1. Introduction
Autoclaved aerated concrete (AAC) is a commonly used wall material with good thermal insulation, which is attributed to its fine, uniform, independent pore structure [
1,
2,
3]. Conventional commercial AAC production usually uses natural silica sand or high pozzolanic active material; fly ash is used as siliceous material to provide Si or Al in hydrothermal synthesis reactions [
4,
5]. Meanwhile, portland cement and aluminum (Al) paste were used as cementitious material and a gas-generating agent, respectively [
5]. Gypsum was treated as autoclaving auxiliary agent to accelerate the formation of target hydration products during the autoclave process [
6]. The most commonly used calcareous material in AAC was quicklime, which not only supplies calcium ions for hydrothermal synthesis reaction in the autoclaving process, but also provides uniform heat, which directly affects the rough-body stability and pre-curing process duration [
4,
5].
Energy consumption and environmental protection are two major issues of great concern to the building materials industry. In order to improve the sustainability of AAC production, research into industrial solid waste substitutes covered almost all of the abovementioned raw materials. Cement was used to generate the initial rough-body strength, which can be replaced by alkali-activated volcanic material [
7]. As a result of the activation effect of autoclaving, the substitution of siliceous solid wastes for traditional siliceous materials, such as blast-furnace slag [
8], coal bottom ash [
9], copper tailing [
10], coal gangue [
11] and iron tailing [
12] has been investigated. Because the main ingredient of phosphogypsum (PG) is calcium sulphate dehydrate (CaSO
4·H
2O), it can be used as autoclaving auxiliary agent. The calcination preparation process determined quicklime to be the most energy-consuming raw material in AAC preparation [
13] and that it can be substituted by carbide slag, the major byproduct of acetylene (C
2H
2) production, which is over 85% calcium hydroxide (Ca(OH)
2) [
14]. Therefore, high-content solid-wastes autoclaved aerated concrete (HCS-AAC) has been proposed not only to transform “waste” into “wealth”, but also to reduce production costs [
6]. The complete substitution of siliceous material and auxiliary material by industrial solid wastes will not affect the formation process of the AAC rough body significantly. However, complete substitution of carbide slag for quick lime, which resulted in the absence of uniform quicklime hydration heat, had a severe effect on the formation of rough-body porosity, and this not only slowed down the reaction rate of Al paste and the slurry foaming rate, but also reduced the thickening speed of the rough body [
15].
Most of the previous studies on carbide slag-based AAC were concentrated on final product performance. Yuli Wang et al. utilized 30–40% high-volume desulfurization fly ash and 8% carbide slag to replace natural gypsum and quicklime, which produced 600 kg/m
3-grade AAC with 3.5 MPa in compressive strength [
16]. Fan Junjie et al. [
17] completely replaced quicklime with carbide slag and prepared AAC products over 600 kg/m
3 with only 2.0 MPa, which cannot satisfy the requirement of B06 A3.5-grade product (bulk density ≤625 kg/m
3, compressive strength ≥3.5 MPa) stipulated in GB 11968-2006, the Chinese national standard. Changlong Wang et al. [
12] prepared 600 kg/m
3, 4.4 MPa-grade AAC products with 60% high silicon iron tailing as the main siliceous material, 25% carbide slag as the calcareous material and discussed the effect of the substituted carbide slag ratio, fineness and pre-curing temperature on the physical-mechanical properties. But the effect of carbide slag substitution on the foaming and thickening processes that determine the formation stability and quality of porous structures were usually neglected, to say nothing of the regulation of the pre-curing process.
In response to the unmatched slurry foaming and thickening process and the overlong pre-curing problem caused by substituting carbide slag for quicklime, the authors introduce fast hardening sulphate aluminum cement (SAC) into AAC production and achieved positive results in foaming and thickening adjustment [
15]. Otherwise, the microwave heating method was used to compensate for the absence of a uniform heat source in carbide slag-based AAC. The slurry foaming rate nearly doubled while the pre-curing duration was shortened by 0.5–1 h [
18]. The gas generation of AAC is mainly affected by slurry temperature and alkalinity while rough-body thickening is mainly affected by the hydration rate of the cementitious materials. Adding a cement accelerator to expedite early-age cement hydration may be another worthwhile approach for modifying the foaming and thickening problem of the AAC rough body. In concrete, industrial, mineral and chemical accelerators have been commonly utilized in a variety of engineering fields to regulate the work performance [
19] such as spray concrete [
20,
21,
22], cellular concrete [
23], urgent repair concrete [
24]. However, the explicit effect of a cement accelerator on an HCS-AAC slurry’s performance, physical-mechanical properties and hydration products is still unclear and needs to be clarified.
This study introduced the frequently used cement accelerator Na2SO4 and Na2O·2.0SiO2 in HCS-AAC as a pre-curing process adjustment agent and mainly discussed the influence on the slurry’s foaming property and time-dependent rheological behavior to evaluate the feasibility of adjusting the foaming and thickening process. Meanwhile, the effects on the physical-mechanical properties were determined by detecting bulk density and compressive strength to represent the side effect of service performance. Additionally, analyses of the mineralogical (XRD) and thermal characteristics (DSC-TG) were conducted to show the microscopic effects of cement accelerators on HCS-AAC. Finally, the influence of various cement coagulants on slurry performance, physical-mechanical properties and hydration products was discussed and analyzed. The results of this paper can provide theoretical and technological guidance for the slurry foaming and rough-body thickening process modification of carbide slag HCS-AAC.
4. Conclusions
This article addressed the problem of HCS-AAC pre-curing caused by the absolute substitution of carbide slag for quicklime, by introducing the commonly used cement accelerators Na2SO4 and Na2O·2.0SiO2 to adjust slurry foaming and thickening. Measuring the feasibility in HCS-AAC by a comprehensive impact analysis of slurry performance, physical-mechanical properties and hydration products. Synthesizing all of the above research, the following conclusions were reached:
(1) Na2SO4 and Na2O·2.0SiO2 can effectively accelerate the slurry foaming process, but the promotion of the slurry thickening process is inconspicuous. Thus, the formation stability of a porous structure rough body is adversely affected. It is recommended that a cement coagulant and rapid hardening cement be used together in an HCS-AAC to realize the common regulation of slurry foaming and thickening.
(2) With the increase in cement coagulant content, the compressive strength of an HCS-AAC obviously fell, which corresponded to the steady fall of bulk density and is mainly ascribed to the acceleration of slurry foaming.
(3) The dosing of Na2SO4 under 0.4% had little effect on the generation of strength-contributing hydration products. Nevertheless, the addition of Na2O·2.0SiO2 in a carbide slag HCS-AAC had a superior accelerating effect on C–S–H generation and crystalization than Na2SO4, which contributed to the high-activity gelatinous SiO2 generated by the reaction between Na2O·2.0SiO2 and Ca(OH)2.