Optimizing Construction Spoil Reactivity for Cementitious Applications: Effects of Thermal Treatment and Alkaline Activation

Abstract

Construction spoil (CS), a prevalent type of construction and demolition waste, is characterized by high production volumes and substantial stockpiles. It contaminates water, soil, and air, and it can also trigger natural disasters such as landslides and debris flows. With the advent of alkali activation technology, utilizing CS as a precursor for alkali-activated materials (AAMs) or supplementary cementitious materials (SCMs) presents a novel approach for managing this waste. Currently, the low reactivity of CS remains a significant constraint to its high-value-added resource utilization in the field of construction materials. Researchers have attempted various methods to enhance its reactivity, including grinding, calcination, and the addition of fluxing agents. However, there is no consensus on the optimal calcination temperature and alkali concentration, which significantly limits the large-scale application of CS. This study investigates the effects of the calcination temperature and alkali concentration on the mechanical properties of CS–cement mortar specimens and the ion dissolution performance of CS in alkali solutions. Mortar strength tests and ICP ion dissolution tests are conducted to quantitatively assess the reactivity of CS. The results indicate that, compared to uncalcined CS, the ion dissolution performance of calcined CS is significantly enhanced. The dissolution amounts of active aluminum, silicon, and calcium are increased by up to 420.06%, 195.81%, and 256.00%, respectively. The optimal calcination temperature for CS is determined to be 750 °C, and the most suitable alkali concentration is found to be 6 M. Furthermore, since the Al O bond is weaker and more easily broken than the Si O bond, the dissolution amount and release rate of active aluminum components in calcined CS are substantially higher than those of active silicon components. This finding indicates significant limitations in using CS solely as a precursor, emphasizing that an adequate supply of silicon and calcium sources is essential when preparing CS-dominated AAMs.

1. Introduction

With the rapid development of China’s emerging markets and the acceleration of urbanization, buildings have mushroomed over the past half century, resulting in the correspondingly massive production of construction and demolition waste (CDW) annually, with a tremendous growth rate and consistently growing stock [1,2,3,4]. It is estimated that the annual production of CDW in China exceeds 2 billion tons, with a stock of up to 10 billion tons, which is eight times the production of household waste and accounts for approximately 40% of the total amount of urban solid waste [5]. Construction spoil (CS), a major component of CDW, is generated from various construction activities, including buildings, roads, subways, and water conservancy projects. CS is characterized by its abundant production and widespread distribution [6,7].

The current management practices for CS, such as temporary storage, external transportation, land reclamation, backfilling, and intensive storage, often lead to environmental issues, including soil and water pollution, air contamination, and soil erosion, as shown in Figure 1. In severe cases, these practices can trigger natural disasters and security incidents like mudslides and landslides [8,9]. The resource utilization rate of CS in China remains unsatisfactory [10], largely due to its insufficient reactivity. This factor significantly influences the microstructure and physical performance of hydration products [6,10,11,12]. To address this challenge, researchers have explored various methods. Mechanical activation has been shown to improve the particle size distribution, specific surface area, and structural properties of CDW [13,14]. Calcination techniques have demonstrated significant environmental and economic benefits in the process of enhancing CS reactivity [7,15]. Additionally, alkaline activation and alkali fusion methods have been proposed as effective approaches for improving CDW and CS reactivity [16,17]. Despite extensive research, a consensus on the optimal thermal treatment temperature and alkali concentration for enhancing CS reactivity remains elusive. Furthermore, quantitative analysis of CS reactivity is still under-represented in the literature. These knowledge gaps present opportunities for further research aimed at optimizing CS utilization and contributing to more sustainable construction practices.

This study investigates the effects of the thermal treatment temperature and alkali concentration on CS reactivity, as well as the underlying thermal treatment mechanism. The morphological and phase composition changes in CS were analyzed using the scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques. Comprehensive evaluations, ranging from macroscopic to microscopic perspectives, were conducted based on mortar strength tests and ion dissolution analyses. The strength activity index and ion dissolution behavior were quantitatively assessed, providing a comprehensive understanding of CS reactivity. The research culminated in proposing the optimal thermal treatment temperature and suitable alkali concentration for enhancing CS reactivity. Furthermore, recommendations for the supply of adequate silicon and calcium sources in the preparation of CS-based AAMs were proposed.

2. Materials and Methods

2.1. Materials

The construction spoil (CS) used in this study was taken from the construction site of the Shenzhen Longgang Metro Line 16 Phase II project, specifically from a depth ranging between 6 and 10 m. The CS was primarily composed of sand, gravel, and clay, characterized by a red–brown appearance and a net-like texture structure. The moisture content was weighted and calculated to be 17.35%. The CS is essentially a residual soil that emerges after the physical and chemical weathering of biotite granite. Through X-ray fluorescence (XRF) analysis, the chemical composition of in situ CS is obtained. As demonstrated in

Table 1. XRF analysis of the construction spoil (wt, %).

Materials	SiO2	Al2O3	CaO	Fe2O3	MgO	K2O	TiO2	Na2O	SO3	P2O5
In situ CS	51.11	39.53	0.02	6.81	0.46	0.66	1.05	0.05	0.13	0.04
Calcined CS	55.00	37.14	0.02	5.63	0.44	0.59	0.80	0.05	0.10	0.04

, the insitu CS samples were rich in SiO₂ and Al₂O₃, with relatively low F₂O₃ and other impurities.

Scanning electron microscopy (SEM), and X-ray diffraction (XRD) were successively used to examine the microstructure and phase composition of in situ CS powders. The results are presented in Figure 2. Small voids and a layered structure can be seen in the SEM images. Kaolinite, quartz and montmorillonite minerals can be found in the XRD patterns.

In the mortar strength test, 32.5 R Portland cement and river sand with particle diameters ranging from 0 to 4.75 mm were used as the fine aggregate. In the ion dissolution test, NaOH pellets with a purity of 99.8% were adopted in the NaOH solution preparation, with the deionized water being the solvent.

2.2. Methods

2.2.1. Thermal Treatment

Figure 3 illustrates the treatment protocol for enhancing the reactivity of CS. For in situ CS, the drying process involved heating in an electric oven at 105 °C for 24 h, followed by sieving through a 1.25 mm mesh, which was used to reduce the moisture content and remove chemically inert small pebbles. The collected samples were then transferred to a cylindrical steel ball milling tank, which was used to achieve a uniform particle size distribution. The PM20L planetary ball mill, operating with a 2.2 kW power output and a rotational speed of 600 r/min for 30 min, was employed in the milling process. Finally, the milled CS powders were thermal treated in a muffle furnace, with the temperature incrementally increased from ambient temperature (25 ± 1 °C) to the target temperatures (600 °C, 650 °C, 700 °C, 750 °C, 800 °C, 900 °C), at a heating rate of 10 °C/min. Once the target temperature was reached, it was maintained for 2 h, followed by cooling to ambient temperature. The thermally treated CS powders were then stored in a cool, dry environment to preserve their integrity and prevent any environmental contamination.

2.2.2. Ion Dissolution Test

In the dissolution test, 0.500 ± 0.001 g of modified CS powder was first dissolved in 20 mL of sodium hydroxide solution. The mixtures were then placed on a magnetic stirrer platform at a constant speed of 300 rpm at ambient temperature for either 24 h or 2 h. Thereafter, the supernatant obtained after precipitation was initially filtered using a vacuum filtration device, followed by further filtration through a 0.22 μm syringe filter. Finally, an inductively coupled plasma mass spectrometer (ICP-MS, Thermo Fisher iCAP RQ, USA) was employed to analyze the resultant solutions. Based on the dissolution performance of the Si, Al, and Ca elements in the modified CS in an alkaline medium, the reactivity of the modified CS can be quantitatively assessed.

A total of 20 samples were prepared in the ion dissolution test, as shown in

Table 2. Ion dissolution test plan for enhancing the reactivity of CS.

No.	Label	T (°C)	C (mol/L)	t (h)
1	${\mathrm{C}\mathrm{A}}_2^{24}$	—	2	24
2	${\mathrm{C}\mathrm{A}}_6^{24}$	—	6	24
3	${\mathrm{C}\mathrm{A}}_{10}^{24}$	—	10	24
4	${\mathrm{C}600\mathrm{A}}_2^{24}$	600	2	24
5	${\mathrm{C}600\mathrm{A}}_6^{24}$	600	6	24
6	${\mathrm{C}600\mathrm{A}}_{10}^{24}$	600	10	24
7	${\mathrm{C}700\mathrm{A}}_2^{24}$	700	2	24
8	${\mathrm{C}700\mathrm{A}}_6^{24}$	700	6	24
9	${\mathrm{C}700\mathrm{A}}_{10}^{24}$	700	10	24
10	${\mathrm{C}800\mathrm{A}}_2^{24}$	800	2	24
11	${\mathrm{C}800\mathrm{A}}_6^{24}$	800	6	24
12	${\mathrm{C}800\mathrm{A}}_{10}^{24}$	800	10	24
13	${\mathrm{C}900\mathrm{A}}_2^{24}$	900	2	24
14	${\mathrm{C}900\mathrm{A}}_6^{24}$	900	6	24
15	${\mathrm{C}900\mathrm{A}}_{10}^{24}$	900	10	24
16	${\mathrm{C}650\mathrm{A}}_6^2$	650	6	2
17	${\mathrm{C}700\mathrm{A}}_6^2$	700	6	2
18	${\mathrm{C}750\mathrm{A}}_6^2$	750	6	2
19	${\mathrm{C}800\mathrm{A}}_6^2$	800	6	2
20	${\mathrm{C}\mathrm{G}750\mathrm{A}}_6^2$	750	6	24

. The main parameters considered were the thermal treatment temperature, alkaline concentration, and immersion time. The labels, such as ${“\mathrm{C}700\mathrm{A}}_6^{24}$” are decoded as follows: “C” denotes construction spoil, “700” indicates a thermal treatment temperature of 700 °C, and ${“\mathrm{A}}_6^{24}$”refers to immersion and stirring in 6 M NaOH solutions for 24 h. ${“\mathrm{C}\mathrm{G}750\mathrm{A}}_6^2$” indicates a mixture of construction spoil (CS) and ground granulated blast furnace slag (GGBS) in a mass ratio of 1:1, followed by thermal treatment at 750 °C, and immersion and stirring in 6 M NaOH solution for 2 h. The codes ${“\mathrm{C}\mathrm{A}}_2^{24}$”, ${“\mathrm{C}\mathrm{A}}_6^{24}$”, and ${“\mathrm{C}\mathrm{A}}_{10}^{24}$” were chosen as control groups. Additional entries are decoded in a similar manner.

2.2.3. Mortar Strength Test

Additionally, the mortar strength test [18] was conducted to determine the optimal thermal treatment temperature, from the perspective of the macro mechanical properties. Based on preliminarily tests, CS powders with 700 °C, 750 °C, and 800 °C thermal treatment temperatures were selected as a partial substitute for cement in the mortar’s preparation, labeled as PM, C700PM, C750PM, and C800PM. As illustrated in

Table 3. Experimental scheme for the strength activity index tests (unit, g).

No.	Designations	32.5R OPC	Modified CS	Sand	Water
1	PM	450	-	1350	225
2	C700PM	315	135	1350	225
3	C750PM	315	135	1350	225
4	C800PM	315	135	1350	225

, designations such as “C700PM” are decoded as follows: “C” signals CS, “700” specifies the 700 °C thermal treatment temperature, “P” denotes Portland cement, and “M” represents mortar. The mass mixing ratio of the thermally treated CS to OPC was 3:7, the specific value of sand/cement ratio was 1:3, and the water to cement ratio was 0.5. Under the standard curing conditions, specimens were tested for flexural strength and compressive strength at predetermined intervals [19].

The strength activity index was adopted for evaluating the thermally treated CS reactivity, which is shown as follows,

$$S\mathrm{A}I={\displaystyle \frac{R}{R_0}}\times 100,$$

(1)

in which SAI represents the strength activity index (%), R indicates the compressive strength of the test specimen at specific maintenance age (MPa), and R₀ denotes the compressive strength of the control specimen at the same age (MPa).

2.2.4. Experimental Procedure

The particle size distribution of the modified CS samples was obtained through the Laser Granulometer (HELOS-OASIS), sourced from Sympatec Gmbh in Germany. The chemical composition of in situ CS and thermally treated CS was checked by X-ray fluorescence (XRF), sourced from Thermo Fisher Scientific in USA. The D8 Advance X-ray diffractometer (XRD), sourced from Brooke in Germany, was used to analyze the crystalline phase. Scanning electron microscopy (SEM), sourced from Zeiss in Germany, was adopted to obtain the microstructure of the modified CS. The inductively coupled plasma mass spectrometer (ICP-MS, Thermo Fisher iCAP RQ, in USA) was employed in the ion dissolution test.

The flexural strength and compressive strength of CS-based mortars at 3-day and 28-day curing ages were successively tested using the CRIMS load testing machine. The loading rates were 6.0 mm/min for the flexural strength test, and 50 N/s for the compressive strength test. A total of 3 prism specimens (40 × 40 × 160 mm³) of each composition were tested for the flexural strength determination, while the left 6 semi-prism specimens of each composition were tested for the compressive strength determination, the average values of which were determined as the final strengths.

3. Results and Discussion

3.1. CS Characterization

The particle size distribution (PSD), microstructure, and phase composition of the thermally treated CS were successively characterized, and they are presented in Figure 4, Figure 5, Figure 6 and Figure 7.

Figure 4 and Figure 5 reveal that after the thermal treatment, CS exhibits a brownish–red appearance, with the PSD being concentrated within the range of 1–20 μm. The specific values are D10 = 0.91 μm, D50 = 3.89 μm, and D90 = 13.50 μm. From Figure 6, it can be seen that a disruption of the layered aluminosilicate structure was formed, and a quantity of microcracks were found in the thermally treated CS. From Figure 7, it can be seen that after the 600 °C thermal treatment, the kaolinite diffraction peak diminishes significantly, indicating the depletion of hydroxyl groups and a transition to an amorphous metakaolinite structure. The minimal change in the XRD pattern of the 800 °C thermally treated CS proves the stability of the metakaolin phase, which has been confirmed by Scrivener [4]. The XRD pattern of the 900 °C thermally treated CS indicates the emergence of diffraction peaks associated with the spinel phase, suggesting that recrystallization occurs under the influence of excessive temperatures.

3.2. Strength Activity Index

The strength activity index (SAI) is of fundamental importance when assessing the reactivity of cementitious material precursors, and it has been widely adopted worldwide. In the Chinese standard GB/T 1596–2017 [18], the SAI is determined based on the precursor’s replacement rate of Portland cement. The flexural strength and compressive strength development of the tested mortar specimens at given curing days (7 d or 28 d) are plotted in Figure 8, respectively. The strength activity index (SAI) of the pre-thermally treated CS (700 °C, 750 °C, and 800 °C) at a curing age of 7 days and 28 days is clearly depicted in Figure 9. The XRD pattern of the CS-OPC hydration products at a curing age of 28 days is plotted in Figure 10.

Figure 8 shows that the mortars incorporating thermally treated CS exhibit a reduction in flexural strength compared to the control groups, irrespective of the curing ages. However, a slight increase in the 7-day compressive strength is observed, whereas the 28-day compressive strength shows a slight decline. These results indicate the feasibility of using thermally treated CS as a supplementary cementitious material [12]. Nevertheless, the material demonstrates limited long-term mechanical performance. This limitation is likely due to the external efflorescence effect, as corroborated by Dassekpo [17].

Figure 9 presents the SAI of the pre-thermally treated CS at both a 7-day and 28-day curing age. The results show that the SAIs of C700 and C750 at a 7-day curing age are greater than 1, indicating strong early strength performance, which aligns with the rapid-setting and high early strength characteristics of geopolymer materials. However, the SAIs at 28 days are lower than those at 7 days, suggesting that the long-term strength development is slower than the early-age strength gain. The calculated SAIs of the pre-thermally treated CS (C700, C750, and C800) at 28 days are 0.88, 0.93, and 0.84, respectively, indicating that the optimal temperature for enhancing CS reactivity is 750 °C. This notably entails lower energy consumption and carbon emissions compared to the optimal thermally treated temperature ranging from 800 °C to 900 °C, as proposed in reference [7,20]. The XRD pattern in Figure 10 confirms the presence of quartz and a significant amount of calcite. The formation of calcite effectively fills the internal voids in the hydration products, contributing to a denser material structure.

3.3. Dissolution Behavior in NaOH Solution

Preliminary tests revealed that the dominant active species of CS in the sodium hydroxide solutions were ²⁷Si, ²⁹Al, and ⁴⁴Ca, while other ions such as Fe, Mg, K, S, and P were present in smaller amounts. Therefore, the dissolution concentrations of these three ions were selected as the indicators for evaluating the dissolution behavior of the pre-thermally treated CS in sodium hydroxide solutions. The ion dissolution results are plotted in Figure 11, where subfigures Figure 11a–c illustrate the concentration distribution of ²⁷Al, ²⁹Si, and ⁴⁴Ca in modified CS following immersion in NaOH solutions of varying concentrations for 24 h. Additionally, subfigure Figure 11d shows the concentration of the three ions in modified CS after immersion in a 6 M NaOH solution for 2 h. The parameters considered include the CS thermal treatment temperatures (600 °C, 650 °C, 700 °C, 750 °C, 800 °C, 900 °C), alkali solution concentrations (2 M, 6 M, 10 M), and magnetically stirring durations (2 h and 24 h).

3.3.1. Effect of Thermal Treatment Temperature

According to Figure 11, the dissolution of ²⁷Si, ²⁹Al, and ⁴⁴Ca in the pre-thermally treated CS significantly improves compared to the insitu CS, regardless of the thermal treatment temperature or alkali solution concentration. The highest dissolution rates for ²⁹Al, ²⁷Si, and ⁴⁴Ca reach 420.06%, 195.81%, and 256.00%, respectively, demonstrating the high efficiency of thermal treatment in enhancing CS reactivity. Additionally, from 600 °C to 900 °C, the dissolution rates show an initial increase followed by a decrease, indicating the existence of an optimal thermal treatment temperature for CS reactivity. The dissolution results for the pre-thermally treated CS (650 °C, 700 °C, 750 °C, and 800 °C) in the 6 M NaOH solution reveal that 750 °C is the optimal temperature. At this temperature, the dissolution contents of active species—aluminum, silicon, and calcium—reach their maximum values. Lower thermal treatment temperatures fail to break the chemical bonds in the insitu CS, while excessively high temperatures may lead to recrystallization, both of which negatively impact CS reactivity. These findings are consistent with Xiao’s results [6].

3.3.2. Effect of Alkali Solution Concentration

As depicted in Figure 11, with a constant thermal treatment temperature, the dissolution of ions within CS initially increases and then decreases as the alkali solution concentration rises from 2 M to 10 M. The optimal dissolution is achieved at an alkali concentration of 6 M. This can be explained by the fact that, at moderate concentrations, the release of OH⁻ ions effectively breaks the chemical bonds within CS. However, at excessively high alkali concentrations, a rapid reaction occurs between the CS and the alkali solution, resulting in the formation of a product layer on the CS surface, which inhibits the dissolution of active species.

In summary, the optimal conditions for enhancing CS reactivity are a thermal treatment temperature of 750 °C and an alkali concentration of 6 M. Additionally, a calcium-enriched thermal treatment of CS, combined with ground granulated blast furnace slag (GGBS) in a 1:1 mass ratio, significantly enhances CS reactivity, demonstrating the feasibility and high efficiency of using GGBS in modifying CS reactivity.

3.3.3. Effect of Dissolution Time

The concentrations of ²⁷Al, ²⁹Si, and ⁴⁴Ca in the thermally treated CS (600 °C, 700 °C, 800 °C) in a 6 M NaOH solution, with dissolution times of 2 h and 24 h, are presented in Figure 12. Throughout the dissolution period, the release amounts and rates of ²⁷Al were significantly higher than those of ²⁹Si and ⁴⁴Ca, indicating an abundance of active aluminum components in the thermally treated CS. Specifically, the total dissolution amounts of ²⁷Al at an early stage were approximately seven to eight times higher than those of ²⁹Si, suggesting that the Al O bond is much weaker than the Si O bond within the glassy structure of CS.

The preferential dissolution of ²⁷Al over ²⁹Si can be attributed to the larger charge-to-radius ratio of Al³⁺ (0.39 Å) compared to Si⁴⁺ (0.26 Å). Consequently, the Al O chemical bond is weaker and more easily broken than the Si O bond, leading to the preferential release of ²⁷Al [21]. Additionally, from 2 h to 24 h of dissolution, the release rates of ²⁷Al and ²⁹Si exhibit non-steady characteristics, with an initially high release rate that gradually stabilizes over time.

For ⁴⁴Ca, the release amounts exhibit relatively low values, indicating a deficiency of active calcium in CS, even with the application of thermal treatment. As a result, when preparing CS-dominated cementitious materials, the formation of calcium silicate hydrate (CSH) becomes challenging, leading to poor strength development.

3.4. Thermal Treatment Mechanism

As observed in Section 3.1, the insitu CS is rich in phases such as kaolinite and montmorillonite, which exhibit low reactivity. Under thermal treatment, the mineral phases within CS undergo varying degrees of transformation. Specifically, as illustrated in Figure 13, in the initial stage, the layered aluminosilicate structure of kaolinite absorbs heat and internal water, leading to water loss. As the temperature rises to 560–580 °C, the aluminum oxide octahedron, which is the fundamental structural unit of kaolinite, undergoes dehydroxylation. Simultaneously, structural water is expelled, resulting in the formation of metakaolinite, a highly reactive phase. The corresponding chemical reaction is shown below [21].

$$\begin{array}{lll}{\mathrm{Al}}_2{(\mathrm{Si}}_2{\mathrm{O}}_5{\left)\right(\mathrm{OH})}_4& \stackrel{560~580℃}{\to }& {\mathrm{Al}}_2{\mathrm{O}}_3·{2\mathrm{SiO}}_2+{2\mathrm{H}}_2\mathrm{O}\\ \hfill \mathrm{Kaolinite}\hfill & & \mathrm{Metakaolinite}\end{array}$$

As the temperature increases, recrystallization within the CS can be observed, as confirmed by previous research [6,21]. This phenomenon explains the decreasing trend in CS reactivity due to excessive thermal treatment. Furthermore, the thermal process increases the kinetic energy of micro-particles within the CS, causing varying degrees of destruction to the vitreous structure. This leads to the breakage of chemical bonds, creating numerous free-end fracture points and releasing a significant amount of active silica-alumina components. Consequently, the thermally treated CS exhibits enhanced reactivity, characterized by a strong pozzolanic effect and a robust micro-aggregate effect.

4. Conclusions

This paper primarily explores the effects of thermal treatment and alkali activation on enhancing the reactivity of CS. The research integrates detailed analyses of the particle size distribution, microstructure, and phase composition, providing a comprehensive understanding of how thermal treatment and alkali activation modify CS’s properties. By utilizing strength activity index tests and monitoring the dissolution of active species in NaOH solutions, this study quantitatively evaluated the CS reactivity. The optimal thermal treatment temperature and optimal alkali solution concentration for maximizing CS reactivity are proposed, suggesting potential applications in cementitious materials. This work contributes to improving the efficiency of construction materials, promoting sustainability by reducing energy consumption and enhancing material properties. Conclusions can be drawn as follows:

(1) Thermal treatment significantly enhances CS reactivity, and the maximum reactivity of CS is achieved at 750 °C calcination for 2 h. Lower temperatures are insufficient to convert the kaolinite phase within CS into metakaolinite, while excessively high temperatures may induce crystallization, which inhibits the release of active silica-alumina components.

(2) The ion dissolution behavior of ²⁷Al, ²⁹Si, and ⁴⁴Ca within thermally treated CS shows optimal performance when immersed in a 6 M NaOH solution. A relatively low alkali concentration is insufficient to break the Al O and Si O bonds, while an excessively high alkali concentration may lead to the rapid formation of surface products on the raw materials, inhibiting the subsequent dissolution of active components. The total dissolution amounts of ²⁷Al are approximately seven to eight times higher than those of ²⁹Si. The dissolution of ⁴⁴Ca, on the other hand, is almost negligible. This is attributed to the smaller charge-to-radium of Si⁴⁺ (0.26 Å) compared to Al³⁺ (0.39 Å), and the deficiency of calcium in CS. Additionally, the dissolution behavior of ²⁷Al and ²⁹Si exhibits a non-steady dissolution characteristic, with rapid dissolution occurring at the early stages, followed by gradual stabilization.

(3) The imbalanced dissolution of active alumina, silica and calcium components in thermally treated CS explains the poor performance in terms of the strength and durability development of CS-dominated hardened systems, as insufficient CHS or NASH gels are formed. This finding highlights the limitations of using CS solely as a precursor for preparing AAMs or GPs. To improve the performance, additional sources of silica and calcium are necessary.