Lightweight materials that have high thermal insulation were increasingly applied to the modern construction and building industry. Lightweight aggregate concrete (LAC) is concrete that contains lightweight aggregate (LA) and has a density < 1900 kg/m3, significantly less than the density of ordinary concrete. Also, LAC has lower thermal conductivity than ordinary concrete because of its greater porosity. Much recent research has focused on reducing the density of LAC and increasing its thermal insulation properties. Techniques include increasing the LA content and using ultralight porous LA. Ke and Gao found that a major obstacle to casting concrete with ultralight LA was that the aggregate floated. If the aggregate density is too great or too small, the aggregate will sink or float. To avoid aggregate sinking or floating, the apparent density of LA should match the dry density of the cementitious matrix.
Foam concrete is another lightweight concrete, in which foam is produced in the cement paste by some suitable foaming agent[,]. Foam concrete has low apparent density and low thermal conductivity; however, it has drawbacks, such as low strength and high drying shrinkage, and it readily absorbs water[,].
Porous lightweight aggregate concrete (PLAC) can be prepared using LA and introducing large numbers of micropores into the concrete. It shrinks less and has less probability of cracking than foam concrete and is less dense and possesses greater heat insulation than LA concrete. Properties of PLAC include good apparent density, high strength, good thermal insulation[,,,] and good sound absorption[13,]. PLAC aggregate can be sintered ceramsite or other non-sintered LA, crushed waste clay bricks, or light aerated concrete waste. Micropores can be introduced using a foaming agent[12,14,15] or an air-entraining agent.
Commercial LAs are sintered at about 1200 ºC, but sintering consumes a lot of clay and energy. Production of non-sintered LA (NLA), which is cured at room temperature or below 100 ºC, reduces clay consumption, saves energy, and reduces emissions. Norlia et al. created a lightweight aggregate with a loose bulk density of 813 kg/m³ and compressive strength of 7.83 MPa using ordinary Portland cement and a foaming agent. Peng et al. created a non-sintered lightweight aggregate from dredging sediment with a bulk density of 850 kg/m³ and compressive strength of 5 MPa. They added a pore-forming agent to the NLA to improve sound absorption and decrease density, but the bulk density remained at about 850 kg/m3. Various measures have been taken to reduce the density of NLA, but the bulk density of NLA is generally > 800 kg/m³. Frankovic et al. produced LAC using 100% NLA and found that it had dry density 1490 kg/m3 and thermal conductivity 0.73 W/m/K, values that are 35% and 46% less than those of normal concrete. The higher density and higher thermal conductivity of NLA, compared to SLA, limit its use in lightweight concrete, so there is correspondingly little research interest in NLA. It is necessary to decrease the bulk density of NLA and improve its thermal insulation properties to make its use viable.
A core-shell non-sintered lightweight aggregate (CNLA) with relatively low bulk density 500–750 kg/m3 has recently been developed. Our thorough literature review showed only one study of low-density CNLA, by Feras. Feras produced CNLA with bulk densities 510–650 kg/m3 and compressive strengths 1.0–2.5 MPa. The CNLA consisted of cores of expanded perlite particles surrounded by outer shells consisting of perlite powders with fly ash and cement. When cement and silica fume were subsequently used to recoat the surface layer, the compressive strength of the CNLA with bulk density 608 kg/m3 increased to 3.55 MPa.
We used a novel CNLA consisting of an expanded polystyrene (EPS) sphere with a diameter of 3–4 mm as the core, surrounded by an outer shell consisting of fly ash and cement, as shown in Fig. 1. The EPS spheres greatly improve the thermal insulation properties of LA. Its bulk density, compressive strength, and other aggregate properties can be set to specified levels by varying core size and adjusting shell performance. The bulk compressive strength of the aggregate was in the range 2.5–7.0 MPa when the density was in the range 580–720 kg/m3. The loose bulk density of NLA can be reduced to approximately 500 kg/m³ if the inner core diameter of the EPS spheres is increased. The properties of CNLA, at the same density or strength, greatly exceed those of the NLA produced by Feras, who used expanded perlite particles as the inner core.
Two sets of samples of porous lightweight aggregate concrete (PLAC) were prepared using core-shell CNLA or SLA. The characteristics and properties of the two sets of samples were analyzed and compared.