Recently, with increased attention to the global warming era, the roadway construction sector has been pressured to be more sustainable to develop clean and green technology. Innovative and sustainable materials based on the concept of alkali-activated material (AAM) have recently been introduced to the sector. They are in the early stages of investigation [1–3]. AAM is the broadest classification of binder systems derived by a chemical reaction of alkaline sources (in solid or dissolved form) with any prime materials [4, 5]. This prime material can be a calcium silicate compound in ordinary Portland cement or a more aluminosilicate-rich precursor (e.g., fly ash, bottom ash, metallurgical slag, naturally pozzolan). AAM is believed to be a green construction material for the future because it produces much less CO2 emission than ordinary Portland cement (OPC). OPC is the most frequently used construction material, and it is the most popular stabilizing agent. AAM could yield sufficient strength and other satisfying manners resulting from the chemical reaction between an alkaline solution and a solid aluminosilicate precursor [6–12]. However, alkali-activated cement technology has apparent limitations. Heat curing is a primary factor of the complete chemical reaction processes to be a significant problem. The difficulty of heat curing hinders its acceptance as an OPC replacement, among other applications. Adopting an AAM with low strength for specific purposes, e.g., unfired bricks or roadway applications, was the challenge in developing a new and green construction material. This study gave a preliminary introduction to relatively low strength AAM as an environmentally friendly road construction material to replace the consumption of OPC. There could help to minimize global CO2 emissions by the cement production industry.
In the manufacturing industry, cement production emits approximately 7% of global CO2 emissions . Every ton of cement production produces approximately one ton of CO2 into the atmosphere . OPC was the most widely used material for almost all kinds of structures. For mitigating the use of OPC, cement substitute materials were introduced [15–17], and AAMs were currently introduced as having the potential to replace OPC with less CO2 emissions. It was claimed that AAMs emitted more than 80% less CO2 compared to OPC . The AAM can be synthesized by mixing by-product materials (e.g., fly ash, bottom ash, or furnace slag) with alkaline solutions (e.g., sodium hydroxide and sodium silicate) [19, 20]. Figure 1 shows the synthesis process of AAM based on the view of this study. The main composition of AAM, based on calcium content, is calcium aluminosilicate hydrate (C-A-S-H). It has a layer structure similar to calcium silicate hydrate (C-S-H) and sodium aluminosilicate hydrate (N-A-S-H), a two-to-three-dimensional structure of Si-O-Al in the matrices [20, 21]. Alkali-activation has two phases, high-calcium alkali-activated and low-calcium alkali-activated, otherwise known as “geopolymers” .
The targeted applications of this relatively low strength AAM may be applied to the roadway with crushed rock as a precursor. Road pavement has an average design life of about 15–30 years . Hence, the number of roads that require rehabilitation and reconstruction has grown to meet the high demand for transporting goods  and people under the current economic circumstances. Therefore, reconstructing and rehabilitating road pavement in an environmentally sustainable and cost-effective manner is an engineering challenge. Typically, road pavement requires a relatively low-strength material compared to other civil engineering structures. To improve the mechanical properties of road base materials, cement stabilization through in-situ pavement recycling was the most popular technique. In this approach, a small amount of cement was added to existing pavement materials; there was no transportation of new road pavement construction materials . In-situ cementitious rehabilitation (i.e., pavement recycling with cement stabilization for road pavements as shown in Fig. 2) is considered a cheaper (35 to 50% of the cost of reconstruction) and potentially environmentally sustainable solution because it recycles existing pavement materials . It was found that the compressive strength of cement-stabilized base materials increases with higher cement content [27, 28]. Currently, OPC is used for more than 80% of road rehabilitation activities worldwide . Although pavement rehabilitation uses only about 2 to 8% (by weight) of OPC compared to 10 to 16% for concrete, the amount of concrete utilized would be relatively small compared to the pavements to be rehabilitated. When in-situ pavement rehabilitation is more widely used, as predicted from the recent growth of the road network worldwide, OPC use for this purpose could vastly increase (estimated to be about 150 tons km− 1) . This higher amount of cement use would cause relatively sharp increases in CO2 emissions owing to cement production. Therefore, alkali-activated stabilizers (binders) for in-situ pavement rehabilitation as a potential environmentally and structurally sustainable solution should be cleverly engineered.
The primary raw material in this study is crushed rock (CR) which is a calcium-rich material. The presence of calcium in the AAM system can provide the basis for gaining strength at room temperature [30–32]. This study has the ultimate goal to explore the possibility of AAM as cementitious material using CR as source material with no other active precursors (e.g., fly ash). The influential factors of AAM for road stabilized were investigated through the compressive strength. Thus, the results introduce the possible use of the crushed rock-based AAM with relatively low strength and more environmentally friendly construction material.
In this study, CR, which is generally used as a road base material, was used as the source material of AAM production. Physical properties of AAM were studied, including the setting times, flowability, and compressive strength. Results were compared with the requirements values for roadway materials. Besides, chemical compositions, morphology, and microstructure of AAM were performed, using X-ray fluorescence (XRF), X-ray diffraction (XRD), and a scanning electron microscope (SEM), respectively.