RSM-optimized serpentine concrete shielding properties: Raw data

The aggregate processing procedure is illustrated in Figure 2.The cementitious materials included P·O 42.5 ordinary Portland cement (OPC),Grade S95 GBFS,High-reactivity silica fume (SF),Calcium-based bentonite (Ca-Bent),Calcined washed kaolin (CWK),and Grade I fly ash (FA). Their physical properties and chemical compositions are listed in Table 2.A polycarboxylate-based high-range water reducer was incorporated at a dosage of 2% by mass of the total cementitious materials.

Tap water was used for all mixing operations.Experimental Design(1) Preparation of Cementitious Paste An excessively high coating W/B ratio results in a slurry that is overly fluid, leading to poor coating adherence and inadequate strengthening. Conversely, an excessively low coating W/B ratio produces a paste that is overly viscous, resulting in uneven coating and thick layers, which also impair the strengthening effect[51].

Based on preliminary experiments, an initial coating W/B W/B of 0.5 was adopted, with the binder consisting of 80% OPC and 20% mineral admixture by mass. (2) Preparation of Slurry-Coated Serpentine Coarse Aggregates  To investigate the effect of different mineral admixtures on aggregate and concrete performance,five types of slurry-coated serpentine aggregates were prepared:Cement–fly ash slurry-coated serpentine(UF),Cement–GBFS slurry-coated serpentine (UB),Cement–silica fume slurry-coated serpentine(US),Cement–Ca-Bent slurry-coated serpentine(UP),Cement–CWK slurry-coated serpentine(UK).The coating process followed the procedure shown in Figure 2. Specifically, cement and admixture were dry-mixed in a mortar mixer for 30 seconds, followed by the addition of water and mixing for 60 seconds to form the slurry.

Serpentine coarse aggregates of various sizes were immersed in the prepared slurry, placed on a vibrating table for 90 seconds, and then left to stand for 30 seconds. This cycle was repeated twice. The coated aggregates were placed on a square-mesh sieve, turned every 10 minutes, and air-dried to prevent clumping.

After slurry coating,the aggregates were cured naturally for 1 day and then transferred to a standard curing chamber (20 ± 2 °C, ≥95% RH) for an additional 6 days to produce the modified serpentine coarse aggregates. Cubic concrete specimens (100 mm × 100 mm × 100 mm) were prepared in accordance with the Chinese standard GB/T 50081-2019, with six specimens cast for each group. The compressive strength was then tested.

The concrete mix proportions are shown in Table 3. (3) Performance Comparison Between Slurry-Coated and Uncoated Serpentine Aggregates and Their Influence on Concrete  The moisture content, water absorption, and crushing value of untreated and slurry-coated serpentine aggregates were compared to evaluate the enhancement effect of the slurry coating treatment. Additionally, the deterioration behavior of concrete prepared with both slurry-coated and uncoated serpentine coarse aggregates was investigated in terms of its compressive strength and crystalline water changes under high temperatures, and the correlation between the enhancement of aggregate properties and the improvement of concrete performance was analyzed.

(4) Response Surface Methodology Design  After identifying the most effective admixture through preliminary screening, a Box–Behnken Design was conducted using Design-Expert software. A multivariate nonlinear regression model was developed with three independent variables—concrete W/B ratio, coating W/B ratio, and admixture dosage—and two response variables: compressive strength and crystalline water content.

The model was used to evaluate the influence of slurry coating parameters on the mechanical and shielding performance of serpentine concrete.  Test

Method

According to NB/T 20378-2016, the benchmark mix design for serpentine concrete was determined after several rounds of preliminary adjustments. The water reducer dosage was fixed at 2% by mass of the binder, and the aggregate gradation curve is presented in Figure 3. Specimen preparation and testing were conducted following the Chinese standard GB/T 50081-2019.

The 28-day compressive strength was measured, and the maximum compressive load was recorded during the test. The average value of three specimens was taken as the final result for each group. To determine the crystalline water content, samples were weighed and placed in an oven at 105 ± 5 °C until a constant mass was achieved.

After natural cooling to room temperature, samples were removed and weighed again. Subsequently, the dried samples were heated in a muffle furnace to 500 °C and 800 °C. After cooling to room temperature, they were weighed to obtain the respective masses. The crystalline water content was calculated using Equation (1), and the average of three specimens was used as the test result.

Where: —Constant weight of the specimen after heating to 800 °C —Constant weight after heating to 500 °C —Constant weight after drying at 105 °C The γ-ray shielding performance of the slurry-coated serpentine concrete was evaluated using a BH1326 nuclear physics testing platform to conduct attenuation tests. The radiation source used was a 137Cs radioactive isotope, emitting monoenergetic photons with an energy of 0.662 MeV, representative of medium-energy γ-rays, allowing realistic simulation of shielding behavior in nuclear radiation protection scenarios.Each concrete specimen was divided into 9 uniformly distributed test points, with 60 seconds of γ-ray exposure at each point.

A NaI scintillation detector, in combination with a multi-channel pulse amplitude analyzer, was used for pulse counting. The initial intensity (I₀) and the transmitted intensities (IF and IB) were recorded and averaged for each specimen.The attenuation behavior followed the Beer–Lambert law. A linear fit was performed between the logarithmic pulse counts and the thickness of the specimens.

The slope of this linear fit was defined as the linear attenuation coefficient (μ) of the concrete material [29, 30].To more intuitively quantify the shielding capacity, the Half-Value Layer (HVL) was introduced. HVL refers to the thickness of material required to reduce the γ-ray intensity to half of its original value. A smaller HVL value indicates a higher shielding efficiency.

[31].                              （2）                            （3）Where: μ — Linear attenuation coefficient x — Thickness of specimen (cm) I₀ — Background count IF — Initial γ-ray pulse count before attenuation IB — Pulse count after γ-ray passes through the concrete HVL — Half-value layer thickness (cm) In addition, the basic physical properties of the serpentine aggregates—crushing value, water absorption, apparent density, and moisture content—were tested in accordance with GB/T 14685-2022.