## 1 Introduction  
In metropolitan cities, ground sinking generally occurs due to the deterioration, cracking, and breakage of sewage pipes (Figure S1), as well as the inadequate compaction of backfill materials (Figure S2). These factors can lead to the formation of void spaces around sewage pipes and culvert boxes. Subsequently, ground sinking occurs as void spaces become larger. [PERSON] et al. [1] reported that the vibration transmitted by vehicles in metropolitan cities could cause the collapse of enlarged underground void spaces. The conventional method involves compacting well-graded soils around sewage pipes or culvert boxes using mechanical equipment (e.g., a vibrator) to prevent ground sinking. However, due to the limitation of mechanical equipment, the area adjacent to sewage pipes and culvert boxes is typically left uncompacted, in some cases leading to ground sinking. The foregoing is recognized as one of the causes of ground sinking in metropolitan cities. Accordingly, the utilization of controlled low-strength materials (CLSMs) has emerged as a highly effective solution for backfilling or void-filling around pipelines or culvert boxes and underneath roads. The American Concrete Institute (ACI) 229 committee defined CLSMs as self-compacting cementitious backfill materials. Other terms used to describe CLSMs include flowable fill, unshrinkable fill, controlled density fill, flowable mortar, plastic soil cement, soil-cement slurry, and K-Krete. CLSMs have been widely used for numerous applications (e.g., backfilling, road bases, pipeline and culvert filling, void filling, tank fills, insulation, and isolation filling) in lieu of conventional compact fillers [2; 3; 4; 5]. Table 1 summarizes the guidelines set out by the ACI 229R-13 that need to be satisfied by each CLSM application. In accordance with these guidelines, CLSMs should exhibit a high flowability, low bleeding, early cementation, and sufficient strength [3; 6; 7]. The ACI 229R-13 document [8] is not only the main guideline used in the USA, but has also been widely referenced in many countries. [PERSON] et al. [9] also noted that apart from the USA, there have been no specific standards or CLSM specifications published by other countries. Therefore, many countries have developed their own CLSM guidelines based on the ACI 229R-13 document. Since the 2000s, previous researchers have developed CLSMs using various waste materials/industrial byproducts (e.g., fly and bottom ashes, ground granulated blast furnace slag, waste foundry sand, cement kiln dust, steel slag, waterworks and paper sludges, waste rubber tires, and red mud) [7; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29]. However, the application of these previously developed CLSMs to underground facility backfilling has been challenging due to three primary reasons: (a) previous CLSMs have relatively long durations for hardening; (b) poorer flowability than grout mortar; (c) large equipment is needed for the placement of CLSMs. In addition, these CLSMs only satisfied the acceptable level of flowability recommended by the ACI 229R-13 (i.e., more than 200 mm spread, as listed in Table 1) [5; 6; 11; 23; 30; 31; 32; 33]. Even though the performance of predeveloped CLSMs satisfied the requirements set out by the ACI 229R-13, especially flowability, those CLSMs could lead to problems of insufficient placement around underground facilities (i.e., pipes and culverts) and cause inconvenience to citizens in metropolitan cities due to the relatively long hardening time [5; 6; 23; 34]. Therefore, there is a need to develop a new CLSM with much higher performance (i.e., high flowability, low bleeding/segregation, and quick setting time) than predeveloped CLSMs. In this study, a new CLSM with high flowability, low segregation, and quick setting time is developed.