementing material systems containing 10% and 15% RHA replacements.

Over the last few decades there has been an increasing interest in low-carbon-foot-print binders that can replace hydraulic cement in concrete mixtures. Given that hydraulic cement is so ingrained in the building materials industry, alternative binders –in addition to offering a low carbon footprint– must exhibit similar or improved consistency and properties in a cost effective manner. A candidate that meets these criteria is activated high-calcium fly ash (AHCFA), which as the name implies uses high-calcium fly ash (HCFA) as main reactive powder along with an activator to increase its hydraulic activity and a setting retarder to regulate the rate of reaction. It is an attempt to get a better understanding of the factors that have greater influence in the reactivity of HCFA. The physical, chemical and crystallographic characterization as well as glass fragility analyses of several HCFA samples was paralleled with the compressive strength of their corresponding AHCFA mortars. Correlations between HCFA characteristics and the compressive strength of the resulting AHCFA mortars were sought. The results suggest that the reactivity of HCFA can be evaluated in terms of glass fragility using non-bridging oxygens per tetrahedron (NBO/T) and alumina saturation index (ASI) as main parameters.

Compared to the hydration process of traditional Portland cements, phosphate-based cements rely on an acid/base reaction process to quickly achieve strong, lightweight and durable binders with lower embodied energy. Since the binding action relies on the chemical composition of the initial components, the rheological and mechanical properties of the resulting ceramic concretes can also be influenced by other mix components including fly ash, fillers and aggregates. This paper reports on an ongoing study examining properties of concretes produced with magnesium potassium phosphate cement binders that incorporate fly ash contents of up to 80% of the total binder mass. Highly flowable mixes were developed with setting times that could be controlled through use of commonly available admixtures. The highest compressive strength of the binders and mortars were achieved when the fly ash content was 50% of the total binder mass. The produced binders and sand mortars had densities of 1800 kg/m3 [3034 lb/yd3] and 2100 kg/m3 [3540 lb/yd3] and compressive strengths of 35 MPa [5.0 ksi] and 60 MPa [8.7 ksi] after 28 days of simple ambient curing. Decreases in both strength and density were observed as the fly ash content was increased further, but remained within practical ranges for common construction applications with high fly ash contents.