Strength activity and microstructure of blended ultra-fine coal bottom ash-cement mortar

Highlights

• Pulverized BA increases the setting time and workability of mortar.

• Pulverized BA strength activity can be retained above 100% at high replacements.

• C-S-H in the fine pulverized bottom ash has lower Ca/Si ratio.

• CH consumption depends on pulverized BA fineness levels.

Abstract

Replacing Portland cement with reutilized coal combustion products can have substantial benefits to the environment as well as infrastructure. In this study, a sub-bituminous coal bottom ash is re-utilized as a cement replacement. The bottom ash was pulverized using a high energy vibratory ball mill at two different milling times to achieve a particle fineness approximately two and three-times finer, respectively, than type I cement. The workability and final setting time were found to increase by 21% and 14%, respectively, and the highest strength activity was observed to be 120% at 90 days. After normalization for particle fineness, age, and binder replacement using a general linear regression model, the data suggested the bottom ash blended cement mortar had 6.8% less strength activity than a comparative fly ash blended cement mortar. Blended bottom ash-cement paste samples were observed to have densely packed CSH product composed of relatively lower Ca/Si ratio (1.37) compared to that of fly ash blended cement paste (1.43) and the control (2.27). The CH content was also observed to be significantly lower in the bottom ash blended cement paste (19.1%) when compared against the control (27.2%) and fly ash blended paste (23.3%). Within the range of experimental conditions considered here, the results suggest that a coal bottom ash can be pulverized with a high energy ball mill to produce a re-utilizable pozzolan that, depending on the powder fineness, can significantly increase the strength activity, improve the microstructure of cement mortar, and increase the cement replacement tolerances without significant reduction to compressive strength activity.

Introduction

In 2015 approximately 117 million tons of coal combustion products (CCPs) were produced in the United States, while only 61 million tons were re-utilized beneficially (52%) [1]. Fly ash (FA), is one of the most common reutilized CCPs (24 million reutilized tons in the U.S. at a 55% reutilization rate), which is used beneficially as a supplementary cement replacement. Coal bottom ash (BA), the incombustible granular and porous residue found in the bottom of the combustion furnace [2], is not reutilized in the U.S at the same rate as FA (4 million tons reutilized beneficially at a 33% reutilization rate). BA is mostly limited to road base substitution or cement feed stock applications [1]. Additional pathways for BA applications are needed to increase the reutilization rate and decrease the amount of landfilled ash.

Reutilizing BA as fine or coarse aggregate has been studied and shown to significantly decrease the concrete unit-weight and yield adequate compressive strength [3], [4], [5]. Coal bottom ash has also been used as an internal curing source due to its high porosity and steep desorption; essentially allowing fluid to permeate from the internal porous reservoirs to the surrounding matrix without requiring significant changes to internal capillary pressure to generate the fluid displacement [6]. The porous BA also has been shown to increase capillary water absorption which tends to extend drying time [7], lower drying shrinkage [8], [9], decrease unit weight, and increase air content [10].

As a cement replacement, pulverized BA can increase the compressive [11] and flexural strength [12], strength activity, and pozzolan reactivity. The mechanical properties of the concrete or mortar, will however, depend largely on the pulverization and particle fineness [13]. Additional thermal treatments can also be applied to the BA to decrease the Loss on Ignition (LOI), decompose the calcium carbonate [14], and increase pozzolan reactivity [15]. Although BA has been reutilized in both cement and aggregate concrete applications, it has been done with varying success. Most of the studies have focused on low cement replacement dosages (<30%) or utilized bottom ash at moderate fineness (∼400 m²/kg). The performance of the coal bottom ash blended cement mortars can be potentially improved, however, by increasing powder fineness through pulverization with high energy ball milling. Improved performance and higher cement replacement dosage tolerances can provide industry with a stronger justification to use coal bottom ash for cementing applications and ultimately serve to increase the ash product reutilization rate.

In this study, a high fineness BA (∼1000 m²/kg), achieved by means of high energy vibratory ball milling, is reutilized in cement mortar applications. At high fineness levels, the particles approach the nanometer range which can alter the microstructure of the blended mortar and increase strength activity, even at high volume replacement dosages. The high fineness BA powders are studied here under a wide range of replacement dosages. The data reported in this study is then combined with data reported from the literature and a general linear model is derived using ANOVA to robustly quantify the experimental variance associated with strength activity using fineness, cement replacement dosage, age, and type of supplemental cementitious material (bottom ash or fly ash) as predictors.